K adsorption on Ag(110): effect on surface structure and surface electronic excitations

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1 Surface Science 424 (1999) adsorption on Ag(110): effect on surface structure and surface electronic excitations F. Moresco a, M. Rocca a,*, T. Hildebrandt b, V. Zielasek b, M. Henzler b a Centro di Fisica delle Superfici e delle Basse Temperature del C.N.R. and Istituto Nazionale per la Fisica della Materia, Dipartimento di Fisica, via Dodecaneso 33, Genova, Italy b Institut für Festkörperphysik, Universität Hannover, Appelstrasse 2, 30167Hannover, Germany Received 30 April 1998; accepted for publication 17 December 1998 Abstract The growth of adlayers on Ag(110) was studied by energy loss spectroscopy low energy electron diffraction ( ELS LEED) which allows the analysis of both elastically and inelastically scattered electrons. Ordered (1 n) missing row reconstructions and disordered adsorption were investigated. We find that extended surface reconstruction takes place already at 3% coverage generating a (1 3) missing row structure. Elastic spot profile analysis shows that a long range correlation between adatoms is present along the 11:0 direction on the reconstructed surface, implying the stabilization of the reconstruction also at long distances from the adatoms. With increasing coverage [(1 2) and (1 3) structures] the distance between the adatoms along 11:0 decreases and the adsorption sites become correlated also along 001 in contrast with the behaviour of on Ni, Cu and Pd. For disordered adsorption we observe that a weak correlation between the adatoms along both directions is already present in the background. The inelastic intensity shows only one marked loss at the surface plasmon frequency, indicating that in the investigated coverage range both -induced s and p levels are empty. Ag surface plasmon dispersion and damping are affected by the presence of in accord with the electronic mechanism generating the reconstruction. For the ordered (1 n) reconstructions the structural anisotropy is enhanced, while the anisotropy of Ag surface plasmon dispersion is reduced. adsorption strongly affects in particular the quadratic term of the dispersion along 001, which drops by 60% already at 3% coverage while the linear term remains initially unaffected independently of surface reconstruction. Above 30% coverage also the linear term decreases and the anisotropy present for the surface plasmon dispersion of the bare Ag(110) is removed Published by Elsevier Science B.V. All rights reserved. eywords: Ag(110); ELS LEED; HREELS; adsorption; SPA LEED; Surface plasmon dispersion 1. Introduction differences are present among the materials, as the alkali coverage necessary to induce the reconstruction Missing row reconstruction induced by alkali depends on the substrate and so not all the metal adsorption is known to occur on the (110) (1 n) structures are reported for all metals. For surfaces of Ag, Pd, Ni and Cu [1 8], and is Ag(110) and Cu(110) the series of (1 n) ordered therefore a general phenomenon for which a missing row structures starts with n=3 and corres- common origin was postulated [9]. However some ponds to the removal of every third 11:0 row. With increasing alkali deposition, a (1 2) reconstruction * Corresponding author. Fax: ; appears, implying the removal of every rocca@genova.infn.it. second substrate row. A second (1 3) structure /99/$ see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S ( 99 )

2 F. Moresco et al. / Surface Science 424 (1999) is present at still higher coverage, and corresponds need a critical coverage. At low coverage the Cu to two out of three top layer 11:0rows missing. surface is observed to be only locally reconstructed, For Pd(110) and Ni(110) only the (1 2) recon- with (1 2) areas concentrated in islands. At a struction forms. The alkali coverage, H, necessary coverage of about 0.2 ML an extended (1 2) to induce such reconstructions depends on the reconstruction forms, whereby the sit in the substrate. For /Ag( 110) the ( 1 3) structure is missing rows at an average distance of 2.5 Cu reported for a coverage, H, of 0.01<H < atoms. Such a study concludes therefore that the 0.05 monolayer (ML), while for H #0.1 ML the reconstruction mechanism is strictly local and (1 2) is observed [1]. For /Cu(110), on the claims that such a mechanism is also active for the other hand, the ( 1 3) structure appears only at other metals. However, such a local reconstruction H =0.13 ML and H =0.2 ML is necessary to model cannot explain the reconstruction on Ag, induce the ( 1 3) and the (1 2) reconstructions Pd and Ni, which takes place at too small an respectively [5 8]. The reconstruction process is amount of, implying a long range mechanism, known to be thermally activated [5], with all where single atoms affect the configuration of systems showing a critical temperature in the range the substrate beyond their nearest neighbors. between 150 and 250, depending also on the Recent X-ray diffraction experiments on the alkali atom coverage. (1 n) structures induced by Cs adsorption on The driving force and the mechanism of alkali Cu(110) [13] and theoretical calculations [14] have induced missing row reconstruction have been shown that the experimentally observed relaxation studied theoretically. Different models exist, all of pattern cannot be explained by just considering which claim to describe all fcc (110) transition the properties of the clean surface atoms, demonmetal surfaces [10 12]. According to Jacobsen and strating that the role of the adsorbed alkali atoms Norskov [10] the effect is local and the driving is more important than suspected so far. force results from an increase in adsorption energy We report here an energy loss spectroscopy which overcompensates the energy required for the low energy electron diffraction ( ELS LEED) reconstruction. A minimum coverage of ca 0.1 ML investigation of the surface geometric structure is required to reconstruct the whole surface. On and of the collective electronic excitations on the the contrary, according to Fu and Ho [12], the (1 n) missing row /Ag(110) reconstructed reconstruction mechanism operates through an electron donation effect, which produces an surface. increased surface electron concentration. They find In the first part of this paper we present a spot that the surface undergoes a missing row reconprofile analysis of the diffraction pattern of the struction as soon as a small amount (#0.05 (1 n) phases. It confirms that the Ag surface electrons per surface atoms) of excess charge is reconstructs completely already at a lower cover- transferred to the surface. age than it is the case for /Cu. The average In a recent scanning tunnelling microscopy distance between the adatoms is smaller than (STM) investigation of the /Cu(110) system expected for a random distribution, implying that [6 8] the nucleation of the reconstruction was they form islands leaving vast reconstructed Ag observed, showing that each single atom areas with no. Increasing the coverage the removes locally two or three substrate atoms out distance between the adatoms decreases monot- of a 11:0 row. The adatom could not be onously. For the (1 2) phases we find that the imaged, but it was supposed to be accommodated position of the atoms is correlated also between in the resulting hole, thus forming a nucleus for neighbouring rows. In the second part of the paper the missing row structure. The long range reconwhich we present a full analysis of the energy loss data struction proceeds then by the coalescence of such were partially reported elsewhere [15]. The nuclei, which interact attractively in the 11: 0 presence of strongly influences the Ag surface direction and repulsively along 001. Nucleation plasmon dispersion and damping independently starts at a small concentration and does not on surface reconstruction thus supporting a long

3 64 F. Moresco et al. / Surface Science 424 (1999) range reconstruction mechanism electronic in origin. 2. Experimental The experiment was performed in an ultra high vacuum ( mbar) on an Ag(110) single crystal prepared in the usual way [17] by ion sputtering and annealing until no traces of impurities could be detected by Auger electron spectroscopy (AES) and by ELS LEED. Potassium was Fig. 1. Ratio of the intensities of the transition at 252 ev and evaporated from a well outgassed commercial of the Ag transition at 352 ev versus exposure time, as obtained getter source (SAES getters) at room temperature by AES. to study the reconstructed phases and at T=100 to inhibit reconstruction. The surface structure and the energy loss spectra the metallic radius of is 2.32 Å [24], were investigated by ELS LEED, a spectrometer H =1 ML corresponds to a coverage of 0.64 which combines the electrostatic deflection unit of atoms per Ag( 110) unit cell, that is, to a spot profile analysis of the low energy electron atoms cm 2. diffraction (SPA LEED) system with beam monochromatization and analysis of high resolution electron energy loss spectroscopy ( HREELS). It 3. Surface structure may be considered either as a HREELS with high momentum resolution, or as a SPA LEED system 3.1. Data presentation with high energy resolution [18]. The transfer width of the instrument is 1500 Å, corresponding A series of (1 n) missing row structures have to a limit in momentum resolution of Å 1. been obtained evaporating on Ag(110) at room The experimental resolution is, however, generally temperature. Sample SPA LEED spectra recorded limited by the quality of the single crystal under along the 001 crystallographic direction at investigation. In the present experiment the different coverage and showing the (1 n) structures are reported in Fig. 2. For H =0.03 ML we momentum resolution was limited to 0.02 Å 1, as determined from the full width at half maximum observe a ( 1 3) reconstruction, while for ( FWHM) of the specular peak in momentum H >0.06 ML the (1 2) reconstruction was space by the quality of the sample. The energy formed. The high coverage (1 3) structure is resolution was tuned to 40 mev to improve the present at H =0.3 ML. The (1 3) and especially signal to noise ratio for the losses. The spectra the ( 1 2) reconstructions present sharp spots were recorded at a crystal temperature of 100 which are indicative of a long range order. and at an impact energy of 65.4 and 51.4 ev. The background of the SPA LEED spectra The evaluation of the coverage was obtained increases with adsorption, while the intensity of by AES from the ratio of the transition at the main spot decreases without changing its profile. 252 ev and of the Ag transition at 352 ev shown This behaviour indicates an increased statistic- in Fig. 1. The deposition was performed at room ally distributed disorder at the surface, connected temperature. The completion of the first monolayer to a random arrangement of point defects [19,20]. of adatoms (i.e. H =1 ML), has been deter- In Fig. 3 the SPA LEED spectra recorded mined by the position of the change in the slope of the dependence of the AES peak ratio versus exposure and occurs at I /I =0.70±0.05. Since Ag along 11:0 for the same set of measurements of Fig. 2 are reported, showing that additional features are observed in the diffuse elastic intensity.

4 F. Moresco et al. / Surface Science 424 (1999) Fig. 2. SPA LEED spectra recorded along 001 for different coverages measured at E =65.4 ev after evaporation at i room temperature. The dotted lines indicate the position of the first order spot in the reciprocal surface lattice along this direction. Fig. 3. SPA LEED spectra recorded along 11:0 for different coverages measured at E =65.4 ev after evaporation at i room temperature. The dotted lines indicate the position of the first order spot in the reciprocal surface lattice along this direction. They shift towards larger exchanged momenta with increasing coverage. This indicates that the position of adatoms changes with H, that is, that their distance diminishes with increasing coverage of. The correlation length between the atoms versus H is reported in Fig. 4 and decreases from 8.6 Å at H =0.003 ML to 4.5 Å at H =0.21 ML and to 4.2 Å at H =0.32 ML. The latter values are smaller than the bulk interatomic spacing but are in good agreement with previous results for /Cu( 110) [5]. Two-dimensional LEED patterns recorded for H =0.0 and 0.21 ML [corresponding to the (1 3) and the (1 2) structures respectively] are reported in Fig. 5. For the (1 3) surface, streaks are present at ca 1/3 of the surface Brillouin zone (SBZ) indicating that the site distribution is uncorrelated between the rows. At H =0.21 ML (Fig. 3b) Fig. 4. Correlation length between the adatoms versus H on the reconstructed surface, as deduced from the SPA LEED data.

5 66 F. Moresco et al. / Surface Science 424 (1999) SPA LEED spectra corresponding to the (1 2) reconstruction at H =0.10 ML are compared in Fig. 6 to the spectra obtained at nearly the same coverage (H =0.11 ML) after deposition at T= 100. As one can see in Fig. 6b, deposition at low temperature inhibits surface reconstruction. In fact, while in the room temperature case the (1 2) superstructures are clearly visible, only weak features are present in the low temperature deposition case, indicating that only a local rearrangement of the substrate takes place around the adatoms. Along 11:0 the additional features due to adsorption are still present, even if their intensity is strongly reduced, indicating a more disordered surface and a worse correlation between the adatoms. Fig. 5. Two-dimensional LEED pattern corresponding to a coverage of: (a) H =0.03 ML and (b) H =0.21 ML. The arrows show the position of the -induced additional features. the streaks break up into spots, showing that the adatom positions become correlated also between the Ag rows. -induced peaks are placed not only in the ( 10) SBZ along C9 Y9 (shown in Fig. 3), but also in the (0, 1/2) SBZ at 1/6 in the same direction. We note therefore that in the case of /Ag(110) a strong correlation between atoms is present Fig. 6. SPA LEED spectra for the reconstructed (1 2) surface in both azimuthal directions, at variance with the at H =0.10 ML and for the unreconstructed surface at case of /Cu [5]. H =0.11 ML along (a) 11:0 and (b) 001.

6 F. Moresco et al. / Surface Science 424 (1999) Discussion of Fig. 5 it is, however, not possible to derive an unique structural model. From the analysis of the SPA LEED data we It is important to note that the measured obtain informations about the position of ada- coverage implies that the single adatom is able toms on the Ag(110) surface. A possible structural to remove Ag atoms also far away from it, so that model is presented in Fig. 7. In Fig. 7a the struc- reconstructed Ag areas without any atom inside ture of the overlayer for the (1 3) phase at must be present. Such behaviour is at variance H =0.03 ML is shown: sits along the missing with the apparently very similar case of /Cu(110) rows every third Ag atom, with no correlation which also presents additional streaks in the recip- between adatoms in different rows. In Fig. 7b we rocal space for the (1 2) phase. For /Cu(110) show the model of the overlayer on the (1 2) the presence of a atom every 2.5 Ag was surface at H =0.21 ML: the distance between two postulated on the whole reconstructed (1 2) surface from the analysis of the STM images [6 8], atoms along the missing rows corresponds then to 1.5 times the Ag lattice parameter. adatoms a conclusion which cannot be extended to the are also correlated along 001, as additional /Ag( 110) case. The induced surface recon- LEED spots are present. From the LEED pattern struction mechanism is therefore different for the two cases. 4. Surface electronic excitations 4.1. Surface plasmon dispersion and damping Fig. 7. Atomistic model of the reconstructed covered Ag(110) surface for (a) H =0.03 ML [(1 3) reconstruction] and for (b) H =0.21 ML [(1 2) reconstruction]. The open circles show the position of the atoms, the gray and black ones the Ag atom positions in the missing row structure. The investigation of surface electronic excitations for /Ag( 110) measurements by ELS LEED were reported separately in two different papers dealing with the effect of on the form of the dispersion of the surface plasmons in the limit of low H [15] and with the excitation spectrum for thick overlayers at low crystal temperature where no reconstruction takes place [16]. Here we will concentrate on the intermediate coverage range in which removes the anisotropy of Ag(110). Sample spectra, recorded for the reconstructed surfaces corresponding to H =006 and 0.32 ML, deposited at room temperature, are reported in Fig. 8 along the two high symmetry crystallographic directions 001 and 11:0. Only one sharp peak is present in the energy loss spectra in this coverage range and corresponds to the excitation of the Ag surface plasmon as, for these scattering conditions, the contribution of the Ag multipole mode is negligible [21]. The surface plasmon energy Bv can thus be inferred from the sp position of the maximum of the loss peak, after the subtraction of an inelastic background due to electron-hole pair excitations. The transferred momentum parallel to the surface, q is then

7 68 F. Moresco et al. / Surface Science 424 (1999) oration at room temperature. Other data were collected after adsorption at 100 for H =0.11 and Surface plasmon energy is reported in Fig. 9 versus q. The data corresponds to the low coverage (1 3), to the (1 2) and to the high coverage (1 3) reconstructed surfaces at different H, along the two high symmetry directions. Continuous lines show the surface plasmon dispersion is little affected by adsorption along 11:0, while it decreases along 001 at large q. The change becomes stronger and the dispersion isotropic at H =0.32 ML, that is, as soon as the (1 3) phase forms. The anisotropy of the Ag(110) surface plasmon dispersion is therefore reduced Fig. 8. Energy loss spectra for the Ag(110) (1 2) and (1 3)II reconstructed surface, corresponding to (a) H =0.06 ML and (b) H =0.32 ML, recorded at E i =65.4 ev along 11:0 and 001. calculated from E, Bv and from the scattering i sp geometry, applying energy and momentum conservation [17]. The dispersion at H =0.06 ML (Fig. 8a) is still anisotropic as was the case for the bare Ag( 110) surface [22,23], but it becomes Fig. 9. Surface plasmon dispersion with q for different cover- ages, on the reconstructed surfaces along (a) 11:0 and (b) nearly isotropic at H =0.32 ML (Fig. 8b). 001, recorded at E =65.4 ev. The continuous lines show the Further measurements were recorded for i dispersion on the Ag(110) clean surface in the two azimuthal H =0.03, 0.08, 0.10, 0.14 and 0.32 ML after evap- directions, from Ref. [24].

8 F. Moresco et al. / Surface Science 424 (1999) with respect to the bare surface case, for the (1 3) and (1 2) reconstructions and it disappears for the high coverage (1 3) structure. The experimental data were fitted with the parabolic form: Bv (q )=Bv (0)+Aq +Bq2 (1) sp d sp d d the best fit parameters and uncertainties were determined by the x2 method, using the MINUIT computing routine, where the uncertainty on q was reported on the dependent variable. The results are collected in Table 1 and Table 2 for all data corresponding to room temperature depos- Fig. 10. Bv (0) versus H for the reconstructed surfaces sp ition of the film for measurements along 001 corresponding to room temperature deposition (%) and to low and 11:0, respectively. The larger errors on the temperature disordered adsorption (#). coefficients determined at H =0.10 and 0.14 ML is due to the smaller data set. A comparison with significantly influence the dielectric response at the coefficients determined for the clean Ag(110) the surface. This result is at variance with the surface is also given. Let us analyse in detail the case of low temperature deposition, where a effect of coverage on the different terms: strong increase of the surface plasmon energy is The surface plasmon energy at vanishing q is present, possibly connected to the formation of a reported versus H in Fig. 10 for both high and Ag interface mode, as discussed at length low temperature deposition. As one can see elsewhere [16]. Bv (0)(0) is weakly reduced by adsorption sp on the reconstructed surface, showing that the presence of a little amount of ordered does not The linear term of surface plasmon dispersion increases slightly at low H for q along 001 and decreases eventually for H >0.14 ML. For Table 1 Surface plasmon dispersion coefficients derived by x2-analysis from Eq. (1) for deposition at room temperature along 001 H (ML) Reconstruction Bv sp (0) (ev) A (ev Å) B (ev Å2) 0 (1 1) 3.700± ± ± (1 3) 3.700± ± ± (1 2) 3.699± ± ± (1 2) 3.698± ± ± (1 2) 3.698± ± ± (1 2) 3.698± ± ± (1 3) 3.693± ± ±0.4 Table 2 Surface plasmon dispersion coefficients derived by x2-analysis from Eq. (1) for deposition at room temperature along 11:0 H (ML) Reconstruction Bv sp (0) (ev) A (ev Å) B (ev Å2) 0 (1 1) 3.702± ± ± (1 2) 3.702± ± ± (1 2) 3.700± ± ± (1 2) 3.703± ± ± (1 2) 3.701± ± ± (1 3) 3.692± ± ±0.7

9 70 F. Moresco et al. / Surface Science 424 (1999) H >0.32 ML it becomes identical to the value At even larger values of H (see the case of observed along 11: ML in Table 3, measured, however, for a film The quadratic term along 001 is reduced by deposited at 100 ), we observe that the quadratic nearly a factor of 2 with respect to the clean term of the dispersion recovers the same value as surface value already at H =0.03 ML and reported for the bare surface case, while the linear decreases then more slowly to 1 ev Å2. Along term is three times smaller and coincides, within 11: 0 it decreases on the contrary almost linearly experimental error, with the value observed on the with increasing coverage. At H =0.32 ML this reconstructed surface at H =0.32 ML. At large term also becomes nearly isotropic. H the linear dispersion term is thus strongly A comparison between surface plasmon disper- decreased. A similar effect was reported by im sion on the unreconstructed (1 1) and on the et al. [25] for Cl/Ag( 111) and ascribed to the reconstructed (1 2) surfaces is reported in reduced sp d polarization for an adsorbate cov- Fig. 11, while the dispersion coefficients are shown ered surface. in Table 3. As one can see the reconstruction only The FWHM of the surface plasmon versus q has a minor effect on the dispersion coefficients, is reported in Fig. 12. The data, deconvoluted with the differences being within experimental error. We respect to energy and angular resolution as demonstrate therefore that the geometrical struc- described in Ref. [17], indicate that: ture does not affect the surface plasmon dispersion The presence of has little effect on surface significantly as the most corrugated case is also plasmon damping for q =0. the least anisotropic. Along 11: 0 the FWHM increases initially weakly with q, while it grows eventually more rapidly above a critical q (q =0.11 Å 1), as C was the case for the clean surface. The additional damping mechanism above q was ascribed to C the opening of a decay channel connected to the excitation of interband transitions involving surface states [17]. On increasing H the damping rate increases both below and above q. C Notably the value of q does not change with C adsorption. A linear fit of the data, for q <q and for q <q, is reported in Table 4. C For H =0.10, 0.14 and 0.32 ML only the data above q the set of data was too small to allow C for such a fine analysis. Fig. 11. Surface plasmon dispersion for q for the reconstructed Along 001 the damping is generally stronger (1 2) and the unreconstructed surface, at H =0.10 and than for the clean surface case. For H = ML, respectively, along 001 and 11:0. The continuous and 0.06 ML the presence of the additional and dashed lines show the dispersion on the Ag(110) bare surdamping mechanisms can still be recognized. face in the two azimuthal directions, as obtained from Refs. [22,23]. The critical q value appear however to be Table 3 Surface plasmon dispersion coefficients derived by x2-analysis from Eq. (1) for deposition at T=100 H (ML) Azimuth Bv sp (0) (ev ) A (ev Å) B (ev Å2) ± ± ± ± ± ± ± ± ±0.8

10 F. Moresco et al. / Surface Science 424 (1999) FWHM versus q with linear forms, above and below q C are collected in Table 5. As one can see the damping rate increases slightly with increasing coverage Discussion Fig. 12. Dispersion with q of the FWHM of the surface plasmon for the same sets of data reported in Fig. 8, along (a) 11:0 and (b) 001. The meaning of quadratic and linear terms in Ag surface plasmon dispersion has been long debated. Within a jellium theory, the first is ascribed to bulk properties and the latter to surface plasmon dispersion with adsorption especially along 001 indicates, however, that also the first is at least partially determined by the surface. As discussed in Ref. [15] the presence of an interband transition between the intrinsic surface state present on Ag(110) at and the empty surface state induced by adsorption, which nearly matches the surface plasmon energy, is responsible for the decrease of the quadratic term. In accord with this explanation, at H >0.5 ML the quadratic term of the dispersion rises again reaching the bare surface value, in accord with the downwards shift of induced surface state with H Ref. [24], which causes a worse matching between the interband transition and the surface plasmon energy at high H. The linear term of the dispersion is little affected at low H and remains thus anisotropic with respect to the azimuthal direction. At high coverage it diminishes and becomes nearly isotropic, in spite of the enhanced geometric anisot- ropy of the reconstructed surface. This effect indicates a strong variation in the electronic structure at the surface, also confirmed by the q and shifted to lower q [q =0.006 Å 1 versus C q =0.11 Å 1 for the clean Ag(110) surface]. C At still larger H, the damping is increased already at small q. The result of a fit of the Table 4 Surface plasmon damping coefficients versus q derived by x2 analysis for deposition at T=100, along 11:0: DBv sp (0) and B, which describes the linear dependence of the FWHM on q H (ML) Reconstruction DBv sp (0) (ev ) B (ev Å) 0.03 (1 3) q <0.11 Å ± ± (1 3) q >0.11 Å ± (1 2) q <0.11 Å ± ± (1 2) q >0.11 Å ± (1 2) q <0.11 Å ± ± (1 2) q <0.11 Å ± ± (1 3) q <0.11 Å ± ±0.05

11 72 F. Moresco et al. / Surface Science 424 (1999) Table 5 Surface plasmon damping coefficients versus q derived by x2 analysis for deposition at T=100, along 001: DBv sp (0) and B, which describes the linear dependence of the FWHM on q H (ML) Reconstruction DBv sp (0) (ev ) B (ev Å) 0.03 (1 3) q <0.05 Å ± ± (1 3) q >0.05 Å ± (1 2) q <0.05 Å ± ± (1 2) q >0.05 Å ± (1 2) q <0.05 Å ± ± (1 2) q >0.05 Å ± (1 2) q <0.05 Å ± ± (1 2) q >0.05 Å ± (1 2) q <0.05 Å ± ± (1 2) q >0.05 Å ± (1 3) q <0.05 Å ± ± (1 3) q >0.05 Å ±0.1 H dependence of surface plasmon damping and is possibly connected to the formation of a surface alloy. 5. Conclusions Acknowledgements This work was partially financed by the Italian Ministry of University and Research under contract In this work we have presented a combined References SPA LEED and HREELS study of the influence of adsorption on Ag(110) surface structure and [1] B.E. Hayden,.H. Prince, P.J. Davie, G. Paolucci, A.M. surface electronic excitations, for coverage in Bradshaw, Solid State Commun. 48 (1983) 325. the range between 0.03 and 0.5 ML. We have [2] J.W.M. Frenken, R.L. rans, J.F. van der Veen, E. Holub- shown that surface reconstruction takes place at a Frappe,. Horn, Phys. Rev. Lett. 59 (1987) [3] C.J. Barnes, M.Q. Ding, M. Litroos, R.D. Diehl, D. ing, very low coverage along 001, while along the Surf. Sci. 162 (1985) 59. unreconstructed 11: 0 direction -induced fea- [4] R.J. Behm, D.. Flynn,.D. Jamison, G. Ertl, P.A. Thiel, tures are present, indicating a correlation between Phys. Rev. B 36 (1987) adatoms. At larger H this correlation is [5] W.C. Fan, A. Ignatiev, Phys. Rev. B 38 (1988) 366. [6] R. Schuster, J.V. Barth, G. Ertl, R.J. Behm, Surf. Sci. 247 stronger and is present along both azimuthal direc- (1991) L229. tions, underlying the very active role played by [7] R. Schuster, J.V. Barth, G. Ertl, R.J. Behm, Phys. Rev. B adatoms in surface reconstruction. 44 (1991) The electronic surface structure is heavily [8] R. Schuster, J.V. Barth, R.J. Behm, G. Ertl, Phys. Rev. affected by adsorption, as evidenced by the Lett. 69 (1992) surface electronic excitation spectrum which shows [9] R.J. Behm, in: H.P. Bonzel, A.M. Bradshaw, G. Ertl (Eds.), Physics and Chemistry of Alkali Metal Adsorption, an increased damping of the surface plasmon Elsevier, Amsterdam, 1989, p associated to the new interband transitions con- [10].W. Jacobsen, J.. Norskov, Phys. Rev. Lett. 60 nected to the -induced states and the removal of (1988) the anisotropy of surface plasmon dispersion. The [11] S.M. Foiles, Surf. Sci. 191 (1987) L779. long range mechanism driving the reconstruction [12] C.L. Fu,.M. Ho, Phys. Rev. Lett. 63 (1989) [13] R. Schuster, I.. Robinson, Phys. Rev. Lett. 76 (1996) must therefore by of electronic origin for /Ag(110) in contrast to the local driving force [14] A.C. Egsgaard Madsen, P. Stoltze,.W. Jacobsen, L.. which is apparently active for /Cu( 110). Norskov, Phys. Rev. Lett. 78 (1997) 158C.

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