Cesium on molybdenum (110)

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ELSEVIER

Cesium on molybdenum (110) D.A. Gorodetsky

a, Yu.P. Melnik *,a, V.A. Usenko a, A.A. Yas’ko a, V.I. Yarigin b aRad~~hys~cs ~~rtment, lyieu Um.oersi~, 252017 Kiev, Ukraine

’ institute

of Physicsand Power Engineering, 249020 Ubninsk, Russian Federation

(Received 29 July 1993; accepted for publication 1.5April 1994)

Abstract Surface structure, work function changes and plasma losses have been studied for the adsorption of Cs on a Mo(ll0) surface. At 77 K, up to the density of 2.2 x lOI cm-‘, Cs is in a two-dimensional liquid state. At higher coverages the adlayer crystallizes, forming domains rotated 19” with respect to each other. The domains possess a hexagonal structure and undergo a rotation of 90” at a coverage of 3 X 1014 cmm2. A full monolayer has a close-packed hexagonal array of adatoms with a density of 5.1 x 1014 cme2. The onset of condensation in the second layer results in the immediate formation of two-dimensional islands with a close-packed hexagonal structure of 4 X lOI cm-* density. The work function passes through a minimum (1.42 eV) at a coverage of 3 x lOI cmP2. After compietion of the first layer the work function continues to grow during Cs condensation in the second layer and levels up to 2.15 eV. The EEL spectrum for clean Mo(llO) possesses a set of peaks which are identified with the surface and volume plasmons. The first feature to appear in the energy plasma loss spectrum due to Cs deposition is the 2.2 eV loss. It appears at the Cs adatom density corresponding to a work function minimum. This signal is interpreted as due to the surface plasmon loss in Cs. The Cs bulk plasmon loss at 4.6 eV appears at the beginning of the second layer.

1. Introduction

ture at room temperature smooth surfaces as W(llO)

The adsorption system of Cs atoms on transition refractory metals has been historically one of the most studied systems since the pioneer work by Langmuir and Kingdon [ll. This study has been stimulated in many cases by the interest in various technologies and the fact that most of the theoretical investigations treating adsorption phenomena deal with alkali atoms. The high mobility of Cs adatoms makes it impossible to study the submonolayer film struc-

* Corresponding ~39-60~/94/$07.00

author. Fax: + 7 044 266 21 29.

0 1994 Etsevier Science B.V. All rights reserved

.%SLM0039-6028(94)00235-2

on

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and Molto). So the most detailed data on the structure of Cs films on W(110) were obtained, in Ref. [2], at 77 K. One could expect to find the analogous Cs film structure as well on the Mo(ll0) surface, which is in geometrical sense identical to the W(110) surface. However, investigations of barium films showed that they have essentially different structures in submonolayers on M~~lO) (31 and W(110) [4]. Cs films on Mo(ll0) were studied earlier [5-91 but only at room temperature and in most cases with coadsorbed oxygen. Thomas and Haas 151 observed a close-packed hexagonal structure in the case of full monolayer coverage.

52

D.A. Gorodetsky et al. /Surface

The electronic properties of Cs atoms adsorbed on transition metal surfaces have been explored mainly for the (100) face [lo-121. A shift of the high-lying d-like surface state of W(100) was observed during Cs adsorption [11,12]. Theoretical calculations showed that this shift can be due to the formation of a polarized covalent bond between the Cs(6s)-derived states and the W(d) surface states. An analogous shift of the surface state was obtained in the case of Cs adsorption on Mo(llO), in Ref. [91, where it was thought it might also be explained by a covalent bond formation. This paper presents a combined experimental study of Cs adsorption on the (110) surface of MO by low-energy electron diffraction (LEED), electron energy loss spectroscopy (EELS), as well as contact potential difference (CPD) methods. Cooling of the sample to 77 K made it possible to obtain data on structure and electronic properties of submonolayer and multilayer Cs films.

2. Experimental The measurements were performed in an ultrahigh-vacuum glass set-up with the residual gas pressure maintained in the 5 X lo-” Torr range. Two set-up types were used. The first one contained a four-grid retarding field analyzer for contact potential difference, electron energy loss and Auger measurements. The second one was a two-grid display-type LEED system which made it possible to obtain more detailed and accurate diffraction patterns in comparison with the analyzer of the four-grid type. Work function changes of the investigated surface were determined by the changes in the contact potential difference between the electron gun cathode and the sample. As a measure of these changes the retarding potential shift was used which was necessary to maintain the low (N lo-’ A) constant value of the sample current. The surface of the molybdenum specimen was oriented with an accuracy of 20’ perpendicular to the (110) direction. Carbon contaminations were removed by oxidating at 1200 K, under an oxygen pressure of lo-’ Torr, and then flashing up to

Science 315 (1994) 51-61

2200 K to remove the oxide. Pure cesium was deposited on the specimen surface from a resistively heated platinum tube filled with a mixture of cesium bichromate and silicon. After activation at 900-1000 K and thorough outgassing the source ensured a stable flux of Cs atoms with an intensity of 10”-10’2 cmp2 s-l. During the Cs-source activity the pressure in general did not rise above 1 x lo-” Torr. For studies of deposition to a thickness of many layers the entire support assembly could be cooled up to 77 K by inserting the specimen holder arm into the liquid nitrogen. For the coverage determination we took into account the correlation of LEED, EELS and CPD data. Estimates of the film thickness were produced by using the amplitude A of the first derivative of the Cs ionization loss line with an energy of 77/79 eV, which is due to the 4d3/2,5/2 doublet excitation. It proved to be more convenient for the MO-Cs system compared to the use of Auger signals. More precise absolute density values for the submonolayer range were derived from LEED patterns, assuming a uniform adatom distribution.

3. Experimental

results

3.1. Film structure and growth mode

Fig. 1 shows the amplitude A of the 77/79 eV Cs ionization energy loss line (4d3/2,5/2) as a function of the condensation time up to a coverage of approximately four monolayers for deposition at 77 K. The curve consists of linear segments with clearly noticeable breaks, which indicate the monolayer-by-monolayer growth mode (Frank-van der Merwe mode). Each break in the A-versus-time plot is interpreted as an indication of the beginning of the following layer formation. So the scale of the relative coverage (or coverage degree) 0 was obtained. In this case we define the coverage degree 13 as the ratio of the atom density to the saturation density in the corresponding atom (monojlayer. The scale of 0 proved to be non-equidistant because the maximum adatom density in the first layer is higher than in the subsequent ones.

DA.

et al. /Surface

lbrhun.)

4(eV)

0

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10

20

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Fig. 1. Cs adsorption on a MofllO) surface at 77 K. Changes with coverage of amplitude A of the Cs ionization loss line (77/79 eV), work function d and intensities of the Cs surface (P,, and volume (P,) plasmon lines.

Fig. 1 also shows the absolute values of the coverage derived form the LEED patterns of the first layer. Such a dete~ination of the absolute adatom density is correct only if the adatoms are in uniformly distributed sites and not in the form of islands. The investigated systems seems to satisfy this requirement because the large dipole moment of cesium adatoms and their high mobility, even at 77 K, ensure an equilibrium state and u~ifo~i~ of the submonolayer. In our case it is also proved by the equality of the Cs vapor fluxes which have been estimated for all diffraction patterns shown in Fig. 2. The obtained absolute density scale is extrapolated into the range of larger coverages because at 77 K the sticking coefficient of Cs atoms remains constant for any coverage as it proceeds from the whole data set. At room temperature deposition no condensation takes place in the second layer, at least for the used fluxes, and so the Cs ionization loss amplitude does not change after the completion of the first monolayer (Fig. 1).

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53

A series of LEED patterns observed during Cs deposition on Mo(ll0) at 77 K is shown in Fig. 2, where Fig. 2a is the pattern of a clean Mo(ll0) face. When Cs is deposited a background intensity rise is observed at the first stage, then diffuse rings (Fig. 2b) appear around the regular MO spots. These rings are due to the existence of a certain short-range order in the adsorbed layer. Using the ring diameter one can evaluate the adatom density IZ. In Fig. 2b this is equal to (1X3-1.7) x 1014 cmp2. Probably, in the case of such an adatom ~pulation the 77 K temperature is not low enough for film crystallization and it is in a two-dimensional liquid state. Analogous patterns were observed for W(llO)-Cs [21, W(llO)Ba [4] and Re(OOOl)-Ba [13] systems. In Ref. [4] we supposed that a deeper cooling (by liquid He) of the sample may bring about the crystallization of such a film so as to transform the diffuse ring to a set of spots. In Ref. [14] this ass~ption was confirmed. As the coverage grows and the distance between the adatoms decreases the rings become larger in diameter and at a density of 2.3 x 1014 cm-’ resolve into separate hexagonally located spots (Fig. 2~1. This means that the adsorbed layer becomes ordered and a ho-dimensional crystal with a hexagonal array is formed on the surface. An analogous situation was observed in the same coverage range in the W(llO)-Cs system 121. However, in Ref. [21 only one hexagon was observed unlike in our case in which there are two groups of spots. In Fig. 2c one can see these hexagons located around the regular substrate reflexes and were rotated by 19” with respect to each other. This means that the Cs adlayer on Mo(ll0) consists of domains oriented in two directions while on W(110) the submonolayer film only has one azimuthal orientation. Note that only first-order additional diffraction spots can be seen in Fig. 2c. We point out this fact and assume that the absence of high-order additional spots as in the W(llO)-Ba system [4] is the result of non-coincidence between the adsorbed film and the substrate lattices. With further Cs deposition the hexagons increase in size and as soon as the density of

Fig. 2. Sequence of LEED patterns obtained by the deposition of Cs at 77 K on Mo(l10). (a) Clean Mo(ll0); (b) Cs low-coverage pattern (density of (1.6-1.7) x 1014 cm-*); (c) n = 2.3 x 1OL4 cmm2; (d) 3 X 10” cm-‘, coverage degree 0 = 0.6: (e) 3.3 X 1014 cm-‘; (f) 4.33 x lOI cm-‘; (g) 5.1 X 1014 cm-*; (h) - 6.1 x 1014 cm-*; (i) two-layer Cs film. n = 9 X 10’” cmm2: (j) two-layer Cs film after heating up to 180 K.

D.A. Corodetsky et al. /Surface

3 x 1014 cmH2 is attained they undergo a rotation of 90”. At a rather narrow coverage interval both orientations coexist (Fig. 2d). The film with the new orientation also consists of two kinds of domains rotated with respect to each other. With the increasing coverage all extra spots move slightly in opposite directions, which is indicative of isotropic adlayer contraction, until the close-packed hexagonal adatom array is attained. During this contraction two patterns (Figs. 2e and 2f) appear one after another. These patterns are due to hexagonal Cs layers which are in registry with the substrate in one direction. That is why the coverage in such films can be calculated with good accuracy. The first structure (Fig. 2e) has a spacing that is twice that of the substrate spacing in the (112) direction; the second one (Fig. 2f) has a twofold spacing in the (170) direction. The corresponding populations in these structures are 3.3 X 1014 and 4.33 X lOi cm-‘, respectively. Calculation of the adatom density according to the patterns of Figs. 2b-2d and a more accurate one according to Figs. 2e and 2f allows us to plot the coverage scale in Fig. 1. It should be noted that the meeting at the (i $) point (Fig. 2e) of two additional spots, which move towards one another in the (112) direction, is possible only because of the coexistence of the two domains with different orientations. For hexagonal films with one orientation (e.g. on W(110) [2]) additional spots do not move in the (112) direction and cannot meet at the (i i) point. Nevertheless, the fact that in Fig. 2e the coalescence of spots belonging to domains of different orientations occurs, counts for nothing for density dete~ination. We can add that the coalescence of additional snots at the (4 f) point means that each domain is tied to the closepacked rows of MO atoms in the (110) plane. When the additional spot motion ceases the closely packed adatom layer is formed and the picture of hexagonally located spots, in Fig. 2g, is observed. The time of its appearance coincides with the time of the first break appearance in the A-versus-time curve (Fig. 1). Adatom density calculation according to Fig. 2g gives the value of 5.1 X 1014 cme2 which is in a good agreement with the obtained scale,

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5s

So at 77 IS on the Mo(ll0) submonolayer Cs film is ordered in the (2.3-5.1) X 1014 cmP2 coverage range, possesses a hexagonal array and consists of domains oriented in two azimuthal directions. After the formation of a close-packed monolayer the cesium condensation in the second layer begins. This results in the appearance of six new spots inside the hexagon of reflexes from the first layer (Fig. 2h). As the coverage grows, the position of the new extra spots does not vary, only a change in their intensity occurs. The brightness of the second-layer spots increases, the first-layer spots becoming less bright (Fig. 2i). The pattern of Fig. 2h was obtained when the summary coverage equaled 6 X 1014 cmw2, i.e. the mean adatom density of the second layer did not exceed 1 X 1Or4 cme2 or 0.25 in terms of coverage degree 8. Meanwhile, the position of the second-layer spots corresponds to a density of 4 X lOi cm-‘, i.e. to the close metallic cesium atom packing. The stability of the second-layer structure throughout its filling indicates that, at the beginning of the condensation, in the second layer Cs atoms form close-packed two-dimensional islands. At a certain temperature the first layer is rearranged by the second one. Fig. 2j shows a diffraction pattern obtained after heating a two-layer film up to 180 K: the first layer became a one-domain structure; a two-domain state of the second layer with an azimuthal disorientation of 19” became distinct. It is interesting to note that the pattern of Fig. 2j gives us a rare chance to observe the reflexes due to triple scattering of electrons by the substrate and by the first- and second-adsorbed Cs layers. Condensation in the second and the third layers is accompanied by a linear growth of the Cs ionization line amplitude. The positions of the second and third breaks in the A-versus-time curve (Fig. I) are in good agreement with the saturation density of Cs atoms in the second and third layers (4 X lOi cm-*) obtained from the diffraction pattern (Fig. 2j) and corresponding to the Cs atomic diameter 5.36 A. If one builds upon the A-versus-time curve it is difficult to trace the filling of the fourth and following layers,

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D.A. Gorodetsky et al. /Surface

Science 315 (1994151-61

but according to Fig. 1 one can state that at 77 K the sticking coefficient of Cs atoms is the same for all layers. 3.2. Characteristic

energy losses

The characteristic energy loss spectra mainly consist of peaks originated by the excitation of collective oscillations and interband transitions. Electron energy losses in adsorbed films were first reported in Ref. [lo] for Cs condensation on W(100). In that study the 1.5 eV energy loss appeared in the secondary emission spectrum when the work function minimum was attained. With further deposition it shifted to 2.5 eV. This signal was interpreted by authors as being due to a two-dimensional plasma loss in the Cs layer, indicating that the Cs overlayer became metallic beyond the work function minimum. However, in Refs. [11,12,15] another interpretation of the energy losses in the W(lOO)-Cs and Mo(lOO)-Cs systems was suggested. Soukiassian et al. [11,123 considered that interband transitions occur for coverages up to a monolayer. This latter interpretation was proved by theoretical calculations 1151 which showed that the electron structure of the Cs monolayer on W(100) is determined by the hybridization of the Cs s state with the W high-lying surface state, and the loss peak at 1.5 eV is rather due to excitation of the hybridized level. Thus, the conclusion about the metallization of Cs submonolayer films on W(100) was questioned. Figs. 3-7 show the changes of the energy.loss spectrum for various Cs coverages on Mo(ll0). The EEL spectrum, N(E), (Fig. 3, curve 1) is typical of a clean MO surface for a primary beam energy value E, > 120 eV (e.g. Ref. [161) and possesses two peaks which are usually attributed to the bulk (22 eV) and surface (10.5 eV> plasmon excitations. However, the electron energy loss function calculated from the optical reflectivity data [17] exhibits a more complex structure. According to Ref. [17] both peaks of Fig. 3 (curve 1) are the result of the superposition of volume and surface plasmon losses. Schubert and Wolf [18] obtained a splitting of the 22 eV peak into two peaks with energies of 20.1 and 23.9 eV. In

40

30

20

10

0 &eVI

Fig. 3. Variations of the EEL spectrum N(E) for Mo(ll0) during Cs deposition. Primary energy E, = 200 eV. The peakto-peak modulation amplitude U,,, = 1 V. (1) Clean Mo(l10); (2) Cs film with 0 = 0.6; (3) Cs monolayer, 0 = 1.0.

Ref. [18] the resolution has been improved by using the second derivative of the energy spectrum (N”(E)). In the present study maxima in the energy distribution curve (fine structure) are also located by the peaks in N”(E) (Fig. 4) for E, = 400 eV. Losses with AE < 13 eV are better resolved in N( El spectra at low E, values ( < 70

i0

i

30

I

I

I

20

10

Of

\E kV,l

Fig. 4. Change of the second derivative of the EEL spectrum (N”(E)) for Mo(ll0) under cesium deposition. E, = 400 eV. 17, = 2 V. (1) Clean Mo(ll0); (2) Cs film with 8 = 0.6.

D.A. Gorodetsky et al. /Surface

i

i

i

I

I

25

20

15

10

5

I

cl b

4

Fig. 5. Change of the EEL spectrum N(E) for Mo(llO) under cesium deposition. E, = 40 eV, U,,, = 0.3 V. (1) Clean Mo(l10); (2) Cs film with 0 = 0.14; (3) 0 = 0.6.

eV> and are shown for E, = 40 eV in Fig. 5. It should be noted that the dete~ination of the loss location with an accuracy of 0.1 eV is rather approximate because the maximum location in the energy spectra depends substantially on the curve slope. The 2.5 and 5.5 eV peaks are attributed [16] to interband transitions involving surface states. Other features of the curves (Figs. 4 and 5) correspond to the maxima in the volume and surface loss functions determined from optical studies [17] and can be identified with the plasmon excitations in molybdenum. One can see a good splitting of the “22 eV peak” in the N”(E) spectrum for clean MO (Fig. 4, curve 1). The energy values of new “high-energy” peaks (20.2 and 23.2 eV) slightly differ from those reported by Schubert and Wolf [HI, but we again emphasize the approximate character of the location of the maxima. In addition to the “high-energy” peaks one can observe in curve 1 (Fig. 4) the “middle-energy” 13.8 and 15.7 eV peaks. Their

Science 315 (1994) 51-61

57

positions are also in agreement with the features of the loss functions in Ref. (171. At the same time the splitting of the “low-energy” 10.5 eV peak has not occurred. This splitting in a 9.7 and a 12.2 eV peak is observed in N(E) spectra with a sufficiently low E, (Fig. 5). The N(E) spectrum for clean Mo(ll0) (Fig. 5, curve 1, E, = 40 eV> includes the whole set of lines whose intensity in the high-energy region is naturally reduced because of a low E,. An extremely remarkable property is inherent in curve 1: a small intensity of the elastic electron peak. There are some considerations to explain this fact. One of them, to our mind the most probable, is connected with the constructive peculiarities of a quasi-spherical retarding field analyzer. As known, the elastic electrons scattered from the perfect periodic lattice produce a set of discrete diffracted beams and the specularly reflected 00 beam among them. For normal incidence of a primary beam and at low electron energy values the possible diffraction angles become too large and the diffracted beams are not in the working cone of a quasi-spherical analyzer. As a result the elastically scattered electrons do not take part in the collector current and in the ideal case the EEL spectrum may not possess an elastic peak. Probably this circumstance allows us to obtain the spectrum with a better resolution at a low E,. Thus, in the spectrum with E, = 40 eV we could split the “low-energy” 10.5 eV peak and distinguish the intensive energy loss of N 2.5 eV. We are to consider the influence of Cs atom condensation upon the EEL spectrum for different E,. The changes of the loss spectrum N(E) for E, = 200 eV are shown in Fig. 3. The height of the 22 eV peak practically does not change, while the magnitude of the 10.5 eV peak decreases and nearly vanishes when 8 + 1. It is known that adsorption of foreign atoms essentially changes the surface electronic structure, primarily suppressing the surface state and surface plasmon excitations. Nevertheless, one can observe the 10.5 eV peak even at a Cs monolayer coverage and this proves the existence of a MO volume loss in this energy region. Figs. 4 and 5 show the changes in the “fine” structure of loss spectra under Cs atom conden-

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D.A. Gorodetsky

et al./Surface

sation. At the initial stage the Cs deposition results in the rapid disappearance of 9.7, 15.7 and 20.2 eV peaks. That is why these losses may be attributed to the excitation of the surface plasmons. Accordingly, the 12.2, 13.8 and 23.2 eV peaks are identified with the volume plasmons because they still remained at coverage 8 = 0.6 in Fig. 4 (curve 2) and began to decrease in the second stage in Fig. 5 (curve 2). The Cs deposition also gives rise to a drastic increase of the elastic peak. According to our viewpoint this happens because the appearance of the surface superstructure with large constant d decreases the diffraction angles 4 according to the relation d sin $=A. As a consequence the elastically scattered electrons begin to reach the collector either in the form of the diffuse ring around the 00 beam (Fig. 2b) or in the form of the hexagonally located spots (Fig. 2~). In the case of E, = 40 eV all the loss lines connected with the collective processes, as well as with the interband transitions, vanish at coverage 8 = 0.6 (n = 3 X 1014 cme2). After that, the spectrum looks like a nearly smooth curve with the elastic peak (Fig. 5, curve 3 and Fig. 6, curve 1). The first feature to appear in the energy plasma loss spectrum due to Cs deposition is the 2.2 eV loss. It appears at 3 X 1014 cme2 in the form of a shoulder on the elastic peak and is shown in Fig. 6 (curve 2) for a monolayer coverage (0 = 1). From the onset of the second layer a new ,loss centered at 4.6 eV begins to grow; the final spectrum of EEL, N(E), for a Cs film with 8 = 2.8 is shown in Fig. 6 (curve 3). Accurate curves of loss intensity versus coverage are difficult to obtain using a sequence of N(E) spectra. Therefore, we again employed the second derivative N”(E) of the EEL spectrum in which the loss line resolution is a lot better (Fig. 7). The heights of the peaks in the N”(E) curves (Fig. 7) were used as a measure of the loss intensities P, and P,; the results are presented in Fig. 1. The 2.2 eV loss appears at 8 = 0.6 and its intensity P, increases almost linearly up to the beginning of condensation in the third layer (Fig.

Science 315 (1994) 51-61

N(E)

E (eV) Fig. 6. Change of the EEL spectrum N(E) for Mo(ll0) under cesium deposition. E, = 40 eV, U, = 0.3 V. The amplification is three times smaller than in Fig. 5. (1) Cs film with fI = 0.6; (2) 0 = 1.0; (3) 0 = 2.8.

0, although the curve slope, as the first layer is filling, differs slightly from the slope for the second layer. After the completion of the second layer the curve slope decreases drastically. The

1

6

4

2

0 AE(ev)

Fig. 7. Change of the second derivative of the EEL spectrum (N”(E)) for Mo(ll0) under cesium deposition. E, = 70 eV, U, = 2 V. (1) Cs film with 0 = 0.6; (2) 0 = 0.7; (3) 0 = 1.0; (4) e = 2.0.

D.A. Gorodetskyet al. /Surface Science 315 (1994) 51-61

intensity P, of the 4.6 eV loss also enhances mainly up to the third layer (Fig. 1). Following Ref. [lo] we consider that the 2.2 eV line is due to the excitation of the surface plasmon in the Cs layer. This loss appears at 8 = 0.6 when the 2.5 eV line connected with the excitation of the MO surface state has already vanished. Moreover, the essential enhancement of the 2.2 eV line intensity takes place during the filling of the second layer, the atoms of which do not make contact with the MO surface atoms. All this does not allow us to associate the origin of the 2.2 eV peak with the excitation of the Mo-Cs-hybridized state [15]. Concerning the 4.6 eV peak one might connect this loss with the second harmonic of the surface plasma oscillation. But the coverage threshold of its appearance (0 = 1) does not coincide with the coverage threshold of the 2.2 eV loss (f3 = 0.6). That is why we prefer to attribute the 4.6 eV peak to the excitation of the volume plasma oscillation in spite of the fact that elementary calculations give the energy value of 3.6 eV for a Cs volume plasmon. As is known for a homogeneous film the surface plasmon energy E,, depends on the adatom density n as

where in general (Y= 3, but for every concrete system (Ycan differ from the 4 value because the adatom electronic state may depend on the coverage. The dependence of the surface plasmon energy on the coverage was observed for the W(lOO)-Cs [lo], Ni(lOO)-K [19], Ir(lOO)-Ba and W(lOO)-Ba [20] systems. But in our work, in the N”(E) spectrum we did not reveal any change in the surface plasmon energy during Cs deposition in the submonolayer region (0.6 < f3< 1). We cannot explain this fact. Maybe there is an influence of the derivation procedure upon the line location in the case when a weak peak is positioned near the intense peak of the elastically reflected electrons, practically on its slope. 3.3. Work function change The work function change upon Cs adsorption is shown in Fig. 1 for the 77 K deposition temper-

59

ature. Continuous measurements have been made during condensation. The curve shows the well known minimum. If we take into consideration the work function value of 5.00 eV for clean Mo(ll0) the work function minimum is 1.42 eV. Beyond the minimum the dependence of 4 on the deposition time makes a break at the point where &I= 2.0 eV. This break is in good agreement with the first break on the A-versus-time curve (Fig. l), i.e. it corresponds to the completion of the first layer. Peculiarities of the +versus-coverage curve are three linear segments in certain coverage ranges and the absence of a maximum at the point of the completion of the first layer. Two of the linear segments are at the initial part of the curve and have different slopes with a break at the point with the coverage value of 1.2 x 1014 cm-*. The third one is within the (3.5-5.1) X 1014 cm-* coverage range. The slope of the initial linear segment is often used for calculating adatom dipole moment p0 in the zero density limit. It is determined according to A+ = 2rnp, and results in the value p,, = 8.8 D for a Cs adatom on Mo(ll0) (in Debye units). Correlation of the $-versus-coverage dependence with the surface structures is clearly seen only at two points. At the work function minimum there occurs a change in the azimuthal orientation of the hexagonal adatom array (Fig. 2d). The break point with coverage 5.1 x 1014 cm-* corresponds to the completion of the first monolayer (Fig. 2g). Beyond the work function minimum the linear segment of the curve is accompanied by an isotropical compression of the adatom array up to the saturation coverage. To associate the initial linear segments with the adlayer rearrangements is impossible because at 77 K the film crystallizes only above the coverage of 2.3 X lOi cm-*. After completion of the first monolayer the work function continues to increase slowly with coverage until the value of 2.15 eV is attained during condensation in the third layer. This behavior does not correspond to the latest point of view on work function changes induced by alkali

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adsorption. According to calculations by Lang [21] a local maximum of $ must occur at 0 = 1 when condensation begins on the first filled layer possessing its native properties. Such work function behavior corresponds also to the earlier views of Smoluchowski [22]. According to Ref. [22] the work function decrease at the beginning of the second-layer filling is connected with the onset of surface roughness. Smoothing of the electron gas surface occurs due to the minimization of kinetic energy and as a result a surface dipole potential appears which lowers the work function. Such an effect has been observed by us for the W(llO)-Ba system [4]. The work function increase during the second-layer filling suggests the first Cs layer on Mo(ll0) does not yet possess bulk properties. Thus, the absence of a work function maximum at the coverage corresponding to the completion of the first layer is connected with a high Cs adatom ionicity even in the full monolayer. This supposition correlates with the continuous rise of the Cs surface plasmon line intensity during the second-layer filling. We understand of course that the interpretation of the peak height in the N”(E) spectrum as a measure of loss intensity needs further proof.

4. Summary The principal properties of a Cs film on Mo(ll0) are similar to those of alkaline element films on other substrates. So the Cs adatom lattice ordered at low temperature has a hexagonal structure similar to the Cs submonolayers on W(110) and W(100) [2,10]. Like in other investigated systems [lo,191 an energy loss connected with surface plasmon excitation starts to develop at submonolayer coverage and a bulk plasmon line appears during condensation in the second layer. Finally, the work function measurements during Cs adsorption produce a well known curve exhibiting a minimum. One interesting result of these studies is the correlation of the work function minimum, the azimuthal reorientation of the overlayer and the appearance of a loss line with an energy of 2.2

Science 315 (1994) 51-61

eV. This correlation can be understood if we take into account the assumption of MacRae et al. [lo] that the 2.2 eV signal is due to a two-dimensional plasma loss in cesium. As it follows from LEED data the Cs adatoms are uniformly distributed over the substrate. When the coverage increases, the nearest-neighbour spacing decreases and at a certain adatom density the overlap of Cs valence electron orbitals becomes possible. As a result the adlayer becomes metallic in nature and cesium plasma excitations begin to appear. After this the adlayer possesses the properties of Cs metal which has a varying density of surface atoms. With increasing coverage this causes the enhancement of the work function up to a value corresponding to a close-packed monolayer. Thus, the work function minimum occurs. It is apparent that the conversion of the layer composed of ionized cesium into a metallic layer is also connected with a drastic change in the interaction of adatoms and substrate atoms. Possibly it is this that causes the change in the azimuthal orientation of an overlayer at the coverage corresponding to the work function minimum. Moreover, it must be emphasized that the film reorientation in the work function minimum is characteristic not only for the Mo(llO)-Cs system [4]. At the same time the investigated system has its own peculiarities: (i) Unlike in the W(llO)-Cs system [2,10] domains of two orientations form a homogeneous ordered layer of Cs adatoms on Mo(ll0). These orientations are connected with the close-packed rows of MO surface atoms with (111) directions. (ii) The intensity of the Cs surface loss line (the 2.2 eV peak in the N”(E) spectrum) rapidly grows with coverage not only when the first layer is filling but also during the condensation in the second layer. Only with the deposition in the third layer the intensity of the surface loss line levels off. The 4.6 eV line, which we interpret as due to Cs volume plasmon excitation, appears after the beginning of condensation in the second layer and its intensity also levels off during the filling of the third layer. (iii) The work function does not decrease after the completion of the first layer but continues to increase during the filling of the whole second

D.A. Gorodetsky et al. /Surface

layer and levels off only during the condensation in the third layer. Together with the Cs surface loss intensity increase within the second-layer growth all this allows one to assume a high Cs adatom ionicity in the first layer.

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