Be(0001)

Surface Science 269/270 (1992) 653-658 North-Holland

surface science

LiK spectra of Li/Be(0001) P . A . Briihwiler ~, G . M . W a t s o n 2 a n d E . W . P l u m m e r Department of Physws and Laboratory for Research on the Structure of Matter, Unu ersay of Pennsyh area, Phdadelphta, PA 19104-6396, USA Recewed 11 September 1991; accepted for pubhcatlon 13 December 1991

We present L~ls core photoemJsslon and absorption spectra as a functton of overlayer density for L~/Be(0001). The combination of these spectroseopies allows us to clearly tdentffy atom~c-hke and substrate-derwed features m the absorption spectra, whsch reflect the unoccupied levels of the core-excited final state Both types of spectra also reflect the change tn the electromc screening associated with the formation of an occupied overlayer band m the mitml state. These measurements thus clearly indicate sigmflcant influence of the chemical environment on core excitation dynamics of metal adlayers on metal substrates

T h e electronic structure of alkali metals adsorbed on metallic substrates is a classic problem, the theoretical study of which began in earnest with the work of Gurney [1]. After extensive effort in the mtervening sex decades much remains to be understood about these systems, even for the simplest case of an sp-bonded substrate such as A1 [2]. In addition, alkah overlayers offer the hope of gaining a foothold on the interesting problem of system dimensionality and electronic properties of a metal, which is given greater importance in light of recent efforts toward understanding the properties of clusters, multilayers, and new materials. With these motivations we have undertaken a broad study of Li/Be(0001) overlayers in the coverage range between zero and the saturated monolayer. Our previous work includes angle-resolved photoemissaon and m-

Previous core level studies of alkali overlayers on sp-metal substrates have concentrated on absorption spectra of the outer p levels of the heawer alkah metals K, Rb, and Cs [3,4]. Systemattc effects were identified, but only a hmited and essentially qualitative understanding of those effects could be obtained It can also be argued that the core lcvcl excitations studied in ref. [3] are likely to be coupled to substrate excitations at similar energies [5], masking some or all of the adsorbate response. The choice of Li for the present study sidesteps some of these complications, since the ls threshold near 55 eV lies well away from substrate excitations. In addition, we have carried out an extenswe set of LEED, photoem~ss~on, inverse photoemission, and electron-energy loss studies of this system as a function of coverage, enabling

stu&es. In this paper we present a combined core level photoem~ssion and photoabsorption study of the Li ls level.

wath the ground state and valence excitation properties. The use of the core photocm~ss~on data allows us to pinpoint the threshold of the absorption spectra and provides a dared measure of the broadening inherent an those data, which has enabled a clearer differentiation between the core- and valence-related aspects of the final state.

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i Present address Uppsala Umverslty, Department of Physics. Box 530, S-751 21 Uppsala, Sweden 2 Present address' Physics Department, Brookhaven National Laboratory, Upton, NY 11973, USA

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P A Bruhwtler et al. / Lt K spectra of Ll / Be(O001)

In particular, we find that there is little intensity at threshold in absorption for low coverages, but instead a peak located some 0.5 eV above threshold, whose lineshape is determined to a good approximation by that of the photoemission line, indicating a small intrinsic width for the one-electron final state. It is possible to explain this state as the unoccupied level of a Be a d a t o m which has been displaced from the surface, consistent with the Z + 1 model for core-excited metals [6]. There is also a feature in the absorption spectra which is due to the substrate density of states. These results point to the importance of final state effects of core transitions of metal adsorbates on metal surfaces. Qualitative and quantitative changes occur in absorption and core photoemission spectra as the Li overlayer develops a new occupied metallic band, indicating the importance of wavefunction overlap betnveen the screening charge and the excited-atom levels. Thus the initial state is also apparent vi,~ the available screening excitations. The data were taken at the U12B T G M [7] at the NSLS. The total yield spectra were obtained by monitoring the current out of the grounded sample as a function of p h o t o n energy. Simultaneous partial yield spectra, and associated core photoemlss~on spectra, were obtained with an angle-resolved analyzer [8] with angular resolution of _+2 °. All data presented here were taken with p-polarized light at 45 ° incidence and normal electron exit angles. The Li films were prepared by evaporation from SAES Getters sources after careful outgassing. Because the calibration f the monochromator can change, we were pardcularly careful to recalibrate during the run, using both second order light and the position of E F in the valence photoemission spectra. This cahbration was found to be constant to within 0.1 eV at the Li K edge over periods of several weeks. The photon energy resolution for the monochromator settings used was 0.3 eV on average in the neighborhood of the L! absorption threshold, much smaller than the intrinsic broadening in the spectra. Fig. 1 contains the raw L~ ls core photoemission spectra taken at hu = 100 eV as a function of L1 coverage 0, defined m terms of the Be(0001)

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1::: - EF (eV) Fig l. LI Is X P S data of Lt/Be(0001) as a function of coverage, at h~, = 100 eV. The spectra have b e e n shifted and m some cases scaled to show the evolution of the hneshape with coverage

surface atomic number density. The saturated monola~el coverage [9] occurs at 0--0.48 [10,11], which corresponds to a LI density of 1.06 × 10 L5 em -2. We have calibrated the coverages for the present study using the work-function variation and the characteristic h u = 40 eV valence photoemission spectrum at 0 = 0.48 [10]. The lowest coverage studied has a Li ls energy distribution with a single asymmetric and very broad line, located at about -57.1 eV relative to E v. As the Li coverage increases, the asymmetry of this line becomes more pronoanced, and at 0 = 0.21 a second line at lower binding energy is clearly apparent. With this coverage change the more highly b o u n d compone~3t shifts in a roughly linear fashion to lower binding energy, following a pattern noted for electropositive metals on many metal substrates [12]. At higher coverages, most noticeably for cover ges near the work-function m l m m u m (9 = 0.25) we find that the a~ymmetry to lower binding energy, visible m the low-cover-

P A Bri~hwder et al / L~ K spectra of Lt / Be(O001)

age Li core photoemission lines has developed into a separate peak. That this feature is absent at the lowest coverages, and grows strong with the formation of the more condensed Li overlayer found at 0 = 0.33 [11] indicates that a close L i - L i proximity is required for its existence. The peak which we associate with relatively isolated Li atoms has fully m e r g e d into the asymmetric tail of the lower-binding-energy peak at 0 = 0.33, the highest coverage for which a (V~- x vf3-)R30° L E E D pattern is observed [11]. We will refer to this as the V~- overiayer. In fig. 2. we present corresponding yield spectra. A background due to second c~rder light-induced substrate features was removed using a procedure described in detail elsewhere [13], or by subtracting data from the clean substrate taken

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Photon Energy (eV) Fig _ L1 Is XAS data determined from total y~eld measurements We have removed one background due to Be absorption of second order light using the procedure described m ref [13] or by simply using the second order spectrum for clean Be(0001), and have also removed a hnear background using the slope of the data below threshold. The data are otherwise unprocessed Also shown is the bulk u ansm~ss~on inelastic electron-energy loss spectrum from ref [15], with a linear background removed

in the same spectral range. We have also subtracted a linear background from each spectrum. These spectra span the coverage range up ta just beyond saturation. For the lower coverages two peaks which shift as a function of coverage are observed near the threshold, which is also shifting with 0 as seen in these and the core photoemission data. Similar features with poorer statistics but empirically better resolution were measured wa constant initml state resonant photoemission simultaneously with the spectra shown in fig. 2, and as briefly discussed in ref [13] and shown m ref. [14] the spectral features m those data are undistort=d by the second order hgh~ Such a spectrum with the imtial state at E v - 3 eV ~s gwen in fig. 3 with the correspondmg total y~eld data, as a verification of our method of reducing total y~eld data with regard to the energies and general lineshape found in the spectra. We now turn to a comparison of the data in figs. 1 and 2.

P A Bruhwder er al / Lz K spectra of Lt,/ Be(O001)

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Fig. 4 Summary of the peak positrons found in the XPS and XAS data, with hnear fits to the data over the coverage ranges where peaks were observed The filled c~rcles indicate the posttlons of the low coverage XPS peaks, the empty circles the h~gher coverage XPS peaks, the filled dtamonds the XAS peak near threshotd, and the empty dmmonds the peak located 2 eV above threshold at low coverages The hnes are provided merely to suggest the correlat,ons mscussed m the text The ' e~tJcal hne at l lu s evaporation time indicates the coverage for whtch the work function was at its mtmmum

The hnear shift with 0 ot primary features toward lower energies in both core photoemission and absorption of the Li overlayers suggests a direct relationship. Fig. 4 summarizes the positions of the various features as a function of 0. There is a good correlation between the first Li ls absorption peak (solid dmmonds in fig. 4) and the maximum of the Li core line (~,,lid circles) in each case until 0 = 0.22, and even better correlation between the core line and the second absorption peak (empty diamonds). At higher coverages the lower-binding-energy hne begins to dominate the profile. In thts coverage ~ange the system approaches the ~ coverage, at which an ordered overlayer and occupied Ll-mduced band first become apparent ,'n L E E D and inverse photoemission, respectively. Inverse photoemission data indicate a d',scontmuous variation m the overlayer charge density approaching th~s cover-

age from below [11]. The peak near thr~ ~,old becomes weakest in this coverage range, and then appears to strengthen again as saturation is approached. A general conclusion at this point is that the spectrum bears little resemblance to the absorption edge of bulk Li [15], 'which we ascribe to chemical effects in the final state. The absorption spectral peaking resembles that of molecular adsorbates [16]. If we make use of the Z + 1 model adopted b~ Bj6rneholm et al. [16] to discuss screening effects for those adsorbates, we can explain the feature near threshold in a very simple, qualitative manner in the following way: At the lowest coverages, the adsorbate atoms are physically isolated from one another on the scale of Li bond lengths. Because the core-excited Li is metallically screened it will strongly resemble neutral Be in the valence region [6], and within the B o r n - O p p e n h e i m e r approximation the spectrum should reflect the unoccupied levels of a single adsorbed Be atom in the adsorption geometry of the ground state Li atom. l.Jpon examination of the unoccupied theoretical densxty-of-states (DOS) of Be [17], one finds that there is a strong sp~ke at E v+2 eV which is verified in the inverse photoem]sslon spectra ot clean Be(0001) [11], and corresponds primarily to p-hkc states. This plhng up of states and the minimum in the total DOS around E~ are duc to an electronic repulsion stemming from the substrate band,'tructure, i.e., non-free-electron-like aspects of Be [17]. It is then to be expected that the -,utward dis;~lacement of a neutral Be atom by 30% of its normal spacing would cause a downward shift of its local electronic DOS in the energy range considered here, in good accord wtth our observations. Simiia.r cons:'deratIons went into the adsorbate vibrational ai~alvsis of Nilsson and Mhrtensson [6]. As noted, for 0 ~ 0.25 there is a broadening of th,s peak, and a similar peaking recurs for higher coverages The nearthreshold absorption peak at low coverage is associated with adatoms isolated on the scale of Li bond lengths, and that at high coverage with core-excited Li in a metallic Li environment. Our stmple arguments could be extended to explain the energy of the absorption peak retat]ve to threshold In terms of a more completely screened

P A Bruhwder et al / Lt K spectra of Lt / Be(O091)

Be valence density of states. This suggests that the lineshape found in core absorption of bulk Li could be understood as the terminus of a continuum of possible lineshapes; however, the nature of the other terminus remains obscure, since the switching on of metallic screening cannot be elucidated using the present system, where a reservoir of electrons is an important precondition. The other strong feature near threshold in the absorption spectra is the peak located at 2.0 + 0.2 eV above the threshold peak. As indicated in fig. 4 this peak follows the threshold given by core photoemission until it broadens and vanishes in the coverage range associated with the development of the overlayer occupied band [11]. The coverage independence of the energy of this feature relative to threshold indicates both that it is associated with the high-binding-energy core level, i.e., with relatively isolated adatoms, and that it is derived from the bulk electronic structure. We associate it with the large DOS peak centered [11,17] at about E v + 2 eV. Thus substrate states have significant weight in the Li core region. The disappearance of the second absorption feature at 0 = 0.33 ~s indicative of a significant reduction in the overlayer-substrate interpenetration found theoretically at the lowest coverages [18], and is well-correlated with our other measurements showing directly the formation of a new electronic band in this coverage range [11]. This is a manifestation of an "initial state effect", since the final state screening is sensitive to the charge distribution near the core-excited Li atom. We take up this issue in greater detail elsewhere [14]. It has been noted that the linear shift with 0 of the core binding energies implies interactions which scale as the square of the inverse of some characteristic length [12]. This long range screening which we discussed above is ascribed in ref. [12] to adsorbate "nteractions via subs~rate bands. Thas as reasonable, because direct screening of a d a t e ~ ! e core excitations by other adatoms at low coverage would proceed via a matrix element-dependent process [19], whose magnitude could be expected to vary inversely with the core binding energy, a trend that is not observed. If we invoke the substrate-inde0endence [20] of the core level shaft proposed h" ref. [12] and limit our

657

considerations to the adsorbaee locations the adsorbate-adsorbate screemng length must bc rather large, because at 0 = 0.03 the adsorbatc~ are on the order of 10 ~, apart, significantly larger than characteristic bond lengths or the Be(000l) surface unit cell. Other excitations vary linearly with 0 for this system for 0 < 0.23, ineluding the work function [11], primaB, inverse photoemission feature [11], and an electron energy loss transition [21]. Because all of these properties are measured as excitations, and recent theory [18] has indicated that final state effects dominate such data, there is no a priort support ]'or seeking an initial state descripnon o f the binding energy shift. The line near threshold in absorption is a strong reason to avoid doing so m its own right, since that cannot be associated with any ground state DOS, even including self-energy effects. There is also strong theoretical [18] and experimental [11] evidence that the valence excitations are strongly influenced by long-range and many-body aspects of the surface potential, lending support to the idea that the substrate bands do not play a direct role in the linear shift of the exotation energies with coverage. Fhat the lower-binding-energy core line shifts m the same direction and with nearly the same slope as the higher-binding-energy line suggests that both long and short range interactions should be co,ls~dered [22]. A picture which encompasses the experimental facts of a long-range adsorbate-adsorbate screening interaction and the substrate-independence of this screening (for metal substrates) is that of the alkali as a surface Friedel impurity. This picture is evident, e.g., in the surface charge distributions calculationed by Ishlda for Na/Al-jellium [23], where one can se". that the p~esence of other adatoms brings a~-out a charge distribution which has a more three-dimens~onai character, from the point of vacw of a gwen adatom tn other words, ,~ gradual charge density increase occurs at ,~ g~,cn adatom site as the coverage increases, due to the metallic screening of the other adatom core potentials. This proposed mechanism, though related to that of Stenborg et al. [12], depends only on the assumption of metallic screening, and n~t on the details of the substrate bandstructure

658

P.A Bruhwder et al / Lt K spectra of Lt / Be(O001)

To summarize, we have found that the core photoemission and absorption spectra of Li/Be(0001) as a function of coverage lead to new possibilities for understanding the core excitations of alkali overlayers. The combined use of these spectroscopies is surprisingly new in this field, and has shown that atomic-like and substrate-derived features are simultaneously present in the spectra, as might be expected from a general point of viev. The deviation between these data and those obtained from bulk Li reflect the importance at the chemical environment for the excitation spectra, in agreement with long-established trends [3,4] and in support of recent work [12]. Overall, the data here are consistent with the existence of a low- and manor ,yer-coverage regime, as are other data for this [10,11,21] and other [2] systems. We have therefore established a gratifyingly consistent pattern among all of the excitation spectra measured for Li/Be(0001), and expect that similar correlations should exist for related cases. The data point to the presence of final state effects which could have a bearing on the classic problem of alkali adsorption on metals, and to mdtrect initial state effects via the charge distribution We would like to acknowledge stimulating conversations with D. Heskett, N. M~rtensson, J.N. Andersen, and A. Nilsson, and we thank O. Bj6rneholm for making avaiiable unpublished results and C. Tarrio and S.E. Schnatterly for communicating the bulk Li absorption spectrum. This work was performed at the National Synchrotron Light Source, which is supported by the US Department of Energy, Division of Materials and Chemical Sciences, u n d e r contract No. DEAC02-76CH00016. The Laboratory for Research on the Structure of Matter ~s supported by the US National Science Foundation under grant No. DMR88-19885.

References [1] R W Gurney, Phys Rex 47 (I935) 47~ [2] T Aruga and Y Murata, Prog Surf Scl 31 (1989) 61 [3] D G~bbs, T - H Chiu, J E Cunnmgham and C P Flynn, Ph~s Re'~ B 32 (198'5) 602

[4] C.P Flynn, Surf. Scl. t58 (1985) 84 [5] W Eberhardt and A Zangwlll, Phys. Rev B 27 (1983) 5960 [6] B. Johansson and N. M~rtensson, Phys. Rev B 21 (1980) 4427; A. Nilsson and N. M~rtensson, Phys. Rev Lett. 63 (1989) 1483. [7] B.P. Tanner, Nucl. lqstrum. Methods 172 (1980) 63. [8] C.L Allyn, T. Gustafsson and E.W. Plummer, Rev Sci. Instrum. 49 (1978) 1197 [9] Saturation here Tmphes only that the monolayer is complete, i.e., further deposition results m formation of a second layer [10] G M. Watson, P.A Briahwller, E.W. Plummer, H.-J. Sagner and K.-H. Frank, Phys. Rev. Lett. 65 (1990) 468. [11] P A. Briihwder, G.M. Watson, E.W. Plummer, H.-J. Sagner and K.-H. Frank, Europhys. Lett 11 (1990)573, and to be pubhshed [12] A. Stenborg, O Bjorneholm, A. Nllsson, N M'~rtensson, J.N Andersen and C. Wlgren, Surf. Scl. 211/212 (1989) 470 [13] P A. Briahwller and E W. Piummer, J Vae. Sci. Technol. A 9 (1991) 1833. [14] P.A Briahwiler and E W Plummet, to be pubhshed [15] J.J. Ritsko, S.E. Schnatterly and P.C Gibbons, Phys Rev. B 10 (1974) 5017 [16] O Bjorneholm, A Sandell, A Ndsson, N M,~rtensson and J.N. Andersen, Phys. Scr., to be pubhshed, W Wurth, D Coulman, A Puschmann, D Menzel and E. Umbach, Phys Rev B 41 (1990) 12933 [1"] M Y. Chou, P.K Lain and M.L. Cohen, Phys. Rev B 28 (1983) 4179, P Blaha and K Schwarz, J I-'h~ F 17 (1987) 899 [18] H lshlda and A Llebsch, to be published [19] K Sturm, E Zaremba and K. Nuroh, Phys Rev B 42 (1990) 6973. [20] The core level shift from 0-1 layer of Li/Ru(0001) ( ~ t 5 eV) is close to that found here, M.-L. Shek, J. Hrbek, T.K Sham and G - Q Xu, Phys Rev. B 41 (1990) 3447 [21] G M Watson, P.A. Bruhwder and E.W. Plummer, to be pubhshed [22] The mtportance of short range interactions can be referred from the difference m the zero-coverage-extrapolated binding energies between the two core lines There ts a dmtlnct possibility that this short range interaction may be the high-adsorbate-denslty extension of the long range interaction, since there is evidence for a dlscontinufly in the charge density when passing through 0 = 0 33 [l 1] for the Ll/Be(00l)l)system From l~g 4 we see that a small variation in the high-coverage slope of the core binding energy shift would yield the ~ame intercept for both core hnes m the zero-coverage hm~t However, the data for intermediate coverages indicate that this is probably not the case, and therefore there are hkeiy two contributions to the screening m the final state [23] H lshlda, Phys Rev B 42 (1990) 10899