Structural studies of alkali metal adsorption and coadsorption on metal surfaces

Structural studies of alkali metal adsorption and coadsorption on metal surfaces

Renee D. Diehl Department of Physics, Pennsylvania State University, University Park, PA 16802, USA

and R6nfin McGrath Interdisciplinary Research Centre in Surface Science and Department of Physics, Liverpool University, P.O. Box 147, Liverpool L69 3BX, UK

N

ELSEVIER Amsterdam-Lausanne-New York-Oxford-Shannon-Tokyo

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Contents 1. Introduction 2. Review 2.1. Aluminum 2.1.1. Na/AI(lll) 2.1.2. K/AI(lll) 2.1.3. Rb/AI(lll) 2.1.4. Cs/Al(lll) 2.1.5. Na/AI(100) 2.1.6. K/AI(100) 2.1.7. Alkali metal mixtures on AI(lll) 2.1.8. Coadsorption of Na and O on AI(lll) 2.2. Beryllium 2.2.1. Li/Be(0001) 2.3. Cobalt 2.3.1. K/Co(1010) 2.3.2. Coadsorption of K and CO on Co(1010) 2.3.3. Coadsorption of K and CO on Co(0001) 2.4. Copper 2.4.1. Li/Cu(lll) 2.4.2. Na/Cu(lll) 2.4.3. K/Cu(lll) 2.4.4. Rb/Cu(111) 2.4.5. Cs/Cu(lll) 2.4.6. Li/Cu(100) 2.4.7. Na/Cu(100) 2.4.8. K/Cu(100) 2.4.9. Cs/Cu(100) 2.4.10. Cs or K / C u ( l l 0 ) 2.4.11. K / C u ( l l 5 ) 2.4.12. Cs/Cu(211) or/Cu(511) 2.4.13. Coadsorption of Cs and O on Cu(111) 2.4.14. Coadsorption of K or Cs and O on Cu(100) 2.4.15. Coadsorption of Na and O on Cu(ll0) 2.4.16. Coadsorption of K or Cs and O on Cu(ll0) 2.5. Gold 2.5.1. Na/Au(lll) 2.5.2. K/Au(lll) 2.5.3. K/Au(100) 2.5.4. K/Au(ll0) 2.5.5. Coadsorption of K or Na and CH3CN on Au(100) 2.6. Graphite 2.6.1. K/graphite 2.6.2. Rb/graphite 2.6.3. Cs/graphite

49 50 51 51 56 58 60 61 62 62 63 63 63 63 63 64 64 65 66 66 67 68 68 69 72 72 73 74 75 76 77 77 77 77 78 78 79 79 80 81 81 83 84 84

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R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

2.7.

2.8.

2.9.

2.10.

2.11.

2.12.

2.13.

Iridium 2.7.1. K/Ir(100) 2.7.2. Cs/Ir(100) Iron 2.8.1. K/Fe(lll) 2.8.2. K/Fe(100) 2.8.3. K/Fe(ll0) 2.8.4. Coadsorption of K and O on Fe(ll0) Lead 2.9.1. K/Pb(ll0) Molybdenum 2.10.1. L i / M o ( l l 0 ) 2.10.2. C s / M o ( l l 0 ) 2.10.3. Na/Mo(100) 2.10.4. K/Mo(100) 2.10.5. Cs/Mo(100) 2.10.6. L i / M o ( l l 2 ) 2.10.7. Na, K or C s / M o ( l l 2 ) 2.10.8. Coadsorption of Na or Cs and O on Mo(100) Nickel 2.11.1. Na/Ni(111) 2.11.2. K / N i ( l l l ) 2.11.3. C s / N i ( l l l ) 2.11.4. Na/Ni(100) 2.11.5. K/Ni(100) 2.11.6. Cs/Ni(100) 2.11.7. N a / N i ( l l 0 ) 2.11.8. K / N i ( l l 0 ) 2.11.9. Coadsorption of K and CO on Ni(lll) 2.11.10. Coadsorption of Na and S on Ni(100) 2.11.11. Coadsorption of K and O on Ni(100) 2.11.12. Coadsorption of K and CO on Ni(100) 2.11.13. Coadsorption of Cs and O on Ni(100) 2.11.14. Coadsorption of K and CO on Ni(ll0) Palladium 2.12.1. K / P d ( l l l ) 2.12.2. K/Pd(100) 2.12.3. Na or Cs on Pd(ll0) 2.12.4. Coadsorption of CO and K on Pd(100) 2.12.5. Coadsorption of Cs and H e on Pd(ll0) Platinum 2.13.1. Na/Pt(111) 2.13.2. K / P t ( l l l ) 2.13.3. C s / P t ( l l l ) 2.13.4. Cs/Pt(100) 2.13.5. Coadsorption of Na and CO on P t ( l l l ) 2.13.6. Coadsorption of K and O on P t ( l l l )

85 85 86 87 87 87 87 87 87 87 89 89 89 90 90 91 91 91 92 93 93 93 97 97 97 102 102 102 102 103 103 105 105 106 107 107 107 107 107 108 108 108 109 110 111 111 111

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2.14.

2.15.

2.16.

2.17.

2.13.7. Coadsorption of K and CO on P t ( l l l ) 2.13.8. Coadsorption of K and O on Pt(755) 2.13.9. Coadsorption of Cs and O on P t ( l l l ) 2.13.10. Coadsorption of Cs and CO on Pt(lll) Rhodium 2.14.1. N a / R h ( l l l ) 2.14.2. K / R h ( l l l ) 2.14.3. R b / R h ( l l 1) 2.14.4. C s / R h ( l l l ) 2.14.5. Cs/Rh(100) Ruthenium 2.15.1. Li/Ru(0001) 2.15.2. Na/Ru(0001) 2.15.3. K/Ru(0001) 2.15.4. Rb/Ru(0001) 2.15.5. Cs/Ru(0001) 2.15.6. K + Cs mixtures on Ru(0001) 2.15.7. K/Ru(1010) 2.15.8. Coadsorption of Li and H 2 0 on the Ru(0001) surface 2.15.9. Coadsorption of Na and H 2 0 on Ru(0001) 2.15.10. Coadsorption of Na and CO on Ru(0001) 2.15.11. Coadsorption of K and O on Ru(0001) 2.15.12. Coadsorption of K and CO on Ru(0001) 2.15.13. Coadsorption of Cs and O on Ru(0001) 2.15.14. Coadsorption of Cs and CO on Ru(0001) 2.15.15. Coadsorption of K and CO on Ru(1010) Silver 2.16.1. K, Rb or Cs on Ag(lll) 2.16.2. K/Ag(100) 2.16.3. Na/Ag(ll0) 2.16.4. K/Ag(ll0) 2.16.5. Cs/Ag(ll0) 2.16.6. Coadsorption of K and O on Ag(lll) 2.16.7. Coadsorption of Rb and O on Ag(lll) 2.16.8. Coadsorption of Cs and 0 2 + C2H 2 on Ag(lll) 2.16.9. Coadsorption of Na and 0 2 on Ag(ll0) Tungsten 2.17.1. Cs/W(100) 2.17.2. L i / W ( l l 0 ) 2.17.3. N a / W ( l l 0 ) 2.17.4. C s / W ( l l 0 ) 2.17.5. Na/W(112) 2.17.6. C s / W ( l l 2 ) 2.17.7. Coadsorption of Cs and H 2 on W(100) 2.17.8. Coadsorption of Cs and O on W(100) 2.17.9. Coadsorption of Na and O on W(ll0) 2.17.10. Coadsorption of Cs and O on W(110) 2.17.11. Coadsorption of Na and O on W(112) 2.17.12. Coadsorption of Cs and O on W(112)

111 112 112 113 113 113 113 114 114 114 115 115 117 119 120 120 121 121 122 123 123 123 124 125 127 128 128 128 133 133 134 134 134 134 134 135 135 135 135 136 136 136 136 136 137 137 137 137 137

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3. General trends 3.1. Phase diagrams - general features 3.2. Sites in alkali adsorption 3.3. Compression within the adsorbed alkali layer with respect to the corresponding bulk metals 3.4. "Charge transfer" and measurements of chemisorption bondlength 3.5. Island formation 3.6. Rotational epitaxy 3.7. Dynamics of alkali metal overlayers 3.8. Alkali metal-induced missing-row reconstruction of fcc (110) d-band metal surfaces 3.9. Coadsorption structures 3.10. Note on structural techniques applied to alkali systems 4. Summary and outlook

138 138 141

Acknowledgements Appendix A: Some general tables References

161 162 164

146 146 149 150 152 155 157 159 160

.......~:~:¢~ii::::ii::::i~::::~i)~i~:~iiiiiji::~:.::.ii::!ii-~!:~ ...........

surface science

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ELSEVIER

Surface Science Reports 23 (1996) 43-171

Structural studies of alkali metal adsorption and coadsorption on metal surfaces Renee D. Diehl a R6n~n McGrath b a Department of Physics, Pennsylvania State University, University Park, PA 16802, USA b Interdisciplinary Research Centre in Surface Science and Department of Physics, Liverpool University, P.O. Box 147, Liverpool L69 3BX, UK

Manuscript received in final form 15 November 1995

Abstract The study of the adsorption and the coadsorption of alkali metals on single-crystal metal surfaces is a very active sub-field of surface science, partly because of the importance of technological applications (promotion of catalytic reactions, enhanced oxidation, increases in electron emission rates), but also because of the fundamental interest in how these "simple" adsorbates modify the surface. In the past few years there has been a marked increase in the number of structural studies of alkali adsorption and coadsorption systems, motivated in part by developments in theoretical models of adsorption. This article reviews recent studies of alkali adsorption and coadsorption using specifically structural techniques, with the intention of highlighting recent developments, providing a useful reference base to the community, and drawing attention to some unifying concepts.

1. Introduction The study of alkali metal adsorption and coadsorption on metal surfaces has been of interest in surface science for nearly 70 years. This is partly due to the importance of technological applications (promotion of catalytic reactions, enhanced oxidation, increases in electron emission rates), but also due to the fundamental interest in how these "simple" adsorbates interact with each other, with coadsorbates and with the surface itself. There have been many previous reviews and monographs addressing different aspects of the subject were written [1-21]. Two early reviews of the subject by Naumovets [3] and Aruga and Murata [6]. Several mini-reviews are included in the proceedings of a 1989 workshop at Bad Honnef, Germany [7] which provided an excellent overview of work in the field at that time. A

0167-5729/96/$32.00 © 1996 Elsevier Science B.V. All rights reserved

SSDI 0167-5729(95)00010-0

50

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

recent volume includes review articles on two-dimensional phase transitions in alkali-metal overlayers [14] and on alkali-induced reconstruction of fcc (110) surfaces [15]. In the 30-year anniversary volume of Surface Science, Naumovets surveyed contributions to the field from the former Soviet Union [16]. Five articles appeared in a recent "spotlight" volume of Surface Review and Letters on theory of alkali adsorption on close-packed surfaces [17], on photoemission investigations of adsorption on AI [18], on adsorption on Cu and Ni surfaces [19], on adsorption on Ru surfaces [20] and on aspects of photoelectron diffraction applied to alkali adsorbates [21]. Two recent reviews address the nature of monolayer condensation transitions [22,23]. With so many reviews on the subject it might be asked why there is need for another. There are three main reasons: (i) there is no comprehensive guide to the research community on the phases and structures formed by alkali adsorption and coadsorption; (ii) there have been several recent developments in structural determinations, including several "unusual" site assignments, the first determinations of coadsorbate structures and the first applications of scanning tunneling microscopy (STM) to alkali systems; (iii) over the past five years there has been a debate over how to describe the bonding of an alkali metal atom to a metallic substrate, which has led to a surge of interest in all aspects of alkali adsorption. Consequently in this article we have attempted to provide a comprehensive guide to all the structural work in this area of which we are aware. In Section 2, the literature is organized by substrate and then further sub-divided into specific adsorbate and coadsorbate systems. The review of each system begins with a discussion of available information on the coverage-temperature phase diagram. The structure of individual phases, plus other topics of specific interest such as transitions between phases and rotational epitaxy are discussed in italicized sub-sections. In the final discussion sections (3.1-3.10), we attempt to highlight recent trends and to draw attention to unifying concepts in both adsorption and coadsorption. A number of points of organization should be noted: (i) Throughout the review we consider the alkali coverage 0alkali as the ratio of the number of alkali atoms to the number of substrate atoms in the first substrate layer, rather than considering the coverage in units of a saturation monolayer of alkali atoms. (ii) The substrate surfaces are described in the order ( l i D , (100), (110), and others; the alkali adsorbates are described in the order L i - N a - K - R b - C s . The coadsorption systems are described in a similar manner. (iii) A list of acronyms used in this review is to be found in Table A.1.

2. Review 2.1. Aluminum

There have been probably as many experimental studies and more theoretical studies of alkali metal adsorption on aluminum than on any other substrate. Much of this work was motivated by the desire to learn more about the fundamental properties of alkali metal adsorption, and of metal adsorption in general. Since aluminum is a simple sp metal, calculation of its behavior as a substrate for alkali metal adsorption is in principle easier than

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

5l

for other metals, although it is clear from what follows that aluminum as a substrate for alkali adsorption is not all that simple. There have been several excellent reviews of alkali metal adsorption on Al surfaces [18,24,25], and while most of the main experimental and theoretical results will be included in this section, the reader is referred to those reviews and references therein for further details. The main common feature among the adsorption systems of Na, K, Rb and Cs on A l ( l l l ) is that, except at very low coverages, they have very different structures at low temperatures (below about 200 K) and at room temperature. This is due to the fact that at room temperature, the adsorption process involves a temperature-activated vacancy formation on the A I ( l l l ) surface, and the adsorbate atoms take advantage of these vacancies to form ordered structures having high adsorbate-substrate coordination. These same structures also occur if the adatoms are adsorbed at low temperature and the substrate subsequently is heated to room temperature. This transition is irreversible, indicating that the low-temperature structures are metastable, i.e. the vacancy site or "substitutional" structures have lower free energies. This has been supported by density-functional theory (DFT) calculations [26-30]. Two other features common to Na, K, Rb and Cs adsorption on A I ( l l l ) at all temperatures up to room temperature are a preference for adsorption in defect sites on the surface, best seen using surface core-level shift spectroscopy (SCLS) [24], and a dispersed phase at low coverages. The dispersed phase is usually characterized by a (1 × 1) low energy electron diffraction (LEED) pattern, indicating a disordered overlayer. Aside from these common results, the progression of phases as a function of coverage is different for each of the alkalis, and a more detailed description of each case is given below.

2.1.1. Na / A l ( l l l ) At 100 K, three distinct phases are produced in the submonolayer range [24,31,32]. The first is the low-density dispersed phase, for which the L E E D pattern continues to be (1 x 1). In this phase the dominant interaction between the adatoms is presumed to be repulsive due to the strong induced dipole moment at the positions of the adatoms. This agrees with DFT calculations for this system, which predict that the repulsive adatom interaction should dominate until a coverage of about 0.1, whereas at higher coverages it is energetically advantageous for the overlayer to form condensed metallic islands [26,30]. These calculations also predict that the preferred site for the adatoms in this dispersed phase is a 3-fold hollow site. Experimentally, a condensed phase begins to appear at a coverage of about 0.15, producing a L E E D pattern which seems to imply an incommensurate overlayer having a unit cell size slightly larger than that of a commensurate p(3 × 3) unit cell. This LEED pattern remains unchanged until the next phase, a (4 x 4) structure, begins to appear at a coverage of about 0.42. The fact that the intermediate phase, which has been referred to as a "3 x 3" structure [24] (see the schematic LEED pattern in Fig. 1) appears to have the same density throughout the coverage range 0.15-0.42 is evidence that this is a condensed phase consisting of islands in which the interactions between adatoms are now attractive. This interpretation is supported by constant Na 2p photoemission binding energies throughout the coverage range of this phase [24] and an almost linear work function dependence on coverage [33]. The density of this phase

52

R.D. Diehl, R. M c G r a t h / Surface Science Reports 23 (1996) 4 3 - 1 7 1 •

O.

•

.0

•

• .

O. •

""

•

•

• •

• •

O.

.0

•

•

O•

•

..

•

.

,O

• •

• •

Fig. 1. Schematic diagram of the "3 x3" structure of Na/AI(lll) [24]. The large dots are substrate spots, smaller dots are superlattice spots. The size of the superlattice spots indicates their approximate average intensity. The unit cell is indicated. was estimated to correspond to a coverage of about 0.45 by measuring the intensity of the Na 2p core level at the highest coverage where the L E E D pattern is present [24]. The "3 x 3" L E E D pattern may be interpreted as being due to a uniform higher-order commensurate overlayer having four atoms per unit cell and a unit cell length slightly larger than 8.58 A, the value expected for a perfect 3 x 3 structure. This structure, however, would place the adatoms in various sites, and there is evidence in the core-level spectroscopy to indicate that each of the adatoms should have similar surroundings to the others. Therefore, it is expected that each of the Na adatoms moves to a higher-symmetry site, creating small Na anti-phase domains which produce the observed splitting of spots in the L E E D pattern. The degree of relaxation of this overlayer into this anti-phase domain structure is not known at this time, but could be determined by various techniques including LEED, surface extended-X-ray absorption fine structure (SEXAFS) and surface X-ray diffraction (SXRD). This intermediatecoverage structure was not observed in earlier L E E D studies of this system [33,34], which indicated a condensation at a coverage of about 0.2 directly into the higher coverage (4 x 4) structure to be discussed next. At a coverage of about 0.42, a (4 x 4) pattern appears in the L E E D pattern and remains up to the monolayer saturation coverage of about 0.56. This coverage corresponds to a higher-order commensurate structure in which there are 9 atoms per (4 x 4) unit cell. A surface extended-Xray absorption fine structure (SEXAFS) analysis of this phase is described below. The situation is quite different when adsorption occurs at room temperature, or if these overlayers are formed at low temperature (below about 200 K) and then heated to near room temperature. The low-coverage dispersed phase appears the same as at low temperature, i.e. the adatoms are truly on top of the surface and not in it [24]. This was predicted by D F T calculations for Na and K on A I ( l l l ) [26,30], where it was found that a certain threshold overlayer density is necessary in order for it to be energetically favorable for the adatoms to occupy vacancy sites. The reason apparently is that the energy gained in increased screening between alkali adatoms at low densities is not sufficient to remove an A1 atom from the surface, therefore the alkali atoms remain adatoms in the dispersed phase.

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

53

At 0Na = 0.15, about the same coverage at which the intermediate-density condensed phase appears at low temperatures, a (v~ x v~-)R30 ° structure appears when adsorption is carried out at room temperature. An STM study of the formation of this phase actually saw evidence for islands of the (v~- x v~)R30 ° structure at coverages as low as 0.06 [35]. The details of this structure were determined by several experimental techniques and are described below. At a coverage of 0.35, a (2 x 2) phase grows in coexistence with the (x/3- x v~-)30 ° structure until an equivalent coverage of about 0.56. This (2 x 2) phase is a surface alloy involving more than one substrate layer [24,32,36,37] and its structure is also described below. The growth of the intermixed phases at room temperature is quite interesting. An STM study [35] has shown that nucleation of the (v~- x v/3-)R30 ° islands occurs preferentially on the upper terrace side of steps, which is consistent with there being an activation barrier for adsorption on the substitutional site. The islands grow by incorporation of additional substituted Na atoms. The mobility of the adatoms in this substitutional phase was found to be significantly lower than that for on-surface adatoms, having an estimated diffusion constant of 2 x 10-13 > D >_ 6 x 10-15 cm 2 s- 1 at 300 K. Diffusion is observed to occur via the migration of the Na atoms within the top AI layer, i.e. the diffusing species is not just the Na atom but the Na atom together with the underlying vacancy. At higher coverages, the coexistence of the (v~- x v~-)R30 ° structure and the (2 x 2) structure is clearly seen in the STM images. A bilayer structure was deduced for the (2 x 2) structure, in agreement with the results of the LEED, SEXAFS and D F T study described below. A1(111)-(4 X 4)-Na LT: A SEXAFS analysis of this phase indicates that the layer is quasihexagonal with a nearest-neighbor N a - N a distance of 3.70 + 0.03 .& [38], and the local N a - N a distance is slightly compressed compared to bulk Na. This structure as deduced from the SEXAFS study is shown in Fig. 2 . The overall structure consists of slightly rotated "clusters" of Na atoms around Na atoms in atop sites. In this phase, three different sites are occupied by Na adatoms: atop, hollow, and an asymmetric site. The perpendicular distances from the surface to the Na atoms vary by more than 0.4 A for the different sites, although the N a - A I bondlengths are nearly the same. The N a - A I bondlengths deduced from the SEXAFS study were 2.8 + 0.1 ,~ for the atop site, 2.81 + 0.05 A for the asymmetric site, and 2.83 + 0.05 A for the 3-fold hollow site [38]. Al(111)-(vc3 x v~)R30°-Na RT: The structure of this phase was first deduced correctly by a SEXAFS study [27], and at the time it was thought to be very unusual because it involved the "substitution" of Na atoms in A I ( l l l ) lattice positions. The structure as deduced from the SEXAFS study is shown in Fig. 3. It is a structure in which one out of every three surface A1 atoms is removed, and these vacancies filled with Na atoms. This structure is generally called a substitutional structure in the literature, a term which is slightly misleading since the centersof-mass of the Na atoms are still well above the surface AI plane. That this structure has a lower energy than hollow-site adsorption structures was shown by D F T calculations [17,26,30], but what allows it to form on A I ( l l l ) is the low energy of vacancy formation of the A I ( l l l ) surface [39]. The SEXAFS study determined the Na-A1 bondlength to be 3.31 + 0.03 ,~, [27]. Interestingly, SEXAFS results from a lower coverage, 0.16, where the L E E D pattern was 1 X 1 produced essentially the same fits. This implies the same adsorption geometry and chemisorption bondlength at the two coverages.

54

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

.°.°

.°°°

••4

o.°.

.q

o.°.

o°°

o°°°

o°"*

r°.,'

c) Fig. 2. Model structures for AI(lll)-(4X4)-Na as determined from SEXAFS [38]: (a) hexagonal close-packed Na layer; (b) Na clusters around the atop-sited Na atoms (corners of the unit cell); (c) "rotated clusters".

A later normal-incidence standing X-ray wave field (NISXW) measurement confirmed the substitutional site and deduced a somewhat different Na-A1 bondlength, 3.10 + 0.06 ~, [36,40]. And finally, a L E E D study reconfirmed the substitutional structure and also elucidated the nature of the surface relaxation [41]. The structural parameterSofrOm this study are given in Table 1. This result gives a N a - A I bondlength of 3.21 + 0.01 A, which is between the t w o experimental studies quoted above. The D F T calculations gave 3.31 A [26]. Al(111)-(2 x 2)-2Na RT: This structure was shown by SCLS work [32] and NISXW [36] to consist of an even higher degree of intermixing, leading to the formation of a bilayer surface alloy rather than three rotated domains of a 2 x 1 structure or a double Na layer, as was

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

55

fcc

hcp

Fig. 3. Adsorption geometry for Al(111)-(v~ x v~-)R30°-Na [27]. Hatched circles are Na atoms; open, dark-shaded and solid circles show the top layer, second layer and third layer of A1.

originally proposed from earlier studies [42-44]. A later detailed structural study combining LEED, SEXAFS and D F T calculations was carried out and concluded that the structure is a four-layer surface alloy, consisting of a N a - A 1 - N a sandwich on a reconstructed A1 layer [37,45]. The structure, shown in Fig. 4, has two Na atoms in the (2 × 2) unit cell, with one Na atom occupying a substitutional site and the second Na atom and an AI atom situated in the 3-fold fcc- and hcp-hollow sites respectively. There are some small differences in the optimum structural parameters determined by LEED, SEXAFS, and DFT, and these are shown in Table 2. Essentially the same geometry was deduced from a photoelectron diffraction study [21,46,47]. The mechanism of the formation of this structure is still not clear, but it was suggested [37] that surface steps act as sources of additional AI atoms for the formation of the sandwich layer.

Table 1 Structural parameters in .~ for Al(lll)-(v/3 × x/3-)30%-Na structure deduced from LEED [41] Angle of incidence 0o

do

d12 d23 uy a UAI,1 UAl,bul k

1.47 + 0,02 2.27 _+0.02 2.32 + 0.02 0.23 + 0.02 0.13 + 0.02 0.10 + 0.01

_

15 °

1.48 _+0.02 2,24 _+0.03 2.34 + 0.03 0.24 + 0.03 0.11 _+0.03 0.10 __.0.02

d o is the N a - A l spacing. The value for d34 was equal to the value for bulk A1. UNa, UAI,1 and /'/Al,bulk a r e the rms vibrational amplitudes for Na atoms, A1 atoms in the first layer, and A1 atoms in the bulk, respectively.

56

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

2.1.2. K/AI(111) At 100 K, potassium adsorption on A I ( l l l ) results in an ordered (v~ x v~)R30 ° structure above a coverage of about 0.1, and a compressed hexagonal phase at coverages above about 0.33 [24]. The L E E D spots for the (v/-3- x v/-3)R30 ° phase are sharp as soon as they appear, consistent with island formation. This island formation indicates that the K - K interactions are attractive at a relatively low coverage, similar to N a / A I ( l l l ) . The structure of this (v~ x v/3)R30 ° phase is discussed below. The attractive interaction, while not typical for alkali adsorption at low coverages on most metals, is consistent with theoretical results for potassium and sodium on A I ( l l l ) [48]. The compressed phase of K / A I ( l l l ) reaches a minimum K - K

(a)

(h'~

Fig. 4. Model of the AI(lll)-(2X2)-Na structure determined by LEED [37]. The top four layers, each of ( 2 x 2 ) periodicity, consist of a Na-A1-Na sandwich on a reconstructed AI layer with a (2 X 2) vacancy structure. The Na atoms in the lower layer of the sandwich are located in substitutional sites in the reconstructed layer. AI atoms in the sandwich layer and Na atoms in the upper layer of the sandwich are located in hcp and fcc sites, respectively, on the reconstructed layer. (a) Top view. (b) Side view.

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

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Table 2 Interlayer spacings dij (,~) determined by LEED, SEXAFS, and DFT analyses of the structure of A1(111)-(2 × 2)-Na [37] Layer Atoms Interlayer spacing LEED SEXAFS DFT

1 Na

2 AI

3 Na

4 A1

5 A1

6 A1

d12

d23

d34

d45

d56

0.85 0.75 0.72

0.55 0.70 0.62

1.52 1.50 1.46

2.25

2.38

2.20

2.32

The layers are numbered from the surface into the bulk. The LEED and DFT analyses also indicate the presence of small (0.04 ,~) lateral displacements of AI atoms in layers 4 and 5, and small (0.05 ,~) vertical displacements of A1 atoms in layers 5 and 6. The interlayer spacings given here are with respect to the midpoints of the layers 5 and 6, which are the first two (almost) perfect layers.

distance at a coverage of about 0.45, and this K - K distance is about 4% smaller than that found in bulk potassium. A recent PhD study of this system was performed in the coverage range 0.05-0.40 at 150 K [49]. The site was found to be on-top and the bondlength was found to increase from 3.10 + 0.06 to 3.27 _+ 0.06 ,~. At room temperature the adsorption of K on A1(111) results in the formation of a dispersed phase at low coverages in which the adatoms are on the surface [24]. At coverages of about 0.1 however, core-level spectroscopy indicates that the site changes, and at the same time, the ordered (f3- x v/-J-)R30° phase is observed in the LEED pattern [24]. A dynamical L E E D study determined that the K atoms in this phase are in the substitutional sites, as described beloiv [28]. Al(lll)-(vC3 x f3)R30°-K LT: The geometry of the (v~- x f3-)R30 ° phase at 90 K has been determined by L E E D [28,50,51]. It consists of the K adatoms in the atop sites with a K-A1 distance of 3.23 _+ 0.05 A, accompanied by a significant rumpling of the AI substrate, as shown in Fig. 5. The rumpling consists mainly of the vertical displacement of the A1 atoms under the K atoms toward the bulk by 0.25 + 0.04 .&. The inward displacement of 1 / 3 of the A1 atoms in the first layer was found to be accompanied by a small outward lateral shift of 0.05 A of the nearest-neighbor AI atoms in the second layer. This analysis also concluded that the vibrations O

0.25 A

Fig. 5. Side view showing the atomic geometry for the low-temperature structure Al(lll)-(v/3 - × VC3)R30°-K [28]. The rumpling of the first AI layer can be seen. Interlayer spacings are indicated in the sketch.

58

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

(a)

(b)

Fig. 6. Atomic geometry of the substitutional site for AI(lll)-(v~-× v~-)R30°-K [28]. K atoms are shown as larger pale grey circles. (a) Side view of the (111) surface on a (101) plane. Values of the interlayer spacings are indicated on the sketch. (b) Top view. of the adsorbed K atoms are anisotropic, with a larger amplitude parallel to the surface than perpendicular to the surface. Al(111)-(v~ x v~)R30°-KRT. • The adsorption site has been determined by L E E D [28,50,51] to be a quasi-substitutional site, similar to the case of N a / o A l ( l l l ) , as shown in Fig. 6. The K atoms are situated at a vertical distance of 2.16 + 0.03 A above the first-layer AI atoms, corresponding to a nearest-neighbor K - A I distance of 3.58 .~. The first interlayer A I - A I spacing was found to be contracted by 2% with respect to the bulk value.

2.1.3. Rb /A1(111) As a function of coverage, R b adsorbed on A I ( l l l ) at 100 K first begins to form islands of a p(2 × 2) phase at a coverage of about 0.08. At a coverage of about 0.28, a (v/-3- X v~-)R30 ° phase begins to grow, at first in coexistence with the p(2 x 2) phase [24,33]. The overlayer remains in

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171 1.0

~

]

,

59

,

{}.9 0.8

4

&

.

O.7 Q

0,6 05 0.4 O3 02

•

= (I.33 ML

,~

= 0 2 5 ML

"¢

0.1 00

• 0 ,

,

i 1 00

. . . .

i 1 50

II 200 Temperature

250

300

{K)

Fig. 7. Relative amount of atop (versus substitutional) site, deduced from intensities in core-level spectra, as a function of annealing temperature for 30 s successive anneals. Filled circles show the results for a (v~ x v~)R30° structure at a coverage of 0.33, and open triangles show results for the p(2 × 2) structure at a coverage of 0.25 [55]. the (v/3- x v~-)R30 ° phase until saturation, at a coverage of about 0.33. NISXW studies of these overlayers indicate that the Rb adsorption site is the same in all of these phases, and that it is the atop site [40,52]. The Rb-A1 perpendicular spacing in this coverage range was found to remain constant to within the experimental precision. A later L E E D study [53] corroborated the atop adsorption site, and the structural details are described below. Initially it was thought that the atop site was also occupied at room temperature [40,52], but later L E E D and SCLS studies concluded that the equilibrium structure, at least in the case of the (v~ x v/3-)R30 ° phase, is in fact the substituted-site structure (see below). Further NISXW experiments involving the annealing of the low-temperature prepared surface to room temperature suggested that only part of the surface may transform to the substituted structure, apparently due to slow kinetics [54]. However, an annealing experiment using SCLS found that the overlayer completely transforms to the substituted structure by a temperature of 250 K [55]. This transition from atop to substituted structures is shown in Fig. 7. The sequence of phases is somewhat different when Rb is adsorbed at room temperature. In this case, instead of the formation of a p(2 x 2) structure, a complicated L E E D pattern, shown in Fig. 8, indicates the presence of a higher-order commensurate structure denoted by the matrix notation (~ _~) [24]. At higher coverages, the overlayer forms a (v~ × V~-)R30 ° structure, described below. Al(111)-Rb disordered LT: NISXW studies [40,52] at 0.12 coverage and 170 K gave essentially the same structural results as those for the p(2 x 2) and (v~-× q~)R30 ° phases described below, i.e. that the Rb adatoms reside in the top sites with a R b - A I bondlength of 3.13 + 0.10 A. (This determination of bondlength assumes no net surface relaxation.) Al(111)-p(2 × 2)-Rb LT: The geometry of this layer has been studied with NISXW [40,52] at 170 K. The NISXW results showed that the Rb adatoms are located in the atop site, with a R b - A I nearest-neighbor distance of 3.13 +_ 0.10 ,~. An interesting finding in this study is that there is apparently a great deal of site disorder in the overlayer. The coherent fraction, which is

60

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

s I ~s

•

.~

•

o/•

T -~.

•

,)

- ~ - .

L

e"

0

~s~

:

•

.

z-

•: : . ~,c~

TM

..

. z ;;--....

Fig. 8. Sketch of the (1_16) LEED pattern formed by Rb and Cs on AI(lll) at room temperature in the coverage region from 0.14 to 0.3 [24]. Large dots are substrate spots. The unit cell of one of the three rotated domains is shown. The thin lines denote streaking observed at low coverages.

a measure of the fraction of adatoms contributing to the ordered site arrangement, was found to be 0.5, indicating a significant amount of (static or dynamic) disorder. Al(111)-(f3 x v/-3)R30°-Rb LT: A NISXW study of this phase at 170 K gave essentially the same structural results as those obtained for the p(2 x 2) phase described above [40,52]. L E E D results at 100 K confirm the top-site occupation by the Rb adatoms, and further determine that there is a rumpling of the top two substrate layers [53]. The R b - A I distance measured by L E E D is 3.35 + 0.03/~. The structure is similar to that found for K / A I ( l l l ) (see Fig. 5), but with the first layer rumpling displacement of 0.265 + 0.02 ~,. The distance of the Rb layer from the midpoint of the rumpled layer is 3.22 + 0.03/~, which is consistent with (but more precise than) the value determined by the NISXW study quoted above. The L E E D study also found evidence for anomalously large-amplitude (0.18 + 0.02 ,~) vibrations of the top-layer AI atoms. It was proposed that this large vibrational amplitude might be the first step in the phase transition between this atop structure and the substitutional site structure discussed below [53]. AI(lll)-(x/3 X f3)R30°-Rb RT." LEED experiments carried out at room temperature [53] indicate that the structure of the (vC3 x v~)R30 ° phase is a vacancy-occupation structure similar to that found for K / A I ( l l l ) at room temperature (see Fig. 5). The perpendicular spacing between the Rb and top layers was found to be 2.41 _+ 0.02 A, corresponding to a R b - A I chemisorption bondlength of 3.74/~. NISXW results [54] are consistent with this result, giving a R b - A I bondlength of 3.72 _+ 0.10 .~. o

2.1.4. Cs /Al(111) Adsorption of Cs onto A I ( l l l ) at 100 K results in a fairly classical progression of adsorbed phases [24,33]. At a very low coverage rings are observed in the L E E D pattern which increase in diameter continuously as the coverage is increased. These rings give way to a p(2 X 2) structure, which eventually is transformed into a (f3- x f3-)R30 ° phase. SCLS results led to the

R.D. Diehl, R. MeGrath /Surface Science Reports 23 (1996) 43-171

61

proposal that the adsorption site is the atop site for both commensurate structures [24]. A LEED analysis of the (v~- X v/3)R30 ° phase is described below. At room temperature, adsorption proceeds similarly to that observed for R b / A I ( l l l ) [24], except that a ring phase is observed at low coverages for Cs, and the saturation structure is a (2v~- x 2V~-)R30 ° phase rather than a (v~ × v~-)R30 ° phase. SCLS results for this phase are consistent with this structure having 4 atoms per unit cell, corresponding to an optimum coverage of 0.33. There appear to be two inequivalent sites for the Cs atoms, but as for the cases of the other systems at room temperature, there appears to be intermixing of the Cs and the AI at the surface [24]. Al(lll)-(v~ x vr3)R30°-Cs LT: A LEED study for this structure led to the conclusion that Cs adsorbs on top sites on a rumpled first A1 layer, just as in the case of thelow-temperature K and Rb structures [25]. The Cs-AI distance was found to be 3.31 + 0.03 A and the rumpling amplitude of the first layer was found to be 0.29 +_ 0.02 A.

2.1.5. Na /Al(lO0) An early room-temperature LEED study indicated that Na forms a c(2 x 2) overlayer for coverages up to 0.5 [43] followed by a hexagonal structure. A more recent study has found that the c(2 × 2) structure grows in an overlayer containing anti-phase domains, which eventually anneal out as the coverage increases toward 0.5, and that the higher-coverage structure is in fact a mixture of the c(2 x 2) structure and a ( lx/-~ × V~-ff)R14° structure [56]. At 100 K, on the other hand, a c(2 × 2) structure initially forms [57], followed by structures having c(7v~x v~)R45 °, c(8 x 2), 3 x 2 and 7 x 7 symmetries [56]. The c(2 x 2) structures formed at 100 K and at room temperature have been shown by SCLS [31], SEXAFS [58] and LEED to be different structures, as also indicated by DFT [29], although there is some disagreement as to the exact room-temperature structure, as described below. Al(lO0)-c(2 x 2)-Na LT: A SEXAFS study of the local geometry of this overlayer indicates that at 140 K the overlayer atoms are indeed in the 4-fold hollow sites, with the Na-A1 bondlength of 3.20 +_ 0.03 A [58]. A subsequent DFT and LEED study found essentia!ly the same result. The LEED results determined that the Na-AI bondlength is 3.27 + 0.01 A, and that the substrate is essentially unrelaxed (Fig. 9a) [56]. Al(lO0)-c(2 x 2)-Na RT: Two early LEED studies of the geometry of the c(2 x 2) structure indicated that the Na adatoms in this phase also reside in the 4-fold hollow sites [59,60]. However, these studies w e r e performed before intermixed overlayers were considered to be likely. Later SCLS studies showed clearly that intermixing does occur above 160 K for this system [31]. A SEXAFS study determined that the overlayer is intermixed, and a structure was deduced in which the Na forms an ordered underlayer beneath a reconstructed surface AI layer [58]. This structure is at variance with DFT calculations for this surface which predicted a substitutional site for coverages above 0.15 [29,30]. However, a more recent photoelectron diffraction (PhD) study has found results which are consistent with the substitutional site [61]. In this case, however, only half of the layer is found to be substituted while the other half is in the 4-fold hollow structure. The Na-AI bondlength was found to be 3.11 .~ in the substitutional site and 2.93 A in the hollow site. The authors speculated that the formation of the complete substitutional overlayer was kinetically hindered [61]. Finally, a LEED study of this system indicated that the structure of this overlayer is wholly substitutional with a nearest-neighbor

62

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

(a)

I

(b)

B

IF,, A top view

top view

5 ± 0.010 ,~

0.030/~ +_0.010 2.m~o ol o A

2 _+0.01 ,~ 2.022 _+0.010 A

side view

0.050 _+0.020 A

side view

Fig. 9. Hollow-site model for the low-temperature structure (a) and the substitutional room-temperature structure (b) for Na adsorption on AI(100). The side views are along the directions A - B [56].

N a - A l bondlength of 3.07 + 0.01 ~, [56]. This structure also was found to accompany a large (9.1%) contraction of the topmost AI interlayer spacing (see Fig. 9b). The irreversible transition between the two structures was also studied by L E E D and found to begin at a temperature of about 180 K and be complete at a temperature of about 260 K [56]. The microscopic interpretation is that A1 atoms are expelled from their normal positions, to be trapped at steps or next to Na adatoms, and the Na adatoms drop into the resulting holes [56].

2.1.6. K/AI(IO0) A LEED study of K/AI(100) indicates that the saturation-coverage structure at 370 K is a higher-order commensurate structure which exhibits split spots at the c(2 x 2) positions of the LEED pattern [62]. This pattern was indexed as (lf]--O/2 x lf]--O/2)R18 ° with two domain orientations at 90 ° to each other. The geometry of this overlayer has not been determined, to our knowledge. 2.1.7. Alkali metal mixtures on A l ( l l l ) LEED and SCLS measurements for mixtures of Na and Rb on A I ( l l l ) at 100 K suggested a net N a - R b repulsion on this surface [63]. However, more recent studies have shown that it is possible to create a mixed Na + X phase, where X is K, Rb or Cs on A I ( l l l ) [64]. Ordered alloys can be formed by adsorbing X onto the (v~ × v~)R30°-Na structure, or by coadsorbing effective 1 / 4 coverages of each of Na and X. SCLS and L E E D analyses of these (2 x 2)-Na + X phases indicate that the structures formed are essentially identical to that described above for

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

63

the A1(111)-(2 × 2)-2Na phase, except that the top layer of Na is replaced by X. The lower alkali layer in this structure is always occupied by Na, not X, irrespective of which alkali is adsorbed first [64].

2.1.8. Coadsorption of Na and 0 on Al(111) Al(111)-(f3 X f3)R30°-(Na + O) RT." A NISXW study of the local geometry of oxygen adsorbed on the AI(lll)(v~- X v~-)R30°-Na surface alloy found results consistent with the O atoms occupying the atop sites on the surface Na atoms [65], which is consistent with previous studies that had suggested that the O bonds directly to the Na adatoms [66]. The N a - O bondlength determined by the NISXW study is 2.03 + 0.12 A, which is low compared to bulk Na oxides, but reasonable when considering that a typical bulk coordination number is 4-6, while in this case the O has only one Na nearest neighbor. The top-site occupation by O is quite unlike the usual case for O adsorption on elemental solids, where coordination is usually maximized. It also appears to contrast with the behavior observed in other alkali-O coadsorption systems studied so far, where generally the O has been found to bond to the substrate atoms. O

2.2. Beryllium 2.2.1. Li /Be(O001) The submonolayer phases of Li/Be(0001) were studied using L E E D and photoemission techniques [67]. At low coverages, Li forms a dispersed phase which is disordered. At a coverage of 0.20, the overlayer condenses into a (v/3- x v~-)R30 ° phase. This condensation was observed at both room temperature and low temperature. The area of the surface occupied by this phase continues to grow until it covers the surface completely at a coverage of 0.33. With increasing coverage, the overlayer then compresses into a hexagonal incommensurate structure, saturating at about 0Li = 1.1. Photoemission and inverse photoemission studies indicate that the condensation transition is accompanied by an abrupt electronic transition from a nonmetallic state to a metallic state [67].

2.3. Cobalt 2.3.1. K / Co(10iO) This system has been studied using LEED [68,69] and Fig. 10 shows the phase diagram [69]. Adsorption at 300 K yields ordered LEED patterns for coverages in excess of 0.28, with pairs of beams symmetrically split in the [1210] direction about the (n + 1/2, m + 1/2) positions. This splitting is shown to vary linearly with coverage in the range 0.25 < 0 < 0.58 and is interpreted as a uniaxially incommensurate phase which has relatively strong domain walls which have either higher or lower density than the c(2 × 2), depending on the coverage. A commensurate c(2 X 2) pattern is observed at a coverage of 0.5 [69] in which the K atoms were found to be in the hollow sites, as described below. A fingerprinting technique using momentum averaging suggested that the K atoms also preferentially occupy the hollow sites in the incommensurate phases [69].

64

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

650 ~

I K/Co(1010)

.....

55(1

= 450 I

E 350 ~. 250

p(lxl) lattice gas

150 0.0

i 0.1

"N ~: div~atrciancy I-~~ ~~[ =~'= structure I~ ~ "~ -=E "~ ~

0.2

I 0.3

0.4

t 0.5

"~

0.6

(9

Fig. 10. Phase diagramfor K on Co(1010) [69]. The phase boundaries are approximate.

Co(10i0)-c(2 x 2)-K RT: The structure of the c(2 x 2) overlayer has been determined with a LEED analysis. A preference was found for adsorption in the maximum-coordination 4-foldhollow site with a K - C o bondlength of 3.12 + 0.05 A and an essentially unreconstructed substrate [68]. 2.3.2. Coadsorption of K and CO on Co(1010) Room temperature adsorption of CO on the OK = 0.5 Co(1010)-c(2 x 2)-K surface resulted in a sharp c(2 x 2) pattern at CO saturation [70]. No ordered or disordered intermediate phases were observed. The stoichiometry was estimated to be 1 : 1 using photoemission. The CO bond axis was shown to be at or close to normal and a model was proposed consisting of K - C O chains. Co(1010)-c(2 x 2)-(K+ CO) RT: A dynamical LEED study [71] determined that the optimum structure, shown in Fig. 11, has the K adatoms in the 4-fold hollow sites as found for Co(1010)-c(2 x 2)-K. The CO molecules are located in sites which are 3-fold coordinated with the K atoms, and are somewhat "off-bridge" with respect to the substrate. The potassium-cobalt bondlength is 3.51 + 0.11 A which gives an effective potassium radius of 2.26 + 0.11 A. The CO molecule was found to be essentially upright, with a tilt angle of 4° + 10°. A comparison of this result to the Co(1010)-c(2 x 2)-K result indicates that the coadsor~tion of CO causes a large shift outward of the K atoms, leading to an increase of about 0.4 A in the effective potassium radius. A complementary near-edge X-ray absorption fine structure (NEXAFS) study of this system [72] deduced a K-induced increase in the CO bondlength of 0.11 ___0.04 ,~ and the polarization dependence of 7r and ~ peaks indicates that the molecule is upright, in agreement with the photoemission [70] and LEED [71] work.

2.3.3. Coadsorption of K and CO on Co(O001) The adsorption of CO and K on Co(0001) at room temperature was characterized using LEED and other techniques [73]. Two ordered coadsorption structures were observed: a (2 x 2)-(CO + K) at 0co = 0.25 and OK = 0.25, and (2 x 2)-(2CO + K) at 0co = 0.5 and OK = 0.25.

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

65

A 012345678

910A

C

© %..++

plan view

A'

section AA'

•......

:.....:

section BB'

section CC'

1.53±0.2

0.75±0.06

O 1.20i0.07 C 1.33±0.07 Co 0.70±0.10 Co ] .45±0.06 Co

Fig. 11. (a) Top and side views of the geometric structure of Co(1010)-c(2x2)-(K+ CO), and (b) the associated geometric parameters [71].

A structural model with K in 3-fold hollow sites and on-top or bridge sites for CO was postulated.

2. 4. Copper Alkali metal adsorption on copper surfaces has been relatively well-studied. A variety of commensurate and incommensurate structures have been observed at submonolayer coverages, and the adsorption geometries have been determined for many of these systems using LEED, SEXAFS, SXRD and SXW techniques. The small lateral energy variation for alkali metals on unreconstructed copper surfaces makes copper an attractive substrate for studying surface phase transitions. K and Cs overlayers on C u ( l l l ) have been found to melt into fluid phases having bond-orientational order. Rotated incommensurate structures have been observed on Cu(100). On the (110) surface there is some evidence from STM studies that the mechanism which leads to the alkali-induced (1 × 2) reconstruction may be related to the intermixed structures observed for alkali adsorption on Al in that substrate atoms are removed and the resulting vacancy is occupied by an alkali atom.

66

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

2

2.15 :

2.08_+0.06

2.64_+0.13

2.03:t0.15

Fig. 12. Schematic drawing of top and side views of the adsorption geometry for Cu(111)-(2×2)-3Li [74]. The coverage is 0.75. The numbers on the Li atoms denote the symmetrically inequivalent Li atoms. Dimensions are in ,~.

2.4.1. Li / Cu(111) Adsorption of Li on C u ( l l l ) at 180 K results in the formation of a Li overlayer, while Li adsorption on C u ( l l l ) at 300 K results in an interesting substitutional structure which is different from those observed so far on A I ( l l l ) [74]. The substitutional structure has (2 x 2) symmetry, and it presumably requires some activation energy similar to that required for substitutional adsorption on A I ( l l l ) [17] since it is not observed at 180 K (see next paragraph). Cu(111)-(2 × 2)-3Li RT.. T h e dynamical L E E D study of this structure [74] has determined that the Li coverage of this structure is 0.75, and that there are three Li atoms per (2 x 2) unit cell, one in a substitutional site and two others in fcc and hcp hollows surrounding the substituted adatom. This geometry is shown in Fig. 12. The effective radius of the substituted Li atoms, 1.34 A, is very close to the metallic radius of Li, while the effective radii of the hollow-site atoms are somewhat smaller. This is a rather unusual arrangement of adatoms, although it resembles the Al(lll)-(2 x 2)-2Na RT structure [37], except that here there are no substrate atoms mixed into the "overlayer". O

2.4.2. Na / Cu(lll) The room temperature phases of N a / C u ( l l l ) have been studied by L E E D [75]. At coverages up to about 0.4, rings a n d / o r broad spots are observed in the L E E D patterns, indicating a

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

67

disordered phase in which the nearest-neighbor separation decreases continuously with increasing coverage. At room temperature no ordered structure forms until the coverage reaches at least 0.4, with the monolayer saturation occurring at 0.44 when a higher-order commensurate structure, having (3 X 3) symmetry with 4 atoms per unit cell, forms. At this coverage, the spacing between Na atoms (3.78 A) is very close to the metallic diameter of Na (see Table A.3). In contrast to these results, a similar LEED study led to the observation of a p(2 x 2) phase at 0.25 coverage [76]. The observation of the different commensurate structures in the two studies is not clear, but it might be related to the exact temperature of observation, which was not specified in either paper. Alternatively, intermixing of the alkali and substrate may occur (see for instance Li/Cu(100)). L E E D studies of thin epitaxial films of Na on C u ( l l l ) [77] indicate that when the substrate is held at liquid nitrogen temperature, the Na layers correspond to hexagonal close-packed planes with the stacking sequence ABAB. At room temperature, on the other hand, Na forms 3D clusters on top of a Na film only a few layers thick.

2.4.3. K / Cu(111) A L E E D study of K / C u ( l l l ) at 80 K [78] indicated similar structures to those observed for C s / C u ( l l l ) (see below). Rings were observed in the LEED pattern up to a coverage of 0.08, followed by modulated rings indicative of a fluid phase having bond-orientational order, followed by a hexagonal incommensurate solid phase aligned along the Cu (1 x 1) directions. The density of the overlayer increased continuously with coverage to a commensurate p(2 x 2) phase at 0.25, which then further compressed to a K - K spacing of about 4.4 A, slightly smaller than the metallic diameter of about 4.76 A. At the higher coverages, a significant amount of frozen-in disorder was observed at temperatures below 120 K, implying that the mobility of the overlayer is restricted at low temperatures due to either the substrate potential or defects [78]. Cu(111)-p(2 x 2)-K LT: A SEXAFS study at 65 K showed that potassium occupies the atop site in the p(2 ox 2) phase, having a K - C u bondlength of 3.05 + 0.02 A and an effective K radius of 1.81 + 0.03 A [79]. A determination of substrate relaxation was not possible in this study due to the large vibrational effects of the K adatoms, even at 65 K (see Section 3.7). A qualitative X-ray photoelectron diffraction study at room temperature supported the top-site identification O

[80]. Melting: The melting of the low-density incommensurate solid was found to occur over a wide temperature range, disordering first into a fluid phase which has significant bond-orientational order and then into an orientationally disordered fluid [78]. This observation is qualitatively similar to the prediction of two-stage melting of the so-called " K T H N Y " [81-83] theory for 2D solids. This theory predicted that melting of a 2D solid should occur via a two-stage process in which the solid first melts into a fluid phase having bond-orientational order (the "hexatic" phase) and then into an isotropic fluid. In diffraction experiments, the signature of the hexatic phase is that of a fluid phase which has strong orientational correlations. It consists of diffraction peaks broadened in both the radial and azimuthal directions. Melting into a hexatic fluid phase was previously observed for rare-gas adsorption [84-87], suggesting that some adsorption systems may be examples of the two-dimensional matter described by the KTHNY theory. The observation of this type of transition for some alkali metals on close-packed substrate surfaces, along with other evidence of a very small corrugation in the adsorption

68

R.D. Diehl, R. McGrath/ Surface ScienceReports 23 (1996)43-171

500 ,-. 400

[Cs/Cu(111) I

p(2x2)

,hcxa:i::rdcr / n

incommensurate

~

300

/71

solid

E

~" 200 I

2nd layer onset

100 0.1

0.2 0.3 0.4 Coverage Fig. 13. Phase diagramdeterminedby LEED for Cs on Cu(lll) [89]. potential energy, suggests that some alkali metal overlayers may also qualify as 2D matter. However, it must be remembered that the observation of a "hexatic" phase is not necessarily due to intrinsic behavior of the overlayer and might be caused by the orienting effects of the substrate structure. 2.4.4. Rb / C u ( l l l ) C u ( l l l ) - p ( 2 x 2)-Rb RT: A LEED and NISXW study of R b / C u ( l l l ) indicated that the

monolayer saturation structure at room temperature is p(2 x 2) [88]. X-ray standing-wave measurements of this phase at room temperature showed that the Rb atoms occupy atop sites with a R b - C u bondlength of 3.07 .~. An interesting aspect of this measurement, which was also present in the standing-wave measurements of R b / A l ( l l 1) [40,52] is that the coherent fraction, which measures what fraction of the adsorbate atoms occupy the same (coherent) position relative to the substrate diffraction planes, was essentially unity (0.99) for the direction perpendicular to the ( l i d plane, but only 0.54 or less in the (111) direction. This indicates that while the Rb atoms lie in a single plane parallel to the surface, there is significant disorder within the plane. In addition, the (111) coherent fraction decreased with time, possibly due to disorder induced by impurity adsorption. 2.4.5. C s / C u ( l l l )

The phase diagram determined by LEED for the adsorption of C s / C u ( l l l ) at 80 K is shown in Fig. 13 [89]. At coverages up to 0.07 rings are observed in the diffraction pattern which vary continuously with coverage, implying a repulsive isotropic disordered phase. At somewhat higher coverages the rings coalesce into spots which are angularly broad and aligned along the substrate (1 x 1) directions. These spots gradually sharpen, indicating an incommensurate hexagonal solid phase, which continues to compress as the coverage is increased until, at a coverage of about 0.25, a commensurate p(2 x 2) phase is formed. At higher coverages the overlayer continues to compress, forming an incommensurate hexagonal solid again until the monolayer saturates at a coverage of about 0.28, corresponding to a nearest-neighbor distance

R.D. Diehl, R. McGrath / Surface Science Reports 23 (19%) 43-171

69

0

of 4.86 A, which is compressed by about 11% relative to the metallic Cs spacing. A similar progression of phases was observed at room temperature by another group, except that the saturation coverage at room temperature was found to be 0.25, at the commensurate p(2 x 2) density [90]. As shown in the phase diagram, the most thermally stable structure is the commensurate p(2 x 2) phase, indicating that while the substrate corrugation is small enough relative to the adatom-adatom interactions to allow incommensurate structures to be formed, it is still large enough to provide significant stability to the commensurate phase. The melting of the hexagonal incommensurate phase was studied by LEED [89]. It was found that the incommensurate overlayer which was dosed at 80 K apparently became better ordered upon heating to about 135 K (depending on coverage), presumably due to the kinetics of either the overlayer ordering or substrate relaxation. This solid then disordered into a fluid phase having bond-orientational order, and then disordered completely at a somewhat higher temperature. This behavior is qualitatively similar to transitions observed for K / C u ( l l l ) , described above [78], and for K / N i ( l l l ) [91]. Cu(lll)-p(2 x 2)-Cs RT." A dynamical LEED study carried out on this p(2 X 2) phase resulted in the first determination of atop adsorption by an alkali metal on a metal substrate [90]. This result is supported by a more recent qualitative photoelectron diffraction study [80]. Substrate relaxation was not included as a fitting parameter in the L E E D study which found the Cs-Cu bondlength to be 3.01 + 0.05 A, giving an effective Cs radius of 1.73 + 0.05 ~,. Aside from the atop site, the main point of interest in this study is that it was found that a scattering potential derived from a cluster calculation provided far better agreement with the experimental spectra than the conventional bulk metal potential. This was attributed to the large deformation of Cs upon adsorption [90].

2.4.6. Li / Cu(lO0) LEED studies of Li/Cu(100) indicate that quite different structures are formed at 180 and 300 K [92] (see Fig. 14). At 180 K, the only ordered structure which forms over most of the submonolayer coverage range, from about 0.25 to 0.55, is the c(2 x 2). This indicates that this overlayer undergoes a condensation transition into this commensurate phase which is similar to alkali adsorption on aluminum surfaces [17,24,30]. The results of a dynamical LEED study of this phase are described below. At higher coverages more complex structures form, and the monolayer saturates at about 0.75. coverage, 0 0.0

180 K 3(X)K

O. 1

0.2

,

,

,

0.3

0.4

0.5

,

i

\

C(2X2)

~ \

0.6

0.7

'Matrix\

0.8

'

rotated ~ 2 n d layer structures \

2xl "N'N-3x3"~ 4x4~ "x,NN,Nstreak, " ~ N,,~" ~

disorder ,

*[ 1/2® O~ \ -1/4® 2/ Fig. 14. Surface structure changes for Li/Cu(100) at 180 and 300 K as a function of Li coverage [92].

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top view

side view Fig. 15. Top and side views of the proposed (2 x 1) structure formed upon Li adsorption on Cu(100) [93]. The open circles denote Cu atoms and the shaded ones denote Li atoms.

At higher temperatures, however, a completely different scenario occurs [92]. The sequence of structures which appears is the same whether the crystal is dosed at 180 K and then heated to room temperature, or dosed at room temperature. At about 0.25 coverage, a (2 x 1) pattern with extra fuzzy streaks appears, followed by a (3 x 3) pattern at 0.45 coverage and a (4 x 4) pattern at about 0.55. Higher coverages result in disorder. Cu(lOO)-streaky (2 x 1)-Li RT: The (2 x 1) plus streaks pattern has been analysed using L E E D [93] and is consistent with a missing-row reconstruction of the Cu(100) surface along with Li adatoms located in the troughs, but disordered across the troughs. There is an accompanying relaxation. This structure is shown in Fig. 15. Lowering the temperature of this surface causes the streaks to coalesce into spots, believed to be due to the ordering of the Li atoms across the troughs [93]. It was not possible to determine the sites of the Li adatoms within the troughs due to low L E E D intensities, but they are expected to be in the high-coordination sites. The missing-row structure itself includes a contraction of the first substrate layer spacing by 7 + 3% and a second-layer expansion of 2 + 4% relative to the bulk spacing. The role of the temperature in the surface reconstruction is thought to be to increase the mobility of surface copper atoms, allowing them to relax into the lower-energy (reconstructed) structure. At least two other alkali adsorption systems on fcc (100) substrates have been interpreted as missing-row structures, namely K/Ag(100) [94,95] and K/Au(100) [96]. It is not understood at this point whether the mechanism for the missing-row structure is related to that which occurs on fcc (110) surfaces, but it does appear to be similar in nature to the intermixing which has been observed to occur for alkali metal adsorption on AI substrates. Further support for this association is given by the observation of two more ordered phases at higher coverages, the (3 x 3) (see below) and the (4 x 4) phases [92]. At even higher coverages, the total amount adsorbed continues to increase as indicated by the Auger signal, but the surface becomes

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(a)

(b)

top view

side view Fig. 16. (a) Top and side views of Cu(100)-(3 x 3)-Li [98]. Gray levels distinguish types of atoms. The numbers identify symmetrically equivalent atoms. Arrows indicate directions of displacements from ideal hollow sites. The line A-A~ in the side view is the base line for heights shown in Table 3. (b) Top view of the first complete Cu(100) layer (which lies just below the quartets), showing lateral relaxation directions.

disordered, and it is conjectured that at this point the Li is diffusing deeper into the surface to form a bulk alloy, and disordering the surface alloy in the process. Cu(lO0)-c(2 × 2)-Li LT: The details of the c(2 x 2) geometry have been determined by a dynamical L E E D analysis, and the Li atoms are found to be in the 4-fold hollow sites [97]. The effective alkali metal radius in this structure is 1.39 + 0.08 A, somewhat less than the metallic radius of Li. No substrate relaxation or reconstruction was detectable in this structure. Cu(100)-(3 x 3)-5Li RT: A dynamical LEED study carried out on this structure found it to be a complex surface alloy [98]. The Li was adsorbed at 300 K and the data were taken at 180 K. The structure is shown in Fig. 16 and consists of small pyramids of four Cu atoms capped by single Li atoms, with additional pairs of Li atoms between the pyramids. The structural parameters are given in Table 3. The height of the Li adatoms above the Cu quartets is 1.91 A, which is essentially the same as that determined for the c(2 x 2) structure described above. The height of the Cu quartet above the average plane of the first complete Cu layer is 1.67 A, which is contracted about 7.5% relative to bulk Cu. The first complete Cu layer is both buckled and laterally relaxed. O

O

o

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Table 3 Optimum parameters of the best-fit structure for Cu(100)-(3 × 3)-Li [98] No.

Lateral displacement

1 2 3 4 5 6

0.30 + 0.26 0.02 + 0.11 0.04 + 0.11 0.08 + 0.08 1.76 + 0.09

Height 5.38 + 0.07 3.88 + 0.08 3.47 + 0.04 1.83 + 0.06 1.77 + 0.05

Interlayer spacing

1.50 0.41 0.06 0.01 0.01

Atom numbers correspond to those in Fig. 16. Lateral displacements refer to displacements from hollow sites. Heights are measured from the plane A - N in Fig. 16.

Cu(100)-(4 × 4)-Li RT." A dynamical LEED analysis has been carried out on this system. The unit cell of the (4 × 4) structure was determined to consist of four Li atoms forming a square cluster located on top of islands of nine Cu atoms, with these islands joined by rows of substituting Li atoms [99]. STM measurements from the same system show a (4 × 4) primitive array of protrusions which are interpreted as being the square clusters of four Li atoms mentioned above [100].

2.4. 7. Na / Cu(lO0) While a structural study of the Na/Cu(100) phases has not yet been published, it is known that the saturated monolayer structure is a c(2 x 2) structure. Diffusion studies using He-atom scattering have determined that at low coverages,° Na diffuses on the surface by a jump mechanism with predominantly single steps of 2.56 A, which corresponds to the separation of the 4-fold hollow sites which presumably are occupied in the c(2 x 2) phase [101]. (The 2.56 also corresponds to the separation of the top sites, which are considered to be unlikely in this case, but does not correspond to the separation of the bridge sites.) The activation energy for diffusion is 51 meV and the effective jump attempt frequency is 0.53 THz. A study of the adatom distribution in the low-coverage disordered phase was carried out for Na/Cu(100) by measuring the diffuse elastic He-atom scattering at low Na coverages. A comparison of these data to extensive molecular dynamics calculations [102] indicates that the interadsorbate potential is significantly weaker than the dipole-dipole potential described by Kohn and Lau [103] for dipoles on a conducting surface. Thin films of epitaxial Na may be grown on Cu(100) at low temperature (60-140 K) [104]. Within the range of 2 to 20 layers of Na, the films exhibit a quasi-hexagonal structure which gradually evolves into a bcc (110) st~cture, witho the nearest-neighbor distance increasing from the substrate-induced value of 3.61 A to 3.66 A, which is the bulk value. 2.4.8. K / Cu(IO0) A rather extensive LEED investigation has been carried out for K/Cu(100) near room temperature. This study at 330 K found that the K overlayer is disordered until 0.18, when a "halo" pattern appears [105]. This diffraction ring is different from those observed in most

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other alkali metal systems at low coverage in that the radius of the ring remained constant up O to a coverage of 0.26. This radius corresponds to a nearest-neighbor distance of 5.6 A, which implies that the overlayer condenses into islands of this phase, which is believed to be a liquid [106] and which eventually covers the surface at a coverage of 0.26. The driving force for this condensation is believed to be the depolarization of the adatoms with increasing coverage, which renders them essentially neutral, making condensation into a metallic phase energetically favorable [106]. A He-atom scattering specular reflectivity study at this coverage observed a phase transition into an ordered phase between temperatures of 123 and 223 K [107]. At a coverage of 0.28 the overlayer crystallizes into a quasi-hexagonal higher-order commensurate structure denoted as (2 5~3) [105]. In this structure there are two equivalent domains rotated 90 ° from each other. At somewhat higher coverages, this solid phase compresses continuously into another quasi-hexagonal higher-order commensurate structure, denoted as (2 3~2) [105]. Finally at a coverage of 0.33, the overlayer compresses into a uniform hexagonal lattice, which is rotated by about 3.3 ° relative to the orientation of the commensurate phases. As the density increases, the rotational angle increases to about 6.0 ° at a coverage of about 0.37, which corresponds to monolayer saturation at this temperature [108]. (See the K/Ni(100) section for a comparison of this rotationalO behavior to various models.) The saturation structure corresponds to a K - K distance of 4.57 A, somewhat smaller than the metallic K diameter of 4.76 A. The film growth mode for K/Cu(100) below room temperature has been studied using He-atom specular reflectivity measurements and has been determined to be of the StranskiiKrastanov type, with the formation of 3D islands after one complete K layer is formed [107]. Cu(IOO)-K disordered RT." Surface X-ray diffraction experiments of the disordered phase [109,110] near room temperature (330 K) indicate that the K adatoms reside in the hollow sites in the disordered phases, which would imply an essentially random occupation of sites until the solid phases are formed. In theo disordered phases the vertical overlayer-substrate spacing remains constant at 2.25 + 0.15 A, corresponding to an effective K radius of 1.6 A. In the solid phases, the observed progression of phases was somewhat different from the earlier L E E D study [106]. The uniaxially incommensurate phase has strong density modulations as measured by the superlattice refections, consistent with a displacement amplitude of 1.3 A [109,110]. This contrasts with the cases of Cs/Cu(100) [109,110] and K/Ni(100) [111,112] where only weak density modulations were observed.

2.4.9. Cs / Cu(lO0) The phases of Cs/Cu(100) have been studied at both room temperature [110,113,114] and 150 K [113]. At both temperatures the overlayer has no long-range order until close to monolayer saturation, although as expected, long-range order appears at a somewhat lower coverage for the low-temperature case. At room temperature the overlayer is disordered until a coverage of approximately 80% of the saturated monolayer, then it orders into two domains of a quasi-hexagonal structure which is uniaxially commensurate, as shown in Fig. 17 [113]. The structures formed at the lower temperature were in fact very similar to those observed at room temperature, except that the quasi-hexagonal layer was observed at a lower coverage, about 70% of the saturation coverage. At this point, a true hexagonal overlayer was observed, with a lattice constant of 5.77 A. Once this layer was compressed to 5.10 A, the commensurate

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R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

(

a l = 5.1A

a2 = 9.13/~

Fig. 17. Quasi-hexagonal structure for Cs/Cu(100) [113].

spacing in Fig. 17, additional increase in coverage caused further compression onloy along the direction perpendicular to the commensurate direction, to a spacing of 8.22 A [113]. At saturation the coverage was 11% higher than for room temperature saturation. Cu(lOO)-Cs disordered RT: From the room temperature LEED study [113] it is not possible to infer much about the disordered phase, but a more recent surface X-ray diffraction study indicates that while there is no long-range order, the adatoms occupy different adsorption sites and possibly form locally ordered islands of the quasi-hexagonal structure which forms at higher coverage [110]. The perpendicular adsorbate-substrate distance was constant (2.94 + 0.15 A) over the whole submonolayer coverage range, which corresponds to an effective Cs radius in the range 1.93-2.18 A. Such a uniaxially incommensurate layer might be expected to have density modulations, but these were found to be very small, in contrast to K/Cu(100) [109,110], as discussed in the previous section.

2.4.10. Cs or K / C u ( l l O ) K or Cs adsorbed onto Cu(ll0) at low temperatures form quasi-hexagonal structures on the surface, but increasing the temperature to room temperature induces a missing-row-type reconstruction of the Cu surface [115,116]. The quasi-hexagonal structure is in fact a uniaxial incommensurate phase, being commensurate in one direction and continuously compressing in the perpendicular direction as the coverage is increased. The overlayer interatomic spacings at the saturated monolayer are 4.3 A for K and 4.7 .~ for Cs, about 5% and 10% compressed relative to their bulk spacings, respectively [115,116]. These structures are more thermally stable at higher coverages as evidenced by higher disordering temperatures [115]. By dosing at room temperature, or by dosing at low temperature and heating the surface, an alkali-induced reconstruction occurs. Temperature-dependent measurements indicate that the reconstruction begins to appear at about 150 K. The symmetry of this reconstruction changes

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75

with coverage: at coverages around 0.12 the symmetry is (1 x 3), at coverages around 0.14 the symmetry became (1 x 2) and then above about 0.3 the (1 x 2) phase changed back to (1 x 3). The L E E D patterns corresponding to intermediate structures are often somewhat streaky [115]. A dynamical L E E D study of the (1 x 2) structure indicated that the most probable structure is a missing-row structure with an 11.7% contraction of the top interlayer spacing and a row-pairing lateral shift of 3% in the second layer [117], similar to that observed for other fcc (110)(1 x 2) reconstructions (see Table 17). A more detailed picture of this reconstruction is provided by a photoelectron diffraction study, described below. If the temperature of this surface is lowered, weak streaks in the pattern which are observed at room temperature break up into a p(2 x 2) modulation which is believed to result from ordering of the K or Cs atoms across the rows in which they reside in the missing-row structure. Further deposition of alkali results in the compression of the alkali atoms within the rows [115]. The (1 x 3) reconstructed structures of Cs or K on Cu(ll0) have been further studied using STM [118-120]. The STM observations at a coverage of 0.13 indicate that this structure also is a missing-row structure, with every third row of Cu atoms missing. The resulting troughs are occupied by alkali atoms. A slight decrease in the coverage leads to the formation of an incommensurate phase with domain walls. The domain walls separate regions of (1 x 3) structure and themselves consist of local (1 x 4) units, where two alkali-filled troughs are separated by three Cu rows. An STM image from this structure for Cs/Cu(110) is shown in Fig. 18 [118]. In this study, the (1 x 3) structure was found to disorder continuously as the coverage was lowered, first by the formation of domain walls into the (incommensurate) structure shown in Fig. 18, and finally into an incommensurate fluid phase, possibly via the creation of free dislocations in the domain walls (see Figs. 18b and 18c) [118]. Further STM studies of K / C u ( l l 0 ) indicate that the K-induced reconstructions proceed in a strictly local nucleation and growth process [119,120]. A single K atom removes on average 2.5 Cu atoms out of a string of unreconstructed (1 x 1) substrate and then is accommodated in the resulting hole. The K adatoms interact by net attractive interactions along the troughs, but by repulsive interactions in all other directions. See Section 3.8 for further discussion. Cu(110)-(1 x 2)-K, 0.2 coverage, RT: The adsorption site of K on the (1 x 2) reconstructed Cu(ll0) surface was determined using photoelectron diffraction for a K coverage of 0.2 [121]. The K atoms were found to reside on the missing-row troughs, in 4-fold hollow sites which would have been occupied by Cu atoms on a perfect (unreconstructed) (110) surface. This substitutional adsorption site is consistent with the STM results described above. The nearestneighbor K - C u bondlength was found to be 3.27 + 0.27 A, and the next Cu neighbor which is in the third Cu layer, is 3.60 + 0.04 ,~ directly below the K adatom. A small accompanying rumpling of the top copper layers was observed, its magnitude smaller than that determined from the LEED study described above [117]. Measurements taken at lower coverages were consistent with the same local structure. O

2.4.11. K / C u ( l l 5 ) He-atom scattering studies of K adsorbed on Cu(ll5) [122] indicate that small amounts of K cause the whole surface to form facets of the (114) orientation. At higher coverages the surface defacets, returning to the flat (115) surface. This faceting-defaceting progression is very similar to the reconstruction-deconstruction transitions observed for alkali metal adsorption on fcc

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l~'ig. 18. (a) STM image of the Cu(ll0) (1 x 3) domain-wall structure at a coverage of 0.12 (795 x 800 A) [118]. (1 x 3) domains are separated by walls, exhibiting local (1 x 4) structure (the Cs-filled troughs are imaged as protrusions). (b) Single and (c) paired dislocations of the wall pattern; dislocations are marked by circles. (110) surfaces (see Section 3.8). The faceting transition is found to be continuous, which is not consistent with a nucleation and growth scenario. At higher coverages a structure with a 2-fold period relative to the (115) surface is observed, which is interpreted as a substrate reconstruction. Interestingly, this structure is only stable under K flux for temperatures above 200 K [122].

2.4.12. Cs/Cu(211) or /Cu(511) L E E D studies of Cs adsorption on Cu(211) and Cu(511) [123] determined that on both substrates, Cs forms quasi-hexagonal structures which correspond to rows of Cs atoms running along the direction of the step edges of the substrate. The Cs atoms along the step edges were

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77

compressed by about 15% relative to their metallic size. On Cu(511), sufficient Cs was also observed to induce a double-step reconstruction of the substrate. This process was thermally activated and was observed to occur at temperatures above 250 K.

2.4.13. Coadsorption of Cs and 0 on Cu(lll) Ordered structures for mixtures of Cs and O on C u ( l l l ) were inferred from L E E D studies [124], which indicated that there were at least two and possibly more ordered coadsorbate structures. This, along with work function results, was interpreted as evidence that oxygen mixes with the Cs on the surface, first occupying positions next to the substrate and at higher coverages forming a second oxygen layer. At certain concentrations ordered structures are formed, but in general, the atomic arrangement is similar to that of a disordered binary solid solution [124]. 2.4.14. Coadsorption of K or Cs and 0 on Cu(lO0) The same general picture has been obtained for Cs and O on Cu(100) [114] and K and O on Cu(100) [125], i.e. that independent of whether oxygen or alkali is adsorbed first, the oxygen initially forms a layer beneath the alkali layer, then forms a layer on top. The K + O/Cu(100) study showed that this model is really only consistent with the data when there is a metallic overlayer of alkali metal to adsorb beneath, and at lower coverages, which essentially means below the work function minimum, a single mixed K and O layer is formed [125]. 2.4.15. Coadsorption of Na and 0 on Cu(llO) The same sort of scenario as described above was observed for (Na + O ) / C u ( l l 0 ) , where it appeared from high-resolution electron energy loss spectroscopy (HREELS) that the oxygen adsorbed on Na-saturated Cu(ll0) was bound directly to the Cu atoms. But in addition, there was experimental evidence that some of the Na may have either migrated into the subsurface region, i.e. that it intermixed with the substrate, or that Na atoms formed a surface sodium superoxide (NaO 2) [126]. Annealing this surface to 645 K produced an ordered ((2v~ × f3-/2)R35.3 °) surface sodium peroxide (Na20 2) structure which exhibited LEED patterns consistent with a stepped surface having average (110) terraces of 15.5 A width and (111) steps 2.64 A high. Further annealing to 935 K converted the sodium peroxide completely into a saturated atomic oxygen overlayer with no sodium present in the surface region [126]. 2.4.16. Coadsorption of K or Cs and 0 on Cu(llO) STM studies were carried out on room-temperature adsorption of oxygen on potassium- and cesium-precovered Cu(110) surfaces [127]. Depending on the precoverage, two different scenarios were observed for the formation of the coadsorbate phases. For alkali precoverages below 0.13 (below the formation of the (1 × 3) reconstruction, see above), O adsorption leads to a contraction of the localized (but not long-range ordered) missing rows into islands of a (1 × 2) structure. After a longer exposure, the reconstruction is locally removed and a new structure, referred to as the (2 × 1) C u - O "added-row" structure, forms. Fig. 19 shows a schematic diagram of a proposed model for this structure. In this model, alkali adatoms, which before O adsorption reside in the missing-row troughs, become incorporated into a C u - O chain structure which runs perpendicular to the trough direction. The O coverage in this structure is 0.5.

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R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

0 1.1-"

[001]

Im

Fig. 19. Model of the Cu(110)-(2 x 1)-alkali + O reconstruction [127]. The small black circles are O atoms, mediumsized circles are Cu atoms and the larger clear circles are alkali metal atoms.

At higher alkali precoverages, where the (1 × 2) reconstruction occurs for alkali adsorption, O atoms are incorporated into the existing (1 × 2) structure until oxygen coverages above 0.5, when complex O - a l k a l i - C u structures are formed. The adsorbate-substrate interactions are considered to be very important in the formation of these structures.

2.5. Gold The adsorption of alkali atoms on gold surfaces does not exhibit the sort of simple phase behavior as for alkalis on Cu or Ni. This is due in part to the existence of reconstructions on the A u ( l l l ) , Au(100) and Au(ll0) surfaces. To our knowledge, no quantitative study has been carried out on alkalis adsorbed on Au surfaces.

2.5.1. Na /Au(111) The A u ( l l l ) surface is reconstructed at room temperature. Domains of a v~- × 22 structure are formed due to uniaxial contraction along the (ll0>-type directions. Gold atoms occupy fcc, hcp and bridge-type stacking positions of the underlying lattice [128]. Additionally, a long-range superstructure is formed by the change in orientation of domains by 120° to form a chevron pattern at intervals of about 220 A [129-131]. Adsorption of Na on this surface up to a coverage of 0.25 was studied using STM [128]. For a Na coverage of 0.15 the periodicity of the clean surface chevron structure is reduced from 220 to 150 A; additionally step edges appear rather frizzled. These changes are assigned to an adsorbate-induced weakening of the coupling of the top-most Au layer to the substrate. A similar effect occurs when the clean surface is annealed to above 600 K [129,131]. For higher Na coverages up to 0.25 the chevron periodicity decreases further until the long-range order is lost and a distorted hexagonal structure is formed. Rather different effects are seen upon adsorption to 0Na = 0.50. Deposition at 85 K leads to quasi-hexagonal structures as seen by LEED [132]. The clean surface reconstruction is main-

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79

Fig. 20. 120X 120 A2 image of the moir6 structure formed on A u ( l l l ) after annealing the 0.5 Na coverage to 600 K [132]. The (13)X(13) and (1.08v~ X 1.08v/3-)R30° unit cells are marked by a rhombus.

tained at this temperature. Adsorption to 0Na = 0.50 at 300 K leads to a surface with irregularly shaped poorly ordered islands of diameter 50-500 ,~ randomly distributed over larger terraces on the surface, with removal of the clean surface reconstruction. The islands are found to consist of a mixed adlayer of both Au and Na atoms, with a hexagonal structure and a (1.08v/-3 × 1.08vr3)R30 ° unit cell. The terraces have a rectangular c(4 × 2) periodic structure existing in three 120° orientational domains. This is interpreted to be a mixed layer with Na atoms substituting every fourth Au surface atom. After annealing to 600 K the c(4 X 2) areas disappear and the whole surface has the (1.08v/3 - x 1.08f3-)R30 ° periodicity, with a long-range 38 A moir6 pattern caused by a lattice mismatch between the substrate and surface layers (Fig. 20). The surface layer is determined to be of mixed NaAu 2 composition with a Na coverage of 0.28, the Au atoms forming a honeycomb lattice with the interstitial sites occupied by Na. The surface atoms occupy different adsorption sites on the second layer. The extra Na atoms are suggested to form a dilute 0Na = 0.2 layer covering this structure which is transparent to the STM [1321. 2.5.2 K/Au(lll) For K adsorption on the A u ( l l l ) surface, a decrease in chevron periodicity is observed, similar to that for the Na/Au(111) system described above [128], although no transformation to a distorted hexagonal phase was observed.

2.5.3. K/Au(IO0) The Au(100) surface has a complex (5 X 20) reconstruction. There have been a number of L E E D studies of the adsorption of K on this surface. Adsorption at room temperature lifts the (5 x 20) reconstruction and a (1 × 2) overlayer is formed, but there is disagreement over the coverage at which this occurs [96,133].

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Fig. 21. STM image of the K/Au(100) system at a K coverage of ~ 0.20 [133]. The bright and dark regions are different level regions of the K-induced (1 x 2) reconstruction while the lines are the remnant ribbons of the Au (5 x 20) reconstruction.

This system has also been studied using STM [133]. In this study, the (5 x 20) reconstruction is maintained upon K adsorption up to 0.1 coverage. Higher K coverages lead to the formation of islands of a K-induced reconstruction whose area grows with increasing coverage, until at OK = 0.25 the (5 x 20) reconstruction is completely lifted and a (1 x 2) phase is formed. The surface density of Au atoms in this phase is 0.5. Upon high-temperature annealing of the K-covered surface a complex intermixed surface phase is formed with a c(6v~ x 2~-)R45 ° unit cell. The growth of the (1 × 2) islands proceeds anisotropically along the ribbons of the (5 x 20) reconstruction and appears to involve mass transport of surface Au atoms over hundreds of ~ngstr6ms. Fig. 21 is an STM image of the surface at a K coverage of 0.2. Rectangular (1 X 2) islands at two levels (corresponding to the bright and dark regions) are formed in the vicinity of the (5 x 20) reconstruction, A potassium-induced missing-row reconstruction is proposed for the (1 x 2) structure [133].

2.5.4. K/Au(llO) The A u ( l l 0 ) surface exhibits a (1 x 2) missing-row reconstruction, where every second [110] row of atoms is absent. An extensive LEED study has been carried out for K adsorption on this surface. A partial phase diagram was obtained (Fig. 22). In this system, the K atoms induce islands of a.reconstructed c(2 x 2) phase which coexist with the missing-row (1 X 2) reconstruction at coverages greater than 0.3 [134]. At a coverage of 0.5 the surface is completely covered by the c(2 × 2) phase. In STM measurements of the system the K atoms are invisible [135]. It was suggested that this is because the alkali atoms are incorporated in the missing rows and that the alkali-metal-

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

81

1ooo -

8(10

6(X)

2O0 t

()

I

I

I

I

I

0.5

I

I

I

I

I

I

I

1,o

Fig. 22. P h a s e d i a g r a m for t h e K / A u ( 1 0 0 ) system, a f t e r Ref. [134].

induced wave functions near the Fermi level have anti-bonding character and do not conduct electrons easily.

2.5.5. Coadsorption of K or Na and CH~CN on Au(lO0) The effects of coadsorption of alkali atoms on the acetonitrile (CH3CN) C - N bondlength has been studied using C K-edge NEXAFS [136]. It was first established, from the angular variation of the intensity of the o-c*N resonance, that the molecular C - C - N axis is almost flat. Then the shift in position of this resonance was used to determine the change in the C - N bondlength, using the so-called "bondlength-with-a-ruler" concept along with a line-shape analysis of the profile of the resonance. The molecular interaction with the alkali atoms was found to reduce the C - N bondlength by 0.04 ,~ (Na) and 0.07 ,~ (K). It was proposed that this contraction is due to a combination of at least three contributions: (i) an electrostatic "through-space" interaction between the molecule and the polarized alkali atom, (ii) a direct chemical interaction between the alkali atom and the molecule and (iii) a chemical interaction between the alkali atom and the molecule mediated through the substrate. 2.6. Graphite While graphite is technically a semi-metal, the behavior of K, Rb and Cs on the basal plane of graphite is very similar to that on metals. At low coverages the adatoms are widely spaced in a phase having no long-range order but having a reasonably well-defined nearest-neighbor distance which varies continuously with coverage. However, for K and Rb, at some critical coverage a condensed solid hexagonal phase begins to form in coexistence with the disordered

82

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171 Ea d

r-

~di

barrier

/

spersed~

/

<

J

I

0.1 1.0 Coverage (no. of monolayers) Fig. 23. Schematic potential energy curve governing the thermodynamic behavior of submonolayer alkalis on graphite [140]. The barrier arises mainly from the band energy term, giving rise to a discontinuous transition with increasing coverage. Inset: schematic diagram showing the p(2 × 2) structure of K adsorbed on graphite.

phase until eventually the whole surface is covered by the condensed phase [137,138]. This type of condensation phenomenon occurs on some, but by no means all, metal substrates. In the case of K adsorption on graphite it has been proposed that the condensation mechanism arises from a change in the electronic band energy which is peculiar to the 2D semi-metal band structure of graphite [139]. The general picture is illustrated schematically in Fig. 23: at low coverages, the energy is a minimum when the adatoms are dispersed due to the dipole-dipole repulsion. As the coverage is increased, the low-density dispersed phase becomes costly in energy due to the increase in band energy, and condensation into the higher-density metallic phase, which does not contribute to the band energy or the electrostatic repulsion, occurs [139,140]. The condensed solid phases for alkalis on graphite form at temperatures well below room temperature; at room temperature the overlayer is disordered, and furthermore, the alkali metal atoms intercalate into the graphite, causing the surface coverage to decrease over normal experimental timescales [141]. The details of the overlayer phases observed for K, Rb and Cs are presented in the following sections and in Fig. 24. The behaviors of Na and Li on graphite are apparently quite different from the heavier

0.02 "ring" 0:015 /

(b)

(~7x"/7)R 19"1* 0.~4 0.16

I t

0.22 ,

0.25 .

I,I,'Irotat ................... _

Fig. 24. Schematic representation of the progression of equilibrium phases as a function of coverage for (a) K and (b) Cs on graphite. The coverage (horizontal) axis range is from 0 to 0.25.

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83

alkalis in that no ordered structures have ever been observed. Photoemission studies were interpreted as indicating that the Na-graphite bond is very weak compared to the heavier alkalis [142]. He-atom scattering studies are consistent with the growth of Na 3D clusters or crystallites rather than a 2D overlayer [143,144]. The fact that Na seems more inclined to bond to itself than to graphite is thought to be related to the fact that Na does not readily intercalate into graphite [142]. Early L E E D studies have indicated that Li also does not form an ordered overlayer on graphite [145], although photoemission studies indicate that the Li-graphite bond appears to be more similar to the heavier alkali metals than to the Na-graphite bond. Therefore it is possible that the absence of the observation of an ordered structure may be due to the experimental temperatures not being low enough to produce an ordered layer. Since Li intercalates into graphite quite rapidly even at temperatures as low as 80 K [142,144], indicating a high adsorbate mobility, the observation of ordered overlayers may require studies at significantly lower temperatures. While L E E D studies of the alkali overlayers on graphite are straightforward and indicate well-ordered phases at the higher coverages, He-atom diffraction intensities are very small from these systems [146], apparently due to the extremely small corrugation of the He-alkali overlayer potential, smaller than would be expected from the superposition of atomic charge densities [146]. However, relatively large peaks due to nearly dispersionless overlayer vibrational modes are observed in the inelastic He-atom scattering spectra [147]. These peaks are attributed to the mostly-in-plane vibration of the alkali adatoms [144,147]. The He-atom scattering intensities from the out-of-plane modes have not been observed due to their lower intensities, which perhaps implies that the in-plane vibrational amplitudes are much larger than out-of-plane vibrations, an observation which has also been made in the NISXW studies of R b / A I ( l l l ) [40,52], R b / C u ( l l l ) [88], in SEXAFS studies of K / N i ( l l l ) [79], and in LEED studies of alkalis on Ru(0001) [20] and alkalis on A g ( l l l ) [148]. It is still not understood why the scattering from the in-plane modes is so much stronger than that from the out-of-plane modes, which normally are much more intense in He-atom scattering [144,147]. The energies of the in-plane modes on graphite were found to be 4.4, 2.8 and 2.3 meV for the p(2 x 2) phases of K, Rb and Cs, respectively [147].

2.6.1. K/graphite Potassium has been the most thoroughly studied of alkali metals on graphite. A very detailed LEED study [137,140] has clearly shown the progression of the dilute phase, followed by a condensation into the p(2 X 2) commensurate phase at OK = 0.02. This progression of phases with increasing coverage is shown in Fig. 24a. A DFT calculation for K/graphite at the p(2 X 2) density has indicated that the lowest-energy configuration has the K atoms in the hollows [149]. This calculation also indicates some buckling of the graphite surface, but the magnitude is not realistic since the model includes only one layer of graphite atoms. An early L E E D study [150] had indicated the existence of a (x/3 × v~-)R30 ° phase at a higher coverage, but subsequent studies showed that the monolayer saturates at the p(2 x 2) phase [137,146]. The p(2 x 2) phase corresponds to a K - K spacing which is expanded by approximately 4% relative to its metallic diameter of 4.76 ,~. The growth of an ordered second layer has not been observed for K/graphite [144]. Structural studies at 30 and 50 K [140] indicate

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that the lateral mobility of potassium at these temperatures is restricted in the solid phase, resulting in the formation of metastable structures. In the dilute fluid phase, however, intercalation and hence mobility was found to be relatively high at temperatures as low as 50 K [141].

2.6.2. Rb / graphite Very little detailed work has been carried out on Rb/graphite, but LEED characterization of the overlayer indicates that at low coverages a dilute phase exists, similar to that observed for K, and that condensed islands begin to grow at a critical coverage at the expense of the dilute phase [143,144]. No phases have been observed having a density higher than the p(2 x 2), which has a lattice constant (4.92/~) only slightly larger than the metallic Rb diameter (4.84 A). At about 80 K it seems to be possible to grow a second layer of Rb, but multilayer growth at low temperature did not occur as for Cs [144]. The progression of the observed phases is probably similar to that shown in Fig. 24a.

2.6.3. Cs ~graphite An early determination of the coverage-temperature phase diagram for Cs on graphite [151,152] indicated that the most stable phase of Cs on graphite is a (v~- x f3-)R30 ° phase which extends to temperatures as high as 350 K. Later experiments in other laboratories failed to observe this phase unless some other gas was coadsorbed [142,143,153]. However, the lower-coverage phases are consistent with later observations. A schematic diagram of the observed phases as a function of coverage is shown in Fig. 24b. In particular, there is a progression of low-density phases at low temperature, starting with the dilute fluid (ring phase) which is observed to a coverage of 0.015 [153]. At higher coverages the overlayer appeared to be disordered according to one study [153] but diffraction rings persisted to higher coverages in another study [144]. In either case, the overlayer is disordered. At a coverage of 0.14, a (v/ff x eft)R19.1 ° phase is observed [152,153]. Between 0.16 and 0.22 coverage, the density of this phase increases continuously and the rotation angle varies in a way which is not quite consistent with the Novaco-McTague (NM) model [154,155] of rotational epitaxy for a Cauchy solid [156]; however, adjusting the ratio of sound velocities to 1.63 from the Cauchy value of f 3 provides a good fit to the data. Fig. 25 shows the rotation angle as a function of lattice misfit for this phase, along with the NM predictions. At coverages near 0.25 the structure is a commensurate p(2 x 2). This appears to be the highest density attained in the monolayer [143,153] and the Cs-Cs spacing of 4.92 A corresponds to an approximately 10% compression relative to the metallic Cs diameter of 5.46 A. At higher coverages, multilayers of Cs grow at low temperatures [144], and ultimately the film, at thicknesses greater than about 5 layers, appears to have the symmetry of the bcc (110) surface [144]. It should be noted that there is no evidence for the type of condensation transition which occurs for K and Rb on graphite. Instead the overlayer density varies much more continuously. Graphite-p(2 x 2)-Cs LT: A dynamical LEED study of this structure has determined that the Cs atoms reside in the hollow sites at a distance of 2.8 ,~ above the unrelaxed graphite basal plane [157], which corresponds to an effective radius of 2.41 A, somewhat smaller than the metallic radius of Cs. O

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

14 I Cs/graphite 1 ,..

1o e.-

" "r"

85

........./"''" ..y..."

...y ,..""

8

.,y" ..,....,."""

6

2 0

0

0.04

(LOg

0.12

0.16

(1.2

0.24

(I.28

Misfit Fig. 25. Epitaxial rotation angle for the hexagonal incommensurate phase of Cs/graphite as a function of lattice misfit relative to the p(2x2) structure, after Ref. [156]. The solid curve indicates the NM prediction with C L / C T = 1.63 and the dotted curve indicates the Cauchy value of C L / C T = v/3. The experimental data are shown as black squares.

2. 7. Iridium Only adsorption on the Ir(100) surface has been investigated. This surface may be prepared as a (5 x 2) reconstructed surface or with a bulk-like (1 x 1) termination [158]. On the unreconstructed (1 x 1) surface, the phase diagram appears to be similar to that for other metals, being dominated by dipole-dipole repulsions. On the reconstructed (5 x 1) surface however, the alkali adatoms sit in the substrate troughs, forming linear chains across the surface.

2.7.1. K / Ir(lO0) The low-temperature (100 K) adsorption of K on the (1 x 1) and (5 X 1) surfaces of Ir(100) was investigated using L E E D [158]. On the bulk-terminated (1 × 1) surface, five well-ordered phases were observed as a function of coverage (0.125 _< OK _< 0.5); these structures developed continuously with increasing coverage. The saturation structure was a c(2 x 2) structure. Real-space structures, shown in Fig. 26, were proposed for the five well-ordered phases based on the assumption that in general the K atoms adsorb in the 4-fold hollow sites. This tendency is balanced by the adatom-adatom repulsion which favors hexagonal overlayers. The resulting proposed real-space structures are therefore quasi-hexagonal. Intermediate phases between these well-ordered structures were interpreted as microscopic mixes of structure elements corresponding to the initial and final structures of the transitions. The (5 x 1) reconstruction of Ir(100) has been determined to be a quasi-hexagonal top layer on the quadratic substrate. To match this hexagonal structure to the underlying quadratic structure, the top layer is rumpled so that linear troughs are formed. The K atoms appear to sit within these troughs and a succession of L E E D patterns is observed as a function of K coverage. The linear chains of K atoms order in such a way as to form quasi-hexagonal arrangements of adatoms [158] (see Fig. 27).

86

T(3-~

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

. . .

c(2v~.4~)R45 °

Q

O = g1 (v~. v~) R arctan ½

_

-(~ -;)

_

. . . .

"111 zI

,'

1 O = Z.

e(4x21

--T:"

O= 2--~

(3x21

[~'\ ",.,.

(~ I) 1

c(2x2) Fig. 26. Proposed structures for K adsorption on Ir(100)-(1 x 1) [158].

2.7.2. Cs /Ir(lO0) Cs adsorption on the (1 × 1) and (1 x 5) surfaces of Ir has been studied using LEED [159]. On the (1 x 1) surface adlayer formation took place with the sample held at 100 K (all

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

87

1--

00

' 1/5

10

0j . . . .

1j

0 =0.24 03 - 215 ~ - -?0

Ol . . . .

3' t"o o

.,,oI o

1j

~

!

o °I

{ .:0,

0j . . . .

12

3n0~/~_~ 00

0

0=0.42 7/10

10

Fig. 27. Proposed structures for K adsorption on Ir(100)-(5 x 1) [158].

structures observed were stable up to room temperature). At low coverage a ring pattern is observed. Increased adsorption leads to a sequence of patterns from a c(4 x 2) to a hexagonal close-packed phase, ending with compressed phases. Structural models are proposed based on mobility of Cs atoms along [10] troughs of the surface, with no reconstruction of the Ir surface. Adsorption at 100 K on the reconstructed Ir(100)(1 x 5) surface leads to a (5 x 5) pattern for the lowest coverage, and then a sequence of patterns as the atom density in the adlayer increases including a close-packed adlayer and a compressed phase. Again, structural models were presented which involve Cs adsorption in the troughs of the reconstructed surface. The Cs atoms move along these troughs as the coverage increases. No lifting of the surface reconstruction was observed [159].

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R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

2.8. Iron There are no low-temperature structural studies for alkalis on iron surfaces. At room temperature, adsorption behavior on the various surfaces is diverse with only the Fe(ll0) surfaces displaying hexagonal patterns. Clearly studies at low temperature would elucidate some of the room-temperature results.

2.8.1. K / F e ( l l l ) Adsorption of K on the F e ( l l l ) surface at room temperature produced just one ordered LEED pattern with (3 x 3) symmetry [160]. This was interpreted as a primitive K overlayer with a coverage of 1/9, the K atoms sitting in high-coordination 3-fold hollow B5 sites. For higher coverages no LEED patterns were observed. The quality of the LEED pattern was improved by annealing to 580 K and then cooling below 300 K.

2.8.2. K/Fe(IO0) No overlayer structures were observed using LEED for K adsorption at room temperature [160], even after thermal annealing to 450 K.

2.8.3. K/Fe(llO) No overlayer structure was observed in the LEED pattern for K adsorption at room temperature until 0 K = 0.31 (close to saturation). Then a hexagonal pattern was observed which was interpreted as indicative of a hexagonal K overlayer [160,161]. At the saturation coverage it was suggested that the K atoms are locked into this hexagonal structure whereas at lower coverage the mobility of the K atoms is too high for ordered structures to be observed.

2.8.4. Coadsorption of K and 0 on Fe(llO) Coadsorption of O and K gave rise to a c(4 × 2) LEED pattern at coverages of O K = 0.28 and 0 o = 0.5. No structure was proposed, and it could not be decided on the basis of spectroscopic data whether the K and O are in a single layer or whether the K forms a layer on top of the oxygen [162].

2.9. Lead Lead is among the least-studied substrates for alkali adsorption, which is perhaps due to difficulties in preparing good surfaces. Interestingly, Pb(ll0), like AI(ll0), does not undergo a (1 x 2) reconstruction upon alkali metal adsorption, as do the d-band fcc (110) metals. However, as described below, an alkali-induced reconstruction having a c(2 x 4) symmetry has been observed on a strained Pb(ll0) surface.

2.9.1. K/Pb(llO) The adsorption of K on Pb(ll0) at 200 K was studied using LEED [163]. Two ordered structures were observed: a c(3 x 2) at a coverage of 0.33 and a c(2 x 2) at a coverage of 0.5. Structures were proposed based on arrays of K atoms with different separations along the [110] direction. The adsorption site was assumed to be the 4-fold site in the case of the c(2 x 2) and a

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

89

mixture of 4-fold and 2-fold bridge in the case of the c(3 x 2). There was no continuous coalescence of the spots between the two structures; rather a fading of the (n/3, 1/2) spots and an emergence of sharp (1/2, 1/2) spots. No alkali-induced reconstruction was observed in this study, which was attributed to the fact that lead is an sp-bonded metal (see Section 3.8). A recent L E E D and low-energy electron microscopy (LEEM) study, however, indicates that reconstruction of this surface does occur on a substrate which is strained [164,165]. K-induced reconstructions having c(2 x 4) and c(5 × 2) symmetries were observed. The c(2 × 4) structure consists of microfacets of (100) and (111) orientations and occurs on the clean strained surface as well. In this study it was suggested that the origin of this reconstruction is shared with the more commonly observed fcc (110) reconstruction, that the reconstruction is an attempt by the surface to get to a lower-free-energy state by producing microfacets of (111) orientation. It is further suggested that the c(2 × 4) forms on Pb instead of the (1 × 2) because the (100) surface has a lower energy than the (111) surface, unlike the other fcc metals.

2.10. Molybdenum A considerable amount of structural work was carried out for alkali metal adsorption on Mo surfaces before much was known about alkali adsorption on the lighter transition metals [3,14]. Although the substrate corrugation appears in general to be larger than that on the lighter transition metals (as evidenced by a greater tendency to form commensurate structures), dipole-dipole repulsion is very important in the formation of ordered structures. In addition, substrate anisotropy on some surfaces leads to one-dimensional chain-like structures.

2.10.1. Li /Mo(llO) Several ordered phases were observed for Li adsorption on this surface at low temperature. These include (3 x 2), c(1 x 3), (3 x 1), (2 x 1) and (5 x 1) phases with increasing coverage. Chain models were proposed to account for some of these structures [166].

2.10.2. Cs /Mo(110) The surface structures of C s / M o ( l l 0 ) have been studied for the sub-monolayer coverages at 77 K [167]. For coverages up to about 0.15 the overlayer is fluid and increases in density with coverage, evidenced by rings of increasing diameter in the L E E D pattern. At higher coverage the overlayer crystallizes and forms an incommensurate hexagonal structure with two rotated domains. At a coverage at about 0.21 the overlayer rather abruptly aligns with the substrate. A further increase in coverage leads to a continuing increase in the density until monolayer saturation occurs at a coverage of 0.36. During this contraction of the overlayer two successive uniaxially incommensurate phases are formed, being locked-in first in the (112) direction and then the (110) direction. The saturation coverage corresponds to a 13% compression relative to the metallic Cs diameter. Continued Cs adsorption leads to the formation of second-layer islands having a local density equivalent to a coverage of 0.25. This corresponds almost exactly to the metallic Cs spacing. Measurements of Cs ionization energy loss lines led to the conclusion that Cs grows layer-bylayer for at least three layers [167].

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[ NalMo(100) I

(a) 3(N)f

(lxl)

250 c(2x2) e~

200 150 100

(b)

0.0

_L_ 1 0.2 0.4 Coverage

0.6

0.8

K/Mo(100)I 300 250 2O0

,50 1O0 ~

I :: I 0.0

O.I 0.2 Coverage

0.3

Fig. 28. (a) Phase diagram for the Na-induced reconstruction of Mo(100) [169]. (7v~ X ~ - ) is the clean-surface reconstruction at low temperatures; IC(2 x 2) refers to the incommensurate continuously varying phase described in the text. (b) Phase diagram for the K-induced reconstruction on Mo(100) [168].

2.10.3. Na /Mo(100) The clean Mo(100) surface reconstructs into a phase having (7V~-X v~-) symmetry at temperatures below 235 K. Adsorption of a minute amount of alkali metal causes a change in this structure [168]. As Na is adsorbed at 80 K, the ( 3 / 7 , 3 / 7 ) and ( 4 / 7 , 4 / 7 ) spots of the reconstruction LEED pattern continuously move toward each other, i.e. toward the c(2 x 2) positions. At a coverage at about 0.15, the c(2 x 2) structure is formed, which is interpreted as an elimination of the domain walls in the (7v~- x v~-) structure. At this coverage no ordering of the Na atoms is observed. This new reconstruction is stable to much higher temperature than the (7x/-2 x V~-) structure and persists until a Na coverage of 0.5. Increased intensity of the c(2 x 2) pattern near 0Na = 0.5 is interpreted as ordering of the overlayer. At higher coverages, a series of more complex patterns was observed due to compression of the overlayer into incommensurate structures and rotational ordering [168]. The first layer is completed at a coverage of 0.80. The phase diagram of the reconstructed phases is shown in Fig. 28 [169].

2.10.4. K /Mo(IO0) Adsorption of K onto the reconstructed (7v~- x v~-) surface at 80 K also causes the split spots of the L E E D pattern to move, except this time they move outward away from the c(2 x 2)

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91

positions first, forming a (5v~ x v~) pattern; then with further K adsorption they move continuously inward toward the c(2 × 2) positions, completing the formation of a c(2 X 2) structure at about 0.16 coverage [168]. As for Na, this is attributed to a reconstruction. The K overlayer forms an ordered (4 x 2) overlayer at 0.25 coverage and a c(2 x 2) at 0.50 coverage, which is also the saturation coverage. A phase diagram for the reconstructed phases is shown in Fig. 28 [168]. It has been proposed for both Na and K adsorption that the changes are caused by a local-bond mechanism rather than the charge-density-wave mechanism [168].

2.10.5. Cs /Mo(lO0) At room temperature, a succession of L E E D patterns is observed including a c(2 x 2) structure at 0.13, a p(2 x 2) structure at 0.25, a rectangular centered mesh at 0.30, for coverages between 0.30 and 0.43 a quasi-hexagonal structure and a true hexagonal structure at 0cs = 0.43 [170]. A similar sequence (with some intensity variations) was observed on a stepped Mo surface [170]. Models for each phase were suggested consisting of a periodic lattice distortion for the c(2 x 2) phase, a primitive p(2 × 2) array with Cs in the 4-fold hollow sites, and hexagonal arrays with inequivalent sites for the higher coverage phases.

2.10.6. Li /Mo(112) The phases formed during adsorption of Li on Mo(l12) at 77 K have been thoroughly characterized [171]. A succession of sub-monolayer phases is formed: a p(1 X 4) at 0.1 coverage, p(1 x 2) at 0.25, an incommensurate structure at about 0.5, a p(1 x 1) at monolayer saturation and a p(1 x 4 / 3 ) at higher coverage. The temperature stability of these phases has also been studied, yielding an effective phase diagram, though not presented as a coverage-temperature plot in Ref. [171]. Li chain structures were proposed for these phases. In these structures, the distance between the chains far exceeds the distance between the adatoms in the chain, apparently due to a strong anisotropy of the substrate. The structures are interpreted as indicating a repulsive interaction across the substrate rows and an attractive interaction within the rows [3]. The diffusion of Li atoms on Mo(l12) was studied using a masking technique during deposition, setting up certain concentration profiles (steps, trenches, stripes) [172]. The diffusion of Li atoms across the surface was measured as a function of annealing time at a constant temperature of 425 K (see Fig. 29). The main finding was of plateaus in the concentration profiles corresponding to a very rarefied Li phase on the surface with a Li-Li distance of at least 30 A.

2.10.7. Na, K or Cs / Mo(ll2) The adsorption of Na onto Mo(l12) at 77 K and at low coverages is similar to that of Li [3,171]. A p(1 X 4) chain structure is formed at about 0.1 coverage, followed by a p(1 x 2) chain structure at about 0.35. This is followed by a one-dimensional compression of the film until monolayer saturation occurs at a coverage of about 0.8. Interestingly, no chain structures are produced by adsorbing K or Cs on Mo(l12) [3,171]. For Cs, a c(2 x 2) structure is observed starting at about 0.2 coverage and remaining until the monolayer saturates at about 0.65 coverage [3].

92

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

i Li/Mo(ll2)]

O.02t

T=425 K

I

0

-0.5

0.5

x(mm) Fig. 29. Evolution of a plateau in the concentration profiles of Li atoms on Mo(112), starting with a step-like concentration profile (1). Annealing times (425 K) are (2) 2 min, (3) 26 min, (3) 65 min. The plateau seen on the right-hand side in (3) and (4) correponds to a continuous transition as a function of Li coverage. The width of the plateau means that the concentration of adatoms is of the order of 1/50 ML coverage, which implies a nearest-neighbor distance of at least 30 .~ [172].

2.10.8. Coadsorption of Na or Cs and 0 on Mo(lO0) The phases formed for Na [173] or Cs [170] and O coadsorption reflect the complexity of these systems, in that the structures formed appear to depend on which species is adsorbed first. The phase diagram for Na-first adsorption at room temperature is shown in Fig. 30. For O adsorption on a Cs monolayer, coexisting c(2 x 2) and p(4 × 1) patterns were initially observed, the c(2 × 2) pattern fading as the dose was increased [170]. For Cs adsorption on a c(2 x 2)-0 overlayer, a sharp c(2 x 2)-Cs-O structure was found [170]. Structural models were proposed in this work for both the c(2 x 2) and p(4 x 1) phases which involve displacement of the Mo atoms in the first layer by Cs and the formation of a top layer of O and Mo atoms. i (Na+O)/Mo(100)I 0.5 0.4

c(2x2)/+2)

0.3 O3

u 0.2

/

/~xl)

(lxl)

o.1 0.0 0.0

0.5

1.0 O Coveragc

1.5

2.0

Fig. 30. Phase diagram for Na + O adsorption on Mo(100) at room temperature [173]. These structures were formed by dosing Na first. The structures are also shown for O-only adsorption (below Na coverage = 0).

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

93

2.11. Nickel There have been many studies of alkali metal adsorption and coadsorption on Ni surfaces and many of these have been reviewed recently [19]. As described in that review, there are strong similarities between the overlayers on Ni and those on the corresponding Cu surfaces, probably owing to the fact that their lattice parameters are only different by a few percent. The difference between the filled and unfilled d-bands of Cu and Ni, respectively, does not seem to have serious consequence for alkali adsorption structures, but may be more important in the coadsorption systems.

2.11.1. Na /Ni(111) An early room temperature study of N a / N i ( l l l ) found that the structures are essentially uniform hexagonal overlayers with density increasing continuously [174], as found for both C s / N i ( l l l ) and K / N i ( l l l ) discussed below. A solid hexagonal phase is not observed until a coverage of 0.33 at room temperature and the onset of the second layer occurs at 0.45, although the monolayer does not saturate until it reaches a density equivalent to 0Na = 0.49 and a N a - N a distance of 3.54 ,~. An interesting feature of this overlayer is that the solid remains aligned with the substrate symmetry directions over the whole coverage range, forming neither epitaxially rotated phases nor rotated commensurate structures, such as the (v~ × v~)R30 ° phase for instance. This is quite different from Na adsorption on other hexagonal substrates and might be due to step pinning, but further experiments would be required to test this.

2.11.2. K / N i ( l l l ) The submonolayer coverage-temperature phase diagram for K / N i ( l l l ) , shown in Fig. 31, was determined by LEED and consists mainly of hexagonal-symmetry overlayers which increase continuously in density with coverage [175-177]. At low temperatures and coverages there is an isotropic phase which appears as rings in the LEED patterns. At coverages above about 0.12 these rings coalesce into modulated rings, indicating a fluid with bond-orientational order, and

500 K/Ni(lll)

p(2x2)

incommensurate solid

400 modulated /

~ 300

N 2oo

m

isotropic fluid

I 0

~ , ~

0.1

"°iset

2nd layer

1 0.3 0.2 Coverage

0.4

Fig. 31. Submonolayercoverage-temperature phase diagramfor K/Ni(111) [176].

94

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

finally into sharp spots indicative of a hexagonal solid phase. This phase compresses continuously, passing through the commensurate p(2 X 2) phase until it reaches its saturation coverage of about 0.31. The commensurate p(2 x 2) phase, discussed below in more detail, is clearly the most stable structure, disordering at about 380 K, while the disordering temperatures of the incommensurate phases at higher and lower densities are generally 300 K or less. The L E E D results are consistent with an overlayer which has strong repulsive interactions over the whole submonolayer density range, i.e. there is no evidence that the overlayer condenses into islands at any coverage [175]. Furthermore, the density modulations of the incommensurate phase, as estimated by the relative intensities of the split spots in the L E E D pattern, appear to be very small, which is also indicative of strong a d a t o m - a d a t o m interactions. However, the substrate does exert an important influence on the overlayer since in the incommensurate phase, the overlayer is always ordered rotationally along a high-symmetry direction of the substrate. This may be an effect of substrate steps, but the relatively high disordering temperature of the commensurate p(2 × 2) phase is indicative that the substrate lateral energy variation is not negligible. To study the overlayer-substrate distance as a function of alkali coverage, a constantmomentum-transfer averaging (CMTA) L E E D study was carried out by measuring the specular L E E D intensity as a function of beam energy at several incidence angles and averaging to reduce the multiple diffraction effects, then transforming the resulting spectra to obtain the interlayer spacing [178]. The measured distance was 2.7 + 0.1 A which is consistent with the K - N i bondlength measured by LEED [176,179] (see below). In that study, the interlayer spacing between the K layer and the top Ni layer was found to be 2.70 or 2.82 A for inequivalent Ni atoms in the surface unit cell. The main result of the CMTA study, however, was that the perpendicular distance does not change between coverages of 0.13 and 0.275. The low-coverage incommensurate solid, between 0.17 and 0.22 coverage, was studied in some detail [91] and found to be similar to those observed for K and Cs on C u ( l l l ) . The solid overlayer disorders into a fluid phase having long-range bond-orientational order. The transition temperature was found to be strongly dependent on the coverage, varying from 130 K at 0.17 coverage to 230 K at 0.21 coverage, as shown in the phase diagram. The radial and angular diffraction profile measurements indicate that this transition is consistent with the predictions made by the KTHNY theory [81-83] for the intermediate hexatic phase in two-dimensional melting, but substrate effects or other intrinsic effects could not be ruled out as a source for the orientational order. The melting transition is reversible and apparently continuous, and the orientationally-ordered fluid phase exists over a very wide temperature range, 50 K or more. Ni(111)-p(2 × 2)-K LT." The geometry of the commensurate p(2 × 2) phase has been studied using no less than three different experimental techniques in four separate studies. First, a LEED study at 120 K showed that the adsorption site is the atop site and that there is a small but significant substrate relaxation [176,179]. This relaxation involves a possible lateral motion of the substrate atoms as well as vertical relaxation in the first and second substrate layers, as shown in Fig. 32. The potassium-nickel bondlength was found to be 2.82 + 0.04 ,~. Second, a SEXAFS study at 70 K confirmed the occupation of the atop site, but the K-Ni bondlength was somewhat longer, 2.92 + 0.02 A [79]. The SEXAFS study was unable to determine substrate relaxation parameters due to the unusually large amount of site "disorder" in the overlayer parallel to the surface, even at 70 K. Third, an angle-resolved photoelectron fine

95

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

A'

Fig. 32. Geometry of the surface region for the Ni(ll 1)-p(2 X 2)-K structure determined by LEED [176]. The atom sizes are reduced for clarity. (a) Top view with arrows showing the direction of the horizontal movements of the top-layer Ni atoms [179]. The magnitude of the movements is 0.06 ,~. (b) Side view of section AX, showing the vertical relaxations of the substrate atoms.

structure (ARPEFS) investigation at 130 K also found that K resides in the atop sites, but with a K - N i bondlength of 3.02 + 0.01 ,~ [180]. While a vertical relaxation of the top Ni layer was observed, no rumpling was observed. And finally, a second photoelectron diffraction (PhD) study at 100 K also found the K adatoms in the atop sites, but with a K - N i bondlength of 2.86 + 0.03 A, and with a significant vertical relaxation of the top substrate layer, but essentially no rumpling of the substrate [181]. These results are summarized in Table 4. It is clear from these results that there is a consensus on the adsorption site of K / N i ( l l l ) in the commensurate phase, but there remains some discrepancy between the values obtained for the K - N i bondlength and the nature of the substrate relaxation. Possible sources for this discrepancy have been discussed in several of the papers referenced above. Ref. [181] points o

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

96

Table 4 Structural parameters deduced from different experiments for Ni(lll)-p(2 × 2)-K Method

dK- Ni

dl2

d23

LEED SEXAFS PhD PhD

2.82+0.04 2.92 + 0.02 3.02+0.01 2.87 + 0.06

1.98+0.03 . . 1.90+0.04 1.86 + 0.06

2.05+0.04 . 1.90+0.04 -

(~)

(i)

(~)

012

023

Ref.

0.12+0.02

0.05+0.03

0.00+0.03 0.01 + 0.09

-

[176,179] [79] [180] [181]

(~)

(A)

.

dK_Ni is the K - N i bondlength; dij spacings for the substrate are taken from the midpoints of rumpled rows; the a's refer to the rumpling amplitudes.

out that LEED measures only the structure of that part of the surface which has long-range order, while PhD and SEXAFS average over all adsorbed K atom sites. This could lead to different results if there is significant site disorder. The observed trend in chemisorption bondlength versus coordination number, shown in Fig. 33, is that the bondlength increases with coordination number. Therefore, based on the above argument, the smallest bondlength would be expected from the LEED study, which is consistent with the measurements. These differences may also account for differences in measured surface relaxations [181]. Since the degree of disorder obviously could vary as a function of surface preparation, variations could be

1.0

0.8

M

o3

0.6

N

M

N

N

0.4

If

0.2

M

0.0

.

0

1

.

.

2

.

,

3

4

5

6

7

Coordination number

Fig. 33. Excess radius versus coordination number of the adsorption bond for the commensurate structures listed in Table 10. The line is a linear fit to the data.

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

97

expected in different experiments using the same technique (either SEXAFS or PhD), and this may explain the large difference between the two photoelectron diffraction results. Because there has often been a low level of agreement between experimental and calculated LEED spectra for alkali metal adsorbates, it was proposed that the addition of a dipole moment to the spherical scattering potential may improve agreement [182]. The addition of this extra parameter can improve agreement somewhat (see Section 2.11.5) [182], but no significant improvement was found for K / N i ( l l l ) [183]. A slight preference was found for a dipole moment of 0.9 D, which is about half the value determined from work function techniques. Recently the validity of this procedure as a means of reducing r-factors has been questioned [184].

2.11.3. Cs/Ni(111) Only one structural study has been carried out for C s / N i ( l l l ) [185], a LEED study which determined that the structures in the submonolayer range are essentially identical to those observed for K / N i ( l l l ) . The submonolayer structures at low temperatures are incommensurate hexagonal over a large density range, progressing from rotationally disordered fluid at low coverages, through a rotationally ordered fluid and then hexagonal solid to a commensurate hexagonal p(2 × 2) solid at 0.25 coverage and finally back to an incommensurate hexagonal solid. The preponderance of incommensurate structures with continuously varying densities suggests that this overlayer system is dominated by the repulsive Cs-Cs interaction. Nevertheless, the p(2 x 2) commensurate structure is the most stable and disorders near 400 K.

2.11.4. Na /Ni(lO0) An early room-temperature LEED study of Na/Ni(100) indicated that at coverages below 0.29 the overlayer is disordered and the adatoms are probably preferentially located in the 4-fold hollow sites on the surface [174]. This is consistent with the diffuse LEED results reported for K/Ni(100) [186,187] (see below). The overlayer then gradually orders as the coverage is increased until at a coverage of 0.5, a full c(2 x 2) structure is formed. This is the saturated monolayer structure. Ni(lO0)-c(2 x 2)-Na RT: Several dynamical LEED studies have been carried out for the c(2 x 2) commensurate phase. A very early L E E D study determined that the Na atoms reside in the 4-fold hollows and that the spacing between the Na and Ni adatoms is 2.87 ,~ [188,189]. It was later shown by the same group [190] and by others [191,192] that the parameter space investigated in the earlier studies had not been sufficient and that the true optimum N i - N a spacing is 2.23 + 0.1 .~. The corresponding N a - N i bondlength is 2.84 + 0.08 A. A more recent study [184] found essentially the same parameters, but with higher precision. In that study the N a - N i bondlengthwas found to be 2.95 + 0.04 A. The first Ni-Ni interlayer spacing was found to be 1.75 + 0.01 A, very close to the bulk value of 1.76 ,~ or the relaxed surface value of 1.77 + 0.01 A [184].

2.11.5. K/Ni(IO0) An early room-temperature L E E D investigation of K/Ni(100) [193] determined that the overlayer was essentially disordered (rings and modulated rings in the L E E D patterns) below a coverage of 0.29, with the density increasing continuously with coverage. These observations are

98

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171 400

.

i

"- ".~ioo

disorderedphase~ 300

diffusion'"or"---@ "" 1

I!

K

=

epitaxialrotation

E

!

200

I

rectangular i hex ~

-~ lsotrop

1 ,

/

/

!

5/3)

7

r-i

I [ii~il 0.3 0.35 0.4 Coverage Fig. 34. Submonolayercoverage-temperature phase diagram for K/Ni(100) [111].

100 / / 0.2

I!ill 0.25

I

essentially in agreement with a later, more detailed study of this system which found that the K overlayer is disordered at room temperature at most coverages [111]. The submonolayer coverage-temperature phase diagram determined from this study is shown in Fig. 34. It shows that the most stable structure is a higher-order commensurate structure at a coverage of 0.30, denoted by (2 5~/3). This structure was also observed for K on Cu(100), Ir(100), and Ag(100). The phase diagram shows that at temperatures above 120 K and coverages below about 0.22, the overlayer is disordered [111]. Later studies revealed that lower-density commensurate phases may also be formed at coverages of 0.125 and 0.2 by further cooling the crystal to 90 K [187]. At a coverage of 0.22 a c(4 x 2) commensurate structure begins to form in coexistence with the lower-coverage phase (either fluid or solid, depending on the temperature), and is fully formed at 0.25 coverage. Two equivalent rotational domains exist for this structure, and the schematic LEED pattern is shown in Fig. 35. This structure was the subject of a dynamical LEED analysis and is discussed below. In order to form this commensurate structure, the "natural" hexagonal configuration of the overlayer must be significantly stretched uniaxally. At higher coverages, the overlayer relieves the resulting strain by compressing in the stretched direction while remaining commensurate in the perpendicular direction, thus forming a continuously compressing uniaxially incommensurate phase. This compression continues until a coverage of 0.32, passing through the higher-order commensurate (2 5~3) phase at 0.30. At 0.32 the overlayer lose its commensuration completely to form an essentially hexagonal overlayer. At the same time, this incommensurate layer rotates with respect to the substrate as shown in Fig. 36 and discussed below. This incommensurate hexagonal structure compresses until monolayer saturation at 0.38 coverage, which corresponds to a K - K distance of about 4.35 A, a 9% compression relative to the metallic K radius. Ni(lO0)-c(4 x 2)-K LT: The geometry of the c(4 x 2) phase was studied using dynamical LEED [194] and the K adatoms were determined to be in the 4-fold hollow sites, with a K - N i

99

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

,i

:i~~

,?

~

:~

•

•

i

l'+

•

='

•

.

"

OK = 0 . 1 4

OK = 0 . 2 5 •

• ..

: .

•

".o°

~:

=:

,.

.

~:

• ~o

•

•

• ... •

OK = 0 . 2 8

:."

..

o°°.

: : • o+

•

..: " ::

.4

•

•

•

o~

• • *o0O

.

:'"

"

• .

•

• :

•

OK = 0 . 3 6

Fig. 35. D i a g r a m s of L E E D p a t t e r n s o b s e r v e d for t h e structures of K / N i ( 1 0 0 )

7 ~)

,~ O-

6

" . ~ " • tx.Q --'7..... •

o

K/Ni(100)

•

K/Cu(100)

•

HOC model

-

GB model

+

t h e o r y N M (r 3 )

~

theory NM (f12)

at 120 K [111].

9

~o O

-~

5

o.

.

""t

.

Vs

5..

4 3

y _-30o 2 1.75

1.80

1.85

1.90

l a t t i c e fit L Fig. 36. R o t a t i o n a n g l e v e r s u s lattice fit for K / N i ( 1 0 0 ) a n d K / C u ( 1 0 0 ) , a l o n g w i t h p r e d i c t i o n s from t h e N M t h e o r y a n d H O C trajectories [111]. L a t t i c e fit is d e f i n e d as t h e ratio of t h e o v e r l a y e r lattice p a r a m e t e r to t h e s u b s t r a t e lattice p a r a m e t e r .

100

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

layer spacing of 2.675 + 0.05 .~ and a K - N i bondlength of 3.20 + 0.05 ,~, corresponding to an effective potassium radius of 1.96 + 0.05 A. The substrate was found to be unrelaxed from the nickel bulk value. Ni(IOO)-K disordered LT: D L E E D was used to study the adsorption geometry for K/Ni(100) over a wide coverage range [187]. The technique used here was to measure the intensities at the positions of the fractional-order beams for the c(4 x 2) phase at coverages where the overlayer was disordered. The resulting I(E) curves were analyzed using conventional LEED I(E) techniques. The conclusion of this study was that the adsorption site of the K adatoms, even at the lowest coverage studied (0.04) is the 4-fold hollow site, and that the vertical overlayer-substrate spacing may actually decrease somewhat from 2.72 to 2.66 A as the coverage is increased to 0.25. The LEED calculations incorporated a dipole moment near the position of the adatoms, and the magnitude of this dipole was found to essentially remain constant (1.1-1.2 D) over the coverage range studied, except at that highest coverage (0.25) where it is somewhat lower (0.86 D) [182]. These values for dipole moment are about 50% of those determined from other techniques. The position of the dipole was 0.37 .~ from the center of the K adatom, toward the substrate [182]. The geometry of the low-coverage layer was also studied by a new holographic LEED technique [186,195]. At a coverage of 0.02 the K atoms were found to be located in the 4-fold hollow sites with a perpendicular distance between 2.2 and 2.4 ,~. This is significantly smaller than the distance determined by the diffuse LEED study described above, but bondlengths measured by this technique have a much lower precision. The strength of the technique is that it can determine the approximate location of the adatom without employing the large search routines or trial-and-error methods required in conventional LEED or D L E E D [186,195]. The overlayer density modulations (domain walls) were studied for the uniaxially incommensurate phase by comparing LEED intensities to a ground-state calculation for the equilibrium structure, assuming dipole-dipole interactions between adatoms and a sinusoidal substrate potential [112,196]. The result of this comparison indicated a rather weak density modulation, as shown in Fig. 37, and the lateral energy variation due to the substrate in this phase was determined to be about 0.10 eV The rotation angle of the high-coverage phase is shown in Fig. 36 along with the corresponding data for K/Cu(100) [108]. Also shown are the predictions of the Novaco-McTague (NM) theory (see Section 3.5) for a repulsive interadsorbate potential and the higher-order commensurate trajectories. The scatter of the data makes it difficult to distinguish between the models. However, the similar sizes of the substrate periods in the two systems would lead us to expect both overlayers to follow the same HOC trajectory, and therefore the observed differences probably arise because the exact form of the interactions is important. The fact that the data for neither system follows exactly one trajectory given by the NM theory indicates that the potential is changing over this coverage range, which is likely, a n d / o r that additional effects not included in the NM theory are important. Nevertheless, the NM theory does correctly reproduce the general behavior of the overlayer. An attempt was made to study the melting of this hexagonal phase of K/Ni(100) because the substrate has a square symmetry and should not impose hexagonal ordering on the fluid phase, which might be the cause of the bond-orientational order in the fluid phases on hexagonal substrates (see Cs or K / C u ( l l l ) , K / N i ( l l l ) ) . No disordering transition was observed at all at a

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

(a)

101

J

(b) Fig. 37. Calculated uniaxial arrays for two different ratios of adsorbate-adsorbate to adsorbate-substrate interaction energies [111]: (a) corresponds to a case similar to K/Cu(100) and (b) corresponds to the K/Ni(100) case.

coverage of 0.37 up to a temperature of 290 K, at which point the density of the overlayer began to decrease. The lack of a melting transition was probably due to the high degree of compression of the K overlayer, which is denser than bulk K. Specular electron reflectivity measurements from low-coverage disordered overlayers of alkalis on Ni(100) were made using incident energies below about 30 eV [197]. The results of these measurements were compared to calculations based on a simple one-dimensional scattering model and reasonably good agreement was obtained. But in order to determine a structural parameter such as the overlayer-substrate spacing, an average potential needed to be guessed. The overlayer-substrate spacings obtained in this way were 3.80 A for Cs, 3.55 ,~ for K and 2.90 A for Na. We now know from more detailed studies of K/Ni(100) and Na/Ni(100) that these number are too large by at least 0.5 A, probably due to the assumption used for the potential. Therefore we will not dwell on this study except to point out that it found smaller overlayer-substrate spacings at very low coverages of Na/Ni(100), a result which, to the extent that these overlayers can be generalized, seems to be in conflict with the diffuse L E E D results for K/Ni(100) presented above [187]. O

102

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

2.11.6. Cs /Ni(lO0) An early LEED and electron reflectivity (see previous section) study performed at room temperature for Cs/Ni(100) [197] found that Cs forms a commensurate p(2 x 2) structure at saturation coverage, with a Cs-Cs distance of 4.98 .& which is about 10% compressed relative to the metallic diameter of Cs. Another LEED study carried out a few years later found two rotational domains of a hexagonal structure, having essentially the same Cs-Cs distance, 5 ,~ [198]. The rotational domains are each oriented along a Ni(100) axis, 90 ° from each other. This experiment apparently has not been repeated and the origin of the discrepancy is not known. However, we note that hexagonal and quasi-hexagonal structures have been much more commonly observed on fcc (100) surfaces than square-symmetry structures.

2.11.7. Na /Ni(110) A temperature-activated Na-induced (1 x 2) reconstruction was found on Ni(ll0) [199]. As for K / N i ( l l 0 ) , which is discussed in more detail below, the reconstruction is lifted at high coverages, close to a saturated hexagonal overlayer.

2.11.8. K/Ni(llO) The structures formed when Cs, K and Na adsorb on Ni(ll0) were studied with LEED [174] at room temperature before it was realized that at room temperature a (1 x 2) missing-row reconstruction is induced (see below). However, the LEED patterns observed in this study are essentially identical to those observed in a later study for K / N i ( l l 0 ) [200]. In the latter study, the adsorption of K on Ni(ll0) was studied both at 90 K and at room temperature. At 90 K, the overlayer remains disordered until a coverage of about 0.26 when it begins to order in an incommensurate structure in which the adatoms apparently occupy the troughs of the (1 × 1) (110) surface, gradually compressing toward but not quite reaching the c(2 x 2) structure. The monolayer saturation coverage is 0.48. The situation is quite different at room temperature [200]. There, a (1 x 2) pattern appears over a wide coverage range, which is due to the substrate reconstruction, and additional coverage-dependent streaking occurs which arises from the ordering of overlayer atoms in the missing-row troughs, as shown in Fig. 38. This structure is denoted as a (2 x 2)-1D structure since, while it is p(2 x 2) with respect to the unreconstructed surface, it consists of 1D rows of K adatoms separated by rows of Ni atoms. At higher coverages, the reconstruction is apparently lifted again, and the overlayer has essentially a hexagonal close-packed structure. Ni(llO)-(1 x 2)-K RT: A medium-energy ion scattering (MEIS) study of the (1 x 2) reconstructed structure of K/Ni(110) [201,202] confirmed that this structure was the postulated missing-row structure and that there is a small expansion of the top Ni layer spacing, d12 by 3% relative to the bulk, accompanied by a 3% contraction of d23 and a 2% expansion of d34. This first-layer expansion is actually quite unusual in these systems, most of which apparently exhibit a first-layer contraction relative to the bulk (see Section 3.8). It is suggested that this unusual behavior arises because of the magnetic properties of nickel [201,202].

2.11.9. Coadsorption of K and CO on Ni(lll) Ni(lll)-(2 X 2)-(K+ 2C0) LT: This structure has been determined using PhD [203]. The overlayer was formed by dosing the Ni(lll)-p(2 x 2)-K overlayer with CO at 100 K. The K

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

(a)

(b)

10

103

10

± ,~,

_

o, I oo

(c

o,

_

i'

_

o'o

_

-

(d) (

. [ooq

- [oo 1

Fig. 38. LEED patterns and real-space structures of c(2 × 2)-like structures for K adsorption on Ni(ll0)-(1 × 1) and (2 × 2)-1D-like structures for K adsorption on Ni(110)-(1 × 2) [200].

atoms remain in the top sites and the CO molecules occupy the 3-fold hollow sites which they occupy in a pure N i ( l l l ) - C O phase. The K - N i bondlength is increased by 0.15 + 0.05 ,~ from that observed for the Ni(lll)-p(2 x 2)-K system. This change seems to be contrary to the expected transfer of charge from the K to the CO 2rr * orbital, which would be expected to cause a decrease in the K - N i bondlength.

2.11.10. Coadsorption of Na and S on Ni(lO0) Ni(lO0)-c(2 × 2) or p(2 × 2)-Na + S: A L E E D study was carried out on mixed layers of sodium and sulfur on Ni(100), determining that when sodium is adsorbed on a surface which is precoated with either a c(2 × 2) or p(2 x 2) overlayer of sulfur, the sulfur always sits 1.3 ,~ above the nickel surface in the 4-fold hollow sites [204]. The sodium bonds to the sulfur, which results in it sitting 2.5 .~ away from the surface. A schematic diagram for this geometry is shown in Fig. 39. 2.11.11. Coadsorption of K and 0 on Ni(lO0) The coadsorption of K and O or Ni(100) was carried out at room temperature using L E E D and STM [205]. For an initial potassium coverage of 0.13, oxygen adsorption results in a p(3 × 3) pattern. This p(3 × 3) pattern was imaged using STM, shown in Fig. 40, and inter-

104

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

4.98 A

121t 1.3

Fig. 39. Section through the centers of the sodium and sulphur atoms with spacings as determined by LEED [204].

preted as a (3 x 3) square array of potassium on top of the oxygen-covered substrate. However recent PhD measurements indicate that the O forms the (3 x 3) pattern with K in a disordered layer on-top of the O layer [206]. L E E D experiments carried out at potassium coverages of 0.25 and 0.37 also resulted in similar p(3 X 3) L E E D patterns. SEXAFS measurements indicate bonding of K to O rather than Ni [207].

Fig. 40. 6 5 x 6 0 _A STM image of the ( 3 x 3 ) overlayer at a K coverage of 0.13 [205]. The upper estimate for the O coverage is 0,5 + 0.1 monolayers,

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

0.30 MLK :.

i c(4x2)

105

"-'-'-" c(2x2) "-'-'-

t

t

0.25 MLCO

0.49 MLCO

c(4x2) 0.13 MLK ~

0.25 MLCO

I

I

I

I

I

I

0

1

2

3

4

5

CO Dose (L) Fig. 41. Schematic diagram showing the succession of LEED patterns observed after adsorption of CO on a K-precovered Ni(100) at room temperature. The K coverage is indicated for each panel on the left side. The coverage of CO after various dosages of CO for each system is indicated under each panel. The open sections on the right-hand side of each panel indicate that the LEED pattern does not appear to change with further dosing [208].

2.11.12. Coadsorption of K and CO on Ni(lO0) A L E E D study for coadsorbed K and CO on Ni(100) indicates that the overlayer structures are different depending on the amount of K on the surface [208]. At low K coverages, a c(4 x 2) structure is observed with CO adsorption ur~ to about 0.4 L, which then changes to a p(2 x 2) structure at higher CO coverages. The situation at higher K coverage is more complex, with CO adsorption first forming a uniaxially incommensurate structure, followed by a c(4 x 2) pattern, which then apparently continuously changes to a c(2 x 2) structure, as depicted in the schematic diagram of Fig. 41. A n STM experiment carried out on the low-K-coverage overlayer indicated that much of the surface was covered with a p(2 x 2) structure [209]. A typical STM image is shown in Fig. 42. While it is impossible to tell the exact nature of this structure from the STM images, it is consistent with the optimal distribution determined by an electrostatic model of this system [210], postulated to explain microcalorimetry data [211], where the surface is covered by islands of a (2 x 2) structure which consists of a p(2 x 2) array of K adatoms, each surrounded by four CO molecules.

2.11.13. Coadsorption of Cs and 0 on Ni(lO0) The adsorption of Cs on an oxygen pre-covered Ni(100) surface resulted in the formation of a p(3 x 3) L E E D pattern [198]. If the oxygen was initially in the p(2 x 2) structure, deposition of Cs to 0.14 coverage resulted in a weak and streaky p(3 x 3) pattern, while deposition of Cs onto the c(2 X 2) oxygen overlayer resulted in a clear and well-defined p(3 x 3) pattern. Increasing the Cs coverage only led to a higher background in the L E E D patterns. This

106

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

.'-

..

~.

. ~

.

::~8~:? . . . .

"

~:i

.

.

.

: ...:,

"

Fig. 42. 200/~ x 200 .~ STM image of (K + CO)/Ni(100) surface showing islands of p(2 x 2) periodicity [209].

10(3 X 3) pattern was observed whether oxygen was dosed onto a Cs-covered surface, or Cs dosed onto an O-covered surface [198,212]. The 10(3 x 3) pattern was interpreted as an ordered O layer on top of the Ni substrate, with a disordered Cs layer on top of the O, similar to the case for K and O coadsorption on Ni(100). At higher O coverages, a c(4 x 2) L E E D pattern was observed, and this was again interpreted as an ordered O layer on the Ni substrate [198,212].

2.11.14. Coadsorption of K and CO on Ni(llO) CO and K coadsorption on Ni(ll0) was ~tudied by LEED, H R E E L S and thermal desorption spectroscopy (TDS) [213]. In these experiments, the surface was predosed with K and then CO was adsorbed. The interpretation of the results in this study is hindered by the fact that the authors were unaware of the K-induced reconstruction and interpreted their LEED results solely based on overlayer structures. However, their main conclusion, that the K and CO forms islands of ordered structures with a fixed stoichiometry, is most likely still valid. The structural model is somewhat similar to that proposed for Ni(100)-(K+ CO) above in that the CO molecules bond close to the K atoms, causing the K adatoms to pull together and form islands of a relatively compressed K - C O structure. The K atoms are believed to remain in the substrate troughs, and the remainder of the "clean" Ni(ll0) surface is occupied by extra CO molecules.

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

107

2.12. Palladium 2.12.1. K / P d ( l l l ) As part of a study of CO 2 desorption [214], K was found to order on P d ( l l l ) at T < 200 K and 0.33 coverage in a (v~- x f3-)R30 ° structure after dosing at 100 K and annealing the dosed surface to 395 K. The saturated monolayer coverage was estimated to be 0.36. Several other patterns were observed as a function of annealing t e m p e r a t u r e but without coverage calibration.

2.12.2. K /Pd(IO0) The adsorption of K on Pd(100) at 250 K produced two well-ordered structures, a p(2 x 2) at 0.25 and c(2 x 2) at 0.5 [215,216]. No long-range order was observed below 0 K = 0.25. The c(2 X 2) saturation phase corresponds to a K - K distance of 3.89 A, which is an unusually large 18% contraction relative to the metallic K diameter. The structural parameters of this phase were d e t e r m i n e d in the L E E D study, described below. Pd(lO0)-c(2 x 2)-K: A dynamical L E E D study of this phase h a s determined that the K atoms adsorb in the 4-fold hollows at a distance of 2.54 + 0.06 A above the surface [217]. This corresponds to a K - P d bondlength of 3.20 A. While the top-layer substrate atoms cannot rumple by symmetry in this structure, the second substrate layer does rumple with an amplitude of 0.04 + 0.02 A. o

2.12.3. Na, Cs /Pd(llO) The adsorption of Cs or Na on Pd(110) was studied by L E E D [218]. R o o m - t e m p e r a t u r e adsorption of either Na or Cs and subsequent annealing to >_ 600 K resulted in a (1 x 2) L E E D pattern which was attributed to substrate reconstruction. The L E E D pattern intensity was maximum for 0y a = 0.21 and 0Cs = 0.09. The fact that the formation of the (1 x 2) was an activated process (implying the importance of Pd self-diffusion) was interpreted as favoring a missing-row reconstruction. Pd(llO)-(1 x 2)-Na, Cs RT: A L E E D analysis [218] favored either a missing-row model or a saw-tooth model, though it was assumed in that analysis that the alkali atoms were randomly distributed over the surface and did not contribute to the I(E) spectra. In a later paper [219], the analysis was extended to include multilayer relaxations. W h e n these were included, the missing-row structure was preferred. The alkali-metal overlayer was assumed to be disordered.

2.12.4. Coadsorption of CO and K on Pd(lO0) This system exhibits several ordered phases as a function of K and CO coverage and annealing t e m p e r a t u r e [216]. K was predosed onto the surface at a t e m p e r a t u r e of 250 K, and exposed to CO after cooling to 100 K. After annealing to 250-400 K, a complex ( 1~-~× f ~ - ) R arctan 2 / 3 was f o r m e d at OK = 0.12 and a (2v~- x 2v~-)R45 ° structure of OK = 0.2-0.5. This latter pattern was strongest at OK = 0.25. W h e n annealed to 500 K, new patterns were observed: (3/2v~- x 3 / 2 v ~ ) R 4 5 ° for OK = 0.12; p(2 x 2) for OK = 0.20-0.25; p(3 X 2) for OK = 0.37; and (v~- x v~-)R45 ° for OK = 0.50. Structural models, based on K adsorption in the 4-fold

108

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

I

Fig. 43. Structural model of the Pd(ll0)-c(4 x 2)Cs/H patches after coadsorption at room temperature. Small open circles are Pd atoms, large open circles are Cs atoms, and solid circles are H atoms [220].

hollow sites, were proposed for the ( 2 ~ - x 2v~-)R45 ° and (v~- x v~-)R45 ° structures. A strong electronic interaction between CO and K / P d was inferred from EELS measurements [216].

2.12.5. Coadsorption of Cs and H 2 on Pd(110) When H 2 is adsorbed on the (1 x 2) Cs structure, the LEED pattern changes to a c(4 x 2) structure [220]. Because the (1 x 2) structure corresponds to a coverage of 0.1 [218,219], the L E E D pattern is assumed to originate from patches of c(4 X 2 ) C s / H structures. The H coverage is estimated from TDS measurements at ~ 0.5, and EELS measurements indicate that the H atoms are located in short-bridge sites. On this basis, and on the assignment of K atoms on Cu(ll0) to 2-fold hollow sites of the open missing-row structure using effective medium theory [221], a structural model in which Cs atoms order in hollow sites within the Pd troughs (see Fig. 43) was proposed. 2.13. Platinum There has been a considerable number of studies of alkali adsorption and in particular coadsorption on platinum surfaces, in part because of the use of Pt as a catalyst material. However, there have not been any successful quantitative studies to date.

2.13.1. Na /Pt(111) The overlayers of Na and Cs on P t ( l l l ) [222] have been studied using LEED. For Na adsorption at room temperature, Na layers were found to be fully disordered, presumably resulting from the high Na mobility and the small atomic corrugation of the P t ( l l l ) surface. However another study [223] reported a (2 x 2) structure. For Na adsorption at 85 K, a succession of L E E D patterns is formed as a function of Na coverage. These are shown in Table 5. The structures from 0-0.33 coverage can be explained by the usual dipole-dipole repulsions in competition with the substrate corrugation. For 0Na > 0.33, there is a coexistence of an aligned (f3- x V~-)R30 ° phase and rotated phases. The behavior of the rotated phases may be compared to the NM theory which predicts that in an incommensurate overlayer, the orientation of the overlayer relative to the substrate which

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

109

Table 5 Succession of L E E D structures for Na adsorption on P t ( l l l ) at 85 K (from Ref. [222]) Coverage

Structures

dNa_Na

(~) Rings p(2 × 2)

< 0.23 0.25

5.55

0.25 < 0 < 0.33

p ( 2 x 2 ) + ( { 3 - × 7'3)R30 °

0.33 0.33 < 0 < 0.59

(f3 × v~)R30 ° Incommensurate hexagons - aligned - rotated

4.81

0.59

3.60

minimizes the overlayer strain energy differs from the main symmetry direction of the substrate. As the coverage increases, the increasing lattice misfit ((d0x/-3- -d)/dov/-3 where d o and d are the atomic distance in the substrate and in the adsorbate layer respectively) is correlated with the increase in the orientation angle relative to the [01] axis of the (v~× x/3-)R30 ° structure. This is shown and compared with the NM theory in Fig. 44. The continued existence of an aligned phase is attributed to step effects.

2.13.2. K/Pt(111) Several L E E D studies of K adsorption on P t ( l l l ) have been reported [224-227]. The most comprehensive of these [224] includes a phase diagram, shown in Fig. 45. In this work K was adsorbed at about 120 K. A succession of hexagonal-symmetry structures occurs as a function of coverage, as shown on the phase diagram. The temperature stability of the ordered phases I

I

I

I

dll

I

o

\

i

--

NM theory Cl./C T = ,13 1)

E

.~ •~

2

g 20

° u

30

i

5

i

10

15

20

30

Orientation angle (deg.) Fig. 44. Variation of the orientation angle of incommensurate domains of Na layers on Pt(111) versus the lattice misfit (squares). This behavior is compared with the predictions of NM theory. The lattice misfit (defined as (dov/3 - d ) / d o v ~ with d o and d the atomic distance in the substrate and in the adsorbate layer respectively) and the orientation angle are measured relative to the (x/3 x vr3)R30 ° commensurate structure [222].

110

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171 500

",

Ist LAYER •DESORPTION

t"

•

400 ~d

DISORDER

300

,5 t

&

t t I

*,_

E

200

-

st

L

10(/

lit

(I

I).1

e/

i i i s

i'
~ ~

0.2

0.3

~

I

II.4

Potassium Coverage O

Fig. 45. Phase diagram for K adsorption on Pt(111) [224]. In the multiphase region a continuous compression of the overlayer results in rotational epitaxy or aligned structures.

increases with coverage. More complicated structures were observed above a coverage of 0.33. The If3- x f3-)R30 ° spots split and the reciprocal lattice vector length grows continuously; at the same time, extra spots appear in a hexagonal configuration. With increasing coverage, a continuous compression of the layer accompanied by a simultaneous rotation from the R30 ° direction was observed. At the saturation coverage of 4 / 9 a (3 x 3) l~attern was observed. With four atoms per unit cell, this corresponds to a K - K spacing of 4.16 A, which is compressed 4% relative to the metallic K diameter. A n o t h e r study reported similar structures [227], with the exception that a 3 x 3 overlayer (not observed in Ref. [224]) was observed at a coverage lower than 0.13. The adsorption temperature in that work was slightly lower (100 K). Studies of multilayer growth carried out at higher temperatures [228] found that the saturation coverage was d e p e n d e n t on annealing temperature. While films up to 6 0 / ~ thick could be grown at lower temperatures, the saturation coverage above 340 K was just one layer [228]. This behavior is consistent with that generally observed for alkali metal adsorption systems.

2.13.3. Cs /Pt(111) At room temperature, Cs adatoms can order at submonolayer coverages, in contrast to Na overlayers [222]. For 0Cs < 0.23 rings were observed with a diameter increasing with coverage. At 0.25 coverage a p(2 X 2) pattern was observed, followed by a Iv/-3- x vt3-)R30° upon increasing the coverage. For coverages 0.24 < 0 < 0.32 a mixture of (2 x 2) and I f 3 - X f 3 ) R 3 0 ° was observed. For coverages greater than 0.33 a hexagonal structure aligned with the substrate was observed up to monolayer saturation (0cs = 0.41). This corresponds to a C s - C s spacing of 4.36 o A, which is a compression of 20% relative to the metallic Cs diameter. At 85 K the same series of structures is observed [222] except that at low coverages the rings are replaced by spots indicating a rotated hexagonal structure, while for coverages > 0.33 an

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

111

aligned hexagonal structure is found mixed with hexagonal structures which are angularly disordered.

2.13.4. Cs /Pt(lO0) An early L E E D study reported a c(4 x 2) LEED pattern after room-temperature adsorption of Cs [229]. An attempt at a dynamical L E E D analysis of the structure was inconclusive, as the adsorption site could not be determined.

2.13.5. Coadsorption of Na and CO on Pt(111) In an early application of the "bondlength-with-a-ruler" concept [230], NEXAFS at the C and O K-edges was used to study the coadsorption of Na and CO on Pt(111) [231]. For Na adsorption at 110 K of an amount corresponding to 0Na < 0.2, followed by dosing with CO, the angular variation in the NEXAFS or and 7r resonances above the C K-edge indicates that CO is oriented perpendicular to the surface. Furthermore, the movement of the tr resonance toward the absorption threshold in the presence of Na is cited as evidence for an increased CO bondlength, estimated at 1.27 + 0.06 A (0.12 _+ 0.03 ,~ longer than the gas-phase value).

2.13.6. Coadsorption of K and 0 on Pt(lll) Four stable ordered K + O overlayers were observed using L E E D at specific oxygen and potassium coverages [226]. Three commensurate structures were found by dosing to OK > 0.54 (about 1.5 layers) and 10 L of 0 2, then annealing to different temperatures. Annealing to 650 K produces a (4 X 4) overlayer, annealing to 700 K an (8 x 2) overlayer, and annealing to 750 K a (10 X 2) overlayer. These are thought to be indicative of ordered domains of "potassium oxide" at less than saturated monolayer coverages. A further incommensurate structure was found by annealing to 750 K with slightly less oxygen coverage. T h i s produced a pattern indicative of a hexagonal overlayer unit cell with a lattice constant of 4.71 A. A K 2 0 stoichiometry is proposed for this phase with each O atom surrounded hexagonally by six K atoms. A separate study of the same system [225] reported the gradual disappearance of the ( f 3 X v~-)R30 ° pattern after room-temperature dosing with 0 2. In agreement with Garfunkel and Somorjai [226] they also found a (4 x 4) structure after annealing the dosed system to 600 K. It was interpreted as ordered quasi-hexagonal domains of the K - O layer. In contrast to the previous study [226], the stoichiometry was found to be KO2.

2.13.7. Coadsorption of K and CO on Pt(lll) This system has been discussed in an earlier review [12]. Several L E E D studies have been made of the coadsorption of K and CO on P t ( l l l ) [232-236]. Investigations performed at room temperature did not report any ordered structures [232-234]. On the other hand, for coadsorption at low temperature, a rich series of ordered structures was reported [235,236]. In the low-temperature study, the phases formed upon adsorption of CO on a K-dosed P t ( l l l ) surface (O K = 0.02, 0.06, 0.11, 0.16, 0.22, 0.25, and 0.30) at 100 K were monitored using L E E D and infra-red reflection absorption spectroscopy (IRAS). For the lowest K coverages, (0.02 and 0.06), no L E E D patterns were observed. A complex series of L E E D patterns is observed for the other K coverages; in all five well-ordered coadsorption overlayers are observed. The same structures are formed if K is dosed on a CO pre-covered surface. The

112

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

•

/~

0K=0.16 2.0m P a . ~ ,

/

I "~

i

,

oo Io

i 0.13mPa.s

~(K+CO)/Pt(lll)] •

•

[\

•

•

o•

• °

5

0K=0.06 ~, 0.27mPa-s . ~ 1 \ . . ~ J ~ / •

-sb

'

I

-4~

'

I

,

0

4'o

,

sb

PohLr Anole 0 (deg.)

Fig. 46. X-ray photoelectron diffractionresults for CO adsorption on the Pt(lll) surface with various pre-coverages of K [234]. The data show the ratio of the intensityof the C ls and O ls levels as a function of polar angle 0. The solid lines are computer-generatedsmoothingof the points. IRAS data in this study indicate probable switching of the CO from atop sites to bridge sites in the presence of K. Kinematic simulations of the diffraction patterns were used to produce models of the structures consisting of linear chains in which K atoms and CO molecules occupy the same adsorption sites with a lateral distance of one lattice constant [236]. X-ray photoelectron diffraction (XPD) was used to study the orientation of CO adsorbed on the clean P t ( l l l ) surface and also coadsorbed in the presence of various coverages of K at room temperature [234,237]. In each case the ratio of the intensity of the C ls and O ls levels of CO was measured as a function of polar angle 0 and a maximum was found near normal emission (Fig. 46). Such an enhancement is interpreted as due to forward scattering of the C is photoelectrons, indicating adsorption of the CO normal to the surface with the C end down. Furthermore, XPD results for the K 2P3/2 core level peak height are similar before and after CO coadsorption, with no CO-induced enhancement, indicating that K remains bonded to the P t ( l l l ) surface and that CO cannot be adsorbed on top of the K atoms. This picture is consistent with the chain models proposed in Ref. [236].

2.13.8. Coadsorption of K and 0 on Pt(755) At room temperature, (4 x 2), (8 x 2) and (10 X 2) patterns similar to those seen for K and O adsorption on P t ( l l l ) were also reported for this surface [226]. No ordered structures were observed for just K adsorption.

2.13.9. Coadsorption of Cs and 0 on Pt(lll) A gradual disappearance of the Cs-induced (2 x 2) structure was observed with LEED upon room-temperature adsorption of 0 2 [238]. CsO 2 compound formation was proposed to explain related work function and photoemission data.

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

00

113

o

Fig. 47. Proposed model for the Pt(111)(2 x 2)-CO + Cs structure [239].

2.13.10. Coadsorption of Cs and CO on Pt(lll) For Cs and CO coadsorption on P t ( l l l ) [239], only two ordered coadsorption structures were observed compared to five for CO adsorption with K. These are the (2 x 2) and (4 X 2) structures. The (2 X 2) structure is stable over a wide coverage range, from Cs coverages of 0.14 to 0.25. The proposed structure for 0Cs = 0.25 is shown in Fig. 47. The fact that this same (2 x 2) pattern is observed at a much lower Cs coverage is interpreted to indicate that there is a net attractive interaction in the overlayer, leading to island formation of this structure.

2.14. Rhodium 2.14.1. Na / R h ( l l l ) Rh(lll)-(v~ x f3)R30°-Na LT: A dynamical L E E D analysis was carried out for this system [240]. The Na atoms were found to be adsorbed in the hcp hollow sites. It should be noted that for Rh, this is not the bulk continuation site (see Table A.2). The N a - R h bondlength was found to be 2.84 + 0.09 ~,, corresponding to an effective Na radius of 1.49 ,~,. No appreciable relaxation of the substrate was observed, although a possible small (0.03 + 0.12) lateral displacement of the top-layer Rh atoms away from the adsorption sites was observed, as well as a possible buckling of the second Rh layer of 0.01 + 0.18 A [240].

2.14.2. K / R h ( l l l ) This system was studied with LEED as part of a multi-technique investigation. The adsorption of K on R h ( l l l ) at low temperature leads to three ordered phases; a p(2 x 2) at a coverage of OK = 0.25, a (f-3 x ¢-3)R30 ° phase at 0 x = 0.33 and a (2 X 2) at OK = 0.5 [241,242]. In this study it was not possible to deduce the structure of the (2 X 2) phase, which might have arisen from rotated domains of a (2 x 1) structure. These phases all appear at coverages higher

R.D. Diehl, R. McGrath /Surface Science Reports 23 (1996) 43-171

114

than that of the minimum in the work function [241,242]. Dynamical LEED studies have been carried out on the p(2 x 2) phase and the (v/-3- x v/3-)R30° phases [243] and are described below. Rh(lll)-p(2 x 2)-K LT: A dynamical LEED study of this structure has shown that the adsorption site is the hcp hollow and that the K - R h bondlength is 3.15 A, corresponding to an effective K radius of 1.80/~ [243]. No substrate rumpling was detected in this study. Rh(111)-(v~ X v~)R30°-KLT. • A dynamical LEED study of this structure has shown that the adsorption site is the hcp hollow, the same as for the p(2 x 2) structure described above, and that the K - R h bondlength is also the same, 3.15 A [243]. O

2.14.3. Rb /Rh(111) Rh(111)-p(2 x 2)-Rb LT: A dynamical LEED study of this structure has determined that the Rb atoms are located in the bridge sites, the only known alkali adsorption system in which the adatoms occupy the bridge site in a primitive structure [243]. The R b - R h bondlength was found to be 3.20 ,~, corresponding to an effective Rb radius of 1.85 .~. In addition, there was a substrate rumple amplitude of about 0.05 A. Rh(111)-v~ x v~)R30°-Rb LT: The adsorption site for this structure was found to be the hcp hollow by dynamical LEED [243], with a R h - R b bondlength of 3.20 A, the same as that found for the p(2 x 2) structure described above, which is in a bridge site. O

o

2.14.4. Cs / R h ( l l l ) Rh(lll)-p(2 x 2)-Cs LT: A dynamical LEED study has determined that the adsorption site O

for this structure is the on-top site and that the Cs-Rh bondlength is 2.95 A, corresponding to an effective Cs radius of 1.6 A [243]. The top layer of the substrate was found to be rumpled by 0.10 A, similar to the results for other top-site structures. Rh(lll)-Vr3 X f3)R30°-Cs LT: The adsorption site for this structure is the hcp hollow site and the Rh-Cs bondlength is 3.25 ~, [243], or 0.3 .~. longer than that found for the p(2 x 2) structure above. This change in bondlength is attributed to the change in coordination of the chemisorption bond from 1 to 3 as the site changes from top to hcp, similar to the change observed earlier for Cs/Ru(0001) [244]. O

2.14.5. Cs /Rh(lO0) This system has been the subject of a previous review [245] and the reader is referred to that work for a detailed discussion. The phase diagram has been determined by LEED [246] and is shown in Fig. 48. The various commensurate structures are interpreted in the usual way by assuming competition between the alkali atoms adsorbing in high symmetry adsorption sites and a repulsive dipole-dipole repulsion between the adatoms, leading to quasi-hexagonal ordered structures. In the case where continuous shifting of superstructure spots is observed (phase (4) in Fig. 48), a "statistical" mixing of microscopic domains of phase (3) (c(4 x 2)) and phase (5) ((3 x 2)-2Cs) with coverage-dependent relative weights is postulated. These authors [246] also succeeded in reproducing qualitatively all of the features of the phase diagram using modeling methods. A first attempt using Monte Carlo methods involving a lattice gas model with equivalent adsorption sites and repulsive dipole-dipole interactions truncated at the third-nearest neighbor distance reproduced only some of the features of the phase diagram. All of the superstructures observed could be reproduced using mean field

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

4(X)

115

i Cs/Rh(100) I (333+5) K f---~.., (Desorpt on)

Lattice Gas 3(X)

~_5~+_5,K / 200

{~

(155+-5)

. . . . 1"-"8 ' F "" "'",

¢ 100

I

0

0.1

I

I

I

I

0 _2

0.3

0.4

0.5

Fig. 48. Experimental phase diagram for the system Cs/Rh(100) [246]. The numbers denote the different phases observed: (1) c(2v~ × 4x/2)R45; (2) (v~ x ~/5)R arctan 1/2, (3) c(4x 2), (4) incommensurate phase with superstructure spots shifting from positions of phase (3) to positions of phase (5), (5) (3 x 2)-2Cs, (6) (x/'5x v~)R arctan 1/2-2Cs, (7) hexagonal IC phase with rotational epitaxy, (8) hexagonal IC phase "c(2v~ x 2x/2/3)R45°''. theory allowing inequivalent adsorption sites and a hard core radius in order to prevent adatoms coming too close to each other. However, this approach did not satisfactorily reproduce the topography of the phase diagram. Rh(lO0)-c(4 x 2)-Cs LT: A L E E D I(E) analysis [247] of the c(4 x 2) phase at 120 K and at 0.25 coveragoe found the Cs adatoms in the 4-fold hollow site with a C s - R h layer distance of 2.87 ___0.07 A corresponding to a C s - R h bondlength of 3.44 _+ 0.06 ,~ and an effective Cs radius of 2.10 + 0.06 A. This structure is shown, along with the L E E D data, in Fig. 49.

2.15. Ruthenium An early L E E D study of Na/Ru(0001) was perhaps the first to indicate the potential for alkali metal overlayers as model 2D systems [248,249]. Now Ru(0001) is one of the most thoroughly studied substrates for alkali metal adsorption and coadsorption, and these adsorption systems have been the subject of a recent review [20]. One interesting observation is that of all the alkali adsorbates, only Cs occupies the on-top site, the others being in the fcc and hcp hollow sites. The Ru(0001) surface was also used in a series of electron-stimulated desorption ion angular distribution (ESDIAD) experiments of coadsorbed systems, looking at the modification of the coadsorbate structure by the addition of the alkali. Several earlier papers [250-252] reported high-temperature "complex" structures for Na, K and Cs adsorption which are now known to be caused by coadsorption [244,253], but since no detailed studies have been carried out on their structures, they are not described here.

2.15.1. Li /Ru(O001) Adsorption of Li on Ru(0001) at 80 K was studied using L E E D [254]. A (2 X 2) structure was observed at a coverage of 0.25 (although a later study did not observe this structure [255]) and a

116

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

Th.

1

~

Exp, 50 100 150 200 250 300

Exp.

........

50 100 150 200 250 300

Zhp .... 50 100 150 200 250 300 Th. l 20 Exp. _ _ 50 100 150 200 250 300

do1 d~2

50 100 150 200 250 300

1_2

O~ 189-~ Rh I 1.~ Rh

Th I 21 Exp. .... 50 100 150 200 250 300

50 100 150 200250300 E(eV)

E (eV)

Fig. 49. Comparison of experimental and best-fit spectra for 4-fold hollow adsorption in the Rh(100)-c(4 x 2)-Cs system [247]. The best-fit structure is also shown.

(v~ X v:3-)R30° structure was observed at 0.33. Laterally compressed and rotated structures developed at higher coverages. The rotation angle of the Li layer relative to the (v~- x vr3-)R30° structure is shown as a function of the overlayer and substrate lattice constant in Fig. 50. The misalignment of the overlayer increases monotonically as the overlayer is compressed. Compared to the same effect on Na, the rotation angle of the Li layer is much greater. I ; P - LI/RU i O 0 1 )

1.8

O -- N a / R u ( 0 0 1 )

m

1.i 0

"o

o 1.6

"o

.

o 0 ir/'C

ga.~...~

E 1.5

e

®%° "n. ~.

ILl

O

1.4 o

,.,I

~e/c.

1.3 1.2 0

5

10 15 20 RELATIVE ORIENTATION

25

30

(o)

Fig. 50. Lattice fit d o / d s (overlayer lattice constant/substrate lattice constant) plotted against the orientation of the overlayer relative to the 30 ° direction of the substrate for Li adsorption on Ru(0001) [254].

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

117

As the overlayer continues to compress, it forms a higher-order commensurate structure at a coverage of 0.64, which has (5 × 5) symmetry. This higher-order commensurate phase has 16 Li atoms per unit cell. Therefore there are several adsorption sites for the Li atoms. A L E E D analysis of this structure [256] indicates that the L i - R u interlayer spacing is 2.17 + 0.10 A. The o Li monolayer saturates at a coverage of 0.78, which corresponds to a Li-Li distance of 3.06 A, which is a 2% contraction compared to the metallic diameter of Li [256]. Films of Li on Ru(0001) have also been studied using L E E D [256]. Dynamical L E E D was applied to films of two and three Li layers. The lateral lattice constant for the Li was found to be 3.34 + 0.04 A, which is close to the value of the (5 x 5) overlayer. The bilayer was found to have interlayer spacings dLi_Ru = 2.35 + 0.10 * and du_L~ = 2.40 + 0.10. The same parameters were found for three layers of Li/Ru(0001), and the stacking sequence was found to be consistent with fcc growth of Li [256]. Ru(OOO1)-(V~ × v~)R30°-Li: A L E E D study of this system showed that Li atoms reside in the 3-fold hollow hcp sites [255]. A L i - R u bondlength of 2.74 + 0.05 ,~ was found, which corresponds to an effective Li radius of 1.39 + 0.05 A. A small rumpling (0.05 A) was observed for the second Ru layer.

2.15.2. Na /Ru(O001) This system has been the subject of a number of studies [248,249,251,257]. For room-temperature adsorption, only one ordered structure, a (3 x 3), was observed, at a coverage close to saturation of the first layer [249]. This structure was interpreted as a higher-order commensurate structure with 4 atoms per unit cell. For low-temperature adsorption, several ordered L E E D patterns were observed with increasing coverage, and a phase diagram was developed from these and thermal desorption measurements (Fig. 51). The patterns were interpreted as resulting from increasingly compressed hexagonal arrangements of adsorbate atoms uniformly spaced due to mutual repulsions. At low coverage, ordered structures are only stable at low

500

I Na/Ru(0001 )1

............ FIRSTLAYER ......... D E S O R P T I O N

,'--2".../

4(X)

DISORDERED

J

,

~

X

t I(312x3/2)

300

/

,

i c!:

~

,, SRM,A~L L tmo

(2x:,) " ~

x

t

_~ .........

:

ilil :;i :ili:: ii ;ili iii

/ ~' S P LIT I ( q 3 x q 3 i R 3 ) '~ (~3xq3)

- - - -\- - r'-

,LARGE~ ' \ t RING+ ~ 1(3/2x3/2) ~ :i:?: : :i: : : : : :i

i> .... . . . . . :i • : !IRREVERS BLE TR:

~_ . . . .

~- 2{}(} I{X)

~ '..

RS0'

TR

LARGE } RING i

i

&- \~ . . . . . . . . . . . . . . . . . . . . . . . . (2x2) + (V3xq3)R30"

l

I

I

I

0.2

0.3

(}.4

0.5

Na Coverage 0 Fig. 51. P h a s e d i a g r a m for N a a d s o r b e d o n Ru(0001) at 80 K [249]. D a s h e d lines i n d i c a t e a p p r o x i m a t e p h a s e boundaries. TR indicates transition region.

118

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

i o Na/Ru II '~ Ar/C | ~ Ne/C l0

20

o

oo

o

,..d 30

- - NMTheory, C I

0

,

i

i

i

i

i

t

l

~ i

J

i

~% I

i

i

1

5 10 15 Orientation Angle (deg.)

t

i

i

i

20

Fig. 52. Orientational ordering of Na on Ru(0001) (circles), compared with Ar on graphite (triangles) [259], Ne on graphite (inverse triangles) [258], and NM theory (solid line). Both the lattice misfit and the orientation angle are relative to the (v~-x f3-)R30° commensurate structure. temperatures, but at higher coverages, patterns are more thermally stable due to the limiting of the Na mobility by a d a t o m - a d a t o m repulsions. The rotational ordering occurring at coverages higher than the (v~- × ~ ) R 3 0 ° was studied at 80 K [248]. With increasing coverage the Na overlayer spots split into three pairs which separate and move away from the ~ location. One pair originates from domains of the hexagonal overlayer which have two possible rotation directions; the other pairs of spots are due to multiple diffraction. The authors showed that the orientation angle of the lattice varies monotonically with the observed lattice misfit in a way similar to that observed for Ar and Ne on graphite [258,259] as shown in Fig. 52. Furthermore, the behavior of the overlayer was shown to be consistent with the NM theory at low coverages. At higher coverages a sharp change takes place in the orientational behavior caused by a reduction in the rigidity of the Na layer. A later L E E D study [257] found results largely consistent with those of Doering and Semancik [249] although a poorly ordered (3 x 3) phase was observed at coverage (0Na = 1/9) at 45 K. If the sample was annealed after Na deposition or if Na was evaporated at temperatures above 200 K, no rotational epitaxy was observed. A commensurate (3 x 3) phase was observed at a coverage of 4 / 9 after adsorption at temperatures above 200 K [257]. The adsorbate-induced beams were attributed to single and multiple scattering events between the substrate and adsorbate layer having a (3 x 3) periodicity. Therefore this is the same structure as reported earlier [249]. Further adsorption led to continuous compression of the (3 x 3) to a (4 x 4) structure at 0 = 0.56. Structural analyses of the p(2 x 2), (v/-3- x v~-)R30 ° and (3 x 3) phases have been carried out and are described below [257]. Ru(O001)-(3 x 3)-Na LT: For the 0Na = 4 / 9 (3 X 3) structure, an adsorption geometry could not be completely determined [257]; two models with atoms located near but not on high-symmetry sites were proposed. Limits were placed on the lateral displacements of the Na atoms from an ideal hexagonal lattice and the buckling of the adsorbate layer. Ru(OOO1)-p(2 X 2)-Na LT and Ru(OOO1)-(v/-3x f3)R30°-Na LT: Structural analyses were

R.D. Diehl, R. McGrath /Surface Science Reports 23 (1996) 43-171

a) Ru(0001 )-p(2x2)-Na

~I ~

i

~

~

•

"1

I

I

I

DI2 D23

DNa.Ru = 2.50 ]~ + 0.04/~ D12 = 2.11 ]~ + 0.03 ]~ D23 = 2.14/~ + 0.05 rNa = 1.60/~ + 0.03 ]~ ODebye = 200 K + 50 K

119

b) Ru(0001 )-(~/3xq3)R30°-Na

~

DI2

~

I D23

DNa.Ru = 2.51 /~ + 0.04/~ D12 = 2.10 • +_0.03 ]~ D23 = 2.14/~ _+0.05 ]~ rNa = 1.60/~ + 0.03/~, ODebye = 150 K + 50 K

Fig. 53. Structural models and parameters for the best-fit arrangement for (a) the Ru(0001)-p(2 × 2)-Na and (b) the Ru(0001)-(vr3 X v~)R30°-Na phases [257]. For (a), the Na atoms adsorb in the fcc site with a Pendry r-factor of 0.28; for (b), the site is the hcp and the r-factor value is 0.25. The effective Na radius is the same, 1.60+_0.03 A in each case.

completed for the (2 x 2) and (vr3 x v~)R30 ° phases. The results are shown in Fig. 53. The Na atoms were found to occupy the fcc hollow sites in the p(2 x 2) phase and the hcp hollow sites in the ( f 3 X v/3)R30 ° phase. The hcp site is the "metallic" site, i.e. that which would be occupied by the addition of an additional Ru substrate layer. No significant lateral or vertical relaxations were found, and importantly, no difference in N a - R u bondlength was found between the 0Na ~- 0.25 and 0Na = 0.33 phases [257].

2.15.3. K/Ru(O001) The phase diagram for K adsorption on Ru(0001) appears to be particularly simple, with no evidence of rotated phases [260-262]. Fig. 54 shows the temperature-coverage phase diagram determined with L E E D [260]. At low coverages, a "ring" phase is observed, followed by a (2 x 2) at close to 0.25 coverage and then a (v~ x f3)R30 ° pattern at 0.33 coverage. One study [261] reported a complex pattern above the coverage corresponding to the (v~-× x/3-)R30° phase. The geometries of the p(2 x 2) and (x/3-x f3-)R30 ° phases were determined using dynamical LEED and are described below. In common with the Cs/Ru(0001) system [244], the site was found to be coverage-dependent. Ru(O001)-(2 x 2)-K LT: The geometry of the (2 x 2) phase was studied using dynamical L E E D [262]. For the p(2 X 2) phase the 3-fold fcc hollow site was found, with a K - R u bondlength of 3.25 +_ 0.05 A corresponding to a K hard-sphere radius of 1.94 _+ 0.05 ,&. This result contrasts with that for Cs adsorption where for the p(2 x 2) phase the adsorption site was on-top. This is explained in terms of the smaller K radius leading to a bigger effective corrugation of the surface and consequently a smaller influence of the repulsive dipole-dipole interaction.

120

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

I K,Ro 000, I 400 -

',

',,Desorption ',

.~ ,

K E

300

-

, ,

2oo-

',

O.1

0.2

0.3

0 Fig. 54. P h a s e d i a g r a m for t h e a d s o r p t i o n o f K o n R u ( 0 0 0 1 ) [260].

Ru(OOO1)-(v~ × v~)R30°-K LT: For this phase the adsorption site was the 3-fold hcp hollow site. A K - R u bondlength of 3.29 + 0.05 A was found corresponding to a K hard-sphere radius of 1.98 + 0.05 A. o

2.15.4. Rb /Ru(O001) Ru(OOO1)-p(2 × 2)-Rb LT: Rb in this structure was found to occupy the fcc hollow sites [263]. The R b - R u interlayer spacing was found to be 3.04 + 0.05 A. The L E E D analysis was refined by using a lateral split-position analysis (corresponding to a mean-square deviation of the Rb atoms of about 0.37 A). A r u m p l i n g of 0.06 + 0.04 A in the top Ru layer WaSoalso found. The R b - R u distance was 3.41 A, corresponding to an effective Rb radius of 2.07 A. Ru(OOO1)-(f3 × ~/3)R30°-Rb LT: Rb was found to occupy the hcp hollow sites [263] in this structure. The R b - R u interlayer spacing was found to be 3.03 + 0.05, which is essentially the same as that found for the p(2 x 2) (above) and therefore the R b - R u bondlength and the effective Rb radius are the same as in that case. As for the p(2 x 2), the introduction of the split-position L E E D formalism significantly improves the agreement between the experiment and calculations.

2.15.5. Cs /Ru(O001) There has been a number of LEED studies of this system [244,252,264]. Over et al. [244] have compiled a phase diagram for the adsorption of Cs on Ru(0001) which is reproduced in Fig. 55. The phase diagram is similar to many other alkali adsorption systems in that a ring phase is observed at low coverage followed by ordered phases as the coverage increases. A p(2 × 2) phase is observed near 0.25 coverage, and a (v~- × v~-)R30 ° phase near 0.33 coverage. In between, complex rotated phases are observed with a rotation angle of about 8°, although no continuous rotation was observed, as for the Li/Ru(0001) [254] and Na/Ru(0001) [248] systems. The one-dimensional sequence of phases as a function of coverage at 80 K reported by Hrbek [264] is largely in agreement with this phase diagram. A previous room temperature study of this system had interpreted splitting of the (2 × 2) diffraction spots in terms of

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

400

v

I/ / / / , ' / .

- I i

121

\ . Ring ~ , \ Desorption / I ", Disorder,,--, U / Z / " , ! ",~.

3o0

K E # 2o0

I(X)

i I i I i

i ! I i

. . . . .

__.j

i i fl ii i I i I

i I iI ,~3~ i i ~ i i ~ i I

¢ ~

~l

~.,

~

J

li

**

i ,

i I ir

i I ~i ,r.,O i

I Ring

~(2x2){1~: ,, ~

i J

f i ,. . . .

. . . . . . . I

0

i l I $ I

0.1

I

0.2

i

I Lf2~t I L i JJ__t_

/

¢v'

i i

i

. . , ' '

~

/

,

~"

z

¢~ ----

~

#

I I _, . . . . . . . . . . . I

0.3

I

0.4

0.5

Coverage 0

Fig. 55. Phase diagram for the adsorption of Cs on Ru(0001). The hash-marked area represents a qualitative result [244]. anti-phase domain formation [252], however, Over et al. showed that these patterns were caused by coadsorption of oxygen [244]. Ru(OOO1)-p(2 x 2)-Cs LT: A LEED analysis of the p(2 x 2) phase indicated that Cs adsorbs in the atop site with a C s - R u bondlength of 3.25 -t- 0.08 A [244]. In addition, there is significant rumpling of the top Ru la),er of 0.10 + 0.04 ,~. The C s - R u bondlength corresponds to an effective Cs radius of 1.90 A. The other structural parameters are shown in Fig. 56a. Ru(OOO1)-(v/-3 x v~)R30°-Cs LT: In this structure, Cs occupies the 3-fold hcp hollow site with a C s - R u bondlength of 3.52 + 0.02 ,~, corresponding to an effective Cs radius of 2.17 ,~ [244]. The details of these results are shown in Fig. 56b. It was noted in this study that the coverage-dependent C s - R u bondlength (implying an increase in the Cs hard-sphere radius from 1.9 to 2.2 A as the coverage increases from 0 = 0.25 to 0 = 0.33) was consistent with the historical Gurney model [265]. However, an interpretation in support of Gurney's model is not straightforward because of the change in adsorption site. The authors refer to a general rule for ionic bonding that a change from 3-fold coordination to 1-fold usually results in a decrease in bondlength by about 0.3 A [266], which suggests that there is no change in the nature of the chemisorption bond between the coverages of 0.25 and 0.33.

2.15.6. K + Cs mixtures on Ru(O001) Intermixing of K and Cs on Ru(0001) was observed using L E E D [20]. When of K + Cs of 0.25 was adsorbed at various Cs : K stoichiometries, it was found from the fcc site of the K-only overlayer to the atop site. For the C s : K ratio commensurate phase was observed having (2v~-x 2v~-)R30 ° symmetry. This atoms and 1 Cs atom per unit cell [20].

a total coverage that K switches of 1:2 a mixed phase has 2 K

2.15. 7. K/Ru(IOIO) This appears to be the only structural study of a Ru surface other than the (0001) plane. In this study, ordered overlayers were observed after saturation dosing at room temperature

122

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171 (a)

(b)

~ Z l ~ u ~'~'~-~~N

r

ZCs

Zcs

D12 D23

D12 D23

VJ,.Jf~k_.a(z~

Zcs = 3.15/~ _+0.08 ZRu = 0.10 ,~ -+ 0.04/~ Dz2 = 2.14/~ _+0.05/~ D23 = 2.13/~ _+0.07 ,~ ®CDS=40 K_+ 10 K V0=-6eV_+ leV

Zcs = 3.15 ,~. _+0.(/3 D12 = 2.10 ~ _+0.04,3, D23 = 2.12/~ _+0.07/~ ®Cs=80K_+IOK Vo = -5 eV _+ 1eV

Fig. 56. Structural models of (a) Ru(0001)-(2 x 2)-Cs and (b) Ru(0001)-(v/3 x v~-)R30°-Cs together with the best-fit structural parameters [244].

followed by annealing to remove K [267]. A commensurate c(2 x 2) was observed at 0 = 0.5. Between 0 = 0.5 and 0.67, patterns with split c(2 x 2) spots were observed which were interpreted as compression of K atoms along the grooves in the (1210) direction, with the K atoms always equally spaced along these grooves (Fig. 57). Below 0 = 0.5, the spots split along the {0001) direction until disappearing at OK = 0.35, implying a periodic change along the (0001) direction. This is proposed to be progressive removal of strips or domains of the c(2 x 2) array, leading to a continuously changing inter-domain spacing of the c(2 x 2) patches.

2.15.8. Coadsorption of Li and 1120 on the Ru(O001) surface This system was the subject of a study using ESDIAD, LEED and other techniques [268]. This study deals with the low-coverage regime (0 u < 0.1) at 80 K. With no Li present, H 2 0

0001

i210"Ca)

OK

(b)

O R. Fig. 57. Structures proposed for K on Ru(1010) [267]. (a) c(2 x 2) phase at OK = 0.5. (b) Saturation coverage structure at 295 K with OK = 0.67.

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

123

adsorption results in a halo-like E S D I A D pattern indicating undissociated azimuthally disordered molecules. With a Li coverage of 0Li > 0.05, the E S D I A D pattern changes to a single lobe, indicating dissociation of the H 2 0 , and OH standing normal to the surface. Upon heating, H 2 0 dissociation is produced for even smaller amounts of adsorbed Li.

2.15.9. Coadsorption of Na and 1-120 on Ru(O001) A structural study for this system was carried out using L E E D and E S D I A D [269]. Water apparently causes the (2 × 2) and (x/-3-× v/-3-)R30° Na structures to disorder. The E S D I A D results may be divided in terms of two sodium coverage ranges: (1) 0ya < 0.25; and (2) 0Na > 0.33. The intermediate coverage regime produces behavior which is a mixture of the behavior at the lower and higher coverages. At 0Na < 0.25, there is little evidence for Na-induced dissociation. There is however a dramatic increase in the H ÷ yield from water in the presence of absorbed Na. This is interpreted as reorientation/deflection of H + from the ionic-like Na atoms. At 0Na < 0.25 a halo is formed indicating that the H 2 0 molecules are randomly oriented azimuthally or are rotating. At 0Na =0.25, when a p(2 X 2) Na pattern forms, H 2 0 adsorption leads to a hexagonal E S D I A D pattern, consistent with the deflection hypothesis. At 0Na >__0.33, the emission angle changes abruptly to normal, and at higher coverages, the emission cone sharpens. The interpretation is that in this higher density structure, Na atoms are less ionized and are able to react with water molecules to form a hydroxide species with the OH bonds oriented normal to the surface, producing intense H ÷ E S D I A D emission. Annealing of the H 2 0 - N a surface produces a succession of complex L E E D patterns. (a) From 80-200 K, the patterns are disordered. (b) From 200-600 K, where molecular water is desorbed from the surface, ordered patterns develop from the w a t e r - N a reaction product. Above the H desorption temperature (600-1000 K), patterns are observed similar to those for coadsorbed O and Na. For T > 1000 K, only O remains giving a familiar (2 × 2) pattern. 2.15.10. Coadsorption of Na and CO on Ru(O001) The coadsorption of CO and Na was studied using E S D I A D and L E E D [270]. CO on the clean Ru(0001) surface is bonded perpendicularly with the C end down. ESDIAD results show that for small Na coverages (0 < 0Na < 0.15) and for saturation coverages of CO, a reorientation of the CO appears to occur with CO tilted away from the normal giving a hexagonal E S D I A D pattern. The authors interpret this as due to a local N a - C O interaction in a coplanar layer. At all other coverages of Na and CO, and at temperature > 80 K, the CO appears to remain normal to the surface. 2.15.11. Coadsorption of K and 0 on Ru(O001) The coadsorption of 0 2 on the (v~- × v~-)R30 ° potassium phase at 85 K was studied using L E E D [271]. Exposure to 0.15 L results in the disappearance of the (v~ X f 3 ) R 3 0 ° L E E D pattern. Over the exposure range 0.35-0.6 L an incommensurate hexagonal rotated L E E D pattern is observed which sharpened upon annealing to 300 K. Higher exposures led to the disappearance of the L E E D pattern. A study by Hrbek [272] is in agreement with these findings.

124

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

Table 6 Ordered structures in the R u ( 0 0 0 1 ) - C O / K coadsorption system (from Ref. [12]) LEED pattern

Temp.

(8 x 2) (7 x 2) p(2X2) diffuse (6 x 2) (5 x 2) (2 × 2) (3 x 3)

80 80 80 80 80 130 130

OK

0co

(CO)/(K)

Ref.

0.09 0.10 0.10 0.15 0.20 0.26 0.33

0.05 0.08-0.15 > 0.02 0.08

1:2 1:1 2:1 1:2

> 0.15

3 :2 1:1

[260] [260] [260] [260] [260] [261] [261]

This system has been studied with a variety of chemical probes [271-276], a detailed description of which is beyond the scope of this review. Many of these studies point to a strong K - O interaction and the formation of K - O compounds.

2.15.12. Coadsorption of K and CO on Ru(O001) This system has been reviewed recently [12]. A summary of ordered structures found as a function of K and CO coverage are shown in Table 6. A model was proposed [260] to explain the observed (n X 2) structures whereby CO molecules and K atoms form rows having long-range K - K repulsion (determining the periodicity in n) and short-range K - C O attraction (determining the ( x 2) periodicity) (Fig. 58). This system was also studied using ESDIAD, LEED and other techniques [277]. CO by itself is perpendicular to the surface. In this study, low coverages of CO and K were studied

O

K-atom

•

CO

Fig. 58. Chain model for the (6 x 2) structure formed by the coadsorption of CO and K on Ru(0001) [260] with 0 = 0.16 and 0co ~ 0.08. Other (n x 2) structures may be formed by varying the separation of the K - C O chains.

R.D. Diehl, R. McGrath /Surface Science Reports 23 (1996) 43-171

125

(01<, 0co < 0.25). The E S D I A D O + yield was strongly suppressed by coadsorption of K; this was interpreted as indicative of a reorientation of the CO so that the molecular axis is strongly inclined or parallel to the surface due to a strong chemical interaction between K and CO. L E E D patterns found in this study were largely consistent with those of Weimer et al. [261]. The opposite conclusion with regard to the orientation of CO was reached by Wurth et al. in a study of angularly resolved Auger lineshapes [278]. A recent L E E D study of this system has determined that in the (2 × 2) phase, the CO occupies the hcp site, a switch from the top site for pure CO adsorption, and the K occupies the top site, a switch from the fcc site occupied for pure K adsorption [253]. This was explained by the bonding of CO to its three coordinated Ru atoms beneath it, weakening the R u - R u bond to the single Ru atom beneath the K atom and allowing this substrate atom to relax. Thus, the CO acts as a softener of the surface, facilitating the top-site occupation by K.

2.15.13. Coadsorption of Cs and 0 on Ru(O001) The adsorption of oxygen on the (v~- x v~-)R30 ° phase of Cs on Ru(0001) at 95 and 220 K [279] was studied using LEED. At 220 K the L E E D pattern changed to a split (2 x 2) at low 0 2 doses. Grobecker et al. [280] reported that this split (2 x 2) phase transformed to a well-ordered ( 2 ~ x 2f~)R10.9 ° phase upon annealing to 300 K. Over et al. [281] also studied coadsorption on the (x/3-x v~-)R30 ° phase, at 100 K. They report the appearance of an unspecified incommensurate superstructure accompanied by the disappearance of the ( f 3 - x f3-)R30 ° structure. Annealing to 310 K gave rise to the reappearance of a ~ coadsorbed phase. The oxygen coverage for this phase was estimated to be 0o = 0.35 + 0.02; giving a 1" 1 0 - C s stoichiometry. This phase was analyzed using L E E D as described below. Trost et al. [282] report a room-temperature fff X x/fiR19 ° phase with a 2"4 C s - O stoichiometry, i.e. with 0 o = 0.57 and 0cs = 0.29. It was also reported that lower Cs coverages resulted in mixed fff x fffR19 ° and fluid phases, which was the subject of the STM study described below. Most recently, Bludau et al. have reported a systematic study of this system, easily the most comprehensive of any alkali-coadsorbate system [283]. They mapped the ordered phases for 0.13 < 0cs < 0.37 and 0.0 < 0 o _< 0.8. The result is shown in Fig. 59. Of these many phases, only two have been studied quantitatively (see below), though the authors suggest structures for several of the other phases. There have been several studies of the chemical interaction of Cs and 0 2 on the Ru(0001) surface. A detailed discussion of these is beyond the scope of this review; readers are referred to the review by Kiskinova [11]. Evidence has been found for (i) strong C s - O interactions and the formation of a C s - O surface compound at room temperature [279,284]; and (ii) the stabilization of a superoxo O 2 species at 90 K [285]. Ru(OOO1)-(v/-3 × V~)R30°-(Cs + 0): In a L E E D study of this coadsorbed phase, Over et al. [281] determined the structure of the (v~ × f3)R30°-(Cs + O) coadsorption phase described above. Both Cs and O were found to occupy hcp hollow sites (Fig. 60). The overlayer may therefore be viewed as coplanar, although the centers of the O atoms are much closer to the Ru surface than the Cs atoms. The hard-sphere radius of the Cs atoms is thus 2.08 +_ 0.05/~, which is smaller by 0.08 .A than in the corresponding oxygen-free system. In turn, the O hard sphere radius is increased by about 0.13 A. The authors interpret this as being due to transfer of charge from the Cs atoms to the O atoms probably via the substrate.

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

126

0.35

weak "d~ - - 7

:weak ~:{ *dltlusc (2x21 , ]

rot. [

~3

he2~1

[

f

,

I ,2,, I

,

rot. ]

x~3

--D,-

]

~3

"~21,

/dt s, ~, (3x2%'3)_~.

I~

~

d~use)

0.30 I I

(2~] [diffuse)

=I

(2x2)

diffuse

Isnlitl

1~7

diffuse Ocs E 2 [(2x21

(2x2) (diffuse)

1

0.25

(2x2)

I

2",3

x39 (weak)

[

[

i

I

J

i

[ \7

[dfffuse,7

I

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I

~:39

I ~7

..7

ldif~\lse ,7

I

diffuse (2.K2)

[ 1

0.20

nng

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! I

0,0

i

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(2x2)

I

i 0,2

0.1

idiffu se ~7

I

",'7

l

I

I

F

Id tffuse

1

I

diffuse ring I

i [I (2x2)

nng

0.13

[

~--i~

I

"v7 I

diffuse ring I

0.15

",39 [weak)

---~

1

q7

[diffuse I

1

diffuse Idtffuse ~ [

[

0.3

0.4

l

0.5

0.6

0,7

0.8

t3o

Fig. 59. Experimentallyderived phase diagram for Cs and subsequently adsorbed O on Ru(0001) at 310 K. Dashed boundaries are approximate [283].

Ru(OOO1)-(2v~ X 2v~)R30°-(Cs + 0): the adsorption of oxygen onto a Ru(0001)-(2 x 2)-Cs phase with 0Cs = 0.25 gives rise to the formation of a Ru(0001)-(2f3 x 2f3)R30°-(Cs + O) coadsorption structure with three Cs and two O adatoms in the unit cell [286]. A dynamical LEED analysis reveals that oxygen is located in 3-fold hollow sites on the Ru surface whereas the Cs atoms remain nearoatOp sites, as in the pure Ru(0001)-p(2 x 2)-Cs phase. The O - R u bondlength of 2.14 + 0.05 A is longer by 0.11 than in the Ru(0001)-(2 x 2)-0 phase, indicating that the R u - O bond is weakened in the presence of coadsorbed Cs. The Cs hard-sphere radius of 1.75 + 0.07 ,~ is close to the ionic Cs radius. Trost et al. [282] reported room-temperature STM images of coexisting (f7- x fff)R19 ° and fluid phases for this system (Fig. 61). Smaller-scale images also show fluctuations in the liquid phase and its boundaries as a function of time. The disordered areas also seem to exhibit some

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

127

Ru(0001)ICslO -(~3 x¢-3)R30* (a) t

Zcs

.

1.52-*O.03A \2.08"-0.02~ )2.1!-* 004 ]k Cs- radius: 2.1~ O- radius: 0.8~

Oxygen

Fig. 60. Structural model and parameters for Ru(0001)-(v/3- x ~/3-)R30°(Cs+ O) determined by LEED [281]. Large circles are the Cs atoms; the small solid circles are the O atoms. (a) Side view (cut along the dash-dot line of (b)). (b) Top view.

granularity reflecting the atomic nature of the fluid. It appears that the detailed interpretation of these images is ambiguous in terms of whether the O or the Cs is being imaged.

2.15.14. Coadsorption of Cs and CO on Ru(O001) As part of a H R E E L S study of the electronic modification of Cs overlayers on Ru(0001), Jacobi et al. [287] studied the adsorption of CO on the (2 x 2)-Cs overlayer. The (2 x 2)-Cs structure was observed to rearrange into a sharp (2 x 2)-(Cs + CO) structure at 300 K, the stoichiometry being 1:1. This structure was also reported by Kondoh and Nozoye [288], who further reported a "rotated-p(2 x 2)" pattern after saturation adsorption of CO on the f3X v~--R30° Ru(0001)-Cs surface. Ru(OOO1)-(2x2)-Cs/nCO, n =1, 2, LT: A L E E D I(E) study has been performed of two coadsorption overlayers of CO and Cs on Ru(0001) [289,290]. The Cs atoms were observed to remain in the top sites, but the CO molecules were found to occupy 3-fold hollow sites, contrary to the occupation of top sites if adsorbed alone. The site-switching effect is attributed to the alkali-induced enhancement of the Ru substrate electron density leading to more pronounced back-donation to CO. The increase in C s - R u bondlength of ~ 0.2 A is similar to that observed for K and CO coadsorption on N i ( l l l ) [203].

128

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

Fig. 61. STM image from a coexisting fluid and v/ff phase with 0cs = 0.21 and 0o = 0.69. Ordered domains of the (Vff x ~-)R19 ° structure are separated by the disordered phase.

2.15.15. Coadsorption of K and CO on Ru(10iO) Surnev and Kiskinova [291] noted a series of complex L E E D patterns for this system, indicative of ordering both along and across the troughs of this surface.

2.16. Silver 2.16.1. K,Rb, Cs / A g ( l l l ) Early studies of Cs/Ag(111) [292,293] determined that Cs forms layers of low-density hexagonal-symmetry structures at low coverages and that the overlayer saturates near the p(2 x 2) phase at a coverage of about 0.25. A later more detailed study [294] indicated that the saturation structure occurs at a coverage of 0.33 and is indeed hexagonal. The phase diagram d e t e r m i n e d by that study, along with those for Rb/Ag(111) and K / A g ( l l l ) , is shown in Fig. 62 [294]. From these it is clear that these overlayers are disordered at room t e m p e r a t u r e for most submonolayer coverages. The stability of these layers increases with coverage and there appears

129

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

400

75

.

300 50 '¢

0

200

o

0/

!~' ".

'-

,,.-7~4 ~

;_< ~_,

~

,~i

_

I

~

]

~

~/.~.I IA

I__. I~/~-',~---~

!

m_-~i

I A [/ /!,'< / I I

=/~. 3 ', IA

_

"

~I

0

..... 0.00

"

I

t ¢~t-I

I

~t.I

I !

J

'

"

~ 0.05

I--

..

des 0

'

.

.....

c~t

..

i

VI~I/I

o.,8..-j//t

I

IA3O °

~

~

~ . ~

~
/

75

200

q

..~L~/!~..-~ HO~i

~

-'"

o.1, ~

I--

I ,

IR

~ ,~ -~ , , , . . . .

, .....

,J,~ J

....................

300

I

I~

.o.1o

c)

i

'desoi;pfiOis

F A C- F

; ,~tl

100

400

0.18

~5 ~/!

,~, ,,, c-i,, F , , , , 1 ~ , ,'*-, . . . .

~ ....

I iI

v

!~ !

0.14

"-..

t IA30

300

IR

'J/'~! ' '

75

~".desor~tion

.

i

r~r-.~ i ILIR~/

....

400

,i

.

I, x

0.10 " ' .

lOO

b) Rb

~/~-¢ /

200

.

o.~o

. I.~. r ~ , 0.10

/I

i

i /-/

i

/

~

~/~

..~v~l

o.~4 - ~ . . ~"Q~ ..~v4:r ~/IA~,I IA ~L.: 0.15

~1

.......

,

0.20

"? ' . . . . - r 0.25

i~o,I

, 1 ~i I_

l fi

\

43

~-~CI, , . . . . 0.30

0.35

I 0.40

coverage IA IA30* IR CoF

incommensurate, aligned with substrate incommensurate, aligned at 30* with substrate incommensurate, rotated coexistence of 2 phases fluid

2,13 ,/7 ",/3 HOC ![~

(243 x 2~/3)R30" (47 x 47)R19.1" (43 x ~/3)R30" i high order commens, phase : commensurate phase

Fig. 62. Coverage-temperature phase diagrams for (a) K / A g ( l l l ) , (b) R b / A g ( l l l ) and (c) Cs/Ag(lll) [294]. The open points indicate the inflection points of the disordering curves (LEED intensity versus temperature plots).

to be only a minor, if any, contribution to the stability from the substrate in the commensurate phases. This, along with the preponderance of incommensurate phases, is an indication that the corrugation of the a l k a l i - A g ( l l l ) potential is very small. While in general there is no condensation of these hexagonal overlayers (i.e. they increase in density continuously as the coverage is increased) there are four commensurate phases observed for K and five for Rb and Cs at T > 40 K [294]. For Cs and Rb, these are a 2(x/-3- x 2v~)R30 ° phase at a coverage of 1/12, a p(3 X 3) phase at a coverage of 1/9, a (vr7- x x/if)R19.1 ° phase at a coverage of 1/7, a p(2 x 2) phase at a coverage of 0.25, and a (x/3 x v/3)R30 ° Cs

130

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

Table 7 Structural parameters deduced from LEED for alkalis on A g ( l l l ) [148] Structure Ag(111)-(2 x 2)-K Ag(111)-(2 × 2)-Rb Ag(111)-(2 x 2)-Cs A g ( l l 1)-(v~ × v/3)R30°-K Ag(111)-(v~ × vC3)Ra0°-Rb

dAlk_ Ag

b

dAgl_Ag2

dAg2_Ag3

dAg3_Ag4

0Agl

0Ag2

(A)

(A)

(A)

(A)

(3`)

(3`)

(3,)

2.69 + 0.03 2.84 _+0.03 3.01 _+0.04 2.83 _+0.03 2.96 _+0.03

3.27+0.03 3.38 + 0.04 3.52+0.04 3.29-!- 0.03 3.40 + 0.03

2.34+0.02 2.34 + 0.03 2.34+0.02 2.36 + 0.02 2.36 + 0.02

2.32+0.03 2.33+0.03 2.32+0.02 2.34+0.03 2.35+0.02

2.35+0.04 2.35+0.03 2.36+0.03 2.35 +0.03 2.34+0.03

0.11+0.03 0.10+0.04 0.09+0.03 0 0

0.03+0.02 0.03+0.03 0.03+0.02 0 0

dAik_Ag is measured from the alkali adatom to the top-most Ag atom(s) in the top layer, dAgl_Ag2, dAg2_Ag3, and dAg3_Ag4 are measured between the bottom-most atom(s) in the upper layer to the top-most atom(s) in the lower layer. The O's refer to the rumpling amplitudes, b is the alkali-Ag bondlength. All dimensions are in 3`.

phase at a coverage of 0.33. For K, the commensurate phases observed are a p(3 x 3) phase at a coverage of 1/9, a (x/ff x v/if)R19.1 ° phase at a coverage of 1/7, a p(2 × 2) phase at a coverage of 0.25 and a (vc3- x v~-)R30 ° phase at a coverage of 0.33. In addition, higher-order commensurate phases were observed at a coverage of about 0.19 for K and Rb and 0.36 for Rb. The temperature stability of these layers increases with coverage, and in general, Cs is marginally more stable than the lighter alkalis at any given coverage. The monolayer-saturation structures correspond to a Cs-Cs distance of 5.01 A (an 8% compression relative to its metallic d!ameter), a R b - R b distance of 4.76 A (a 7% compression) and a K - K distance of 4.54 A (a 5% compression). An early SEXAFS measurement of the bondlength of C s / A g ( l l l ) at two different coverages found bondlengths of 3.2 + 0.03 .~ at 0.15 coverage and 3.5 + 0.03 A at 0.30 coverage [295,296]. This was taken to be "proof" of an ionic to covalent chemisorption bond transition, but such a bondlength change has not been observed in any other alkali adsorption systems in this coverage range unless accompanied by a site change (see chemisorption bond section 3.4 below). A site change has in fact been observed for K and Rb on A g ( l l l ) , from fcc to hcp hollows between the p(2 × 2) and (v~- × v~-)R30 ° phases, respectively (see structure determinations below) but the bondlength does not change appreciably in these cases [148]. Table 7 gives the structural parameters deduced from LEED. For each of the overlayers studied, an epitaxially rotated incommensurate phase exists at densities between the (f7- x v~-)R19.1 ° phase and the p(2 x 2) phase. The rotation angles of these overlayers at 35 K as a function of overlayer misfit with respect to the p(2 × 2) phase are shown in Fig. 63. (Misfit is defined a s ( d a - d s ) / d s, where d a is the adsorbate spacing and d s is two times the substrate spacing. Thus the commensurate p(2 X 2) structure has a misfit of zero.) In the density range described here, the a d a t o m - a d a t o m spacing ranges from 5.78 to 7.65 A. These spacings are larger than the diameters of the alkalis (see Table A.3). The primary interaction between the adatoms is therefore expected to be the dipole-dipole repulsion arising from the strong polarization of the adatoms. Any deviations from this potential form would be expected to be largest for Cs at the higher densities since Cs comes closest to the point of orbital overlap and intralayer metallic bonding. The behaviors of the three different alkalis are in fact relatively similar at low densities (high misfits) but differ at the higher densities (low misfits). Fig. 64 shows the same data in comparison to various models for the rotation. It can

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171 25

---

-

~

-

T

r

-

131

-

20

o~°

=~'~

'r

e,.-

I

#

o

5

0

Am •

o

, z.,'~

•

Rb

~

Cs

"

zx

!

a e~

~'~l(Z~ I ~ 0.0 0.1

K

. . . . . . . . . . . . . . .

0.2

0.3

0.4

misfit (relative to (2x2)) Fig. 63. R o t a t i o n a n g l e v e r s u s misfit for K, R b a n d C s / A g ( l l l ) p(2 X 2) p h a s e .

[294]. Misfit a n d a n g l e a r e m e a s u r e d relative to t h e

readily be seen that none of these models fit the data exactly. Furthermore, there is a strong temperature dependence of these rotation angles, as shown for R b / A g ( l l l ) in Fig. 65. This temperature dependence is discussed in Section 3.6 along with a more detailed comparison to the models and to other systems. Ag(lll)-p(2 x 2)-K LT: A dynamical LEED study at 40 K [148] showed that K is in the fcc hollow sites with a K-;Ag bondlength of 3.27 + 0.03 .~, which corresponds to an effective K radius of 1.82 + 0.03 A. A substrate rumpling was observed which consists primarily in the three nearest-neighbor Ag atoms being pushed down by 0.11 + 0.03 A with a corresponding smaller rumpling of Ag atoms in the second layer. This LEED analysis included the possibility of anisotropic vibrations of the overlayer atoms and the best fit corresponded to an rms amplitude of 0.15 A perpendicular to the surface and 0.50 A parallel to the surface. Ag(lll)-(v~- × v~)R30°-KLT: A dynamical L E E D study [148] indicated that the adsorption site for this structure is the hcp hollow, with a K-zAg bondlength of 3.29 + 0.03 A, which corresponds to an effective K radius of 1.84 + 0.03 A. No rumpling of the substrate and very little relaxation was observed. The anisotropic vibration analysis gave rms amplitudes of 0.16 and 0.24 ~, for vibrations perpendicular and parallel to the surface, respectively. Ag(lll)-p(2 x 2)-Rb LT: A dynamical L E E D study [148] showed that Rb is in the fcc hollow sites with a R b - A g bondlength of 3.38 + 0.03 A, which corresponds to an effective Rb radius of 1.93 + 0.03 A. A substrate rumpling was observed which consists primarily in the three nearest-neighbor Ag atoms being pushed down by 0.10 + 0.04 ~, with a corresponding smaller rumpling of Ag atoms in the second layer. The vibrational amplitudes deduced from this study were 0.15 and 0.50 A perpendicular and parallel to the surface, respectively. Ag(lll)-(vr3 - × v~)R30°-Rb LT: A dynamical LEED study of this structure found that the adsorption site is hcp hollow, with a R b - A g bondlength of 3.40 5- 0.03 A, corresponding to a Rb effective radius of 1.95 + 0.03 A. There is no surface rumpling and very little relaxation. o

o

o

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

132

25 ~ . : ~ f..-.:~.3.7:.:,~7-....... .,, f•..:. ' > . " .:...-... . . " - - - ""~.:: :.-:..-.:;...-...~?..:, .... ..",'>:. ..~t...~.:................... : . . . . • .. , ~,. -..~. ...... . :. .... [" :.. :'. :.., " . :i::-~./.. . - .......... ;.. . . . . . . !- . ; . " 20 r . , ~ : - . . ' . " , . . , .: . . : . ~ : - ~ ~ :":"

g~

" "" .......

':"

Z '"::

° I' : g

lo

t':

~

:." ' . ' " ...../. . .'"

I,: : I, . :

" .." ." -. "/

,"

./ / ~ , / / . ..: .~'"

.

"-

•

i-

j

~

i

0.0

'

~

0.1

"~

,>;i;;>;')> " ' :" •

"-.'-

..<"

• "..."

"..~.../"

....

~:.----~.. . • .:

....

.

"

-. '" 0

.:.

,,-i

. ' ~ ,

.,-:":- . ~

_ '

".. : : - ~ > , , , ~ ' ~ < ~ 1 ~ "

f~> ..,'. ~,~ -..1: : , : ~ ! ~ ~~ r

15

~ .~7:: :~., ~: ". ,'" '.".. -:..,.....:. .~ / . ....'. . " ..--'.5

' "

..

"

l • :~

I

. CS'.

r

i

0.2

0.3

0.4

misfit (relative to (2x2))

- -

BG

urJA ---- 60

. . . .

----

BG q~A = 30°

-----

BG q~s = 60°

___

BG qJs = 30 °

-- --

--

N M dipole potential N M L e n n a r d - J o n e s potential Shiba

__J

Fig. 64. R o t a t i o n d a t a s h o w n w i t h t h e p r e d i c t i o n s o f t h e B G a n d H O C m o d e l s a n d t h e N M a n d S h i b a t h e o r i e s . T h e s m a l l d o t s a r e t h e l o c a t i o n s o f h i g h e r - o r d e r c o m m e n s u r a t e s t r u c t u r e s h a v i n g p e r i o d s s h o r t e r t h a n 40 .A.

20

I

~

I

I

|DD [] []0"157 0.127

(D

[]

15

IR OOQOOQOOOmOOil~ fO

g

~= 10 0.199

O

0000

5 40

00

°~°o~

I

I

I

60

80

100

-~

120

T (K) Fig. 65. T e m p e r a t u r e

dependence

of the rotation angles for Rb/Ag(111)

[294].

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

133

The vibrational amplitudes deduced from this study were 0.18 A perpendicular to the surface and 0.48 .~ parallel to the surface. Ag(lll)-p(2 x 2)-Cs LT: A L E E D study of this structure found that the Cs atoms adsorb in the fcc hollow sites with a C s - A g bondlength of 3.52 + 0.04 A, corresponding to an effective Cs radius of 2.08 + 0.04 A. The substrate rumpling amplitude for this structure was 0.09 + 0.03 A. The vibrational amplitudes deduced from this study were 0.14 A perpendicular to the surface and 1.0 * parallel to the surface. O

2.16.2. K /Ag(IO0) Potassium adsorption studies on Ag(100) have led to somewhat conflicting views of the nature of the surface structures. One study which included LEED, soft X-ray photoemission and photoabsorption measurements found that K condenses on the surface at low coverage to form 2D islands [297]. The phase transition from a dispersed disordered phase into this condensed phase was observed to be driven by temperature and was irreversible, leading to the conclusion that the dispersed phase is metastable. At 90 K, the adsorption of potassium resulted in the formation of a dispersed disordered phase, followed successively by commensurate phases: a c(4 × 2) a t O K = 0.25, a (3 X 2) at OK = 0.33 and a c(2 × 2) at O K = 0.50. The K - K spacing at saturation was 4.08 A, which is 14% smaller than the K metallic diameter. It is notable that no ordered incommensurate structures were observed. Heating these overlayers to 220 K (or alternatively, adsorbing at 220 K) caused the structure to change. The L E E D pattern was (1 × 1). Further annealing to 400 K and cooling led to the observation of a (5 × 10) pattern for coverages below saturation and a (5 x 4) pattern for saturation coverage. These overlaxers were interpreted as a quasi-hexagonal overlayer with an average K - K bondlength of 4.15 A. Quite different adsorption structures were observed in a different study [94,95]. At low temperatures (125-150 K) the succession of phases was essentially the same as that found in the study described above except that incommensurate structures were also observed. The succession of structures were: a dispersed phase followed by a c(4 × 2), then a c(3 × 2), then a hexagonal layer, and finally a c(2 × 2). Adsorption at room temperature, however, led to completely different L E E D patterns being observed: a (2 × 1) at OK= 0.1 followed by a metastable (3 × 1) (which seemed to require second-layer K atoms to form). These structures were interpreted as K-induced substrate reconstructions. It was suggested that the most probable structure of these reconstructions are missing-row structures [94,95]. At low temperatures an additional c(4 × 3) pattern superimposed on the (2 × 1) pattern was interpreted as ordering of the K atoms on the reconstructed substrate. It should be noted that the room-temperature results from these two separate studies of K/Ag(100) are very different in nature and not just in interpretation. Therefore further studies are required before this system can be considered to be understood.

2.16.3. Na /Ag(llO) Sodium adsorption on Ag(ll0) at 300 K led to a distinct sharpening of the (1 x 1) diffraction beams, followed by a (1 x 2) L E E D pattern and then a (1 x 3) L E E D pattern [298]. Following an early paper by Hayden et al. [299], it is now understood that this succession of phases is consistent with the formation of the missing-row reconstructions which have been observed in other studies, including K / A g ( l l 0 ) [300] and C s / A g ( l l 0 ) [301], described below.

134

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

2.16.4. K /Ag(110) The adsorption of various (O K from 0.13 to 0.39) amounts of K onto Ag(ll0) induces a (1 x 2) structure [300]. This structure was determined to be a surface reconstruction and was the subject of the MEIS study described below. A g ( l l 0 ) - ( l × 2)-K: A MEIS study [300] determined that the structure of this reconstructed surface is a missing-row reconstruction. Relaxation parameters for the substrate were found to be - 9 + 2% for the top layer and - 1 + 2% for the second layer. The K adatoms which did not contribute were assumed to be disordered. 2.16.5. Cs /Ag(llO) The adsorption of Cs onto Ag(110) leads to a (1 x 2) reconstruction which was determined to be a missing-row structure using dynamical L E E D (see below) [301]. The position of the Cs atoms on this reconstruction was determined by X-ray diffraction for two different Cs coverages, 0.2 and 0.3 [302]. The Cs atoms were found to be adsorbed in incommensurate chains in the troughs of the missing row, with an average height of 1.7 ,~ for 0.2 coverage and 1.4 .~ for 0.3 coverage. It was proposed that the greater average Cs height in the low-coverage phase was due to the existence of some Cs atoms in a commensurate structure in which the adatoms have a higher coordination [302]. Ag(ll0)(1 X 2)-Cs LT: The (1 × 2) reconstruction induced by Cs adsorption was found to have the missing-row structure by a dynamical L E E D study [301]. The surface relaxations of the three outmost layers were found to be - 1 1 % , - 2 % , and - 9 % , which compare to - 7 % , + 1% and - 2 % for the clean surface. In addition, the L E E D study found a rowpairing in the second layer of 0.10 + 0.10 ,~ and a rumpling of the third layer of 0.10 + 0.05 A. This system was also studied using SXRD [302]. The missing-row reconstruction was confirmed for Cs coverages of 0.2 and 0.3 though the construction of d12 was less than that found by L E E D [301]. 2.16.6. Coadsorption of K and 0 on Ag(111) No ordered overlayers were observed for O adsorption at room temperature on a K-dosed surface in the sub-monolayer range. In the multilayer regime, two ordered structures were observed which were postulated to be associated with the growth of KO2 [303]. 2.16.7. Coadsorption of Rb and 0 on A g ( l l l ) Again, no ordered L E E D patterns were observed for sub-monolayer Rb coverages. At higher coverages, L E E D patterns associated with the growth of various planes of bulk Rb oxides were observed [304]. 2.16.8. Coadsorption of Cs and 02 + Cell 2 on Ag(111) Under certain reaction conditions, the coadsorption of oxygen and ethylene with Cs/Ag(111) led to the formation of CsO x compounds on the surface [292,293]. This CsO x surface compound gives a sharp (2v~- x 21if)R30 ° L E E D pattern. Since this structure occurs for a wide range of initial Cs coverages, it is suggested that this structure forms as two-dimensional islands [292]. This L E E D pattern could also be generated by simple dosing and heating of oxygen on Cs-precoated A g ( l l l ) surface [292].

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

2.16.9. Coadsorption of Na and

0 2 on

135

Ag(llO)

Adsorption of 0 2 o n Ag(ll0) in the presence of Na led to the formation of a (4 x 1) LEED pattern [298]. It was believed that this structure included either subsurface Na alone, or both Na and O below the surface.

2.1 Z Tungsten Studies of coadsorption of alkalis and oxygen on tungsten were among the pioneering works of Langmuir [305]. Thereafter, structural aspects of adsorption received attention especially from groups in the Soviet Union in the early 1970's and detailed coadsorption studies were carried out by Chen and Papageorgopoulos at around the same time. Since then there has been sporadic activity in mapping out phase information, but no quantitative work has appeared.

2.17.1. Cs/W(lO0) This system is interesting as the clean surface reconstructs to a c(2 x 2) overlayer with zig-zag chains of atoms along one of the two diagonals of the unit cell [245]. This system was extensively studied theoretically as a model system to study the nature of alkali bonding at surfaces [306,307]. In an early LEED study [308,309], c(2 x 2), p(2 × 2) and hexagonal LEED patterns were observed for Cs adsorption on this surface at room temperature. The initial c(2 x 2) coverage results were later thought to be due to the coadsorption of Cs and H on this surface [310,311]. The later study [311,312] also observed a weak p(2 x 2) phase at 0cs = 0.25 (corresponding to the work function minimum) and, at saturation coverage 0cs = 0.43, a hexagonal pattern. Simple structural models were proposed. A phase diagram for this system was later determined by LEED [245] and is shown in Fig. 66; structural models for the observed phases were proposed in this work based on Cs adsorption in the troughs of this surface.

2.17.2. Li / W(llO) A study was made of the low-temperature phases formed when Li adsorbs on W(ll0) [313]. (2 x 3), c(2 x 2), and c(3 × 1) phases were found at increasing Li coverages. These overlayers

s00l-Cs/W(100)

,,,

400 -DisorderedCs + /

/f "I

E 3oo-."t o a. I--200

T I~,Cs-Desorplion .

~

"

/1\\/

1.-" "

c(2.2)W * Disordered Cs ....

,

"ring ';~ [

_'_~E~ . . . . . . .

100

"\

F--T~I : ,,-.. ~'I~2.2)Wt c(2.2 W : ! " ,T * T ÷ " ;-o ~: / Ip(2*2)Csl rncommens. Cs(11) i ~ ~,

1,I~"_

~ _ _ ~

. . . . . . . .

•

,~8,

:

i u~-J°

o

_- _

;. . . . .

c(2~2}W + Incommens

Cs

(I} hex

0

'

' 0.1

0 2i Cesium

'

' 0,3

' O.Z,

'

i OS

Coveroge

Fig. 66. Experimental phase diagram for the adsorption of Cs on W(100) [245].

136

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

disordered upon bombardment by low-energy electrons (E > 54 eV). This effect deserved further study. A similar effect was found for O / R h ( l l l ) but was later found to be an artifact caused by CO coadsorption coming from the filament [253].

2.17.3. Na /W(llO) The ordered phases formed by Na adsorption on this surface have been studied using LEED [314]. Several ordered phases were found at 77 K; real-space structural models based on a d a t o m - a d a t o m repulsion were proposed. Phase transitions in the system were originally modeled using Monte Carlo methods by the same group [315] and the complete phase diagram has been modeled by Roelofs and Kriebel also using Monte Carlo simulations [316]. These simulations found some stable phases which corresponded to the experimentally determined ones, and some measure of qualitative agreement was found for the phase diagram. For room-temperature adsorption of Na no ordered phases were observed [317].

2.17.4. Cs/W(11o) The adsorption of Cs on W ( l l 0 ) at 77 K was studied using L E E D and other techniques by Fedorus and Naumovets [318]. Several ordered phases were observed as a function of coverage. In this case, contrary to L i / W ( l l 0 ) [313] and N a / W ( l l 0 ) [314], a d a t o m - a d a t o m repulsions appear to dominate and the overlayers do not appear to be in registry with the underlying surface. A metastable hexagonal commensurate phase was later observed using reflection high-energy electron diffraction ( R H E E D ) at a coverage close to the completion of the first Cs layer [319].

2.17.5. Na / W(112) The adsorption of Na on W(l12) at room temperature was studied using L E E D [320]. In the coverage regime 0 < 0Na < 0.5, a p(2 × 1) pattern was observed with maximum intensity at ~ 0.4-0.5 coverage. A model was proposed with Na atoms in 4-fold sites in the troughs of the surface. In the range 0.5 < 0Na < 0.8, the L E E D patterns indicated a continuous compression of the Na atoms along these troughs as coverage increases. The minimum spacing of the Na atoms at 0.8 coverage in the troughs was estimated to be 3.54 + 0.05 A, about 7% smaller than the metallic Na diameter. Coverages higher than 0.8 led to second layer formation. O

2.17.6. Cs /W(112) Adsorption of Cs on W(l12) at room temperature [321] produced weak (h + 1/3, k + 1/2) and (h + 2 / 3 , k + 1/2) beams which continuously converged until a c(2 × 2) pattern was formed. Increasing the coverage caused the half-order beams to split again until saturation. T o explain this continuous convergence of L E E D spots, a gradual mixing of structures with the Cs atoms adsorbed in 4-fold hollow sites in the [110] direction was proposed. However, the coverages quoted for some of these sub-monolayer overlayers (0.3 < 0 < 0.6) are undoubtedly higher than that of the completed first layer. 2.17. 7. Coadsorption of Cs and H 2 on W(IO0) A thorough room-temperature study of this system was made [311], motivated in part by the need to establish the behavior of Cs on the clean W(100) surface (see Section 2.17.1 above).

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

137

Both Cs adsorption on a hydrogen-covered surface and H 2 adsorption on a Cs-covered surface were studied. This early work is affected by the inability to establish a Cs overlayer without the presence of some H (giving rise to extra diffraction spots in c(2 x 2) positions). Cs adsorption on a H surface appears to increase the sharpness of the H c(2 x 2) pattern, due to a Cs-induced increase in ordering and binding energy of the H atoms. In all cases studied the H atoms were assumed to sit underneath the Cs atoms, as was the case for Cs and O on W(100) (see below).

2.17.8. Coadsorption of Cs and 0 on W(IO0) Both Cs adsorption on an O-covered surface and O adsorption on a Cs-covered surface were studied using LEED and work function measurements [312]. Oxygen adsorption produced a (4 x 1) pattern with an estimated coverage of 0 o = 0.5. Cs adsorption on this structure caused a change to a c(2 x 2) pattern, after deposition of 0cs = 0.25. It was proposed that the (4 x 1) structure was due to molecular O 2 and that Cs induced its dissociation to produce the c(2 x 2), with a disordered Cs layer on top of the underlying O layer. Continued adsorption of Cs caused the evolution of weak (4 x 4) or (6 x 6) patterns, depending on the initial coverage of O. Oxygen adsorption on Cs-covered surfaces produced similar patterns. The interpretation is that O is underneath the Cs layer in each case, until 0 o = 1, and that beyond that coverage oxygen atoms adsorb on top of the Cs layer. Other EELS and work function results appear to be consistent with this conclusion [322,323].

2.17.9. Coadsorption of Na and 0 on W(llO) Room-temperature deposition on Na did not produce any ordered structures. Oxygen coadsorption on Na precovered surfaces did not produce any ordered overlayers [317].

2.17.10. Coadsorption of Cs and 0 on W(llO) Cesium adsorption on oxygen-covered surfaces was studied by LEED [324,325]. A number of ordered structures were observed, but as the structure of the oxygenated surfaces was not known, the proposed structures were quite speculative. Oxygen adsorption on Cs overlayers was studied later [317]. Cs adsorption at room temperature produced a ring pattern at low coverage and a hexagonal pattern at high coverage. For 0cs < 0.3 only (1 x 1) spots were observed, and for coverages between 0.3 and 0.6 weak extra spots were seen, due to oxygen exposures up to 2 L. Above 0.64, the hexagonal spots due to Cs are maximized and some additional spots due to O are seen. For higher coverages, only a (1 x 1) phase was seen [317].

2.17.11. Coadsorption of Na and 0 on 141(112) The coadsorption of Na and 0 2 o n W(112) was studied using LEED and work function measurements [320,326,327]. A variety of LEED patterns were observed depending on the O and Na coverages; the interpretation was that the O atoms were beneath the Na atoms, independent of the order of dosing, and form ordered overlayers. The Na overlayer orders on top of the ordered O layer.

2.17.12. Coadsorption of Cs and 0 on W(l12) A complex series of ordered structures were reported depending on 0 o and 0cs for Cs adsorption on O-covered surfaces [321]. Cs coverages quoted in this work are too high for

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sub-monolayer systems (the first Cs layer should be complete at 0cs = 0.4; hence the proposed structures are erroneous. At higher coverages, both L E E D and work function measurements were interpreted as W - O - C s double layers.

3. General trends 3.1. Phase diagrams - general features

Phase diagrams provide a useful tool for understanding and comparing alkali metal adsorption systems. While there are many more equilibrium coverage-temperature phase diagrams for alkali metal adsorption systems than there were a few years ago, there are still few compared to the number of adsorption systems which have been studied. This is partly because alkali metal adsorption often induces a reconstruction of the substrate, which can depend both on the alkali coverage and on the annealing temperature. In addition, the propensity for alkali metals to burrow into some substrates makes the issue of determining the "equilibrium structure" a tricky one. These difficulties have prevented the mapping of equilibrium phase diagrams for alkalis on aluminum and graphite in particular, and in certain other cases of alkali adsorption. Only a few coverage-temperature phase diagrams have been mapped out for alkali metal adsorption on reconstructed surfaces and these are rather complex. In cases where equilibrium phase diagrams are difficult to determine, it is useful at least to have isothermal non-equilibrium structure versus coverage diagrams, such as those provided for alkali adsorption on aluminum, for instance [24]. In spite of these difficulties, there are a great number of alkali adsorption systems which do not suffer the problems of intermixing and massive substrate reconstruction. The systems for which equilibrium phase diagrams have been determined are shown in Table 8. The phase diagrams for alkali metal overlayers on flat metal surfaces have several common elements. In the next few paragraphs we will over-generalize slightly in order to describe several aspects which are common to many of these adsorption systems. First, at coverages significantly lower than monolayer saturation, ordered phases only occur below room temperature. The exceptions to this arise in cases where the adsorption potential is more corrugated, producing a higher degree of site stabilization. At very low coverages the dominant alkali-alkali interaction is repulsive, causing phases with widely spaced adatoms. Typically, if the temperature is low enough, diffraction rings are observed at low coverages. These rings indicate that there is a reasonably well-defined nearest-neighbor distance but no long-range rotational correlations in these dispersed overlayers. Rings have been observed at very low coverages, even in most cases where the alkali overlayer condenses at some critical coverage into islands. The formation of these uniform-density overlayers depends on a strong repulsive interaction between the alkali adatoms, and rings corresponding to a d a t o m - a d a t o m spacings as large as 60 ~, have been observed [137]. All of the available evidence indicates that these dispersed "ring" phases are repulsive fluids except at very low temperatures. The high mobility of the adatoms in these dispersed phases has been the bane of experiments seeking to determine low-coverage adsorption sites on fcc (111) surfaces [79]. On substrates of somewhat

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Table 8 Experimentally determined coverage-temperature phase diagrams for alkali metal overlayers System

K/Ag(lll) Rb/Ag(111) Cs/Ag(111) K/Au(100) K/Cu(ll0) Cs/Cu(110) Cs/Cu(111) K/Co(10]0) Na/Mo(100) K/Mo(100) K/Ni(100) K/Ni(lll) Cs/Ni(lll) K/Pt(Ill) Cs/Rh(100) Na/Ru(0001) Cs/Ru(0001) K/Ru(0001) Cs/W(100)

Fig. 62a 62b 62c 22 13 10 28a 28b 34 31 45 48 51 55 54 66

Ref. [2941 [2941 [294] [134] [115,116] [115,1161 [89] [691 [169] [168] [111] [175,176] [185] [224] [246] [249] [244] [260] [245]

This is an up-dated version of a table which appeared in an earlier review [14].

higher corrugation such as fcc (100), some site stabilization has been observed for very low coverages at temperatures near 100 K [187]. At intermediate coverages these low-density fluids solidify. In some cases there is a condensation into a dense phase (see Section 3.5), but in most cases the overlayer forms a low-density solid phase. The temperature stability of these solids increases rapidly as their density increases, suggesting that if the temperature is lowered sufficiently, these solid phases can be produced at extremely low densities. The symmetry of alkali metal overlayers, even on surfaces having square or rectangular symmetry, is almost always hexagonal or quasi-hexagonal, except in cases of exceptional substrate corrugation. Commensurate structures are generally more stable than incommensurate structures of a similar density. In general, more open substrate surfaces have a larger corrugation for alkali adsorption than close-packed ones, and the larger alkali metal atoms are affected less by any given substrate periodicity than the smaller ones. The refractory metals provide a larger corrugation than the lighter transition metals. The importance of the substrate corrugation for any particular system is evident by the relative stability of the commensurate phases, both to changes in coverage and to increasing temperature. Since the adatom-adatom and adatom-substrate interactions change with the coverage, the corrugation of the chemisorption potential can also change with coverage, even causing the equilibrium adsorption site to change with coverage [20]. On more corrugated substrates, the coverage-driven transitions between commensurate phases tend to be first-order transitions, in which the intermediate coverage phases consist of coexisting commensurate structures and the diffraction pattern indicates the presence of both

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commensurate phases. On very smooth substrates such as the fcc (111) surfaces, incommensurate phases tend to dominate the phase diagram, and the stability of these incommensurate phases depends largely on the strength of the a d a t o m - a d a t o m interactions which stabilize them. Intermediate substrate corrugation causes the formation of density modulations, or domain walls in these incommensurate phases, and a further increase in the corrugation can lead eventually to the formation of microscopic domains of two (or more) commensurate structures, and finally to the type of phase coexistence described above. The corrugation of the substrate also provides the field which leads to the non-symmetry rotation of incommensurate overlayers, as described in Section 3.6. Other substrate effects can arise from defects in the substrate structure. Substrate steps can cause pinning of the epitaxial rotation of the overlayer. Step pinning of incommensurate phases seems to be more prevalent at low coverages than at high coverage, and it has been demonstrated that alkali metal atoms at low coverage tend to adsorb at step sites [328]. This tendency to align along the steps is usually overriden at higher coverages, presumably by the increasingly important collective interactions as the density increases. Step pinning not only causes incommensurate overlayers to align along the step direction, it can cause or prevent the formation of particular commensurate phases. For instance, in the low-coverage potassium overlayer on C u ( l l l ) or Ni(111), the (f7x f7)R19.1 ° phase was not observed, possibly due to step pinning along the 0 ° direction. The saturation of the monolayer for most of these overlayers occurs when the a d a t o m - a d a t o m spacing is smaller than that dictated by their metallic diameters. In several cases it has been observed that the density of the monolayer does not saturate until part of the second layer is adsorbed. This monolayer compression effect is described in more detail in Section 3.3. So far there have not been very many studies on the disordering of alkali metal overlayers. There have been a few studies of the melting of the low-density incommensurate structures on Cu(111) and Ni(111), which melt into fluids which are highly correlated. The highly correlated nearest-neighbor distance in these fluids should not come as a surprise since the appearance of diffraction rings at even lower coverages indicates strong a d a t o m - a d a t o m interactions. However, rotational correlations are observed in these overlayers over a wide temperature range and are interesting in light of the KTHNY theory for 2D melting, as described in Section 2.4.3 and for comparison to models of systems having repulsive interactions [91]. Since there are several phase diagrams determined so far for hexagonal surfaces, we can begin to use them to compare these overlayers. Most of the commensurate structures and their disordering temperatures are shown in Table 9. The fact which is most apparent from this table is that the disordering temperatures of these structures depend most on their densities. This is evident both from the observation that the disordering temperature for a given structure of a given alkali decreases with substrate lattice parameter, and from the observation that on any particular substrate, the larger alkalis are more stable. There are no systems which seem to buck this trend, leading to the conclusion that at least for many systems, the primary parameter affecting the stability of the overlayer is its density relative to its normal metallic density. This means that it is very difficult to extract a comparison of the substrate corrugation from different substrates. However, by comparing substrates with very similar lattice spacings, Cu and Ni for example, it can readily be seen that Ni appears to provide somewhat more stability than Cu to the p(2 x 2) phase, possibly because of a higher degree of d-electron participation in the chemisorption bond [329]. Unfortunately, there are insufficient data at this time to carry these

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Table 9 D i s o r d e r i n g t e m p e r a t u r e s for c o m m e n s u r a t e s t r u c t u r e s o f alkali m e t a l o v e r l a y e r s o n h e x a g o n a l s u b s t r a t e s System

NNsu b

p(3 x 3)

(v~- x V~-)R19.1 °

p(2 x 2)

(v~- x v~-)R30 °

(3 × 3)

(.&)

(K)

(K)

(K)

(K)

(K)

Na/Ru(0001)

2.70

-

-

100

260

440

K/Ni(111) K/Ru(0001) K/Pt(111)

2.49 2.70 2.77

-

-

370 275 225

420

-

K/Ag(111 ) R b / A g ( l l 1) Cs/Ni(111)

2.89 2.89 2.49

42 43 -

58 65 -

160 178 430

250 320 -

-

Cs/Cu(111) Cs/Ru(0001) Cs/Ag(lll)

2.52 2.70 2.89

58

65

360 325 188

370 -

-

NNsu b is t h e n e a r e s t - n e i g h b o r s p a c i n g o f t h e s u b s t r a t e .

comparisons further, although we remark in passing that there does seem to be enhanced commensurate stability on substrates with partially filled d-bands as opposed to the noble metals. Monte Carlo calculation of the phase diagrams for some alkali metal adsorption systems has been somewhat successful for surfaces having relatively large corrugations, such as Cs/Rh(100) [246] and N a / W ( l l 0 ) [316]. However, coverage dependence of the adsorption site and substrate rumpling (see Section 3.2.) clearly would cause difficulties in a simple application of Monte Carlo techniques to study the phase diagrams of alkalis on most hexagonal substrates.

3.2. Sites in alkali adsorption All of the alkali adsorption sites determined so far for relatively simple (primitive) commensurate overlayer structures are given in Table 10. It is interesting to note that only four of these site determinations were made before 1991. If we ignore the systems in which a significant substrate reconstruction occurs, some generalizations can be drawn. First, alkalis adsorbed on the substrates of square and rectangular symmetry are in the high-coordination 4-fold hollow sites. All available evidence indicates that these are the preferred sites over the whole coverage range [109,110,187]. The situation on the hexagonal substrates is more interesting. In some cases the equilibrium adsorption site is in the highest-coordination adsorption hollow site (either hcp or fcc), while for others it is the lowest-coordination top site, and for one system it is the bridge site. The natural question to ask is what are the important factors in determining which site is occupied? In general we have expected monatomic adsorbates on metal surfaces to adsorb in the high-coordination sites, and indeed this holds for most alkali metal adsorption systems. What is different about the systems where top sites are observed? First, we note that so far, top sites have been observed only on hexagonal-symmetry subtrates. Such substrates will have a naturally smaller component in the corrugation potential due to the hard-wall repulsion. Beyond that it has been shown using D F T calculations that a necessary component of the

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Table 10 Experimentally determined commensurate structure parameters for alkali metal adsorption systems (only primitive structures are listed) System

Structure

Temp. (K)

Site

Method

Cs/Ag(lll) C s / A I ( l l 1) Cs/C(0001) Cs/Cu(lll) Cs/Rh(lll) Cs/Rh(lll) Cs/Rh(100) Cs/Ru(0001) Cs/Ru(0001) Rb/Ag(111) Rb/Ag(lll) Rb/Al(111) Rb/Al(111) Rb/Al(111) Rb/Al(lll)

p(2 × 2) (Vc3 × x/3-)R30 ° p(2×2) p(2 x 2) p(2 x 2) ( f 3 × V~-)R30 ° c(4 X 2) p(2 × 2) (v/-3 x v~-)R30 ° p(2 x 2) (x/3- × v/3-)R30 ° p(2 X 2) (v~-×v~)R30 ° (v~ x v/3)R30 ° (v~- x ~/3-)R30°

50/200 100 80 300 50 50 120 80 80 40/300 40/300 170 170 150 100

fcc Top Hollow Top Top hcp Hollow Top hcp fcc hcp Top Top Top Top

LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED SXW SXW SXW LEED

Rb/A1(111) Rb/Al(lll) Rb/Cu(111) Rb/Rh(lll) Rb/Rh(lll) Rb/Ru(0001) Rb/Ru(0001) K/Ag(111) K/Ag(lll) K/AI(lll)

(v/3-×v~-)R30 ° (Vc3 x f3-)R30 ° p(2 × 2) p(2 × 2) (v;3- × x/3)R30 ° p(2 × 2)

300 100/300 300 50 50 50

Subst. Subst. Top Bridge hcp fcc

(v/3 × x/3)R30 ° p(2 × 2) (V~- × ~/3)R30 ° (v~ × v~-)R30 ° (V~ x v/3-)R30 ° c(2 × 2) p(2 × 2) (1 × 2) p(2 x 2) p(2 × 2) p(2 × 2) p(2 × 2) c(4 x 2) c(2 x 2) p(2 × 2) (vC3 × Vc3-)R30° (v/3 × v~-)R30 ° p(2 × 2) (x/~- × v~-)R30 ° (v~ × x/3-)R30 ° (v/-3- × v~-)R30 °

50 40/300 40/300 90 300 300 65/300 100/300 120/400 70/300 130/300 100/300 80 300 50 50 80 80 100/300 300 170/300

hcp fcc hcp Top Subst. Hollow Top Subst. Top Top Top Top Hollow Hollow hcp hcp hcp fcc Subst. Subst. Subst.

K/AI(lll) K/Co(1010) K/Cu(lll) K/Cu(110) K/Ni(lll) K/Ni(lll) K/Ni(lll) K/Ni(lll) K/Ni(100) K/Pd(100) K/Rh(lll) K/Rh(lll) K/Ru(0001) K/Ru(0001) Ya/Al(111) Ya/Al(lll) Na/AI(lll)

Bondlength

Effective r

Exc. r

N

Ref.

(A)

(A)

(.~)

3.52_+0.03 3.45 + 0.03 3.1 3.01 + 0.05 3.25 + 0.05 2.95 + 0.05 3.44 + 0.06 3.25 + 0.08 3.52 + 0.02 3.38 + 0.03 3.40 + 0.03 3.13 + 0.10 3.13+0.10 3.23+0.10 3.36 + 0.03

2.08_+0.03 2.03 + 0.03 2.41 1.73 + 0.05 1.90 + 0.05 1.60 + 0.05 2.10 + 0.06 1.90 + 0.08 2.17 + 0.02 1.93 + 0.03 1.95 + 0.03 1.70 + 0.10 1.70+0.10 1.80+0.10 1.93 + 0.03

0.41 0.36 0.74 0.06 0.23 0.53 0.43 0.23 0.50 0.45 0.47 0.22 0.22 0.32 0.45

3 1 6 1 1 3 4 1 3 3 3 1 1 1 1

[148] [25] [157] [90] [243] [243] [247] [244] [244] [148] [148] [40] [40] [54] [53]

SXW LEED SXW LEED LEED LEED

3.72+0.10 3.74 + 0.02 3.07 + 0.01 3.20 + 0.05 3.20 + 0.05 3,41 + 0.05

2.29+0.10 2.27 + 0.02 1.79 + 0.01 1.85 + 0.05 1.85 + 0.05 2.07 + 0.05

0.81 0.79 0.31 0.37 0.37 0.59

6 6 1 3 3 3

[54] [53] [88] [243] [243] [263]

LEED LEED LEED LEED LEED LEED SEXAFS PhD LEED SEXAFS PhD PhD LEED LEED LEED LEED LEED LEED LEED SEXAFS NISXW

3,41 + 0.05 3,27 + 0.03 3.29_+0.03 3,23 + 0.05 3.58 + 0.03 3.12 + 0.05 3.05 + 0.02 3.27 + 0.27 2.82 + 0.04 2.92 + 0.02 3.02 + 0.01 2.86 +0.03 3.20 + 0.05 3.20 + 0.05 3.15 + 0.05 3.15 + 0.05 3.29 + 0.05 3,25 + 0.05 3.21 + 0.01 3.31 + 0.03 3.10 + 0.06

2.07 + 0.05 1.82 + 0.03 1.84_+0.03 1.80 + 0.05 2.10 + 0.03 1.87 + 0.05 1.77 + 0.03 1.99 + 0.27 1.57 + 0.04 1.67 + 0.03 1.77 + 0.01 1.62 + 0.03 1.96 + 0.05 1.82 + 0.05 1.80 + 0.05 1.80 + 0.05 1.98-/- 0.05 1.94 + 0.05 1.78 + 0.01 1.88 + 0.03 1.67 + 0.06

0.59 0.49 0.51 0.47 0.73 0.54 0.44 0.66 0.24 0.34 0.44 0.29 0.62 0.49 0.46 0.46 0.65 0.61 0.81 0.91 0.70

3 3 3 1 6 * 1 * 1 1 1 1 4 4 3 3 3 3 6 6 6

[263] [148] [148] [28] [28] [68] [79] [121] [179] [79] [180] [181] [194] [217] [243] [243] [262] [262] [41] [27] [36]

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Table 10 (continued) System Na/AI(100) Na/AI(100) Na/AI(100) Na/Ni(100) Na/Ni(100) Na/Rh(lll) Na/Ru(0001) Na/Ru(0001) Li/Cu(100) Li/Ru(0001)

Structure

Temp.(K)

c(2×2) 140 c(2 × 2) 100 c(2 × 2) 100/300 c(2×2) 300 c(2X 2) 320 (v~×v~-)R30 ° 30 p(2×2) 50 ( ~ x v ~ ) R 3 0 ° 50 c(2×2) 180 (x/3 × v/3)30 ° 50

Site Hollow Hollow Subst. Hollow Hollow hcp fcc hcp Hollow hcp

Method SEXAFS LEED LEED LEED LEED LEED LEED LEED LEED LEED

Bondlength

Effective r

Exc. r

(~,)

(~)

(~)

3.20+0.03 3.27_+0.01 3.07_+0.01 2.84_+0.08 2.95_+0.04 2.84_+0.09 2.95_+0.04 2.95_+0.04 2.67+0.08 2.74_+0.05

1.76_+0.03 1.83_+0.01 1.63_+0.01 1.59_+0.08 1.72_+0.04 1.49_+0.09 1.60_+0.04 1.60_+0.04 1.39_+0.08 1.39_+0.05

0.79 0.86 0.66 0.62 0.75 0.52 0.63 0.63 0.86 0.86

N

Ref.

4 4 4 4 4 3 3 3 4 3

[58] [56] [56] [190,191] [184] [240] [257] [257] [97] [255]

The temperatures quoted are measurement temperature/dosing or annealing temperature, where it is indicated to be different from the measurement temperature. The bondlength quoted is the chemisorption bondlength, effective r is the chemisorption bondlength minus the metallic radius of the substrate atom, excess r is the effective r minus the ionic radius of the alkali atom. N is the coordination number of the alkali adatom. A coordination number denoted as * indicates that due to surface reconstruction, an unambiguous assignment cannot be made.

top-site structure for K/Al(111) is a small rumpling of the substrate. Indeed, in the LEED studies of the top-site systems where substrate relaxations are included in the analysis, all of them show some degree of rumpling. The geometry of this rumpling for a p(2 × 2) structure is shown in Fig. 67. It can readily be seen that the main effect of this rumpling is to move the alkali adatoms closer to the surface, thus slightly decreasing their nearest-neighbor distance as well as the next-nearest-neighbor distance. Just how close do the adatoms get to these next-nearest

(a)

X-

Y

Fig. 67. Schematic diagram showing the geometry of the rumpling on top-site surfaces. (a) Top view, the shaded circle denotes the adatom. (b) Side view through the plane XY. The rumpling amplitude is denoted by ,9.

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Table 11 Adsorption sites for alkali overlayers and substrate bulk moduli (in units of 10la N m 2 [266]) System Struct. Site rmet/NNsu b Bulk modulus Cs/AI(lll) Rb/AI(111) K/AI(111) Cs/Ag(111) Rb/Ag(111) K/Ag(111) Cs/Cu(111) Rb/Cu(111) K/Cu(111) K/Ni(111)

(v~ x f3-)R30° (v~ x ¢3-)R30° (v~- × v~)R30° p(2 × 2) p(2 × 2) p(2 x 2) p(2 x 2) p(2 x 2) p(2 × 2) p(2 x 2)

Top Top Top fcc fcc fcc Top Top Top Top

0.92 0.85 0.83 0.94 0.88 0.82 1.08 0.96 0.94 0.96

0.722

Cs/Rh(111 ) Rb/Rh(111) K/Rh( 11I) Cs/Ru(0001) Rb/Ru(0001) K/Ru(0001) Na/Ru(0001)

p(2 × 2) p(2 x 2) p(2 × 2) p(2 x 2) p(2 × 2) p(2 x 2) p(2 x 2)

Top Bridge hcp Top fcc fcc fcc

1.01 0.95 0.88 1.01 0.94 0.88 0.71

2.70

1.007

1.37

1.86

3.21

Rumple 0.29 0.265 0.25 0.09 0.10 0.11 N N N 0.12 0.10 0.05 0 0.10 0.06 < 0.05 0

Dimension units are in _A. N indicates that the parameter was not measured. References to the original work are in Table 10.

neighbors? Taking the case of R b / A g ( l l l ) , which hasO the largest rumpling amplitude, we see that the next-nearest-neighbor distance is about 4.2 A, giving an effective R b bond radius of 2.75 ,~, which is only 0.2 A larger than the metallic R b radius and 0.2 ,~ smaller than it would have been without relaxation. This relaxation may be considered to cause an effective increase in coordination of the alkali. But is this relaxation a n e c e s s a r y feature for top-site adsorption to occur? Assuming that this is so, it has b e e n proposed that whether an atom occupies a top site may d e p e n d on how easily the substrate can be deformed to accommodate the rumpling [20], and that therefore a substrate which cannot be easily deformed is less likely to exhibit top-site adsorption. Table 11 lists the adsorption systems on hexagonal substrates along with the substrate bulk moduli. (Note that rumpling is not allowed by symmetry for hollow-site adsorption in the (f3x v ~ ) R 3 0 ° structure, and these cases are therefore excluded from this list.) F r o m the table it appears that while bulk modulus gives some indication for the magnitude of surface rumpling, it alone is not an indicator for adsorption site. The ratio of the metallic alkali radius to the nearest-neighbor spacing of the substrate surface gives a crude comparison of surface corrugations: the larger the number, the smaller the expected corrugation. It can be seen that this alone is also not a good indicator of adsorption site, at least for these systems. However, each of these factors, surface deformability and corrugation, must surely be components of the balance of energy which determines whether a top-site structure occurs. Unfortunately we do not yet have a quantitative relationship which would allow us to predict adsorption sites.

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D F T calculations have predicted that for K / A I ( l l l ) there is a crossover from top site to hollow site as the coverage decreases, and that a single adatom should be in the hollow site [28]. It has also been suggested that top-site structures (and rumpling) are related to a possible increased screening effect for certain symmetries [244], implying that top-site occupation is a collective effect and would not happen for a single adatom. One outstanding experiment left in the field of alkali metal adsorption is to measure the adsorption site of the first adatom in a system which exhibits top-site adsorption at a higher coverage. Of the lowest-coverage site determinations for such a system, the NISXW study of R b / A I ( l l l ) [40,52], indicates that the top site is still occupied at a coverage of 0.12 in the disordered phase. A very recent PhD experiment for K/Al(111) purports to indicate that the top site is occupied in the coverage range 0.05-0.40 [49]. This clearly is not in agreement with the D F T result [28]. More experiments are needed to further understand the coverage-dependence of adsorption sites. Extending experiments to extremely low coverages is not easy to accomplish, however, both due to low signal at low coverages, and to the high mobility of alkali adatoms. An STM investigation of K / C u ( l l l ) at 4 K was not successful apparently due to migration of K atoms onto the STM tip [330]. At higher coverages, a change in adsorption site with coverage has in fact been observed on three substrates, Ru(0001) [20], A g ( l l l ) [148] and R h ( l l l ) [243]. In the cases of Cs/Ru(0001) and Cs/Rh(111), the site changes from the top site in the p(2 x 2) structure to the hcp site in the (f3- x v/3-)R30 ° structure. It was proposed [244] that for Cs/Ru(0001) this change is due to the screening afforded by the surface geometry of the atop site in the p(2 x 2) phase but not the atop site in the (v/3- x v~-)R30 ° phase. In the cases of K/Ru(0001), Rb/Ru(0001), K / A g ( l l l ) and R b / A g ( l l l ) , the adatoms are adsorbed in the fcc sites in the p(2 x 2) phase and in the hcp sites in the (v~- x vr3)R30 ° phase. For adsorption on Ru(0001) it was proposed that the hcp site at higher coverage occurs because the hcp site is the natural "metallic" site on Ru(0001), but this is clearly not the case on A g ( l l l ) . Since this site change has been observed now on both an hcp and an fcc substrate, a likely cause of the change may be coordination. In the p(2 x 2) structure, adatoms in either hollow site effectively increase their coordination when the substrate rumples, and the choice of fcc site may be related to the rumpling energy. In the (v~ x v~)R30 ° structure, however, rumpling cannot occur by symmetry for either hollow site, therefore the coordination is a maximum when the second-layer substrate atom is directly beneath the adatom. The case of R b / R h ( 1 1 1 ) is so far unique in that its site changes from bridge in the p(2 X 2) structure to hcp in the ( f 3 x v/-3)R30 ° structure. No explanation for the bridge site has been proposed at the time of this writing. The other surprising adsorption sites for alkali metals are the so-called substitutional sites. At the simplest, these structures involve adatoms which occupy vacancies that are apparently induced by the alkali adsorption. These in fact are a weak case of alkali-induced reconstruction which is common on the fcc (110) surfaces (see Section 3.8). More complex multilayer intermixed structures have also been observed and characterized. The substitutional or even more complex adsorption sites have been determined for alkalis on A I ( l l l ) , AI(100) and Cu(ll0), and for Li on C u ( l l l ) and Cu(100). For the case of A1(111), D F T calculations have indicated that the reason a substitutional structure is so favorable is that the energy for vacancy formation on A I ( l l l ) is so small (0.36 eV) [17,39]. It was also suggested that these substitutional structures provide a way for the alkali adatom to form a more ionic bond with the substrate, the

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evidence of this being the increase of (mostly) substrate charge density between the alkali adatoms [17]. A low vacancy-formation energy was also predicted for A g ( l l l ) (0.67 eV) and C u ( l l l ) (0.92 eV) [39], but so far no substitutional sites have been observed on those surfaces except for Li adsorption. The intermixing of Li with metal surfaces may in fact be a common occurrence, but so far very few Li structural studies have been carried out. These studies are intrinsically more difficult because the tendency toward diffusion of Li into the crystals makes characterization much more difficult.

3.3. Compression within the adsorbed alkali layer with respect to the corresponding bulk metals The nearest-neighbor distances of alkali metals in their saturated monolayers are generally smaller t h a n their metallic diameters derived from the bulk solids. Table 12 shows the lateral nearest-neighbor distances at monolayer saturation for many alkali overlayers along with the ratios of the monolayer nearest-neighbor spacing to the bulk metallic alkali diameter. While it has been occasionally suggested that this is due to charge transfer in the chemisorption bond which makes the effective size of the alkali smaller, calculations for a free-standing Cs monolayer indicate that it is due to the strong intralayer metallic bonding [331]. Such a compression is a consequence of the reduced coordination of the atoms in the 2D solid, similar to the shorter bondlengths observed for the lower coordination bonds in general [332]. It should be noted, however, that for systems which form islands at low coverages, the equilibrium spacing in these "unconstrained" layers below saturation coverage is generally larger than the bulk spacing [57]. This is an indication that the adsorbate-substrate interaction is considerably more important at the lower coverages than it is at monolayer saturation. This compression effect at monolayer saturation has been observed in other metallic adsorption systems, for example Pb/Cu(100) [333] or Hg/Cu(100) [334]. Looking at overlayers which are commensurate at saturation coverage, it is clear that the presence of a commensurate phase can stabilize the structure at a density extraordinarily compressed relative to the normal bulk alkali spacing, or, in some cases, expanded relative to the bulk. However, it is the incommensurate structures which give us a clearer view of the "true" two-dimensional compression effect. There is some scatter in the measured values for the incommensurate lattice spacings, but there do not seem to be any clear trends which depend on either the substrate material or its symmetry, which is consistent with the notion that this compression is more a property of the 2D layer than of the chemisorption bond. However, the heavier alkali metals experience a larger compression than the lighter ones. From the data we have available, the mean measured compressions (for incommensurate monolayers) are 2% for Li, 5% for Na, 8% for K, 7% for Rb and 15% for Cs. The calculated value for the free-standing Cs monolayer was 11% [331].

3.4. "Charge transfer" and measurements of chemisorption bondlength In the past few years there has been a lively debate on whether alkali adsorption can be discussed using traditional bonding terms such as "ionic", "covalent" and "metallic". Put another way, the question is whether there is actual charge transfer from the alkali atoms to the substrate, as in the traditional Gurney model [265], or whether the charge rearrangement is better described as dipole formation within the alkali atom itself. In the Gurney picture the

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Table 12 Monolayer-saturation lattice constants for alkali overlayers in ,~ System

NN distance

NN/dmetaUic

C/I

Li/Cu(100) Li/Ru(0001)

2.94 3.06

0.94 0.98

C I

Na/AI(lll) Na/Cu(100) Na/Cu(111) Na/Ni(lll) Na/Ni(100) Na/Ru(0001) Na/W(ll2)

3.82 3.61 3.78 3.54 3.52 3.67 3.54

1.00 0.95 0.99 0.93 0.92 0.94 0.93

C C I I C I I

K/C(0001) K/Pd(100) K/AI(lll) K/Cu(lll) K/Cu(100) K/Cu(ll0) K/Ni(lll) K/Ni(100) K/Pd(lll) K/Pd(100) K/Pt(lll) K/Ru(1010) K/Ag(lll) K/Ag(100)

4.92 3.89 4.26 4.4 4.57 4.3 4.47 4,35 4,58 4.30 4,16 4.02 4.54 4.08

1.03 0.82 0.90 0.92 0.96 0.90 0.94 0.91 0.96 0.90 0.87 0.84 0.95 0.86

C C I I I I I I I C I I I C

Rb/AI(lll) Rb/Cu(111) Rb/C(000m) Rb/Ag(lll) Rb/Ru(0001)

4.96 5.10 4.92 4.76 4.69

0.97 1.00 0.96 0.93 0.97

C C C I C

Cs/AI(lll) Cs/C(0001) Cs/Ag(lll) Cs/Cu(lll) Cs/Cu(100) Cs/Cu(110) Cs/Ir(lO0) Cs/Mo(110) Cs/Ni(111) Cs/Ni(lO0) Cs/Pt(111) Cs/Ru(O001) Cs/W(100) Cs/W(110)

4.95 4.92 5.01 4.86 4.57 4.7 4.04 4.74 4.71 5.0 4.36 4.69 4.81 4.70

0.91 0,90 0,92 0.89 0.84 0.86 0.74 0.87 0.86 0.92 0.80 0.89 0.88 0.86

C C C I I I I I I I I C I I

C or I denotes whether the monolayer saturation structure is commensurate or incommensurate. References can be found in the subsections appropriate to the adsorption system.

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Table 13 Coverage dependence of chemisorption bondlengths System

A0

A 0 / 0sat

Ab

Ref.

Na/AI(lll) K/AI(111) Rb/AI(lll) K/Ni(111) K/Ni(100) K/Rh(111) Rb/Rh(ll 1) Cs/Rh(111) Na/Ru(0001) K/Ru(0001) Rb/Ru(0001) Cs/Ru(0001)

0.16-0.33 0.05-0.40 0.12-0.33 0.13-0.275 0.08-0.50 0.25-0.33 0.25-0.33 0.25-0.33 0.25-0.33 0.25-0.33 0.25-0.33 0.25-0.33 0.25-0.33 0.25-0.33 0.15-0.30

0.32-0.66 0.11-0.89 0.36-1.0 0.42-0.89 0.16-1.00 N/A N/A N/A 0.48-0.63 0.76-1.00 0.76-1.00 0.76-1.00 0.64-0.85 0.69-0.92 0.5-1.00

0 +0.03 0.17 + 0.12 0 +0.1 0 +0.1 (d±) - 0.075 + 0.03 0 + 0.05 0 + 0.05 0.30 + 0.05 0 + 0.04 0.04 +0.05 0 + 0.05 0.27 +0.08 0.02 + 0.03 0.02 + 0.03 0.30 +0.03

[27] [491 [40,52] [178] [187] [243] [243] [243] [257] [262] [263] [244] [148] [148] [295]

K/Ag(lll) Rb/Ag(lll) Cs/Ag(lll)

A0 is the coverage range over which measurements were taken, AO/Osa t is this coverage range relative to the saturation coverage. Ab is the change in chemisorption bondlength in A. N/A indicates that the information was not available when this was written.

valence s-orbital of the alkali atom is b r o a d e n e d and lowered in energy upon interaction with the substrate, resulting in a partial electron transfer to the substrate and a partially ionic state for the alkali at low coverage. At higher coverages the net charge transfer per adatom decreases due to mutual depolarization of the alkali dipoles. It was proposed that as the density becomes high enough to cause orbital overlap of the adatoms, that the overlayer becomes metallic and the chemisorption bond ceases to be ionic. The alternate picture suggested that the nature of the alkali chemisorption bond does not change as the coverage is increased. At this time it appears there is an emerging consensus that much of the previous discussion arose from semantic differences rather than real ones, though there are still unresolved questions about some aspects of the problem [17,25,30,335]. While most of the work pertaining directly to this debate has centered on experimental and theoretical studies of electronic structure, several structural studies have attempted to address this issue, mainly by measuring bondlength changes as a function of coverage. As the bondlength in ionic alkali compounds is substantially less than that for covalent or metallic compounds, it was expected to provide an indication of the type of bonding in these systems. In light of the theoretical interpretation of alkali chemisorption on A I ( l l l ) , this approach is certainly not without pitfalls. For Na or K on A I ( l l l ) for example, the bonding was interpreted from D F T studies [17] as being ionic in the substitutional-adsorption case where the coordination of the adatom was 6-fold and the effective alkali radius was near its bulk metallic radius, whereas in the case of top-site structures the overlayer was characterized as metallic even though the effective chemisorption-bond radii were closer to the ionic values. Nevertheless, as a means of gaining more information about the chemisorption bonds, bondlengths have been measured as a function of coverage for several systems and are shown in Table 13.

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149

The perpendicular overlayer-substrate spacing for K/Ni(111) was measured using a specular L E E D analysis and was found to be constant over the coverage range from 0.13 to 0.275 at 2.7_+ 0.1 A [178]. A SEXAFS study of Na/AI(111) found that the site for Na at room temperature was the substitutional site at coverages of 0.16 and 0.33 and that the bondlength remained the same [27]. A NISXW study of the R b / A I ( l l l ) system also found the perpendicular distance, which was also the bondlength since the adsorption site is atop, to be constant over the coverage range 0.12 to 0.33, at 3.13 +_ 0.10 ,~ [52]. A diffuse L E E D study of K/Ni(100) determined both that the adsorption site remained the same and that the K - N i bondlength remained essentially constant in the coverage range from 0.08 to 0.25. A change in bondlength of 0.27 A was measured in the Cs/Ru(0001) system, between the p(2 × 2) and (Vr3- x x/-3)R30 ° phases [244]. This change, however, was accompanied by a change in adsorption site from the top to the hcp hollow and a corresponding change in coordination from 1 to 3. No significant change in bondlength was observed for Na [257], K [262] or Rb [263] on Ru(0001) between the p(2 x 2) and (vr3- x v~-)R30 ° coverages where their sites change but their coordination with substrate atoms does not. In contrast to these studies, a bondlength change of 0.3 ,~ was found in the SEXAFS study of Cs/Ag(111). However, the adsorption site was not determined in that study [295]. The only case where the site stays the same (atop) and there is a definite increase in bondlength (0.17 ~,) is the recent study of K/AI(111) [49]. Corroboration of this result is desirable as it disagrees with the D F T calculations for the same system [28]. It is fair to state there there is no conclusive evidence for a change in bondlength as a function of coverage where the alkali atom has been shown to remain in the same site. (Where different sites occur, a change in bondlength can be expected because of the difference in the number of neighbors. A graph of the measured excess radii given in Table 10 as a function of the coordination of the chemisorption bond is shown in Fig. 33.) This hypothesis could be further tested by measurements of very low coverage alkali systems, perhaps using third-generation synchrotron sources and synchrotron techniques (SXW, SEXAFS, PhD). In addition, since the corrugation of the chemisorption potential is apparently very small where top sites are observed, it must be assumed that very low temperatures are required to restrict the mobility of the adatoms: for the N i ( l l l ) - K system, no SEXAFS oscillations could be observed at low coverage at 60 K apparently because of the high mobility of the K atoms [79].

3.5. Island formation For certain alkali-substrate combinations, the alkali overlayers undergo a condensation transition at a relatively low coverage instead of compressing uniformly as the coverage is increased. This condensation transition is intriguing because it resembles a nonmetal-to-metal transition such as those observed in other metal adsorption systems [22,23]. The structural result of this condensation is the formation of 2D islands of alkali metal, presumably in equilibrium with a 2D vapor phase. In all cases but one, these islands are solid at room temperature, and their melting has not been studied. The alkali adsorption systems for which condensation has been observed are shown in Table 14. This table neglects any condensation p h e n o m e n a which might be associated with large reconstructions such as those which occur on fcc (110) surfaces because of lack of detailed characterization.

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Table 14 Alkali adsorption systems which undergo condensation, along with the coverages at which they condense, the structure they condense into, and the temperature System Coverage Structure Temp. NN Ref.

(3,) Na/Al(lll) Na/Al(111) Na/AI(ll 1) K/AI(I 11) Rb/AI(111) Rb/AI(111) Na/Al(100) Na/Al(100) Li/Be(0001) Na/Cu(111) Li/Cu(100) K/Cu(100) K/C(0001) Rb/C(0001) K/Ag(100)

0.15 0.06 0.15 0.1 0.08 0.1 0.25 0.2 0.2 0.18 0.25 0.18 0.02 0.02 0.08

"3 ×3" HOC (v~- x v~-)R30° (~- x v/3-)R30° (7~ × g~-)R30° p(2 x 2) (~ _ ~6)HOC c(2 x 2) c(2 x 2) (v/3 x v~)R30° (3 × 3)HOC c(2 x 2) Liquid p(2 × 2) p(2 x 2) c(2 × 2)

LT RT RT LT,RT LT RT RT LT RT LT LT RT LT LT RT

~ 4.3 4.96 4.96 4.96 5.73 5.5 4.05 4.05 3.97 6.01 3.61 5.6 4.92 4.92 4.09

[24] [35] [24] [24] [24] [24] [56] [571 [67] [359] [92] [106] [137] [144] [297]

The condensation of Na on A I ( l l l ) has been studied theoretically [30,48]. This D F T study indicates that at low coverage the a d s o r b a t e - s u b s t r a t e interaction dominates the overlayer, while at higher coverages the attractive, metallic a d s o r b a t e - a d s o r b a t e bonds are more important and cause the condensation of the overlayer [30,48]. The reason that condensation occurs for some alkali overlayers and not others is apparently a result of the balance b e t w e e n the a d s o r b a t e - a d s o r b a t e and a d s o r b a t e - s u b s t r a t e interactions. The stronger dipole moments of the heavier alkali atoms upon adsorption and their lower cohesive energies presumably are related to the fact that they have less of a tendency to condense, compared to the lighter ones. In all cases reported here, alkali metals condense into commensurate or higher-order commensurate phases, suggesting that the extra energy gained by forming a commensurate structure may tip the balance in favor of condensation. This energy must be quite large in the case of alkalis on A I ( l l l ) at room temperature where the site is substitutional. The case of K / C u ( 1 0 0 ) where K has been observed to condense into a liquid phase at 330 K is quite different from the other systems and deserves further study. 3.6. R o t a t i o n a l epitaxy

The rotational epitaxy of incommensurate layers has been studied for many years in a diverse range of systems, and many models have been devised to describe these systems [336]. In this section we will describe a few of the models which are possibly applicable to alkali overlayers. A geometrical model developed by Bohr and Grey [336] and applied with success to various metal interfaces, proposes that the overlayer will tend to align itself so that its domain walls (which arise from the static distortion of the lattice) are oriented along a symmetry direction of the substrate. Thus various trajectories of rotational angle versus lattice misfit are predicted for

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171

151

incommensurate overlayers, depending on their symmetry. A second geometrical model, first applied to alkali metals by Doering [337], proposes that the overlayer may form higher-order commensurate (HOC) structures, where the repeat distance of the overlayer relative to the substrate is large compared to the lattice spacing. For a finite system, there are several main trajectories of H O C structures in angle-misfit space, which an overlayer might follow quasicontinuously as its density (and misfit) changes. A more extensive theory was developed by Novaco and McTague (NM) [154,155] in which the a d a t o m - a d a t o m interactions are explicitly taken into account in the form of elastic constants. This first-order theory calculates the response of an elastic overlayer to a small-amplitude substrate potential corrugation. Since, in general, transverse overlayer deformations cost less energy than longitudinal ones, the incommensurate overlayer relaxes by rotating relative to the substrate, i.e. the rotation is essentially a static transverse distortion wave in the overlayer. For any given misfit there is at least one minimum-energy overlayer angle. Since elastic properties depend on the details of the a d a t o m - a d a t o m interactions, one might expect different rotation angles for different a d a t o m - a d a t o m interactions. Fig. 68 shows plots of the angle-misfit trajectories for these models. The reason for the correspondence between the geometrical models and the harmonic NM theory is not completely obvious since the models have quite different origins, but in essence, the HOC and domain-wall alignment models pick out low-energy trajectories, just as the NM theory does. However, the NM theory actually selects the lowest-energy trajectory for a given interadsorbate interaction, whereas the geometric models cannot distinguish between the different trajectories. The NM theory (and the geometric models) is expected to break down close to commensurate structures where the periodic effects of the substrate on the overlayer are more important. For this regime, a domain-wall theory was developed by Shiba [338,339] which predicts that close to a commensurate structure where domain walls in the overlayer are relatively sharp, the overlayer (and the domain walls) will remain aligned along a substrate direction until a certain critical misfit is reached. The critical misfit occurs at a point where the domain walls become too weak for the overlayer to sustain the alignment. At this point the overlayer crosses over from a "domain wall" to a "modulated" regime, and the equilibrium angle of rotation moves toward that predicted by the NM theory for a "uniform" overlayer. In the geometrical models, this crossover behavior of the overlayer could be accommodated by the jumping from one trajectory to another, but of course there is no way in these models to predict when or how this might occur. Table 15 indicates which alkali overlayers have been observed to form rotated incommensurate structures and the density range and angle range over which they have been observed. In general all of the systems behave similarly at the same coverages. On the hexagonal substrates the data in most cases qualitatively follow the predictions of the NM theory for a repulsive dipole interaction. There are deviations from this behavior though, and in some cases (notably Li/Ru(0001), Fig. 50)), the geometrical theories may provide a better description. It should be noted, however, that a marked temperature dependence was observed in the rotation angle for alkalis on A g ( l l l ) [294], and such a change with temperature may make comparisons to the models difficult. It rules out a simple application of the geometrical models, since a continuous off-trajectory change would not be consistent with their predictions. The temperature dependence of the rotation angles for alkalis on A g ( l l l ) was studied in some detail [294]. For most coverages, the rotation angle decreases toward 0° as the tempera-

152

McGrath / Surface ScienceReports 2 3

R.D. Diehl, R.

(1996) 43-171

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F i g . 68. Diagram showing the predictions of various models describing rotational epitaxy. (a) Hexagonal substrate. (b) Square substrate.

ture is increased. The move toward alignment with the substrate is a consequence of the loss of adatom-adatom correlations as the temperature increases. The temperature dependence of this rotation angle has been shown to be consistent with the power law behavior predicted by a mean-field theory of epitaxial rotation [340]. This theory could not be applied to the data at low coverages, however. Near the ( f 7 x v~)R19.1 ° phase the rotation angle was found to increase, possibly toward 30 °, but this could not be confirmed because the rotation transition was pre-empted by melting of the overlayer. There are fewer data for rotated alkali layers on square-symmetry substrates, but the data which are available indicate that these systems also agree qualitatively with the NM prediction for repulsive dipoles. 3. 7. D y n a m i c s

of alkali metal overlayers

An unusually large degree of in-plane disorder has been observed in some commensurate alkali metal overlayers, even at temperatures as low as 35 K. This has been observed with

R.D. Diehl, K McGrath / Surface Science Reports 23 (1996) 43-171

153

Table 15 Rotational parameters for alkali metal overlayers System

Coverage range

Angle range (deg)

Hexagonal on hexagonal Cs/C(0001) Na/Pt(111) K/Pt(lll) Li/Ru(0001) Na/Ru(0001) K/Ag(111) Rb/Ag(111 ) Cs/Ag(111 )

0.16-0.22 0.33-0.59 0.33-0.44 0.33-0.46 0.33-0.53 0.14-0.20 0.14-0.21 0.14-0.23

5 -0 30 -18 30 -20 30 -13 30 -14 19.1-0 19.1-0 19.1-0

Hexagonal on square K/Cu(100) K/Ni(100) Cs/Rh(100)

0.33-0.37 0.32-0.38 0.33-0.43

3.3-6.0 3.3-6.0 0 -?

References may be found in the appropriate subsections for the adsorption systems.

several techniques, including NISXW, SEXAFS and LEED. While these techniques cannot directly determine whether this site disorder is static or dynamic, it is most likely to be dynamic disorder that arises from the very small corrugation in the adsorption potentials for these systems. Indeed, it has only been explicitly noted for adsorption on close-packed substrates. A high degree of site disorder has been observed for both top-site and hollow-site adsorption, but not for substitutional adsorption. Table 16 indicates the adsorption systems in which a high degree of site disorder has been explicitly noted. In the case of the NISXW studies, the disorder appears in the analyzed data as a low coherent fraction, a parameter which is basically the fraction of adatoms occupying sites. The Table 16 Alkali adsorption systems with a high degree of site disorder System

Structure

Site

Temp.

Technique

Observation

NISXW NISXW NISXW SEXAFS LEED LEED LEED LEED LEED LEED LEED

Low coherent fraction Low coherent fraction LOw coherent fraction Low-k peak in NNN amplitude Fitted split-position model Fitted split-position model Fitted split-position model Fitted split-position model Fitted large in-plane amplitude Fitted large in-plane amplitude Fitted large in-plane amplitude

(K) Rb/Al(111) Rb/AI(111) Rb/Cu(111) K/Ni(111) Rb/Ru(0001) Rb/Ru(0001) Cs/Ru(0001) Cs/Ru(0001) K/Ag(111) Rb/Ag(111) Cs/Ag(111)

p(2 X 2) (v~- x ¢3)R30 ° p(2 x 2) p(2 x 2) p(2 x 2) (v~- x v/3)R30 ° p(2 x 2) (v/3- x v/3-)R30° p(2 X 2) p(2 X 2) p(2 x 2)

Top Top Top Top fcc hcp Top hcp fcc fcc fcc

170 170 300 70 50 50 80 80 50 40 40

References may be found in the subsections appropriate to the adsorption systems.

154

R.D. Diehl, R. McGrath / Surface Science Reports 23 (1996) 43-171 0.2 - -

2

3

1st Shell

4

- - - 2nd Shell

5 6 k (~-1)

7

8

Fig. 69. Individual first- and s e c o n d - n e i g h b o r contributions to the S E X A F S amplitude from K / N i ( l l l ) the significantly shifted amplitude function of the second shell due to e n h a n c e d vibrational motions.

[79]. N o t e

coherent fractions determined at 170 K for R b / A I ( l l l ) are 0.5 in the p(2 X 2) phase and 0.8 in the (v~ x v~)R30 ° phase. By assuming that the latter coherent fraction is due solely to in-plane vibrational order, a vibrational amplitude of 0.18 ~, was deduced [40,52]. A similar result was obtained for R b / C u ( l l l ) , except in this case the temperature was higher [88], and the site coherent fraction was found to be only 0.54. The SEXAFS study of K / N i ( l l l ) is notable because the in-plane disorder is observed directly from a comparison of the k-dependent EXAFS amplitudes for the nearest-neighbor (NN) and next-nearest-neighbor (NNN) substrate atoms [79]. These amplitudes are shown in Fig. 69 and clearly show that the amplitude of the NNN EXAFS is peaked at a much lower k-value than the NN EXAFS. This stronger k-dependent damping of the NNN EXAFS indicates more disorder in the NNN bond. Since the adsorption site in this case is the top site, the NN EXAFS amplitude is only affected by disorder perpendicular to the surface, whereas the NNN EXAFS amplitude contains a combination of both parallel and perpendicular disorder. This allows the separation of the two directions, and clearly indicates a much higher degree of in-plane disorder. While a SEXAFS measurement at one temperature cannot distinguish between the types of disorder, an analysis involving several temperatures deduced that the in-plane disorder does not appear to be normal Debye-Waller type (harmonic) vibrational disorder. In the LEED studies of alkalis on Ru(0001) [20] and Ag(111) [294], the in-plane disorder appeared as damping in the LEED spectra and was treated by fitting to spectra which were calculated assuming large in-plane vibrations. In the case of adsorption on Ru(0001), the technique used was a split-position technique, where the adatoms are assumed to reside most of the time at two positions on either site of the equilibrium site. This improved the agreement between the experimental and calculated spectra and resulted in a mean-square vibrational amplitude of 0.37 A for the p(2 x 2) structure of Rb/Ru(0001). The in-plane vibrational amplitudes deduced from the LEED studies of the p(2 x 2) structures of alkalis on A g ( l l l )

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were 0.5, 0.5 and 1 ,~ for K, Rb and Cs, respectively. It is known [148] that the calculation technique used in these studies somewhat overestimates the amplitudes. In-plane vibrational frequencies for alkalis on graphite [144,147], Cu(ll5) [341] and Cu(100) [342] have been directly measured using the helium-atom scattering. For the case of graphite, these vibrational energies are in the range 2.3-4.4 meV for Cs, Rb and K, corresponding to in-plane vibrational amplitudes of about 0.2 A, which agrees reasonably well with the experimental values deduced from the experiments above. Static disorder or anharmonic vibrational behavior, which have not been considered in the LEED studies cited above, would both lead to an overestimate of the vibrational amplitudes. In order to separate these effects it is desirable to do temperature-dependent structural studies of alkali adsorbates. A tensor LEED study was carried out for the temperature dependence of the anisotropic vibrations of Ni(100)-c(4 X 2)-K between 90 and 200 K [343]. The vibration amplitude parallel to the surface was found to be about 10 times that perpendicular to the surface at all temperatures studied. Unfortunately the temperature range studied was not large enough to see a significant temperature effect on the vibrational amplitudes. A larger effect was observed for the temperature dependence of the vibrational amplitudes of the uniaxial incommensurate phase of Cs/Cu(100) using surface X-ray diffraction [344]. In this case, the vibrations were observed to be anisotropic in the plane, having a larger amplitude in the [010] direction (incommensurate) than the [100] (commensurate) direction. The rms vibrational amplitudes were observed to approximately double for both directions between 190 and 300 K, from about 0.16 to 0.29/~ for the [010] direction and from about 0.10 to 0.18 A for the [100] direction. The overlayer Debye temperature derived from this study is comparable to the bulk Debye temperature of Cs [344]. o

3.8. Alkali metal-induced missing-row reconstruction of fcc (110) d-band metal surfaces This topic has already been covered in a recent review [15], and the reader is referred to that work for an extensive discussion. Here we attempt to summarize the current understanding of this fairly general phenomenon and to provide an up-date of some recent studies. The (110) face has the highest surface energy of the low-index phases and indeed the (110) faces of the 5d transition metals Au, Pt and Ir spontaneously reconstruct when clean to give a (1 x 2) periodicity in the [110] direction. The 3d and 4d metals do not undergo this clean surface reconstruction, but when small (sub-monolayer) amounts of alkali metals are added to the surface, reconstruction to a (1 x 2) phase (and under certain circumstances to (1 x 3) and (! x 5) phases) occurs. This does not appear to occur for the sp metals AI and Pb [163]. (But see Section 2.9.1 which discussed reconstruction of a strained Pb(ll0) surface.) These phases were at first thought to represent ordered structures of the alkali atoms [174] but it was argued first by Hayden et al. [299] that this involved unphysically short alkali-alkali distances and that in fact reconstruction of the metal surface in the presence of the alkali was taking place. There is now widespread agreement that the (1 x n) reconstructions (both clean surface and alkali-induced) are missing-row reconstructions, which also involve some degree of subsurface relaxation and reconstruction. This is shown in Fig. 70. Ad12 and Ad23 are the changes in the first and second interlayer spacings while or describes a second-row pairing and ~ a third-row buckling for atoms beneath the missing row. Experimentally determined parameters are listed

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156

a + Pairing (o)

b I

"4

*l

I

I I

dl2

t

d23

#6

Id34

Fig. 70. Schematic diagram of the missing-row model with multilayer r e c o n s t r u c t i o n indicating relevant geometric p a r a m e t e r s . T h e s e include a second row pairing described by or a n d a third row buckling described by ~ [15].

in Table 17 for both clean and alkali-induced reconstructed surfaces. It is apparent that in general the d12 and d23 interlayers are contracted relative to the bulk value. This table does not include any reference to the positions of the alkali atoms in these systems. At room temperature, alkali atoms are highly mobile and do not appear to contribute

T a b l e 17 Experimentally d e t e r m i n e d relaxation p a r a m e t e r s for fee (110) r e c o n s t r u c t e d surfaces Reconstruction

Au(1 x 2) Au(1 x 2) Ir(1 x 2) Pt(1 × 2) Pt(1 x 2) K / A g ( 1 x 2) C s / A g ( 1 x 2) C s / A g ( 1 x 2L) C s / A g ( 1 x 2H) C s / A u ( 1 x 3) K, C s / C u ( 1 x 2) K / C u ( 1 x 2) C s / C u ( 1 x 2) C s / C u ( 1 x 3L) C s / C u ( 1 x 3H) K / N i ( 1 x 2) C s / P d ( 1 x 2)

0al k 0 0 0 0 0 0.16 0.14 0.2 0.3 ~ 0.19 0.15 0.20

0.25 0.08

Ad12

Ad23

or

t~

(%)

(%)

(3,)

(M

-

+4 - 6 - 12 +4 - 24 - 1 - 2

0.14 0.07 < 0.04 0.13 0.10

0.2 0.24 0.23 0.10 0.32 0.10

+ +

0.10 - 0.04

0.14 < 0.02 0.05

Small 0.10

+ 0.03 0.10

18 20 12 16 18.4 - 9 - 11 - 8 - 6 - 22 - 12 - 11 - 6 - 20 +3 - 9

9 4 to 0 1.5 1 0 - 9 - 3 - 1

Method

Ref.

MEIS LEED LEED MEIS LEED MEIS LEED SXRD SXRD MEIS LEED PhD SXRD SXRD SXRD MEIS LEED

[360] [361] [362] [363] [364] [300] [301] [302] [302] [365] [117] [121] [366] [366] [366] [201,202] [219]

Ad12 a n d Ad23 are the changes in the first and second interlayer spacings while or describes a second row pairing a n d t~ a third row buckling for atoms b e n e a t h the missing row. T h e distinction b e t w e e n ( I x 3 L ) a n d ( l X 3 H ) refers to the light (two missing rows) a n d heavy (one missing row) (1 × 3) structures.

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157

to diffraction or ion scattering results (apart from some streaking) upon which most detailed determinations are based. It is generally thought that the alkali atoms are situated in the missing rows with an average separation along the row determined mainly by the alkali-alkali repulsion [15]. Jacobsen and N0rskov showed that this is the case for K adatoms on Cu(110) in an effective-medium theory calculation [221]. This has now been demonstrated experimentally for K / C u ( l l 0 ) using PhD [121] and for C s / A g ( l l 0 ) using SXRD [302]. There does not appear to be a critical alkali coverage for the reconstruction to occur. Indeed STM studies of K on Cu(ll0) have shown that the reconstruction is local in nature and is initiated by a single alkali atom, taking the place of top layer silver atoms [120]. Second-harmonic generation results from C s / A g ( l l 0 ) [345] are also interpreted as indicating thermally activated local reconstruction as a precursor to long-range (1 x 2) order. We note however that this picture of a localized mechanism has been challenged by recent results where the removal of the Pd(ll0)-(1 x 2) clean surface reconstruction occurred upon adsorption of small amounts of K [364]. This study concluded that the transition is Ising in character, suggesting a global effect of the adsorbate on the surface energy. The energetics of the reconstruction has been discussed by several authors, in particular Behm et al. [200] and Barnes [15]. Barnes [15] tabulates the results of calculations for several metals which in general indicate that the energy difference between the two phases is small, indicating a delicate balance between the phases (a few % of the surface energy). There is a thermal activation barrier for the formation of the (1 x 2) phase due to the necessity of removing the rows of metal atoms which is evidenced by the fact that low-temperature adsorption must be followed by annealing to induce the reconstruction. Once the barrier is overcome, the missing-row adsorption site is more stable as the chemisorption energy of the alkali on the reconstructed phase is higher than on the clean unreconstructed surface. This difference in chemisorption energy has been attributed [200] to the polar nature of the alkali bond to the surface which is maximized when the alkali atoms are embedded in the surface. The evidence for this is that at higher alkali coverages where the bond formed is less polar, a return to the (1 X 1) geometry is obtained [200]. Behm et al. also proposed a microscopic mechanism for the initiation and propagation of the reconstruction [200]. The removal of a single metal atom is thermally activated in the presence of K at 300 K. Then further substrate atoms adjacent to the already reconstructed area are removed and expelled to the surface where they diffuse away on the smooth surface. Confirmation of this scenario might be achieved either by sophisticated molecular dynamics simulations or by time-lapse STM microscopy.

3.9. Coadsorption structures This review shows that there is now a large database of phase information for alkali coadsorption systems. There are an enormous variety of phases formed as a function of alkali and coadsorbate coverage and temperature. So far there are no complete Oalk--Ocoad--T phase diagrams, as most workers present fixed-temperature slices of the phase diagram at constant alkali coverage, varying the coadsorbate dose. The most complete study [283] presents a 0c,-0 o phase diagram (Fig. 59) at a fixed temperature of 310 K for 0.13 < 0c, < 0.35 and 0 < 0 o < 0.8.

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Table 18 Experimentally determined commensurate structure parameters for alkali metal coadsorption systems System

AI(lll)-(V~-×V~-)R30°-

(Na + O) Ru(0001)-(v~-×v~)R30 °(Cs + O)

Site

T

Alkali Coad.

(K)

Subst.

Method B

(,~)

AB

Eft. r (alk) Exc. r N Ref.

(,~)

(,~)

(,~)

Top (Na) RT NISXW 2.03_+0.12

[65]

(Na-O) hcp

Ru(0001)-(2v~-×27~-)R30 °- Top

hcp

100 LEED

Hollow

100 LEED

3.10_+0.07

2.08+0.05

3 [281]

1.75_+0.07 0.08

1 [286]

(Cs + O) Co(1010)-c(2×2)-(K+CO) Ni(111)-(2×2)-(K+2CO) Ru(0001)-(2 × 2)-(K+ CO) Ru(0001)-(2X 2)-(Cs+CO) Ru(0001)-(2× 2)-(Cs+2CO)

Hollow Top Top Top Top

Lowsym. RT LEED hcp/fcc 100 PhD hcp 50 LEED hcp 50 LEED bcp/fcc 50 LEED

3.51+0.11 +0.40 2.26+0.11 3.02_+0.05 +0.15 1.77+0.01 3.12+0.06 3.16_+0.06

0.93 0.44

4 1 1 1 1

[71] [203] [253] [289] [289]

The temperature quoted is the dosing or annealing temperature; often measurements are taken at considerably lower temperatures. The bondlength quoted (except where noted) is the alkali chemisorption bondlength, AB refers to the bondlength change upon coadsorbing gas, effective r is the alkali chemisorption bondlength minus the metallic radius of the substrate atom, excess r is the effective r minus the ionic radius of the alkali metal atom. N is the coordination number of the alkali adatom.

At present there do not appear to be any general unifying concepts in categorizing the range and symmetry of structures formed. Detailed structural information is available only for a handful of systems (see Table 18). In the case of alkali and CO coadsorption on close-packed surfaces, the structures determined for coadsorption with Cs on Ru(0001) and N i ( l l l ) were identical: alkali atoms in top sites and CO molecules occupying 3-fold coordinated hcp hollow sites with the C-end down [203,289]. For Ru(0001) [289] this involved a site-switching of the CO from the top site occupied in the pure CO-Ru(0001) system. It was suggested that the reason for this is e n h a n c e d back-donation to the CO 2rr * anti-bonding orbital in the 3-fold site in the presence of the alkali atom. Indeed, e n h a n c e m e n t in CO heat of adsorption in the presence of an alkali has been demonstrated using microcalorimetry [211]. The sites for K + CO on Ru(0001) were different [253]. In that case, the clean-surface site for K was fcc, but upon CO adsorption it switches to the top site, while the CO occupies the hcp sites, as for Cs coadsorption. This unexpected result was explained in terms of the CO bonding to its three coordinated Ru atoms weakening the bonds to the fourth Ru atom, which is occupied by the K atom. This weakening of the Ru bonds allows the substrate to relax and therefore top-site occupation is preferred. For the Co(1010)c(2 × 2)-(K + CO) system [71], K atoms reside in 4-fold hollow sites with 3-fold coordination to CO molecules. The K site is the same as that for the purely Co(1010)-c(2 × 2)-K system. There does not appear to be a clear trend in the change in the alkali-substrate bondlength. In cases where the site stays the same, large increases are found for Co(10i0)-c(2 x 2)-(K + CO) [71] and N i ( l l l ) - ( 2 x 2 ) - ( K + C O ) [203]. For R u ( 0 0 0 1 ) - ( 2 X 2 ) - ( C s + n C O ) , (n = 1, 2), the bondlength is almost unchanged from that of the pure alkali system. In the case of O and alkali coadsorption, attractive alkali-O interactions lead to the

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159

formation of ordered coadsorption systems. That charge transfer occurs is evidenced by the reduction in effective alkali radius rcs = 2.17 .& to rcs = 2.08/~ in the Ru(0001)-(v~- x v~-)R30 °C s / O system, indicating a change in the alkali to a more ionic state [281]. The structure of the overlayer has both Cs and O in 3-fold hollow sites. The situation appears to be different in the Ni(100)-(3 x 3)-K/O system where it appears that a K - O bilayer is formed with a (3 x 3) O structure underneath a disordered K layer [205-207]. For AI(111)-(v~ x x/-3-)-(Na + O), there is a different site with O atoms sitting atop of Na atoms in the structure [65]. In summary, it appears that the complex chemistry inherent in these three-component systems means that it is difficult to point to general trends. Perhaps these will emerge when a larger database of structures is available. 3.10. Note on structural techniques applied to alkali systems

This section is included to summarize the relative usage of the different structural techniques and to point out some of their advantages and disadvantages as applied to alkali adsorption systems. The reader is referred below to other sources for fuller descriptions. Almost all coverage-temperature phase diagrams (partial and complete) in the case of alkali adsorption and coverage-coverage phase diagrams in the case of coadsorption have been established using L E E D [347]. The relatively low cost and high speed of data acquisition means that it is well-suited for establishing phase information for these reactive systems. Table 10 shows that dynamical LEED is also the most-often applied quantitative technique. Tensor L E E D codes have improved the efficiency of the technique to the extent that model calculations can in certain cases be completed on a PC notebook computer [240]. The only restrictions would appear to be the size of the unit cell and the necessity for well-ordered phases. Even this latter restriction can now be overcome using the D L E E D technique [187]. Holographic LEED has also been applied to disordered alkali overlayers [186], though its use is unlikely to become widespread because of the relative complexity of the measurements and analysis and the low precision of structural parameters obtained. However, it may be useful as a starting point for studies using other techniques. STM [348] occupies a unique position in that it can provide both phase information and limited structural information. As variable temperature STMs become more widely available, it will be possible to explore more of the phase diagram than is presently accessible. One of the drawbacks of STM for alkali adsorption studies is the necessity to withdraw the sample from the tip during the adsorption process and then re-establish tunneling afterwards. However this restriction could be lifted by applying a purpose-built STM such as those developed for in-situ monitoring of molecular beam epitaxy [349]. The STM also provides qualitative structural information although in many cases alkali atoms appear to be "invisible" to the instrument [119] and it is sometimes difficult to unambiguously assign observed features to specific atoms, particularly in the case of coadsorption [205,209]. The migration of alkali adatoms onto the STM tip can also be problematic [330]. Despite these drawbacks, the structural information gained from STM can often be used as the starting point for more quantitative techniques. Energy-scanned PhD [350] is a structural tool which is not confined to ordered structures because of the local nature of the scattering process. With the recent development of multiple-scattering codes and direct methods structures can now be determined with a high

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degree of accuracy [350]. It is particularly attractive when applied to coadsorption systems where information can be independently collected on the local environment of the alkali atoms and the coadsorbates to provide a complete picture [203]. SEXAFS provides information in a very direct way and again is not restricted to ordered phases. It is particularly effective in cases where the adsorption is atop, because of the polarization dependence of the amplitude [79]. Its other strength is in measuring bondlengths, and it has been applied to measure these as a function of coverage [27,79]. The main restriction is the difficulty in extracting information on substrate buckling and reconstruction that sometimes accompanies alkali adsorption. NISXW [351] has been successfully applied to a number of systems, in particular in measuring bondlengths as a function of coverage [52]. The relative simplicity of the technique makes it attractive and a site determination is possible if absorption excited by standing waves from two or more substrate planes allows triangulation. A useful parameter yielded by this technique is the "coherent fraction", i.e. the fraction of scatterers in the ordered site. The main drawback is that since the positions of atoms are measured relative to the bulk planes, accurate surface positions are unattainable if surface relaxation and reconstruction persist after adsorption. On the other hand, since chemical-specific atom positions can be determined, it could be very useful if applied to measuring bondlength changes within coadsorbed molecules. There have only been a small number of alkali systems studied using SXRD [352]. Because of the grazing-incidence angle, this technique is well-suited to determine atomic positions in the surface plane. Conversely, the precision perpendicular to the surface is less than that of most other techniques. An attraction of the technique is that multiple-scattering calculations are not required to model the data so that analysis is in principle more straightforward than for LEED. Similarly, there have been few studies using MEIS [353]. This is in part because of the need for an ion accelerator makes the technique relatively expensive. Again, an advantage is that analysis of data involves relatively simple Monte Carlo simulations of the scattering. Finally, other techniques have provided useful information on particular aspects of adsorbate and coadsorbate structure without providing full structural determinations. These include ESDIAD [354,355] and NEXAFS [356]. Core-level spectroscopy [18,24], vibrational spectroscopy (primarily HREELS [357,358] and He-atom scattering [357,358]) have also provided indirect information on the adsorption site and bond. It is pertinent to look at the consistency of the above techniques. In cases where multiple techniques have been applied to the same systems (see Table 10 for examples) it appears that site determinations are usually the same, but that bondlengths and other parameters are usually dispersed, often much wider than the appropriate combination of experimental errors. Possible reasons for these discrepancies have already been suggested in Section 2.11.2.

4. Summary and outlook We have made much progress in the past five years in the understanding of alkali metal adsorption. We now understand that alkali metal adsorption systems are not as simple as we

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161

once supposed, and furthermore, there is tremendous variety in the phases which occur on the various substrates. This variety hinges on the competing interactions which occur upon adsorption, and we now know to some extent which parameters are important in determining the equilibrium phases in these overlayers. As a result of the competing interactions, many of the structures which occur are the result of a fine energy balance which can be tipped by small changes in adsorbate size or electropositivity, for instance, or even a small amount of impurity. These energy balances are evident in many phenomena described in this review, including the adsorption sites, overlayer condensation, density modulations in incommensurate structures and substrate reconstructions. It is these fine energy balances that make alkali adsorption systems so interesting from a fundamental point of view and also make them so difficult to study theoretically. At this point, not many first-principles calculations have been carried out for alkali metal adsorption because of their complexity. These overlayer systems change their properties dramatically as the coverage is changed, and yet the energy differences between completely different structures is often on the order of only 10-100 meV. Nevertheless, progress has been made in the calculation of the fundamental adsorption properties in certain systems, most notably for Na and K on A I ( l l l ) [17,30]. These calculations give insight into why certain adsorption sites are occupied, why condensation occurs, why intermixing occurs, and whether there is indeed charge transfer. However, what we still lack is any means of generalizing this insight to other adsorption systems enough to be able to correctly predict their adsorption behavior. There are still surprises with almost every new experiment on alkali metal adsorption. While we are now getting a handle on what details are important with respect to the behavior of alkali metal adsorption systems, progress in understanding the differences between alkali coadsorption systems has been slower. Because they are more complex and thus more difficult to characterize in the first place, fewer experiments have been done to determine the structures of the coadsorbed layers and therefore the experimental database is much smaller. And while we have begun to gain some insight from the cluster calculations which have been carried out for some alkali coadsorption system [9,245], there is a long way to go before we can generalize or predict alkali coadsorption behavior.

Acknowledgements M. Scantlebury is thanked for assistance in organizing the reference database and in preparing the figures. The authors would like to acknowledge support from NATO (Grant CRG 920276), from the UK Engineering and Physical Sciences Research Council through its funding of the IRC in Surface Science in Liverpool, and from NSF (Grant Nos. DMR-9022681 and GER-9450147). The authors would like to thank J.N. Anderson, C.J. Barnes, W. Allison, D. Heskett, B. Kasemo, B.E. Hayden, J. Lahtinen, A.G. Naumovets, A.M. Bradshaw, D.P. Woodruff, H. Over, G. Ertl, J.P. Toennies, J. Braun, G. Witte, C. Stampfl, M. Scheffler, P.A. Dowben, D.L. Adams, M.S. Altman, H.-J. Ernst and I.K. Robinson for supplying reprints/preprints/communications from their work. We would also like to thank G.S. Leatherman, H. Over, J.N. Andersen and G.P. Lopinski for critically reading the manuscript.

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Appendix A: Some general tables

Table A.1 List of acronyms found in the text ARPEFS CMTA DLEED DFT ESDIAD HOC (HR)EELS IRAS KTHNY LEED LEEM L LT MEIS NEXAFS (NI)SXW NM (N)NN PhD RHEED RT SCLS SEXAFS STM SXRD TDS XPD

Angle-resolved photoelectron fine structure Constant-momentum transfer averaging Diffuse LEED Density-functional theory Electron-stimulated desorption ion-angular distribution Higher-order commensurate (High-resolution) electron energy-loss spectroscopy Infrared reflection absorption spectroscopy Kosterlitz, Thouless, Halperin, Nelson, Young (theory of melting of a 2D solid) Low-energy electron diffraction Low-energy electron microscopy Langmuir (exposure unit, 1 L = 10- 6 Torr. s) Low temperature (usually _<100 K) Medium-energy ion scattering Near-edge extended X-ray absorption fine structure (Normal incidence) standing X-ray wavefield absorption Novaco-McTague (theory of rotational epitaxy for a 2D solid) (Next) nearest neighbor Photoelectron diffraction Reflection high-energy electron diffraction Room temperature Surface core-level spectroscopy Surface extended X-ray-absorption fine structure Scanning tunneling microscopy Surface X-ray diffraction Thermal desorption spectroscopy X-ray photoelectron diffraction

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Table A.2 Lattice spacings of metal surfaces: the lattice constant of the bulk metal (a 0) [266], the nearest-neighbor spacing in the bulk metal, and the nearest-neighbor spacings in commonly observed overlayer structures ~ c metal

AI Cu Au Ir Ni Pd Pt Rh Ag Pb bcc metal

Fe Mo W hcp metal

Be C Co Ru

ao

NN

(A)

(A)

4.05 3.61 4.08 3.84 3.52 3.89 3.92 3.80 4.09 4.95

2.86 2.55 2.88 2.72 2.49 2.75 2.77 2.69 2.89 3.50

(111)

(100)

p(2×2)

(~f3×v~-)R30 °

c(4x2)

c(2x2)

(A)

(A)

(A)

(A)

5.73 5.10 5.77 5.43 4.98 5.50 5.54 5.37 5.78 7.00

4.96 4.42 5.00 4.70 4.31 4.76 4.80 4.65 5.01 6.06

5.73 5.10 5.77 5.43 4.98 5.50 5.54 5.37 5.78 7.00

4.05 3.61 4.08 3.84 3.52 3.89 3.92 3.80 4.09 4.95

ao

NN

(100)

(A)

(A)

c(4 X 2)

c(2 × 2)

p(2 × 2)

c(2 ×

(A)

(A)

(A)

(X)

2.87 3.15 3.16

2.49 2.73 2.74

5.74 6.30 6.32

4.06 4.45 4.47

4.97 5.46 5.47

2.49 2.73 2.74

ao

NN

(A)

(A)

2.29 2.46 2.51 2.70

(110)

(0001)

2.29 1.42 2.51 2.70

p(2 x 2)

(¢3- × v~-)R30 °

(A)

(A)

4.58 4.92 5.02 5.40

3.97 4.26 4.35 4.68

Table A.3 Ionic and metallic radii of alkali metals and their melting temperatures [266] Metal Li Na K Rb Cs

Ionic

Metallic

Tm

(A)

(A)

(K)

0.68 0.97 1.33 1.48 1.67

1.56 1.91 2.38 2.55 2.73

453.7 371.0 336.3 312.6 301.6

2)

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