A Comment on the Letter by D. M. Riffe et al. Phys. Rev. Lett. 64, 571 (1990)

Citrin, P. H.

Riffe, D. Mark

Wertheim, G. K.

English

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VOLUME 71, NUMBER 1 PHYSICAL REVIEW LETTERS 5 JULY 1993lonicity of Alkali-Metal AdsorbatesRiffe, Wertheim, and Citrin [1] reported the core levelbinding energies (BE's) of surface layer W atoms whenalkali metals are adsorbed on W(110). For low coverageand for first-layer saturation coverage, they find only verysmall shifts from the BE values for a clean surface. Theyconclude that there is little, if any, charge transfer fromthe adsorbed alkali to %, even in the limit of low cover-age. At low coverages, adsorbed alkali atoms have longbeen believed to be fully ionic [2]. In this Comment, weshow that these small BE shifts are fully consistent withlarge ionic character of an adsorbate.We used idealized point charge (PC) models of ionicadsorbates. A PC =+ 1 represents a cationic adsor-bate, the character [3] that we have found for K/Cu(100). A PC= —1 represents an anionic adsorbate,the character that has been found [4] for F/Ag(111).We examine the BE shifts for a Cu surface using thesame cluster model for Cu(100) as in our previous study[3] of K/Cu(100); the eA'ects shown for Cu should holdfor other metal surfaces. The PC is in a fourfold siteabove a Cuqs cluster [3]; it is placed near the equilibriumadsorbate distance, z, found for CuzsK, z(PC=+1)=3.0 A, and CuzsF, z(PC = —1) =1.8 A. Self-consistent-field (SCF) wave functions were obtained forCu25PC+, where PC+ =+1, and CupgPC; the chargeon Cu25 maintains the proper neutrality for substrate(Cuzs) plus adsorbate (PC).We use initial state BE's given by Koopmans' theorem[5] (KT). Final state relaxation, which screens the corehole, is neglected because there is a large body of evi-dence [5] which suggests that this neglect may not beserious. In Table I, the shifts of the KT BE's, AEg(nl),for the 3s and 3p core levels of the four Cu atoms nearestthe adsorbate are given under the heading SCF. TheAE~(nl) are the diA'erence of the BE's for Cu2sPC orCu25 PC+ from neutral Cu25,AEg (nl ) =Eg (nl; Cu 2sPC) —Eg (nl; Cu 25 ) .For both Cu25PC+ and Cu25PC, the AEq are small;for Cu25PC+, the values are somewhat larger than theextremely small shifts reported by RiA'e, Wertheim, andCitrin [1]. It is important to stress that the key feature ofour result is that the BED are small; the absolute value oreven the sign is less important. This is because there areother eAects [5] which could lead to changes of —0. 1 eVin the hE~. One of these is a diAerential final state relax-ation energy between Cu25PC and Cu25. Another is limi-tations in the representation of the Cu surface conductionband given by the Cu25 cluster. The main point is thatthis calculated hE8~0. 1 eV is much smaller than ex-pected from the large electrostatic potential due to ionicadsor bates.%'e estimate this electrostatic eAect by fixing the sub-strate density as that for the isolated charged Cu2q clus-ter and adding the point charge to this frozen orbitalTABLE I. SCF and FO BE shifts for core levels of the ad-sorption site Cu atoms; see Eq. (1).Cu25 PC+SCF FOCu25PCSCF FOAEs(3s) (eV)~E, (3&) (eV)—0.13—0.12+1.56+1.56+0.02+0.02—2.99—3.00Paul S. Bagus and Gianfranco PacchioniIBM Research DivisionAlmaden Research Center650 Harry RoadSan Jose, California 95120Received 6 March 1992PACS numbers: 79.60.Dp, 68.55.0i[Il D. M. Riffe, G. K. Wertheim, and P. H. Citrin, Phys.Rev. Lett. 64, 571 (1990),[21 I. Langmuir and K. H. Kingdon, Phys. Rev. 21, 381(1923).[3l G. Pacchioni and P. S. Bagus, Surf. Sci. 269/270, 669(1992), and references therein.[41 P. S. Bagus, G. Pacchioni, and M. R. Philpott, J. Chem.Phys. 90, 4287 (1989).[51 P. S. Bagus, D. Coolbaugh, S. P. KowaIczyk, G. Pac-chioni, and F. Parmigiani, J. Electron. Spectrosc. Relat.Phenom. 51, 69 (1990).[6] The case of BE shifts for 0 adsorbates, referred to in Ref.[I], is not modeled by PC= —1 which represents anidealized anionic adsorbate. The bonding of oxygen to ametal surface has considerable covalent character asshown, for 0/Cu, by P. S. Bagus and F. Illas, Phys. Rev.B 42, 10852 (1990).(FO) cluster. The /5Eg for the FO CuzsPC clusters rep-resent only the eAect of the electrostatic potential arisingfrom the PC; they do not include polarization of the sub-strate charge to screen the potential of the PC. These FOBED, given in Table I, are quite large. The electrostaticpotential due to a cation, PC =+1, increases the Cu corelevel BE's by —1.5 eV, while that that due to an anion[6], PC= —1, lowers the Cu BE's by —3.0 eV. Thelarger magnitude of the FO BED for PC = —1 occurs be-cause the PC= —1, which models F/Cu, is closer to theCu(100) surface, by 1.2 A, than is PC =+1, which mod-els K/Cu. The contribution of the response of the highlypolarizable metal surface to the presence of the PC isgiven by the diAerence between the FO and SCF AEp.This polarization contribution is almost equal in rnagni-tude and opposite in sign to the electrostatic contribution.EfI'ectively, the polarization screens the point charge andthis explains why the BED due to an ionic adsorbate willbe very small.In conclusion, the small surface W(4f) BE shifts found[1] for low coverage of alkali metals on W(110) are quiteconsistent with the strong evidence [2,3] for the ionicityof adsorbed alkali atoms.206 0031-9007/93/71 (1)/206 (1)$06.00Qc 1993 The American Physical Society

Ionicity of Alkali Metal Adsorbates, Reply

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Ionicity of Alkali Metal Adsorbates, Reply

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