Oxygen chemisorption and surface reconstruction on Ag(110) investigated with second-harmonic generation

surface s c i e n c e ELSEVIER

Surface Science 345 (1996) 281-289

Oxygen chemisorption and surface reconstruction on Ag(110) investigated with second-harmonic generation S. Reiff *, J.H. Block Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany

Received 13 July 1995; accepted for publication 30 August 1995

Abstract

The adsorption of oxygen on Ag(ll0) at temperatures ranging from 300 to 500 K is investigated by second-harmonic generation (SHG), low-energy electron diffraction (LEED) and work function change measurements (A~). For low oxygen doses the SH signal decreases in accord with the free electron model. Above Oo=0.15 the SH intensity starts to increase, and at the saturation coverage Oo =0.5 it reaches a level nearly 10 times that of the clean surface value. This enhancemenf is explained in terms of laser radiation field resonance with electronic transitions between bulk states and pr-like surface states at the ~r-point of the surface BriUouin zone. This effect is most pronounced for p-polarized excitation. The results compare very favorably with band structure calculations assuming an oxygen-induced added-row reconstruction model. Keywords: Chemisorption; Oxygen; Second harmonic generation; Silver; Surface reconstruction

I. Introduction

The a d s o r p t i o n of oxygen on the transition metal surfaces Ag, C u and Ni has been a matter of nonabating interest for m a n y years (see e.g. Ref. 1-1]). A m o n g other topics, the (110) faces of these metals serve as m o d e l systems for the study of restructuring phases because they easily reconstruct u p o n oxygen [ 1 ] as well as u p o n alkali metal adsorption [ 2 ] . For Ag(110) in particular, oxygen a d s o r p t i o n was investigated because of its relevance as a "realworld" catalyst for selective ethylene epoxidation. I n this connection the role of surface reconstruction is still uncertain. Possibly the oxygen-induced reconstruction m a y prepare the surface in such a way that other m o r e reactive oxygen species can be dispensed u n d e r reaction conditions [ 3 ] . * Corresponding author. 0039-6028/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0039-6028(95)00886-1

At r o o m temperature, molecular oxygen adsorbs dissociatively on A g ( l l 0 ) and forms a series of ordered p ( n x 1) structures, with the integer n decreasing gradually from 7 to 2 as the coverage increases (ideal: O o = l/n) [ 4 ] . Recently an additional (8 x 1) superstructure was observed with He diffraction [ 5 ] . A l t h o u g h these structures have been investigated with a variety of techniques [ 3 - 1 9 ] , it was not until 1989 that an inelastic H e scattering study [ 6 ] for the (2 X I ) - O phase could provide strong evidence of a reconstruction of the a d d e d - r o w type, which was also supported by later S E X A F S measurements [ 7 ] . D u r i n g the formation of the (2 x 1) reconstruction, A g - O rows along the [ 0 0 1 ] direction are added on top of the (1 x 1)Ag(110) surface with the atomic oxygen adsorbed in the long bridge sites at a height c o m p a r a b l e to that of the neighboring Ag a t o m s (see Fig. 1). As for (2 x 1), an a d d e d - r o w reconstruction model is

282

S. Reiff, J..H. Block~Surface Science 345 (1996) 281-289

Clean Ag(110)

(2xl)-O/Ag(110)

[oo111 [1 10]

[1i01

Fig. 1. Schematic surface structure of clean Ag(ll0) (left) and added-row reconstructed (2 x 1)-O/Ag(ll0) (right). The large open, grey and dark grey circles represent first, second and third layer Ag atoms, respectively. The oxygen atoms, occupying the long bridge sites, are outlined by black dots.

also assumed for the (3 x 1)-O structure [-83, the added A g - O rows here being separated from each other by three [ 110] lattice constants. Recent work using scanning tunnel microscopy (STM) [-3] seems to provide strong, though not conclusive evidence that the remaining (n x l)-O structures are due to an added-row mechanism as well. In this contribution we report second-harmonic generation (SHG) data as a function of oxygen coverage on Ag(110). This method probes the nonlinear optical response of a surface, i.e., the emission of light at frequency 203 under laser irradiation at frequency 03. The surface sensitivity of S H G stems from the fact that the second-order surface susceptibility Z(s2), responsible for the generation of the second-harmonic light intensity I2~, vanishes in the b u l k of centrosymmetric media. Within the surface region, however, this symmetry is broken and (beside the bulk magnetic dipole and electric quadrupole contributions, which are often negligible) a second-order electric dipole contribution to I2~o appears. The usefulness of S H G as a surface science tool was demonstrated by many authors (for a list of references see Ref. [-20]); it enjoys a number of advantages: S H G is non-destructive, accesses even buried (e.g. solid/liquid) interfaces and monitors surface processes on a time resolution only dependent on the laser repetition rate. Furthermore surface excitations can be examined spectroscopically with the use of a tunable laser [-21]. Since S H G is extremely sensitive to the

charge density profile of the outermost atomic layer, this method is able to probe surface reorganization processes such as relaxation or reconstruction [22]. In the present work, second-harmonic generation, combined with thermal desorption and titration experiments, was used to study oxygen chemisorption as well as oxygen-induced reconstruction of the A g ( l l 0 ) surface. Low,energy electron diffraction and measurements of the work function change were used in addition to provide a reliable coverage calibration.

2. Experimental The experiments were performed in an U H V chamber with a base pressure of 1 x 10-1°mbar. The apparatus is fitted for thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), argon ion sputtering, work function measurements via a Kelvin probe (A~) and second-harmonic generation (SHG). The A g ( l l 0 ) specimen was prepared from a 99.999% purity single crystal by cutting and polishing to a mirror finish. It was mounted on a liquid nitrogen cold finger and could be heated resistively via supporting Ta wires. The sample was cleaned using repeated cycles of 500 eV argon ion sputtering at room temperature followed by annealing to 750 K and heating in an O2 atmosphere at 500 K to remove carbon residues [-9]. Cleanliness was confirmed by L E E D and by AES. The clean and well-ordered Ag(110) surface exhibited a sharp (1 x 1) L E E D pattern and showed the whole series of (n x 1)-O superstructures, with n ranging from 7 to 2, during the exposure to oxygen E4]. Brief sputtering and annealing to 950 K was carried out routinely prior to each experiment. A Q-switched Nd:YAG laser producing 5 ns pulses at a repetition rate of 10 Hz and a wavelength of 1064 nm was used as a light source for the S H G measurements. The linearly polarized beam was passed through a 2/2 plate and SH filter and impinged on the sample at an angle of 0i~ = 60 ° relative to the surface normal. The illuminated area was 1 0 m m 2, i.e. about 20% of the total

283

S. Reiff, J.H. Block~Surface Science 345 (1996) 281-289

A g ( l l 0 ) crystal surface. The reflected SH radiation at 532 nm was detected after proper filtering and without further analysis of its polarization state by employing an RCA photomultiplier tube with a gated integrator technique. The [ 110] direction of the crystal was aligned parallel to the plane of incidence to within 5 °. The laser intensity was kept below 10 MW/cm 2 to preclude any light-induced surface damage. A second-order dependence of the SH signal on the laser intensity, as expected for two-photon processes, could be verified below 12 MW/cm 2. Oxygen gas with better than 99.998% purity was dosed by backfilling at pressures between 10 .6 and 10 .5 mbar. The extremely low sticking probability of oxygen on A g ( l l 0 ) (s= 10 .3 . . . . . 10 - 4 [4,10]) makes such high pressures necessary to limit the experiments to a reasonable duration. Thus, to complete the ( 2 x 1)-O structure at the saturation coverage Oo = 0.5, a typical exposure of 10000 L (langmuir) is needed, a fact that makes the O/Ag(110) system susceptible to impurities such as COz. At room temperature, CO2 adsorbs on O/Ag(110) via the formation of surface carbonate [9,11]. The carbonate can be thermally decomposed, liberating CO2 at ~ 4 6 0 K and leaving behind surface oxygen [12]. The oxygen adsorption experiments were therefore performed at crystal temperatures between 300 and 500 K (i.e. in the range including the surface carbonate decomposition temperature) to eliminate the possibility of affecting the S H G data with residual CO2 as delivered from chamber walls or from filaments during oxygen exposure.

3. Results Fig. 2a shows the normalized SH intensity I2o~/I2oo,clea n for the p-polarized 1064 nm excitation

during the exposure of Ag(110) to oxygen. The crystal temperature was held at 500 K. Oxygen, as an electronegative adsorbate, tends to localize free electrons withdrawn from the metal surface and consequently decreases the surface polarizability. Thus the observed SH intensity shown in Fig. 2a as a function of the 0 2 dose exhibits the expected initial decline [13,23]. However, this trend is

a.)

A

.....

b.)

O/Ag(llO)

6 4 2

0] +oo

c.)

d.)

++

400 p-

==

200 0

.

•

4

,

8

..

,

I .t--r'7,

. . . .

12

540 560 580 600 620

O2EXPOSURE / 103L

TEMPERATURE/ K

Fig. 2. SHG and A ~ results obtained during adsorption (left) and subsequent thermal desorption (right) of oxygen on Ag(ll0). The adsorption temperature was 500 K. (a) Normalized SH intensity as a function of 02 exposure, and (b) as a function of temperature for the p-polarized excitation at 2 = 1064 nm. (c) and (d): Exposure and temperature dependence of the oxygen-induced work function change. The appearance of (n × 1)-O LEED structures are indicated by arrows. In (d), the thermal desorption spectrum for mass-to-charge ratio = 32 is shown for comparison.

already reversed at 400 L Oz, where the SH intensity passes through a minimum, followed by a substantial increase as the 02 dose is continued. Finally, at saturation coverage (above 10000 L), an unusual, more than eightfold SH enhancement over the clean surface signal level is reached. Artefacts due to a possible contamination of the vacuum entrance and exit windows can be excluded, as the effect is reversible if oxygen is thermally desorbed from the crystal. This experiment is depicted in Fig. 2b. The desorption SH signal exactly mirrors the adsorption trace on a reversed time scale. For comparison, an Oz thermal desorption curve recorded by a differentially pumped quadrupole mass spectrometer is shown in Fig. 2d. With a 0.5 K/s heating rate, the maximum 0 2 desorption rate occurs at 575 K, the point of steepest SH intensity decrease in Fig. 2b. In this work, the proportionality between the change in work function A ~ and the oxygen coverage Oo [14] was utilized to calibrate the

284

S. Reiff J..H. Block~Surface Science 345 (1996) 281-289

coverage scale. In contrast to TDS where relative coverages can be obtained by an integration of the T D curves, A ~ measurements via the Kelvin probe method have the advantage of being nondestructive. The maximum A ~ at saturation (Oo = 0.5) reported by different authors ranges from 600 to 800 meV [4,15-17]. The dependence of zX~b on Oz exposure obtained in this work is displayed in Fig. 2c. The data were taken under conditions identical to those in Fig. 2a. The saturation Aq~ = 650meV attained above 10000 L corresponds to 0 0 = 0 . 5 . Thus, intermediate coverages can be related to the respective A~ simply by linear interpolation. The reliability of this calibration procedure is verified by L E E D observations. The oxygen-induced (n x 1) patterns appear at less than 10% below the ideal coverages Oo = l/n, a deviation that could be explained by island formation [12]. In Fig. 2d the A ~ decrease during the thermal desorption of oxygen is also shown. The point of inflection coincides well with the T D maximum at 575 K, consistent with the considerations above. To ensure the relative accuracy of the dose versus coverage transformation, the SI-IG measurements were repeated for different adsorption temperatures (between 300 and 500 K) and different 02 pressures. For each adsorption experiment at a different (T, p) set, several subsequent runs of S H G and A~ measurements were recorded under identical conditions. In all cases good agreement was achieved between the time dependence of the S H G signal and the respective A ~ data. As an example, the initial sticking probability of oxygen on A g ( l l 0 ) was determined from the initial slope of the A~b curves to be 1.8 x 10 - 3 at 325 K and 0.6 x 10 -3 at 500 K. This trend is also reflected in the S H G traces. Since the influence of residual CO2 on the SH intensity can be ruled out by this comparison, one can justifiably attribute the variation of the SH signal entirely to the changes in the oxygen coverage. For this reason the dependence of the normalized SH intensity on the oxygen coverage can be evaluated as presented in Fig. 3. The S H G minimum (I2~,/12~,c1~ ~0.5) is reached at Oo=0.15, a coverage that coincides roughly with the onset of the (7 x 1)-O phase. For higher coverages the SH intensity increases considerably and reaches a lim-

10

.

.

.

.

I

.

.

.

.

I

,

,

,

,

I

i

,

,

,

I

.

.

8

.

.~

O/Ag(110) --

.

I,~ = 8.4 MW/cm 2, p-pol. Z = 1064 nm, 0Enc = 60°

,

~

6

z LU I-z I a LU

4

< 2~ nO z

2

.

.

.

.

I

0,1

. . . . .

I

0,2

.

.

.

.

I

.

0,3

OXYGEN COVERAGE

.

.

.

I

0,4

.

.

.

.

0,5

OO

Fig. 3. Dependence of the normalized SH intensity from A g ( l l 0 ) on the oxygen coverage upon p-polarized excitation at 2 = 1064 nm. The solid line serves as a visual aid. The dashed line indicates the results obtained by Heskett et al. [13] at a fundamental wavelength 2 = 532 nm.

iting value of Izo/I2~,,dean~9.5 at 0 0 = 0 . 5 . This S H G enhancement is somewhat higher than the data of Fig. 2a suggest. The discrepancy can be explained by laser-induced desorption that cannot be neglected at 500 K and at long exposure times. Consequently saturation coverage, Oo = 0.5, is not completed under laser irradiation. However, if the laser is switched off during exposure a final 9.5-fold SH amplification is observed. For comparison, the expected S H G curve according to the predictions of the free electron model [13,23] is sketched in Fig. 3 (dashed line). The monotonic decrease is in contrast to our results. The reason for the unusual S H G enhancement observed in this work becomes clear upon taking into account radiation field resonance at co or at 2co with optical transitions between surface states. The S H G yield of the oxygen-covered Ag(110) surface is strongly dependent on the rotational angle of the polarization vector. This is illustrated in Fig. 4 for different oxygen coverages. The angle of rotation, denoted as 7~, is 0 ° (90 °) if the incident light is linearly polarized parallel (perpendicular) to the plane of incidence. The I2o~(p)/Iz~(S) ratio is

S. Reiff J.H. Block~Surface Science 345 (1996) 281-289

P 10

p

s

'- ' ' ' ' ',~1 ~' ~

'

'

'

. . . . ' ' ' t ' O/Ag(110)

'

'

'

e o = 0.45

60 "1-

F

4"

IJJ N

-J <

2

nz0

= 0.24 =0

~ o

(9O = 0.15 0

30

• 60

90

120

ANGLE OF ROTATION

150

180

~ / o

Fig. 4. SH rotational anisotropy of O / A g ( l l 0 ) at 2 = 1064 a m for various oxygen coverages. The angle of incidence was 60 ° with respect to the surface normal and the plane of incidence was along FX. The angle of rotation gt = 0 o (90 o) corresponds to p- (s-) polarized incident light. The magnified scale of the insert reveals the existence of two minima for Oo~0.15, replacing the single central minimum at higher coverages.

,-,4 for clean A g ( l l 0 ) and ~ 18 for 0 0 = 0 . 4 5 . This underlines the finding that the oxygen-mediated S H G enhancement is most pronounced for p-polarized excitation. Note that the I2,o(7t) graphs exhibit two minima only for Oo ~<0.15, namely at g t = ( 6 5 _ 5 ) ° and g t = ( l 1 5 + 5 ) °, instead of one minimum at 7t = 90 °. This anisotropy feature presumably stems from the non-linear free electron response rather than from symmetry properties of the Ag(110) surface. By use of a density-functional approach, Liebsch [24] calculated the variation of the SH signal with polarization of the incident radiation at 1.17 eV (i.e. 1064 nm) for a jellium surface using the electron density and dielectric function of silver. With a Rudnick and Stern parameter [25] that is proportional to the integrated normal component of the SH surface polarization a = - - 1 5 - 3 i , he found the minimum to occur at about gJ = 70 ° (and 110 °, respectively). In contrast, for l a [ ~ l , Izo~(gt) decreases monotonically until g" = 90 °. This theoretical result corresponds to our experimental findings if one relates the low-coverage data to [a[ >>1. Adsorbed oxygen

285

tends to bind the nearly free conduction electrons of the metal and thus leads to a reduced polarization parameter a. Some effort was made to investigate the influence of subsurface oxygen on our S H G results. Employing isotope experiments, Backx et al. [18] showed that adsorbed oxygen on Ag(110) diffuses to subsurface sites at temperatures above 423 K. The subsurface oxygen remains kinetically trapped until at a temperature around 550 K exchange with adsorbed atomic oxygen becomes possible. This was demonstrated by Campbell and Paffet [12] in a titration experiment. At room temperature the entire oxygen surface coverage can be removed by a reaction w i t h carbon monoxide, where CO2 is formed and released into the gas phase [9,11] while the subsurface oxygen remains non-reactive until the crystal is heated to ,--550 K. Although Campbell and Paffet [123 applied relatively high oxygen pressures (65 mbar) to accumulate subsurface oxygen, minute amounts of the subsurface species may h a v e also been present at the moderate conditions used in the present work and could thus affect the magnitude of the SH signal. This was investigated by means of the following procedure. Oxygen was dosed to Ag(110) at 500 K and 1.5 x 10 -s mbar, long enough to give a (2 x 1) saturation coverage. The SH signal was recorded during exposure according to Fig. 2a, and a CO pressure of 5 x 10 .6 mbar was subsequently maintained for 100 s. During the first 50 s the evolution of CO2 was registered by the mass spectrometer. Alternatively, the SH signal could be monitored. Similar to the desorption curve in Fig. 2b the SH intensity dropped from I2o,/I2o,.dean~9.5 to a minimum and saturated at I2o,/I2o,,ole~ ~ 1 as long as the CO2 production proceeded. This procedure was repeated for titration temperatures of 500, 400 and 325 K. The results can be considered to show no pronounced temperature dependence with the exception of a longer reaction time at the lower temperatures. At least for a titration temperature of 325 K, where no exchange between surface and subsurface oxygen occurs [ 18], the effect, if any, of subsurface oxygen on the SH signal should have been detected. To summarize, an influence of potentially present subsurface oxygen on the S H G response from

286

S, Reiff, J.H. Block~Surface Science 345 (1996) 281-289

O/Ag(llO) can be neglected within experimental error.

4. Discussion The oxygen-induced added-row reconstruction plays an important role for an understanding of the observed enhanced SH signal from O/Ag(110). In order to examine the possible origin of the described phenomenon it is suitable to quote some second-harmonic generation results obtained by other scientists on related adsorption systems. Bloch et al. studied O / C u ( l l l ) at elevated temperatures and found that oxygen adsorption leads at first to a substantial decrease of the SH intensity, followed by a minimum, increase and saturation [26]. The minimum and saturation signal levels are I2o/I2~,elean=0.1 and 0.2, respectively. The authors attribute the slight S H G increase beyond the minimum to a reversible insertion of oxygen to subsurface sites. They conclude that the breaking of the inversion symmetry, solely due to the presence of the surface, is enhanced by subsurface oxygen and the resulting outward relaxation of the C u ( l l l ) surface. With the above-described titration experiments we attempted to show that subsurface oxygen is not responsible for the S H G amplification on O / A g ( l l 0 ) , at least under the experimental conditions used. There was no significant change in the observed SH signal, whether the adsorption and/or titration were performed at 500 or at 325 K, i.e. above and below the temperature of oxygen diffusion into subsurface sites [ 18]. Note that we observed a nearly tenfold SHG enhancement, while subsurface oxygen on Cu(111) induces signal variations of 10% relative to the clean surface value. Thus, within the limits of our experimental resolution, the main contribution to the SH signal from O / A g ( l l 0 ) does not arise from subsurface oxygen. The subsurface argument was not necessary for the interpretation of the S H G data of O/Ag(110) reported by Heskett et al. [13]. They found a monotonic decrease of the SH intensity upon a O2 dose with a saturation of /2co/_/2o,clean=0.25 at O o = 0.5. The authors discuss this behavior in fight of the argument of Bloembergen et al. [23] that

the non-linear surface susceptibility Z~s2) of a metal surface depends primarily on the polarizability of the free electrons in the selvedge region. Z~s2) in turn determines the magnitude of the SH intensity according to [27] 1

I2`0 °C g(fO)gl/2(2C0 ) × IL*(2eo)"Z~s2)(eo;2co):L(~o)L(a~)le .

(1)

Here, e is the linear dielectric constant of the metal and the tensor L is a local field correction factor that takes into account the Fresnel factor and the beam polarization. An adsorbate/substrate bond like O/metal which localizes free electrons consequently reduces their polarizability and leads to a reduction of Z{sz) and 12,o. The decisive difference between the experiments of Heskett et al. and the present work is the choice of the laser wavelength, which was 532 nm in their case and 1064 nm in ours. The drastic influence of the wavelength on the S H G response is a clear indication of the presence of a resonance effect which can be excited at 2 = 1 0 6 4 n m (i.e. at a photon energy he)= 1.17 eV) but not at 2 = 532 nm (i.e. hco=2.34 eV). Urbach et al. [28] demonstrated the influence of the linear optical properties on the S H G output. Using a tunable laser they found for clean Ag(110) a pronounced SHG enhancement at he) = 1.95 eV, which could be attributed to a zero-point in e(2co) due to interband transitions at 3.8 eV (cf. Eq. (1)). However, e(co) and e(2eo) are smooth near hco= 1.17 eV; thus the observed SHG behavior should not be affected by this linear effect in the present work. On the other hand, ;~sz) itself will be influenced when transitions between electronic states are excited by the laser or the SH radiation fields. In the single-particle picture, the relationship between Z{s2) and the surface electronic band structure can be expressed as follows [-27]: Z!2k)(CO,2CO)~ ~' (air~lc)(clrslb)(blrkla ~ a,b,c

x (2heo--E~--ih3~ca) -1 × (ho9 - Eba -- ihyba)- 1,

(2)

where ri is the cartesian coordinate operator, and la), [b) and It) represent the initial, intermediate

287

S. Reiff, J..H. Block~Surface Science 345 (1996) 281-289

and upper states, respectively. The other symbols have their conventional meanings. The symmetry selection rules are contained in the matrix elements in the numerator. Obviously, Z(s2) will be resonantly enhanced if either he) or 2hco coincide with the energy of an optical transition between two singleparticle states. In the following, we will show that surface states with the appropriate energy separations are provided by the adsorption of oxygen on Ag(ll0). The electronic surface band structure of clean and oxygen-covered A g ( l l 0 ) was calculated by Tjeng et al. [ 19] using a tight-binding method and the results were found to agree well with the available angle-resolved photoemission (ARUPS and ARIPS) data. The result for the first surface Brillouin zone (SBZ) of the clean surface is sketched in Fig. 5. The hatched areas are a projection of the bulk density of states. Surface states within the two sp-like band gaps around Y and as well as surface resonances are indicated by thick lines. The lower empty surface state at ,X lies close to the b o t t o m of this gap at about 2 eV and would be difficult to distinguish from the projected bulk band. The higher state at X is approximately 5 eV above the Fermi energy and quite out of range for an excitation with he) or 2hoe used in our studies. Calc. Band Structure: ( l x l ) - A g ( 1 1 0 )

The two surface state bands around the Y-point, labelled by Me and Ma, are experimentally determined to lie 0.1 eV below and 1.65 eV above the Fermi level [29]. Urbach et al. [28] dearly demonstrated that an enhancement in the SH intensity at h e ) = 1.74 eV can be assigned to a transition between the occupied Me and the unoccupied M3 band. However, the photon energies he)= 1.17 eV and 2hco=2.34 eV used in the present work are definitely off-resonance with any possible transitions for clean A g ( l l 0 ) . This situation is considerably changed when oxygen adsorption and reconstruction comes into play. Fig. 6 displays the relevant sections of the SBZ for O/Ag(110) with Oo = 0.5, calculated under the assumption that the surface is (2 x 1)-buckledrow and (2 x 1)-added-row reconstructed, respectively [19]. Several features are evident: (1) ME and M 3 are shifted to higher energies with respect to the clean surface. This effect is most pronounced for the added-row structure where at ~r, the now empty M2 state is located 1.2 eV above E r. (2) Antibonding oxygen-derived 2p states show up within a bandwidth of 3 eV below and 2 eV above E F. Again, for the added-row reconstruction the oxygen band denoted as py lies higher in energy at (about 0.5 eV) than for the buckled-row case, where py coincides with the Fermi level. (3) The bulk sd band gap around S is projected onto

(2xl)-O/Ag(110)

(2xl)-O/Ag(110)

buckled-row reconstructed

added-row reconstructed

4

>

~iiii

0) LL

uJ

0

....

-2 " 1

2

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

irr]

g -4

::i ~i 'i%~N~,

-8

g

~,

7

£

~,

2

g

Fig. 5. Surface band structure of the clean Ag(ll0) surface [19]. Surface states and resonances are indicated by thick lines, and the bulk-projected DOS by the hatched area. The insert shows one quarter of the SBZ of (1 x 1)-Ag(ll0), F Y X, and of (2 x 1)-O/Ag(ll0), F ~ S' X', respectively.

c(

[001] =

-0 , --1

F

-1

?

[001] =

V

Fig. 6. Section of the surface band structure of (2xl)O/Ag(ll0) for two reconstruction models: buckled-row and added-row [191. Oxygen-derivedstates are labelled by Px and py. The laser photon energy is indicated by the length of the thick arrow.

288

S. Reiff, £H. Block~Surface Science 345 (1996)281-289

due to the reduction of the SBZ. The bulk band dispersed negatively around Y is situated 0.3 eV below E v. These results lead to the following conclusion: Optical transitions from this occupied bulk band to either M2 or py can be excited by photons with an energy he)-- 1.17 eV for the addedrow reconstructed surface if one takes into account the accuracy of the calculation: 0.2 eV for states below the Fermi level and 0.4 eV for states above the Fermi level [ 19]. Furthermore, the same arguments confirm that the formation of a buckledrow r e c o n s t r u c t i o n is very unlikely because of the considerable misfit between the energy spacings and he). It remains to show that the abovementioned transitions fulfill the symmetry selection rules. The transition matrix elements included in Eq. (2) can be treated in terms of the familiar photoexcitation selection rules. These assert that the photoexcitation matrix element ( ~ f l A . P l g J i ) must be totally symmetric for allowed g~i~ ~f transitions. The suggested initial state, i.e. the occupied bulk band at Y, stems from a projection of the Q_band that shows dispersion between the W - a n d L-point of the bulk Brillouin zone [30]. This Q_band is mainly y-like, as are both Mz and py, which are considered the final states. Using the coordinate convention in which x, y and z are taken along [110], [001] and the normal to the surface, respectively (cf. Fig. 5), and noting that the plane of incidence in our experiments is the x - z mirror plane, we conclude that ( g t f l A . P [ ~ i ) is antisymmetric for s-polarized light and therefore vanishes. On the other hand, the matrix element is totally symmetric for p-polarized light, thus making photoexcitation possible. This fits the experimental findings that the SHG enhancement was observed for p-polarized but not for s=polarized excitation. Consequently it is most likely that the SH intensity from added-row reconstructed (2 x 1)-O/Ag(ll0) is resonantly enhanced due to an excitation of optical transitions between bulk states and py-like surface states at Y by the p-polarized laser radiation at he)= 1.17 eV. Next, the question arises how the smooth S H G behavior for intermediate oxygen coverages can be explained. For the clean A g ( l l 0 ) surface a resonance effect can be excluded at the photon energies

used. The initial S H G decrease between O o = 0 and Oo = 0.15 is thus attributed to a charge transfer from metal ("nearly free" electron) states to the adsorbate atom. This is in good agreement with metastable deexcitation spectroscopy (MDS) data which reveal the depletion of sp metal states near the Fermi edge with a parallel growth of oxygeninduced states as the coverage increases [5]. For Oo>0.15, surface states like M2 and py appear at energies that are increasingly near-resonant with laser-induced transitions. Two scenarios are possible: either the surface states under consideration are shifted continuously to higher energy or, once formed, their energetic position is fixed and only the density of states increases. The latter case is supported by photoemission data for O / A g ( l l 0 ) [5], which show the occurrence of a coverageindependent O-2px state below EF, as well as by a SH spectroscopy study on O / C u ( l l 0 ) [31] where no energetic shift in the resonance peaks was observed upon 02 dosing. However, a nonreconstructed ( 7 x l ) ... ( 3 x l ) series is very unlikely if one considers the buckled-row reconstruction as being a relaxed rather than a reconstructed surface configuration: the corresponding band structures presumably resemble that obtained for the ( 2 x 1)-O buckled-row case (Fig. 6), i.e. without the energy separations necessary for resonance. Hence the continuous increase of the SH signal beyond Oo=0.15 indicates that the addedrow reconstruction model is valid for all (n x 1)-O structures starting at (7 x 1) around O o = 0.15. This is compatible with the idea that oxygen atoms within the added O-Ag rows see more or less identical surroundings and feel only a minor influence from the next O-Ag row, although a repulsive interaction among the rows must exist to create long-range ordered phases.

5. Summary Optical second-harmonic generation was utilized to study the adsorption of oxygen on Ag(ll0). The coverage was monitored by A ~ measurements and checked by L E E D observations. The main conclusion can be summarized as follows: The decrease in SH intensity upon small Oz

S. Reiff, J.H. Block~Surface Science 345 (1996) 281-289

doses was assigned to the ability of a d s o r b e d a t o m i c oxygen to extract electrons from the Ag substrate. F o r O o > 0 . 1 5 the SH signal increases a g a i n a n d at s a t u r a t i o n coverage reaches a n e a r l y tenfold e n h a n c e m e n t over the clean surface value. Q u o t i n g surface b a n d structure calculations, this effect was a t t r i b u t e d to a g r o w i n g r e s o n a n c e of electronic t r a n s i t i o n s with the laser r a d i a t i o n field at the Y - p o i n t of the surface Brillouin zone. T h e electronic b a n d s i n v o l v e d are m o s t likely a projected b u l k b a n d as the initial state a n d py-like Ag a n d oxygen-derived surface b a n d s as the final states. C o n s i s t e n t with o u r findings, each of the w e l l - k n o w n (n x 1)-O phases with n r a n g i n g from 7 to 2 is t h o u g h t to be of the a d d e d - r o w type. O u r a r g u m e n t s c o n c e r n i n g the responsibility of certain electronic states for the r e s o n a n c e p h e n o m e n o n are, of course, n o t a conclusive p r o o f of the u n d e r l y i n g m e c h a n i s m , because o u r i n v e s t i g a t i o n was restricted to a fixed laser wavelength. Thus, further S H G experiments e m p l o y i n g a t u n a b l e light source, k n o w n as s e c o n d - h a r m o n i c g e n e r a t i o n spectroscopy [ 2 1 ] , are needed to further identify the n a t u r e of the discussed electronic states. F o r example, S H G spectroscopy s h o u l d yield a steplike b e h a v i o r if surface-projected b u l k b a n d s are i n v o l v e d i n the transitions. I n addition, b a n d structure calculations of the i n t e r m e d i a t e (n x 1)O / A g ( 1 1 0 ) phases w o u l d be helpful in c o m b i n a t i o n with S H G spectroscopy, for c o n f i r m i n g the validity of the described model.

Acknowledgements We wish to t h a n k K. H o r n for helpful discussion a n d T. Rebitzki for p r o o f r e a d i n g the m a n u s c r i p t .

References [1] F. Besenbacher and J.K. Norskov, Prog. Surf. Scil 44 (1993) 5. [2,1 R.J. Behm, in: Physics and Chemistry of Alkali Metal Adsorption, Eds. H.P. Bonzel, A.M. Bradsliaw and G. Erfl (Elsevier, Amsterdam, 1989) p. 111. [3,1 T. Hashizume, M. Taniguchi, K. Motai, H. Lu, K. Tanaka and T. Sakurai, Surf. Sci. 266 (1992) 282.

289

[4,1 H.A. Engelhardt and D. Menzel, Surf. Sci. 57 (1976) 591. [5,1 M. Canepa, P. Cantini, L. Mattera, S. Terreni and F. Valdenazzi, Phys. Scr. T41 (1992) 226. [6,1 L. Yang, T.S. Rahman, G. Bracco and R. Tatarek, Phys. Rev. B 40 (1989) 12271. [7,1 L. Becker, S. Aminpirooz, A. Schmalz,B. Hillert, M. Pedio and J. Haase, Phys. Rev. B 44 (1991) 13655. [83 G. Bracco, R. Tatarek and G. Vandoni, Phys. Rev. B 42 (1990) 1852. [9,1 M. Bowker, M.A. Barteau and R.J. Madix, Surf. Sci. 92 (1980) 528. [10,1 C.T. Campbell, Surf. Sci. 157 (1985) 43. [11] M.A. Bartean and R.J. Madix, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1982) Vol. 4, p. 95. [123 C.T. Campbell and M.T. Paffet, Surf. Sci. 143 (1984) 517. [13-1 D. Heskett, K.J. Song, A. Burns, E.W. Plummet and H.L. Dai, J. Chem. Phys. 85 (1986) 7490; D. Heskett, L.E. Urbach, K.J. Song, E.W. Plummer and H.L. Dai, Surf. Sci. 197 (1988) 225. [14,1 W. Heiland, F. Iberl, E. Taglauer and D. Menzel, Surf. Sci. 53 (1975) 383. [15,1 G. Rovida and F. Pratesi, Surf. Sci. 52 (1975) 542. [16] C. Backx, C.P.M. de Groot, P. Biloen and W.M.H. Sachtler, Surf. Sci. 128 (.1983) 81. [17] R. Sporken, P.A. Thiry, J.L Pireaux, R. Caudano and A. Adnot, Surf. Sci. 160 (1985) 443. [183 C. Backx, C.P.M. de Groot and P. Biloen, Surf. Sci. 104 (1981) 300. [19] L.H. Tjeng, M.B.J. Meinders and G.A. Sawatzky, Surf. Sci. 236 (1990) 341. [20] G.L. Richmond, J.M. Robinson and V.L. Shannon, Prog. Surf. Sci. 28 (1988) 1. [21,1 A. Goldmann, 129th WE-Heraeus-Seminar, submitted. [22,1 S. Reiff, W. Drachsel and J.H. Block, Surf. Sci. 304 (1994) L420. [23,1 N. Bloembergen, R.K. Chang, S.S. Jha and C.H. Lee, Phys. Rev. 174 (1968) 813. [24] A. Liebsch, Phys. Rev. Lett. 61 (1988) 1233. [25] J. Rudnick and E.A. Stern, Phys. Rev. B 4 (1971) 4272. [26] J. Bloch, D.J. Bottomley, S. Janz and H.M. van Driel, Surf. Sci. 257 (1991) 328; J. Bloch, D.J. Bottomley, S. Janz, H.M. van Driel and R.S. Timsit, J. Chem. Phys. 98 (1993) 9167. [27,1 Y.R. Shen, in: The Principles of Nonlinear Optics (Wiley, New York, 1984). [28] L.E. Urbach, K.L. Percival, J.M. Hicks, E.W. Plummet and H.-L. Dai, Phys. Rev. B 45 (1992) 3769. [29] R.A. Bartynski and T. Gustafsson, Phys. Rev. B 33 (1986) 6588; B. Reihl, R.R. Schlittler and H. Neff, Phys. Rev. Lett. 52 (1984) 1826. [30] B. Segall, Phys. Rev. 125 (1962) 109. [31] J. Woll, G. Meister, U. Barjenbruch and A. Goldmann, Appl. Phys. A 60 (1995) 173.