High-resolution 4f  photoemission spectra from clean W(110) show that the natural lifetime  width and the (electron-hole)-pair singularity index are both larger in  the first atomic layer than in the bulk. Phonon broadening for the  surface and bulk components are smaller than theoretical estimates, and  little excess broadening is detected in the surface layer. These  findings are very different from the conventional picture of  surface-atom core-level line shapes and have implications extending to  other systems

Citrin, P. H.

Riffe, D. Mark

Wertheim, G. K.

English

DigitalCommons@USU

VOLUME 63, NUMBER 18 PHYSICAL REVIEW LETTERS 30 OCTOBER 1989Di8'erent Core-Hole Lifetime and Screening in the Surface of W(110)D. M. Riffe, G. K. Wertheim, and P. H. CitrinATcf T Bell Laboratories, Murray Hill, New Jersey 07974(Received 14 June 1989)High-resolution 4f photoemission spectra from clean W(110) show that the natural lifetime width andthe (electron-hole)-pair singularity index are both larger in the first atomic layer than in the bulk. Pho-non broadening for the surface and bulk components are smaller than theoretical estimates, and little ex-cess broadening is detected in the surface layer. These findings are very diA'erent from the conventionalpicture of surface-atom core-level line shapes and have implications extending to other systems.PACS numbers: 73.20.At, 79.60.CnIt is now well established that core-level photoemissionis a sensitive probe for studying the effects of the reducedcoordination at the surface. ' With this spectroscopy theprimary focus has been on the shift of the surface-atomcore-level binding energy, whose sign and magnitudehave been shown to be directly related to the narrowingand shape of the surface density of states. " Studies ofthis shift as a function of material, crystal orientation,reconstruction, depth below the surface (i.e., subsurfacelayer shifts), adsorbate, and coverage exemplify the widerange of activity and interest in this area. ' ' However,the investigation of other manifestations of the reducedcoordination at the surface, such as changes in core-holelifetime, conduction-electron screening, and phononbroadening has been hampered by a lack of adequate ex-perimental resolution and statistics. Indeed, while theexcess width of the surface-atom core level in just a sin-gle metal, W, has been tentatively ascribed to additionalsurface phonon broadening, crystal-field splitting, recon-struction, and subsurface contributions, no measure-ment has yet succeeded in clearly identifying thebroadening mechanisms operative at the metal surface.In this Letter we report on high-intensity, high-res-olution core-level photoemission measurements from aclean W(110) surface. We find that the major contribu-tion to the increased surface linewidth in this metal in-volves none of the previous suggestions, but is, instead,enhanced lifetime broadening resulting from the more lo-calized 5d electrons in the narrowed surface conductionband. In addition, we find differences in core-holescreening between the surface and bulk componentswhich are manifest as a measurable difference in theshape of the (e-h)-pair excitation tail of the photopeak.These novel results are not specific to W, and should ap-ply to a large number of systems in which conductionelectrons dominate the core-hole decay.The single-crystal sample of dimensions 4x20X0.2mm was cleaned by resistive heating to 1520 K in 10Torr of oxygen followed by flashing to 2300 K in a vacu-um of 1 & 10 ' Torr. After cooling to either 210 or 310K, which took only seconds due to the large difference inthermal mass between the sample and its mount, photo-emission data were collected using the ATILT Bell La-boratories 6-m toroidal-grating monochromator (TGM)on the vacuum-ultraviolet ring of the National Synchrot-ron Light Source. A 100-mm voltage standing-wavehemispherical analyzer was used with a pass energy of 2V, which gives a resolution of 40 meV (FWHM). Thehigh intensity of the monochromatized radiation (70 eVfor all measurements reported here) allowed single 2.5-eV scans with high statistics to be obtained in only 4min, during which time the average surface H contam-ination is estimated not to exceed 0.01 monolayer.The total instrumental resolution was determined fromthirteen independent measurements of the Fermi cutoffof the W(110) crystal, yielding an average value of83+'3 meV FWHM. (The error bars quoted here andthroughout the paper represent a 95%-confidence level,based on deviations from the mean of results obtainedfrom fitting a large number of independent data sets. )Su.btraction of the electron analyzer width yields a pho-ton resolution of 73 meV, which is consistent with earlierdeterminations based on the much sharper Fermi edge ofCu at 90 K. Analysis of both Cu and W Fermi edgesconfirms that the resolution function is very well approx-imated by a Gaussian.The clean W(110) surface is ideally suited for study-ing the broadening mechanisms of surface-atom core lev-els. The 4f states are long lived (i.e., the photopeaks arenarrow) and there is no surface reconstruction. More-over, for this close-packed surface, photoemission fromsubsurface layers should be indistinguishable from thatfrom the bulk because the nearest- and next-nearest-neighbor coordinations of the subsurface atoms are iden-tical to the bulk values. A complete photoemission spec-trum of the W 4f lines from a freshly cleaned surface(see inset in Fig. 1) shows four lines. The )2 eV sepa-ration of the j 2 and 2 spin-orbit doublet allows theindividual components to be analyzed separately. Furth-ermore, the bulk and surface lines in the 4f7y2 data areso well resolved and have such high statistics that in-dependent values of the natural Lorentzian lifetimewidth, the Gaussian phonon and instrumental broaden-ing, and the core-hole screening singularity index foreach of the lines can be readily obtained.The fit to the 4f7y2 data is shown in Fig. l. Its quality1976 1989 The American Physical SocietyVOLUME 63, NUMBER 18 YSICAL REVIEW LE R 30 OCTOBER 19892.0—120—EXPT:W (110) 4f rga5OC)z 1.0COLLJz0.5—BUl KSURFACET-DEPENDENCE100 — ——OHp $10—.—OHp= 250Kt— 80—z'.DZ.'O60—II2—U II1—I' g'I'LlJ -2—32.0 31.5Il3INDING ENERGY (eV)FIG. l. Analysis of the W 4~7g2 hotoee f ' p t mission spectrum of ap oosurface. The data, taken withinear ackground and two flines, representing the fully independentsur ace (right) and bulk ~1tions. The residuals of the fi( eft) contribu-Inset: A scan encome t are shown in tho passing the W 4e bottom panel.qyq and 4f 7' components.30.5can be critically assessed by the resi ud s, ea ic eviations and show onI the 6d s a istics. This imh a one cannot hope to isponent, viz. , the siolate a third com-gnal from the second ata ysis because its bindin -enh 11 th th Gan e aussian width. Ah'v' ""'""d'"' 1e ourse ves to a two-cornEighteen independent dponent analysis.dn ent ata sets were analen oniach-Sunjic' DSn ent aussians to test the re rodthe output paramete Thra eters. e resultin widt'" for th' bulkber of surprises.e and surface lines contain a num-(1) The surface-atom core-level shift is.h.-. -lue .d-d dl 1, p g oaine rom earlier data wit"e experiment we found that this sd db dobdh d ''h—50 meV/L 1 L (Le y rogen initiall changmuir) 10 Tposure], demonstrating that reating at g care must be exercised40—I I I I100 200 500TEMPERATURE (K)FIG. 2. Expenmental phonon widthsurface W 4f7 1s of the bulk and (110)7/2 levels compared withtheoretical calculationsthe results of two200to maintain a clean surface durin(2) We 6ndce n g the measurement.that the independentl detian widths of the by ermined Gauss-ulk and surface cornsimilar at 210 K.mponents are very94.5 ~ 2.7 and 96.0 ~Alarger, 100.8+the differencer, 4.4 meV for the bulk andi r ce is only slightlgsur ace. Subtractin the iua rature from the total Gaussiansuits in an excess width for thet bulk (surface) of 45.4at 310 K.5.5) at 210 K and 57.3 ~ 8.7 (65.6+'4.2)The temperature de endd 6 h dhyese wi ths, which are plotted in Fing due to phonons d d'hg e photoemission pro-s excite urin te room-temperature valuessmaller than the semlem ire i pirical estimate of 109 meV, 'o va ues are smaller than the thf '"d I4me for bulk W at the(1 hAlthou hs a sos own in Fig. 2).g the measured phonon broadeninthat expected from eithening is less thanf h t d der calculation abovedifference in surface d b 1ure epen ence, alon with twith an estimat' bace an ulk values areion ase on the bulk and suoth consistentm of W(110). U ising the bulk width at 310 Ke re erence point and a bulk Deb e te as ed line in Fi . 2.1 t th to th e phonons constant but 1owering1977VOLUME 63, NUMBER 18 PHYSICAL REVIEW LETTERS 30 OCTOBER 1989the Debye temperature to 250 K, a value appropriate forthe W(110) surface, ' produces the dot-dashed curvewhich just intersects the error bars on the surface data.Thus, although the difference in phonon broadening istoo small to discern any significant difference betweenthe surface and the bulk, the data are consistent with asimple interpretation of a Debye-phonon excitation spec-trum in the limit of a Gaussian line shape.(3) In contrast to the small difference observed in thephonon broadening, the data clearly show that the core-hole lifetime width is distinctly larger at the surface thanin the bulk. Figure 3 demonstrates that there is no dif-ficulty in separating the Gaussian and Lorentzian contri-X~ 0.08—Cl&- 0.06—CC~ 0.04—z 0.02Cf)120—)CD«00-80—60—CC40—I—z 20I60I I80 100GAUSSIAN WIDTH (meV)120FIG. 3. Distinguishing between Gaussian and Lorentzianbroadening. The bottom panel shows a cut through the g sur-face of a least-squares analysis of the data in Fig. 1. For thepurpose of this demonstration only, a common Gaussian widthwas used for both surface and bulk photopeaks, leaving theother parameters unconstrained. (The use of a commonGaussian width in these data is justified by the fact that thebulk and surface lines are so well separated. ) The correspond-ing natural Lorentzian widths and singularity indices areshown above. The clearly defined minimum in g demonstratesthat the Lorentzian lifetime and Gaussian phonon broadeningsare readily distinguishable in these data.1978butions to the linewidth. Analysis of the eighteen datasets yields a width of 60 ~ 3 meV for the bulk and 84 ~ 3meV for the surface. The value for the lifetime width ofthe bulk line is in satisfactory agreement with earlierdeterminations, which were 55 and 61 meV.Since the 4f7y2 hole state decays almost entirely by%704 504 5 Auger transitions [the fluorescence yield isonly 7 x 10 (Ref. 17)],one can look for an explanationof the increased width at the surface by focusing on theproperties of the metal 5d band. Starting with the freeW atom, there will be two opposing effects on the 4f7yqlifetime in forming the metal. On the one hand, theAuger decay rate will be enhanced because the d-bandoccupancy is increased both by reconfiguration in the ini-tial state and by metallic Sd core-hole screening in thefinal state. On the other hand, the Auger decay rate willbe reduced by the delocalization of the Sd conductionelectrons, which decreases the overlap between the 4fand Sd orbitals. The larger lifetime width observed inthe surface relative to the bulk, then, clearly implies thatthe reduced delocalization in the narrowed, more atomic-like surface Sd band is the dominant mechanism.(4) The singularity index, a, of the bulk and surfacelines are also different, 0.035 ~ 0.003 and 0.063+ 0.003,respectively (the rms deviations from the mean are smallbecause the coupling of a to the other parameters is veryweak). The small absolute values of a, compared to,e.g., the simple metals Na, Li, Mg, and Al, ' are aconsequence of the larger fraction of screening chargeresiding in orbitals of higher angular momentum. Thebulk value of a is close to the minimum value of 3'2 ob-tainable for screening by s, p, d, and f orbitals. Byshowing that the valence-band excitation spectrum is ap-proximately constant from 0 to 5 eV, Sebilleau et aI. 'have demonstrated that the DS line shape is appropriatefor W. Their calculated values for a are, however, ap-proximately the same for bulk and surface and are inagreement only with our surface value.Our results for the W(110) surface have significantimplications on some of the earlier interpretations ofsurface-atom core-level measurements. ' For exam-ple, the present findings indicate that the excess width ofthe surface signal from Ta and other W surfaces is duealmost solely to the increased natural width of the corehole, not the other broadening mechanisms which havebeen proposed. For the W(111) surface we have con-firmed that the extant data are compatible with this con-clusion, but the resolution of the data does not allow in-dependent Gaussian- and Lorentzian-width parametersto be determined for the bulk and surface lines. In gen-eral, it should certainly not be assumed, as has been donein the past, that the natural width is the same in the bulkand surface, especially when it is determined primarilyby Auger processes involving valence electrons. More-over, the singularity index should not be assumed to beunchanged at the surface, even when the absolute bulkvalue is small.VOLUME 63, NUMBER 18 PHYSICAL REVIEW LETTERS 30 OCTOBER 1989In summary, we have shown that the modification ofthe electronic properties at the surface readily accountsfor the changes in the core-electron photoemission spec-tra from the first atomic layer of W(110). There is noanomalous phonon broadening, no evidence for crystal-field splitting, and no need to postulate unresolved linesfrom subsurface layers. The important phenomena atthe surface are the changes in the natural width and theconduction-electron screening. These conclusions departfrom all previous interpretations of surface-atom core-level spectra and are not limited to W(110). We antici-pate that the higher-energy resolution and beam intensi-ties now obtainable from improved synchrotron sourcesand beam-line optics should open up a variety of systemsfrom which new conclusions about surface photoemissionwill emerge.We are indebted to Professor P. J. Estrup for the loanof the tungsten sample used in these experiments. TheNational Synchrotron Light Source at Brookhaven Na-tional Laboratory is supported by the U.S. Departmentof Energy, Division of Materials Sciences and Division ofChemical Sciences (DOE Contract No. DE-AC02-76CH00016).For reviews, see P. H. Citrin and G. K. Wertheim, Phys.Rev. B 27, 3167 (1983); D. Spanjaard, C. Guillot, M. C. Des-jonqueres, G. Treglia, and J. Lecante, Surf. Sci. Rep. 5, 1(1985).P. H. Citrin, G. K. Wertheim, and Y. Baer, Phys. Rev.Lett. 41, 1425 (1978); Phys. Rev. B 27, 3160 (1983).P. J. Feibelman and D. R. Hamann, Solid State Commun.31, 413 (1979);Phys. Rev. B 20, 1433 (1979).4B. Johansson and N. Martensson, Phys. Rev. B 21, 4427(1980); Helv. Phys. Acta 56, 405 (1983).5J. F. van der Veen, F. J. Himpsel, and D. E. Eastman, Phys.Rev. Lett. 44, 189 (1980); Solid State Commun. 37, 555(1981);40, 57 (1981);Phys. Rev. B 25, 7388 (1982).G. Treglia, M. C. Desjonqueres, D. Spanjaard, Y. Lassailly,C. Guillot, Y. Jugnet, T. M. Duc, and J. Lecante, J. Phys. C14, 3463 (1981);C. Guillot et al. , J. Phys. C 15, 4023 (1982);16, 1555 (1983); Solid State Commun. 50, 393 (1984); Phys.Rev. B 30, 5487 (1984).7G. K. Wertheirn, P. K. Citrin, and J. F. van der Veen, Phys.Rev. B 30, 4343 (1984); 3$, 7820 (1988).K. G. Purcell, J. Jupille, G. P. Derby, and D. A. King, Phys.Rev. B 36, 1288 (1987); J. Jupille et al. , Phys. Rev. B 39, 6871(1989).9M. A. van Hove and S. Y. Tong, Surf Sci. 5. 4, 91 (1976); J.W. Chung, S. C. Ying, and P. W. Estrup, Phys. Rev. Lett. 56,749 (1986).'OS. Doniach and M. Sunjic, J. Phys. C 34, 285 (1970).'T.-M. Duc, C. Guillot, Y. Lassailly, J. Lecante, Y. Jugnet,and J. C. Vedrine, Phys. Rev. Lett. 43, 789 (1979).P. H. Citrin, G. K. Wertheirn, and Y. Baer, Phys. Rev. B16, 4256 (1977).'3C. P. Flynn, Phys. Rev. Lett. 37, 1445 (1976).' D. Sebilleau, G. Treglia, M. C. Desjonqueres, C. Guillot, D.Chauveau, and D. Spanjaard, J. Phys. C 20, 2647 (1987).' N. W. Ashcroft and N. D. Merrnin, Solid State Physics(Saunders, Philadelphia, 1976), p. 461.'sD. P. Jackson, Surf. Sci. 43, 431 (1974).'7E. J. McGuire, Phys. Rev. A 9, 1840 (1974).1979

Different Core-Hole Lifetime and Screening in the Surface of W(110)

https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2087&amp;context=physics_facpub

Different Core-Hole Lifetime and Screening in the Surface of W(110)

Abstract

Similar works

Full text

Available Versions

DigitalCommons@USU