摘要:

FARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY NO. 58 1974 A General Discussion of the Faraday Division of the Chemical Society in collaboration with the Deutsche Bunsengesellschaft fur Physikalische Chemie and the SociCtC de Chimie Physique. Ph o to-e$ec t s in Adsor bed Species THE FARADAY DIVISION CHEMICAL SOCIETY LONDONISBN 0 85186 848 7 ISSN 0 301 7249 0 The Chemical Society and Contributors 1975 Printed in Great Britain at the University Press, AberdeenA GENERAL DISCUSSION ON Photo-eJects in Adsorbed Species 10th-12th September 1974 A GENERAL DISCUSSION on Photo-Effects in Adsorbed Species was held at the University of Cambridge on the loth, 1 lth and 12th September, 1974. The Confer- ence was a unique occasion in that it was the first of a series of joint meetings of the Faraday Division, the Deutsche Bunsengesellschaft fur Physikalische Chemie and the Sociktk de Chimie Physique that will be organised every two years, the second being scheduled to take place in Germany in 1976.The present topic and the detailed scientific programme were planned by an organizing committee comprising representatives of each of the three societies, members of which contributed nearly three-quarters of the papers. Among the overseas visitors were : From France Dr. M. Breysse Dr. J. Bourdon Prof. B. Caroli Prof. B. Claude1 Dr. G. Dalmai-Imelik Mr. M. Formenti Mr. C. Guillot Miss Y. Jugnet Prof. J. Lavorel Dr. R. Lesclaux Dr. P. Meriaudeau Prof. B. Pouyet Dr. C. Riou Dr. J. Robillard Mr. J. Roussel Prof. S. J. Teichner Dr. C. Troyanowsky Mr. J-G.Villar From West Germany Dr. W. Bauer Prof. H. P. Boehm Mr. E. Bohn Dr. A. M. Bradshaw Prof. H. D. Breuer Dr. J. C. Fuggle Prof. H. Gerischer Dr. W. Gopel Dr. K. Gottlieb Mr. R. Harborth Prof. R. A. W. Haul Prof. G. Heiland Prof. W. Hirschwald Dr. E. V. Hoefs Dr. K. Jacobi Dr. H. Killesreiter Dr. J. Kiippers Mr. E. E. Latta Dr. R. Memming Prof. D. Menzel Prof. H. Moesta Mr. A. Plagge Dr. K. Richter Dr. N. Rosch Prof. F. Steinbach Mr. M. Steinkilberg Dr. H-J. Traenckner Prof. G. Wedler 3In addition, the overseas visitors from eleven other countries attended : Dr. H. Beens, The Netherlands Dr. P. Biloen, The Netherlands Dr. A. H. Boonstra, The Netherlands Mr. D. Brkic, Italy Dr. S. Coluccia, Italy Prof. J. Cunningham, Eire Dr. B. Feuerbacher, The Netherlands Dr.B. Fitton, The Netherlands Dr. P. Forzatti, Italy Prof. J. Haber, Poland Dr. H. D. Hagstrum, U.S.A. Dr. R. Hubin, Belgium Dr. R. E. Jesse, The Netherlands Mrs. E. Lelental, U.S.A. Mr. M. Lelental, U.S.A. Dr. I. Marklund, Sweden Dr. C. Morterra, Italy Dr. D. B. Matthews, Australia Prof. C . A. McDowell, Canada Dr. P. J. Marller, Denmark Dr. R. C. Nelson, U.S.A. Prof. W. Palczewska, Poland Dr. D. Penn, U.S.A. Prof. M. J. Sparnaay, The Netherlands Dr. D. Spears, Sweden Prof. K. Tamaru, Japan Mrs. C. Tenret-Noel, Belgium Dr. F. Trifiro, Italy Dr. R. F. Willis, Tlze Netherlands Dr. A. N. Wright, U.S.A. The attendance was 155 of which over 70 were from abroad. The President of the Faraday Division of the Chemical Society, Prof. T. M. Sugden F.R.S. welcomed our Continental colleagues and overseas visitors and, at the conclusion of the meeting, expressed, on behalf of the Faraday Division, his immense satisfaction that this first corroborative effort had been so successful, both scientifically and socially.page 7 19 28 35 46 59 62 80 90 97 106 116 125 143 CONTENTS Introductory Paper by T.B. Grimley A. VALENCE-ELECTRON LEVELS OF ADSORBED MOLECULES Theory of the Angular Dependence of the Photoemission Line Shape from an Adsorbate by A. Liebsch and E. W. Plummer Calculation of the Electronic Structure of Ethylene Bonded to Diatomic Nickel and Correlation with Ni-C2H4 Photo-emission Data by N. Rosch and T. N. Rhodin Ultra-violet Photoemission Studies of CO, N2 and C Adsorbed on W(100) by W. F. Egelhoff, J. W.Linnett and D. L. Perry Ultra-violet Photoelectron Spectroscopy of Adsorbed Oxygen by A. M. Bradshaw, D. Menzel and M. Steinkilberg GENERAL DrscussIoN-Dr. T. B. Grimley, Dr. C . R. Brundle, Dr. H. D. Hagstrum, Dr. A. M. Bradshaw, Dr. W. F. Egelhoff, Prof. J. W. Linnett, Dr. D. L. Perry Ultra-violet and X-ray Photoelectron Spectroscopy (UPS and XPS) of CO, C02, O2 and H20 on Molybdenum and Gold Films by S. J. Atkinson, C . R. Brundle and M. W. Roberts Ultra-violet Photoelectron Spectroscopy Studies of Gas Adsorption on Trans- ition Metals by P. J. Page and P. M. Williams GENERAL DIscussroN-Dr. C . M. Quinn, Dr. C. R. Brundle, Dr. R. W. Joyner, Dr. D. Menzel, Dr. S. Evans, Dr. M. J. Tricker, Prof. M. W. Roberts, Prof. J. M. Thomas, Dr. D. E. Parry, Dr. P. M. Williams Ultra-violet and X-ray Photoelectron Spectroscopy Studies of Oxygen Chemi- sorption on Copper, Silver and Gold by S.Evans, E. L. Evans, D. E. Parry, M. J. Tricker, M. J. Walters and J. M. Thomas He1 Photoelectron Spectroscopy of Small Molecules Adsorbed on Metal Surfaces by P. Biloen and A. A. Holscher Photoemission Spectra of Adsorbed Layers on Pd Surfaces by H. Conrad, G. Ertl, J. Kiippers and E. E. Latta GENERAL DIscussIoN-Dr. C. R. Brundle, Dr. S . Evans, Prof. M. W. Roberts, Dr. D. Briggs, Dr. J. C . Fuggle, Prof. D. Menzel, Dr. F. R. Smith, Prof. J. M. Thomas, Dr. D. E. Parry, Dr. A. M. Bradshaw, Dr. P. Biloen, Dr. E. W. Plummer, Dr. T. B. Grimley, Dr. D. R. Lloyd, Dr. R. W. Joyner, Dr. J. Kiippers, Dr. H. Killesreiter B. PHOTO-ADSORPTION, PHOTO-DESORPTION AND PHOTO-REACTIONS AT SURFACES Oxidation of CO Desorption of Oxygen by Ultra-violet Irradiation of ZnO Single Crystals under Ultra-high Vacuum Conditions by F.Steinbach and R. Harborth6 CONTENTS 151 Interaction of Ultra-uiolet Radiation with Surfuce Energy Levels of the ZnO-Li20-02 System as Revealed by Electron Spin Resonance Spectroscopy by J. Haber, K. KosiIiski and M. Rusiecka I60 Photo-assisted Surface Reactions Studied by Dynamic Mass Spectroscopy by J. Cunningham, E. Finn and N. Samman 175 GENERAL DiscussIoN-Dr. R. 1. Bickley, Prof. G. Heiland, Dr. W. Hirsch- wald, Dr. E. Thull, Prof. F. Steinbach, Mr. R. Harborth, Dr. W. Bauer, Prof. A. Hausmann, Dr. A. J. Tench, Prof. J. Cunningham, Prof. M. W. Roberts, Dr. T. B. Grimley, Prof. Q. Menzel, Prof.H. P. Boehm, Dr D. E. Parry 185 Photointeraction on the Surface of Titunium Dioxide between Oxygen and Alkanes by N . Djeghri, M. Formenti, F. Juillet and S. J. Teichner 194 Photo-adsorption and Photo-catalysis on Titanium Dioxide Surfaces by R. I . Bickley and R. K. M. Jayanty 205 Luminescence during Oxygen Adsorption on Thorium Oxide by M . Breysse, B. Claudel, L. Faure and H. Latreille 215 GENERAL DiscussroN-Prof. H. P. Boehm, Prof. S. J. Teichner, Prof. F. S. Stone, Dr. R. 1. Bickley, Prof. J. M. Thomas, Qr. 1. Marklund, Prof. B. Claudel C. PHOTOCHEMISTRY OF ADSORBED SPECIES 219 Photochemistry of Adsorbed Species by H . Gerischer 237 Absorption Spectroscopy and Photo-reactions of Adsorbed Molecules by W. Bach and H. D. Brewer 244 Photochemistry and a Model for Adsorption of some Conjugated Hydrocarbons by H. Moesta 2 5 3 Spectroscopic Studies of 0 rien ta tional Interact ions bet ween Straigh t- chnin Alkanes and Aromatic Hydrocarbons by M . Lamotte, R. Lesclaux, A. M. Merle and J. Joussot-Dubien 261 Photochemical Processes in Monomolecular Dye Layers on SnO, by R. Memming 27 1 Sensitizing Eflciency of Dimers Adsorbed on Molecular Crystals by H . Killesreiter 281 Photosensitization of the Charge Transfer Across ZnO Interfaces by Binary Dye Mixtures by K. Hauffe and U. Bode 292 Surface Photopolymerization of Tetrafluoroethylene by D. €3. Maylotte and A. N. Wright 301 GENERAL DIscussIoN-Dr. T. B. Grimley, Prof. F. S. Stone, Prof. H. P. Boehm, Dr. J. Bourdon, Prof. T. G. Truscott, Prof. F. Steinbach, Dr. W. Bauer, Prof. J. Haber Concluding Remarks by F. S. Stone 305 309 AUTHOR INDEX

ISSN:0301-7249    

DOI:10.1039/DC9745800001

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Introductory Paper Theoretical Aspects of Photoemission, Photodesorption and Photochemistry of Adsorbates BY T. B. GRIMLEY Donnan Laboratories, University of Liverpool, P.O. Box 147, Liverpool L69 3BX Received 10th September, I974 1. INTRODUCTION Photons which interact with solid surfaces, and with adsorbates, can be scattered elastically or inelastically, or they can be absorbed. Scattering is of little interest to us at this Discussion. Elastic scattering is studied experimentally to find the optical constants of the materials (substrate and adsorbed layers) ; the inelastic scattering of optical or U.V. photons is the Raman effect. The absorption of photons by isolated atoms in their ground states leads only to electronic excitation or to photoemission, but photon absorption at surfaces, or in adsorbates, may have many additional consequences ; adsorption, desorption, chemical attack on substrate, chemical changes in the ambient gas, and so on.Just over one half of the papers at this Discussion are concerned with such photochemical phenomena, and just under one half are concerned with photoemission from substrates, and adsorbates, that is to say with UPS and XPS. In spite of its historical importance in the development of quantum theory, only now are microscopic theories of photoemission being developed. The aim of these theories is to relate the experimental data (the energy-resolved angular distribution of the emitted electrons) to the electronic structures of surfaces and adsorbates. I shall discuss some of the problems involved in this task below, but it is important to remind ourselves now, that UPS is only surface sensitive because of the short (- 1 nm) mean free path of 5-100 eV electrons in solids,'* that this short mean free path is a many- body phenomenon, and that consequently a proper many-body theory of photo- emission is the ultimate goal.In the meantime, however, experimental workers are developing photoemission as an empirical diagnostic tool in chemisorption studies. This is amply demonstrated by the papers on photoemission at this Discussion. One of the problems in the theory of photoemission, which is present also when we turn to other photo-effects in adsorbed species, concerns the value of (or the operator for, if we use quantum theory) the vector potential of the radiation field at the surface, or in the adsorbed species; I shall refer to this in section 3.But there are many other problems to be solved, and the fundamental theory of photochemical reactions at surfaces is in no better state than that for homogeneous photochemical reactions. Thus, an ab initio calculation of the rate of a simple process like the photodesorption of CO from tungsten has not yet been attempted because, although the formal theory is straightforward, at least one simplification of doubtful validity is needed to reduce the problem to a simple form involving the familiar potential energy curves of pseudo diatomic molecules (see section 4). 78 INTRODUCTORY PAPER Most of the fundamental theoretical problems in the study of chemical reactions at surfaces remain untouched.As in the theoretical study of gas phase reactions, progress hinges on the calculation of electronically adiabatic potential energy hyper- surfaces for the reaction. This is the task of chemisorption theory. 2. PHOTOEMISSION In view of Lippmann's generalization of Ehrenfest's theorem, the photocurrent can be written in terms of a transition rate. Thus, the transition rate wF to a final state IF) of energy EF is given by WF = h-'xl (Fl TI 1) I 'a(&? - EI) (1) I where T = Y+ VG Y is the T-matrix corresponding to the electron-photon interaction V, and G is the stationary propagator (i.e., the Fourier time transform of the propa- gator) for the electron-photon system ; G(E) = G(E+iO) = (E+iO-H)-I = (E+iO-H, - Y)-', Since Y is small, it is sufficient to replace T by Y in eqn (1).Also, if we treat the radiation field classically or, after evaluating matrix elements of Boson operators otherwise, we can write eqn (1) as a transition rate involving initial and final ezectronic states [ i ) and If) HoIn = &IF), Hot0 = W). Wf = h-'Cl(f[ Vli)l"(&~ +Ao -Ef) (2) i Here hco is the photon energy, and Y = (ieh/mc)V*A, where A is the vector potential of the radiation field. The final state If) in eqn (2) describing photoemission in the presence of condensed matter needs a little thought. This is best illustrated by considering photoemission from a free electron ~ e t a l . ~ - ~ Of course a perfectly free electron cannot absorb a photon because energy and momentum cannot be conserved in the process; the matrix element in eqn (2) vanishes unless momentum is conserved, and the 8-function conserves energy. But the situation is different if the electron is confined to the half- space z < 0 say, by a step potential - V, if z < 0 but zero otherwise, because now the matrix element in eqn (2) acts only to conserve the component of momentum parallel to the surface z = 0.The initial state li) consists of incoming and outgoing plane waves in the metal, and an exponentially decaying state in the vacuum ( z > 0) with the same component of electron momentum parallel to the surface (fig. 1). For a given parallel momentum hfil there are two linearly independent final states with energy cf and orthogonal to li). If we choose them in the forms (3) z > o z t o describing an incoming state in the metal, and both incoming, and outgoing states in the vacuum, and 2 3 0 z < o (4) describing both incoming and outgoing states in the metal, and only an incoming state in the vacuum (fig.l), then since only If) contains the outgoing state exp(i(fz+ fll-p)) describing the photoelectron in the vacuum, this is the final state to be used in9 eqn (2). Had we chosen the two states If) and If’) differently, both would have contained the outgoing state in the vacuum, and the transition rate to both would have had to be calculated to obtain the photocurrent. In eqn (3) and (4) T. B . GRIMLEY if we choose our energy zero to correspond to electrons at rest in vacuum. If > If’> FIG. 1.-Initial and final states for photoemission from a free electron metal.For the step potential, the matrix element in eqn (2) is proportional to the triple product of the values at z = 0 of the initial and final state wavefunctions, and the vector potential. Consequently, for the free electron model, photoemission is a surface effect. This is because it is only at the surface that the potential energy function in Schrodinger’s equation changes its value, and we may note that, because the electron still moves perfectly freely parallel to the surface, there can be no photo- emission if the electric vector in the radiation field is parallel to the surface. If we add a periodic crystal potential to the free electron model, there is, in addition to the surface effect, a “ volume effect ” depending on the energy band structure.This volume effect is unphysical unless allowance is made for the finite mean free path of the electrons in the solid. The simplest way to do this is to use a complex k in eqn (3).5-7 2.1 PHOTOEMISSION FROM ADSORBATES Strictly speaking there is no such thing as photoemission from adsorbates. What we can observe, and derive an expression for (see below), is the difference in the emis- sion with, and without, the adsorbate. It is generally believed that the energy- resolved angular distribution of this difference in photoemission can provide informa- tion on the local geometry of the adsorption site, and on the orbitals used in forming the chemisorption bond. The grounds for this belief are most easily demonstrated by looking at the theory of photoemission from an isolated atom to plane wave final states,* and remembering that in a molecular orbital theory of chemisorption, the atomic orbitals on the adatom will be chosen to belong to irreducible representations of the point group describing the symmetry of the adsorption site.For a plane wave final state the matrix element in eqn (2) is If) = Ik) = exp(ik*r) < f I Vli) = - (ek/mc)Ao(e-k)(k - qli). (6) (7) Here i?A0 exp(i4.r) is the vector potential of the radiation with propagation vector 410 INTRODUCTORY PAPER and polarization specified by the unit vector e. the form For a free atom, the initial state has I9 = f W Ylrn(8, 4) (8) where Ylm is the usual spherical harmonic, so to evaluate the scalar product on the right in eqn (7) we expand the plane wave Ik - q ) in terms of spherical Bessel functions and spherical harmonics ; CQ + L L = O h f = -I, exp{i(k- 4) 1.1 = 4.n 1 iLjL(lk- q1r) x C Y L h f ( f ) Y t M ( & ) .(9) Then Except at low photoelectron energies (less than about eV for the 21.2 eV helium lamp) we can neglect q in comparison with k , and consequently the angular dependence of the emission is described by the terms The first factor simply gives a (cosine)2 distribution about the direction of polarization of the incident radiation, and is the same for all atomic orbitals. The second factor shows how the angular dependence of the emission depends on the I-value of the atomic orbital. For an adsorbed atom, the atomic orbitals will be chosen to belong to the various irreducible representations of the point group describing the symmetry of the adsorp- tion site.Consider, for example, the adsorption of a chalcogen at a site on a crystal where the symmetry is described by the group C41, (fig. 2 is an example). Z t 0 cubic The x FIG. 2.-Adsorption at a site of C4v symmetry. x' adatom orbital p z is now distinguished from the degenerate pair p x and p,,. If 8 is the angle between the direction of the emitted electrons and the surface normal (Lee, the polar angle of i), then since Y,, - cos 8, the angular dependence of the emission from p z is described by (e.i)2 C O S ~ 8. (1 1 ) If the incident radiation is polarized in the plane of incidence, the angular dependence of the emission from p z in the plane of incidence is described by sin2(B+8,) COS* 8 (12) where 8, is the angle of incidence (fig.3). This dependence is shown in fig. 4 forT. R. GRIMLEY z FIG 3.-Photoemission in the plane of incidence by radiation polarized in the plane of incidence. I FIG. 4.-Intensity (arbitrary units) of photoemission from a pz orbital as a function of the final polar angle 6. Angle of incidence of radiation ; (1) 0", (2) 30", (3) 60". 0 30 60 90 flldeg FIG. 5.-Intensity (arbitrary units) of photoemission from the pair of orbitalsp, andp, as a function of the final polar angle 6. Angle of incidence of radiation ; (1) O", (2) 30", (3) 60"12 INTRODUCTORY PAPER 8, = 0 (normal incidence), 30" and 60". The angular dependence of the emission from the degenerate pair of orbitals px and p,, is described by the factors (i%P)2 sin2 e and is shown in fig.5 for the experimental arrangement of fig. 3. If all three orbitals were involved equally in the chemisorption bond, the angular dependence of the photo- emission would reduce to the uninteresting factor (S-r2>2. But, except at tetrahedral, octahedral or icosahedral sites, which we do not in any case expect to encounter in adsorption, equal involvement of p x , p y and pz in the bonding must be " accidental ", and for the adsorption geometry of fig. 2 for example, we expect the emission from pz with the angular distribution of fig. 4 to show at lower electron energies than that from p, and py with the angular distribution of fig. 5. Of course p z is distinguished from p x andp,, at any 4fold site, at the centre of a square for example, and now the emission from px and p,, might well show at lower electron energies than that from pz.Thus, energy resolved angular distribution work is expected to distinguish one local geo- metry from another; it should of course distinguish between C,, and C2, sites because, at the latter, the degeneracy betweenp, and p y is lifted. We note incidentally, that the dependence of the emission on the azimuthal angle 4 (the azimuthal angle of k, the propagation vector of the final state) is not interesting for p-states at a 4-fold site, since it comes entirely from the factor (8&)2. Of course this is not always the case. At an adsorption site with C,, symmetry for example, the angular dependences are and of course all three orbitals are resolved energetically.To summarize the above simple discussion, we can say that, if the measured dzflerence emission is divided by the angular dependent factor ( $ ~ k ) ~ , the remaining energy-resolved angular dependence seems capable of providing information both on the adsorption geometry, and on the adatom orbitals used in forming the chemi- sorption bond. However, there are some important modifications to be made before we can be sure that photoemission is a useful tool in this connection. In particular, both initial and final states are modified by the presence of the substrate, and the final state is also modified by interaction of the ejected electron with the hole it leaves behind. This electron-hole interaction is a feature of atomic photoemission too. The modification of the initial state is of course due to the formation of a chemisorp- tion bond between the adsorbate and the substrate.An electron in the final state contributes little to this bond, and the important modification to make here is to replace the plane wave state by a correct eigenstate of the Hamiltonian for an electron moving in the field of the substrate in the half-space z < 0, as in eqn (3) for example. The scattering of an electron in this state by the adsorbate can be included through an appropriate T-matrix, and in this way an important part of the electron-hole inter- action referred to above can be included. The importance of choosing a final state with the correct Bloch character in the substrate has been demonstrated by Lieb~ch,~ and by Liebsch and Plummer (this Discussion). When this is done, the angular dependence of the emission even from an s-orbital, which would be the trivial ( 2 4 ~ ) ~ factor for a plane wave final state, yields information on the symmetry of the adsorp- tion site, because this information is now contained in the behaviour of the final state wavefunction over the region of space occupied by the adatom.T.B . GRIMLEY 13 The formation of a chemisorption bond is easy to describe in a molecular orbital theory, although other descriptions are possible.1°-12 The molecular orbitals GP of the system adatom+ substrate are written as linear combinations of the adatom orbitals #A and the substrate orbitals 4k A k Of course the substrate orbitals +k are themselves a complete set so the adatom orbitals are not really needed in eqn (14).They are included so that we can get a good approximation to the initial state in the presence of the adatom without including unbound substrate states in the k-summa tion. In practical calculations, the orbitals +k may be linear combinations of a suitable basis set of atomic orbitals on the sub- strate atoms. Using the form (14) for the initial state in eqn (2) we find 1 - = -- Im grS(& + i0) n where @ is the Greenian matrix for use l3 with the non-orthogonal basis set ( A , k ) . The diagonal elements prr(e) of the spectral density matrix are net densities of states. The terms in eqn (15) with r = s = A give the contribution to the transition rate which depends explicitly on the net densities of states on the adatom.This contribution, with PAA approximated either by a single &function at the atomic level or, by a Gaussian distribution, is considered by Liebsch and Plummer (this Discussion). The single &function approximation to PAA is more appropriate to physisorption than to chemisorption, except of course for lone pair orbitals on chemisorbed N, P, As, etc. For orbitals involved in chemisorption, PAA can develop considerable stmcture,14 and for strongly chemisorbed material it will be possible to identify the peaks in PAA below the Fermi level with the orbital energies of electrons in the chemisorption bond. These peaks may however show splitting due to correlation effects. The calculation of pAA is a basic task of chemisorption theory. Grimley and Pisani l6 have obtained results for hydrogen on some cubic tight binding solids using a local Hartree-Fock model, and employing Dyson’s equation to draw the atoms of the semi-infinite substrate into the local self-consistency problem ; more calculations on these lines are in progress.Also at Liverpool, Mr. Newton has (unpublished) results for hydrogen on the nearly free electron metal aluminium, using essentially Anderson’s Hamiltonian, but with overcompleteness of the basis set allowed for. Eqn (1 7) underlines the importance of these calculations in predicting the energy dependence of the emission. Conversely, if the matrix element in eqn (17) can be calculated, the experimental data may provide a method for determining PAA because the angular dependence of the contribution (17) is in the matrix element not in PAA.However, (17) is only one of five contributions to the emission. The other four involve the spectral densities Pkk, PAB, P k l and PAk. From the term involving P k k we can isolate the emission from the clean substrate by using Dyson’s equation14 INTRODUCTORY PAPER 3 = gf + @‘-lrg to express gs, the substrate portion of the Greenian matrix in terms of the interaction matrix V‘ forming the chemisorption bond, and the Greenian matrix @f for the clean substrate. I shall not give the details here (see for example ref. (16)), but in this way we get an expression for the total contribution which the adatom makes to the transition rate to the final state If). This determines the dzflerence photoemission, and as well as the contribution (17) there are contributions involving PAB, Pkl and P& Dr.Bernasconi is working on practical methods of esti- mating the importance of these “ unwanted ” contributions, all of which are energy and angular dependent, because until this can be done, theory cannot tell us whether the “ useful ” contribution (17) can be isolated from the experimental data. Turning now to the final state If) in eqn (15) and (17), since its energy is above the vacuum level, it seems an obvious approximation to take If) to be the same whether there is an adatom on the surface or not, and this is what Liebsch and Plummer (this Discussion) do. But in many cases, and particularly if the initial state hole is well localized on the adsorbate, it may be necessary to correct the final state for scattering by the adsorbate.This is achieved formally by replacing Y in eqn (1 5) by (1 + TG) V where T is the T-matrix corresponding to the adsorbate scattering potential V A ; T = VA+ VAGT where G(Ef+iO) is the stationary propagator. Evidently there is much theoretical work to be done on photoemission. 3. THE PHOTON FIELD In the foregoing discussion, I have assumed that the vector potential A(r, o) is known. For high photon energies (for XPS) the substrate is transparent and A can be assumed to have its vacuum value everywhere. To improve on the approximation, we might consider calculating the (classical) field using the macroscopic electromagnetic constants E , CT and p. But photoemission results from the p A interaction between electrons and photons which also contributes to the electromagnetic constants, so the question arises : How much of the electron-photon interaction can we include in the electromagnetic constants ? By considering the interaction of a radiation field and an electron field we see the problem quite clearly, and actually go some way towards solving it.3.1 THE ELECTRON-PHOTON INTERACTION I N PHOTOEMISSION Plasmon contributions of the substrate to the electromagnetic constants, or even those of an adsorbed overlayer, are allowed for by using an appropriate permittivity E(Z, o) in the usual Helmholtz equation for A . The exact Heisenberg equations of motion of the coupled Boson field A and the Fermion field $ describing the electrons are (1 8) (1 9) 1 V x V x A+,UOEOA = p0.l J = (eh/2im)[$tV$ - (V$)$t] - (e2/m)A$$f and ih$ = - (A2/2m)V2$ + V$ + (ieA/m)AaV$ -k (e2/2m)A2$.In eqn (19) the electron-electron interaction is included in the operator product V@. To make progress with these coupled equations we need to approximate. In the zero order approximation we put J = 0 in eqn (18) so that A is simply the vacuum field, and no account is taken of the presence of condensed matter. We improve on this approximation by calculating J using ground state expectation values of Fermion operators, J -” -(e2/m)n(r)A (20)T . B. GRIMLEY 15 where n(r) is the electron density. time transforms, we find Using this in eqn (18) for A(r, t ) and taking Fourier V x V x A(r, co) - co2pos(r, w)A(r, o) = 0 (21) where E(r, w) = E,[ 1 - m3r)/w2] w:(r) = e2n(r)lmc,. Thus, a position and frequency dependent permittivity depending on the local plasma frequency w, replaces so in the Helmholtz eqn (21) for A .After solving this equation we obtain a vector potential A(r, t ) to use in the perturbing term A.V$ in eqn (19) (the term in A2 can be dropped as usual). In our problem, the substrate occupies the half-space z < 0, and consequently in the first approximation, n(r), and hence E(Y, co), is simply a step function of z at z = 0. We learn from this approximate solution of the coupled eqn (18) and (19) that the inclusion of the plasma contribution to the permittivity in calculating the vector potential to use in the p-A interaction is well- grounded. Further developments are possible but I do not pause to consider them here because even the above simple development, whilst of some fundamental interest, does not seem to have important consequences in our present field of interest. It affects the magnitude, and the spatial variation of A at the surface.The magnitude of A affects the absolute value of the photocurrent in photoemission, and of course the absolute rate of absorption of photons in any photochemical reaction at the surface. The spatial variation of A will be masked in UPS by the short mean free path in the substrate of the ejected electron. 4. PHOTODESORPTION The formal theory of photodesorption is straightforward. Consider the photo- desorption of a single adsorbed molecule. The particles (electrons and nuclei of the substrate and the adsorbate) and the electromagnetic field are in the initial state linit) when the field-particle coupling is switched on.We require the transition rate to final states Ifin) in which there is either an outgoing adsorbate molecule or ion. This is (cf. eqn (1)) w = (finlTlinit)} 2S(Efi, - Einit) (23) where T is the T-matrix corresponding to the field-particle interaction V. Since V is small, T = Vis a good approximation. To make progress we employ the adiabatic approximation for the electronic motion, and write the particle wavefunction Yinit for example as in ternis of the electronic and nuclear coordinates Y and R. If we evaluate the matrix element of the electronic transition i-+fat the equilibrium nuclear configuration in the initial state Ro then (fin1 Ylinit) 21 ( F ( l ) ( f ; R,I Veli; R,) where V , is the p A interaction between electrons and photons, and (FII) is the familiar Franck-Condon factor between initial and final states of the nuclear motion on the potential energy hypersurfaces E,(R) and E,(R) associated with the electronic states t,hi and t,hf.Consequently y i n i t ( r 7 R) = $i(r ; R)XiI(R) w = h-'I(Fll)(f; RolVJi; R,)l26(Ei1+fiW--EfF) (24)16 INTRODUCTORY PAPER where E,, and Efp are the initial and final state energies of theparticles. Further progress seems to me to be quite difficult. If we could introduce a reaction coordinate s which changed only slowly on the time scale of all other nuclear coordinates Q so that x(R) 21 4MQ ; s) we could arrive without difficulty at the concept of potential energy curves Ef(s) and E,(s) along the reaction coordinate.18 It seems quite impossible to justify this concept in our problem, but I employ it simply to draw fig.6 to illustrate photo- desorption. The lowest excited electronic state will often correspond to charge (a) cations (b) neutrals FIG. 6.-Photodesorption of cations and neutrals. transfer between the adsorbate and the substrate, so according to eqn (24), ions are photodesorbed. The threshold frequency for this process on a metal will be ( I - 4 + D)/h for cations, and (6 - A + D)/h for anions. Z and A are the ionization potential and electron affinity of the adsorbate, D is its binding energy on the metal with work function 4. The threshold for photodesorption of neutrals is of course D/h but the process is a second order one requiring T N V+ VGo Y in eqn (23) so that w = 6+ 1 6(EiI + Ao - (25) The charge transfer states giving photodesorbed ions in first order are included in the summation over intermediate states in eqn (25).The second order process with threshold D/h is depicted in fig. 6. Of course neutrals are photodesorbed in first order, but they are in an excited electronic state, and the threshold exceeds D/h by this excitation frequency. For the photodesorption of CO from polycrystalline tungsten there is a threshold near 250 nm (about 4.8 eV) corresponding apparently to CO- ions in the final state.lg It is one task of chemisorption theory to calculate the potential energy hypersur- faces Ei(R) and Ef(R) but I do not think it useful to discuss the problem further because experimental data on photodesorption is scarce, and chemisorption theory is not yet developed to the point where potential energy curves on extended substrates can be calculated to the required accuracy. <FIJ><JlO<f; RolblA ROXA RolVeli; Ro) j J - E j JT.B . GRIMLEY 17 5. PHOTOCHEMICAL REACTIONS ON SURFACES I am not competent to make anything more than some general remarks, and to pose some no doubt obvious questions, on this subject. A review paper will be presented later by Prof. Gerischer. Owing no doubt to our inadequate knowledge of the electronic and geometric structures of surfaces and adsorbates, most of the fundamental theoretical problems in surface photochemistry remain untouched. Consider for example the process CO +2CH4+ Ci +T~co+CH,O+C,H, + Cf where Ci and Cf represent initial and final states of the condensed matter (substrate plus adsorbed species) on which the reaction takes place.The thermal reaction (26) in the homogeneous gas phase is endothermic (AH;98 = 60 kJ/mol CH20) but the photochemical reaction probably takes place on nickel (Bach and Breuer, this Dis- cussion). The fundamental theoretical problem can be posed in connection with any reaction like (26), namely to calculate the rate of reactive transitions out of an initial state /init> to a final state Ifin). Less fundamental but still very challenging problems are involved in the following questions : (a) Where (insofar as this question can be asked in quantum theory) in the condensed matter is the photon absorbed, and what becomes of the hole? (b) Can we begin to calculate electronically adiabatic potential energy hyper- surfaces for the reaction? (c) Can any other coordinates be treated adiabatically? Experiment can help theory here by identifying reaction intermediates, proposing mechanisms, and suggesting a critical elementary step in the overall reaction.As already mentioned, efforts are now being made to develop the theory of chemisorption on extended substates to an accuracy, comparable at least to that obtained in treating small molecules. The possibility of calculating the rate of an elementary hetero- geneous reaction on electronically adiabatic potential energy hypersurfaces depends on the outcome of these efforts. I am grateful to Dr. Bernasconi for some valuable discussions on the theory of photoemission. L. Hedin and S. Lundquist, Solid State Phys., 1969, 21, 1. C. B. Duke, Proc. Int. School of Physics " Enrico Fermi " Course LVIII (1973). To be published in Nuovo Cimento. €3. A. Lippmana, Phys. Rev. Letters, 1965, 15, 11 ; 1966, 16, 135. I. Adawi, Phys. Rev., 1964, 134, A788. G. ID. Mahan, Phys. Rev. By 1970,2,4334. W, L. Schaich and N. W. Ashcroft, Phys. Rev. B, 1971,3,2452. ' D. R. Penn, Phys. Rev. Letters, 1972, 28, 1041. * see for example L. I. S e w , Quantum Mechanics (McGraw-Hill, New York, 1968), p. 420 ; J. W. Gadzuk (J. Vac. Sci. Tech., 1974, 11, 275 and unpublished work) has considered photoemission from adatoms into plane wave final states. A. Liebsch, Phys. Rev. Letters, 1974,32, 1203. K. F. Wojciechowski, Acta Phys. Polonica, 1966,29, 119 ; 1968,33, 363. Physics " Enrico Fermi " Course LVIII (1973). To be published in Nu0z.o Cimenta. LVIII (1973). To be published in Nuavo Cimento. Okamoto, M. Onchi and Y. Tarnai (Mawzen, Tokyo, 1974), p. 72. lo T . Toya, J. Res. Int. Catalysis (Japan), 1958, 6, 308 ; 1960, 8, 209. l2 J. R. Schrieffer and R. Gomer, Surface Sci., 1971,25,315 ; J. R. Schrieffer, Proc. Int. School of l3 T. B. Grimley, J. Phys. C, 1970,3,1934 ; Proc. Int. School of Physics " Enrico Fermi " Course l4 T. B. Grimley, Structure and Properties of Metal Surfaces, ed. S . Shimodaira, M. Maeda, G.18 INTRODUCTORY PAPER W. Brenig and K. Schonhammer, 2. Phys., 1974, 9, 1. l6 T. B. Grimley and C. Pisani, J. Phys. C, 1974, 7, 2831. l7 P. W. Anderson, Phys. Rev., 1961, 124, 41. J. 0. Hirschfelder and E. P. Wigner, J. Chem. Phys., 1939, 7, 616; R. D. Levine, Quurzturn Mechanics of Molecular Rate Processes (University Press, Oxford, 1969), p. 215. P. Kronauer and D. Menzel, Ansor~tion-Desorptin Phenomena, ed. F. Ricca (Academic Press, London, 1972), p. 313.

ISSN:0301-7249    

DOI:10.1039/DC9745800007

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

A. VALENCE-ELECTRON LEVELS OF ADSORBED MOLECULES Theory of the Angular Dependence of the Photoemission Line Shape from an Adsorbate BY A. LIBBSCH AND E. W. PLUMMER" Department of Physics, University of Pennsylvania, Philadelphia, Pa. 191 74 Received 17 June 1974 A theory of the angular-resolved photoemission from localized adsorbate orbitals is presented in which the effects of the final state are discussed in detail. This theory is applied to the problem of line shapes and peak positions in a photoemission energy distribution from an orbital localized at the surface. The interference of the wave scattered elastically from the periodic potential of the solid with the wave emitted directly from the adsorbate can cause apparent splitting of a single adsorbate level and l e v shifts in peak positions.In a previous paper a " one-step '' model for photoemission was developed and applied to photoemission from ads0rbates.l The essential ingredient of this model was an appropriate description of the Bloch character of the final state of the excited electron by using a multiple-scattering theory such as has been applied in EEED calculations.2 The purpose of the original paper was to demonstrate that the angular distribution of the photoemitted electrons from a localized level is dominated by the symmetry of the adsorption site as well as the symmetry of the bonding orbital^.^ In this paper the theory, which will be described below, will be applied to a more practical problem: the line shape and position of the peak in the photoemissioii spectrum from an adsorbate as a function of the angle of detection and photon energy. Recent experimental and theoretical l * work has stimulated considerable interest in angular-resolved photoemission from surface adsorbates.The angular dependence of the emission from a localized surface orbital may well yield information about the symmetry of the bonding site as well as the symmetry of each 0rbita1.~ On the other hand, if an experiment is conducted with a fixed angle of collection these effects may become a liability when comparing other experimental data or theoretical calculations of the ground-state orbitals. What one measures will depend upon the angle of collection as well as the incident photon energy. Calculations for a simple model of a hydrogenic adsorbate on a cubic crystal predict that the measured photo- emission spectra from this localized surface orbital may have its peak shifted by 1 eV or in some cases appear as a double peak, depending upon the final-state energy, adatoni position and angle of collection.These types of effects seem to have been observed in two cases : (1) CO adsorption on Ni( 11 1) where the CO peak splits as the crystal is rotated, and (2) Egelhoff seems to see a splitting in the hydrogen level at 5 eV below the Fermi energy on (1OO)W when he collects 14" off normal, whereas Pluinmer and Waclawski do not see a splitting when collecting over all azimuthal angles from zero to 45" in polar angle.4 THEORY The photoemission process is characterized in our microscopic " one-step '* model by the description of the initial and the final state of the system.It is con- venient to use a linear combination of atomic orbitals for the initial state as has been 1920 ANGULAR DEPENDENCE OF THE PHOTOEMISSION LINE done previously by several other This approach permits a simple incorporation of the adsorption geometry and can easily be extended to hybridized adsorbate-substrate orbitals as well as LCAO's appropriate to the substrate itself The new feature of the model lies in the description of the final state. As it is to be expected on the basis of low-energy electron diffraction experiments, the coherent elastic scattering of the excited electron by the lattice potential is strong in the energy range of interest in photoemission. This fact indicates that the final state must be described by its full Bloch wave-function, since the plane-wave final state approxima- tion does not incorporate the multiple scattering.Furthermore, the electronic mean free path, Ze, at these energies is of the order of a few lattice spacings as a consequence of strong inelastic electron-electron interactions. We therefore employ a multiple scattering formalism similar to those that are currently being used in analysis of LEED intensities.2 The matrix elements between the initial and the complete final state are then evaluated numerically without any further assumptions. Several points regarding the adequacy of the above model and its relation to existing photoemission calculations are noteworthy. First, no attempt has been made at this point to include the presence of the hole that is left behind by the excited electon.Thus, possible relaxation effects as well as exciton-like interactons of the outgoing electron with the bole are so far not taken into consideration. Secondly, for infinite systems (i.e., extended initial states) and in the limit of weak electronic damping, the present model becomes similar to band structure calculations of photoemission intensities.' Since most materials, however, show rather short or medium mean free paths and since the band-structure approach is by its very nature limited to bulk photoemission, the above described model is, in principle, more appropriate. VACUUM SOU D 0 0 0 e 0 1" FIG. 1 direct .-Illustration of two processes contributing to photoemission from adsorbate pz orbital : (1) emission into plane-wave final state, (2) indirect emission via backscattering from substrate.Only single scattering from the first layer is indicated. In order to illustrate the influence of the coherent scattering, we consider the excitation of an electron from a localized s or p orbital adsorbed on the substrate. The two processes that contribute to the scattering amplitude for the adsorbate signal are the direct emission from the orbital into a plane-wave final state, lkf>, and the indirect emission from the orbital via backscattering from the substrate lattice potential. Both are indicated schematically in fig. 1. For a given polarizationA . LIEBSCH AND E. W. PLUMMER 21 vector A inside the solid and a photon energy ha, the cross-section is a function of final electron energy Ef and detector angles 0, and Qf.It is given by the expression Here, 4R is the wave function of the orbital adsorbed at the site R and Ei its energy level. In the case of surface molecule^,^ (bR is to be replaced by the linear combination of orbitals appropriate to the adsorbed atom or molecule and one or several substrate atoms. T = VL+ VLGT is the T-matrix corresponding to the lattice potential VL and G is a free-electron propagator with an appropriate self-energy inserted to include inelastic effects. The first term of the matrix element in eqn (l), i.e., the direct emission in the absence of scattering, is simply proportional to the Fourier transform of the initial state : (2) where Fn,, and Ylm are the radial and the angular part of 4(kf), respectively. The second term of the matrix element, i.e., the indirect emission from the adsorbate via backscattering from the substrate, can be evaluated for the general case of multiple elastic scattering and arbitrary atomic potentials. For simplicity, however, we present here only the result for single scattering from s-wave scatterers : 4 E f , @,, Qf) I(kfl(1 + T a p = A I 4 R ) l 2 ~ ( E f - - E i - ~ ~ ) .(1) YIP A I ~ R ) = exp(-ikf R )hkf A F A ~ ~ I ) Ylrn(k,>, ( ~ ~ I T G P AIW + ~ X P (-ikf * ~)~,l(IkfI)t(IkfI>(l/a~) C Y,rn(kfll +O, - k f l ( g ) > 0 where The g are the reciprocal lattice vectors of the Bravais net parallel to the surface and a is the lattice constant. The normal and parallel position of the adatom relative to the substrate are denoted by dl and dll, respectively.The quantity t( lkfl) represents the s-wave component of a single-site scattering vertex in the substrate, Vo is the inner potential and T(E) is the imaginary part of the optical potential. The above result exhibits the following physical features. (1) The contribution to the matrix element due to backscattering is no longer proportional to the angular part of the Fourier transform Yzrn(kf) of the initial state. Instead, the expression involves the Fourier components corresponding to directions in k-space that differ from kf by a reciprocal lattice vector parallel to the surface and that point into the crystal rather than to the detector. (2) Similarly, the matrix element is no longer pro- portional to kf * A as in the absence of scattering but rather involves the factors (k,ll +g, -kfL(g)) .A.(3) For a given detector angle, the structure in the intensity as function of final energy Ef is determined by the band structure of the substrate, In the specific case considered in eqn (3), resonance energies occur for Re rc(g)a = 2nn, y1 integer, i.e., where En = (li2~nfa)~/2m, Eg = (lig)'/2m, and Eli = E sin2 Of. These energies coincide with band crossings in the corresponding free electron band structure. (4) The geometry of the adsorption site enters the matrix element only via two phase- factors, one for the normal position dL and one for the parallel displacement d , ~ of the Ef + Yo = Ell + (En + Ee + kf 11 gh2 Jm)2 J4En, ( 5 )22 ANGULAR DEPENDENCE OF THE PHOTOEMISSION LINE adatom relative to the underlying substrate.This is a particularly attractive feature since it permits the separation of adsorbate and substrate geometry. The positions at which extrema in the intensity occur are entirely determined by the symmetry of the substrate whereas the relative intensities of these extrema are determined by the adsorption site. RESULTS In order to illustrate some of the consequences of the theory outlined above, we show in fig. 2 the intensity as function of final energy for emission along the surface normal from an s orbital bound in two configurations.8 The curves represent the intensity for the case of single scattering (solid curves), multiple scattering (dotted curves), and in the absence of scattering (dashed curves), The three-dimensional FIG.^.-Intensity (arbitrary units) as function of final electron energy for emission from an s orbital adsorbed in position (0, 0, a), (a) ; and ( 4 2 , a/2, a/2), (6) relative to the substiate : Single scattering (solid curves), multiple scattering (dotted curves), and no scattering (dashed curves). The arrows indicate the resonance energies specified by the reciprocal lattice vectors, and the corresponding band crossings in the free-electron band structure at the tap of the figure.I I I 1 + S ORBITAL Ef =6eV A =(0,0,1) (b) CENTRES - 0.5 - 0 30 60 0 30 60 90 final polar angle 6'' FIG. 3.-Photoemission intcnsity (arbitrary units) as function of final polar angle for an s orbital ad- sorbed in top (a) and centre (b) positions ; single scattering (solid curves), multiple scattering (dotted curves), and no scattering (dashed curves).2.5 2 .o * h 1.5 5 .- v) * .- E: .C( *i 1 .o 0.5 '3.4 4.4 5.4 6.4 ' final energy/eV I FIG. 4.-Photoemission intensity (arbitrary units) as function of final electron energy for an s orbital adsorbed in top position. The solid curves represent the intensity along four different detector angles : 6''. The dashed curve shows the intensity in the absence of scattering. The arrows indicate the effective peak position caused by final-state effects.24 ANGULAR DEPENDENCE OF THE PHOTOEMISSION LINE reciprocal lattice vectors specify the resonance energies, eqn (9, and the corresponding band crossings as indicated in the free-electron band structure at the top of the figure.The effect of backscattering from the substrate is seen to be of the order of 20-50 % relative to the intensities in the absence of ~cattering.~ It is, however, rather remarkable that the multiple scattering intensities agree so closely with the single scattering intensities. We believe this to be a consequence of the fact that the single scattering resonances coincide not only with the reflection point of the first band, i.e., the Bragg energies as is the case in LEED, but also with the intersections of the first band with all higher bands.1° This is due to the circumstance that the adatom acts as a spherical source in contrast to the incoming plane wave in LEED. While this point requires further detaiIed study, it might prove to be of considerable practical interest in that single- or double-scattering approximations to the final state in photoemission from adsorbates are much more adequate than in LEED.Comparing panels (a) and (b), we notice that the change of the adsorption site from (0, 0, a) to (a/2, 4 2 , a/2) inverts some of the maxima to minima due to the presence of the phase factors as described above. It is apparent from eqn (5) that in normal direction all resonances are degenerate with regard to various vectors g whose components have opposite signs. At finite angles 0, and @,, however, these resonances split very rapidly indicating that the photoemission intensity exhibits a strong angular dependence. As an example we show in fig. 3 a figure from ref. (1) in which the intensity is plotted as function of polar angle 0, for two adsorption sites at a fixed energy E,.In the absence of S OR81TAL W = 2 e V 4= 40" + f = 0 3.4 4.4 5.4 6.4 7.4 - - . _ _ kina1 energy/eV angle is 4Q" ; otherwise as in fig. 4. FIG. 5.-Intensity as function of final energy for two different electronic mean free path. The detectorA . LIEBSCH AND E. W. PLUMMER 25 scattering, the intensity is a smooth function proportional to k, A (dashed curves). In the limit of single scattering (solid curves), the maxima that are seen in panel (a) for the top position are inverted into minima in panel (b) for the centre position because of the phase factor associated with the parallel displacement. In both cases, the effect of multiple scattering (dotted curves) tends to smooth out the single-scattering intensities.The intensity also varies with azimuthal angle and the type of orbital chosen for the ads0rbate.l In the example above and in ref. (I), we have assumed that the initial state has a sharp energy level, Ef. For a given photon energy, hco, we accordingly observe only electrons with a kinetic energy given by Ef = Ei+ttcv (no relaxation). We now consider the case where the initial level can be described by a Gaussian distribution centred around Ei with a full-width at half-maximum given by W. For a particular photon energy, we then observe emitted electrons with kinetic energies approximately in the range E,+ko- W < Ef 5 Ei+ ha+ W. Because of the coherent scattering in the final state, the photoemission intensities exhibit a considerable amount of structure over this energy range, thus causing the observed peak to deviate in shape and position from the Gaussian distribution centred around Ei + hco that one would obtain for plane-wave final states.This effect is shown in fig. 4, in which intensities are plotted for emission from an s orbital for four different detector directions 0, (solid curves). The dashed curve gives the corresponding distributioii in the absence of scattering, i.e., for plane-wave final states. Over the indicated range of 60°, the peak is seen to greatly vary its shape and to change its position by about 2 eV. At I .5 CI A I .o * 8 -g .- c .I E .I UI 0 . 5 n ,---1- v 3.4 4.4 5.4 6.4 7.4 final energy/eV detector angle is 40" ; otherwise as in fig. 4.FIG. 6.-Intensity as function of final electron energy for two different initial state widths. The26 ANGULAR DEPENDENCE OF THE PHOTOEMISSION LINE 0, = 60°, a weak splitting of the peak takes place. Fig. 5 illustrates the dependence of the observed peak on the electron damping length. Since strong damping (i.e., small Ze) diminishes the importance of scattering, the peak distortion and shift are less pronounced for Ze = 12 A than for Ze = 20 A. 2.5 2 .o x -3 1.5 8 c.. .* c c 0 .I-# v1 3 1.0 0.5 n - S ORBITAL U - 12 - 10 -8 initial energy/eV detector angle is 40"; otherwise as in fig. 4. FIG. 7.--lntensity as function of initial electron energy for three different photon energies. The We show in fig. 6 the influence of the width of the initial level on the observed peak.It is obvious that the final state effects will be more significant for broad levels than for narrow ones. (For infinitely sharp levels, the peak position is unaffected and only the relative intensities are modified.) Finally, in fig. 7 we show the effect of changing the photon energy. The scattering effects depend upon the final energy so they will change as the photon energy is varied, as is seen in fig. 7. This points out an obvious check for initial or final state effects in line shape. In summary, we have shown that : (a) the correct final state can cause considerable structure in the measured intensities, as a function of the angle of detection ; (b) this structure is closely related to the substrate bonding symmetry; and (c) for a reasonably wide adsorbate level (1-2 eV) the peak position and peak shape in the photoemission spectra depends on the angle of detection, the electron escape depth and the final-state energy.A. Liebsch, Phys. Rev. Letters, 1974, 32, 1203. For a review of LEED theories see, for example, G. E. Laramore, J. Yuc. Sci. Tech., 1972,9,625.A . LIEBSCH AND E . W. PLUMMER 37 For photoemission from surface molecules into plane-wave final states, Gadzuk (J. Vac. Sci. Tech., 1974, 11, 275, and Solid State Comm. (in press)) has demonstrated that the angular re- solved intensity is highly anisotropic reflecting both the adsorption geometry and the orientation of the chemisorption orbitals. While we believe the adequate description of the initial state to be crucial for a quantitative analysis of photoemission spectra, we restrict our discussion in the present paper to localized adatom levels in order to isolate the angular anisotropy due to the Bloch character of the final state.U. Gerhardt and E. Dietz, Phys. Rev. Letters, 1971, 26,1477 ; T. Gustafsson, P. 0. Nilson, anti L. Wallden,Phys. Letters, 1971,27A, 121 ; N. V.Smith and M. M.Traum,Phys. Rec. Letters, 1973, 21, 1247 ; L. Wallden and T. Gustafsson, Physica Scripta, 1972, 6, 73 ; R. Y. Koyama and I-. R. Hughey, Phys. Rev. Lett, 1972,29,1518 ; R. H. Williams, J. M. Thomas, M. Barber and N. Alford, Chem. Phys. Lett., 1972, 17, 142. Also, private com~nunications with B. J. Waclawski (0, on (100) W) and W. F. Egelhoff (HZ on (100) W). D. E. Eastman and J. E. Demuth, to be published in J. Vuc. Sci. Tech. W. L. Schaich and N. W. Ashcroft, Phys. Rev. B, 1971,3,2452 ; and D. R. Penn, Piiys. Rev. Let- ters, 1972, 28, 1041. ' See, for example, N. V. Smith, Phys. Rev. Letters, 1969,23, 1222 ; N. E. Cristensen and B, 0. Seraphin, Php. Rev. B, 1971, 4, 3321 ; and A. R. Williams, J. F. Janak, and V. L. Morruzzi, Phys. Rev. Letters, 1972, 28, 671. * A11 calculations are for the (001) surface of a simple cubic crystal and, unless otherwise specified, the following parameters are used : Yo = 10 eV (inner potential), 1, = 6A (mean free path), 6, = 77/2 (s-wave phase shift), a = 4&lattice constant), and A = (O,O,l). All curves are divided by [kflF&(lkfl) so that the intensity in the absence of scattering is energy independent. The initial energy can then be taken as arbitrary. For realistic potentials, thz magnitude of these effects is likely to be even higher. ances. ' O The remaining crossings between higher-lying bands correspond to multiple-scattering reson -

ISSN:0301-7249    

DOI:10.1039/DC9745800019

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Calculation of the Electronic Structure of Ethylene Bonded to Diatomic Nickel and Correlation with Ni-C, H, Photo-Emis- sion Data* BY N. R~SCH AND T. N. RHODIN School of Applied and Engineering Physics, CorneIl University, Ithaca, N.Y. 14850 Receiued 30th May, 1974 The electronic structure of a model “ surface complex ” consisting of a Ni-diatom and an ethylene molecule has been calculated using the SCF-Xa scattered-wave method. Comparison of calculated n-orbital bonding shifts with recent photoemission spectra for chemisorption of ethylene of nickel favours a n-bonded over a o-diadsorbed complex. Charge distributions of various molecular orbitals indicate that the C-C double-bond is weakened more for the latter molecular arrangement suggesting such a complex as a likely intermediate in heterogeneous reactions.Reliable microscopic information on the interaction of transition metals with unsaturated hydrocarbons is becoming available. 9 Photoemission (UPS) studies of acetylene, ethylene and benzene chemisorbed on a Ni( 1 1 ])-surface have yielded the first direct observation of n-d bonding. Using the self-consistent field-Xa scattered wave (Xa-SW) m e t h ~ d , ~ a reasonably accurate calculation of the electronic structure of Zeise’s anion and especially of the Pt-ethylene bond has been given re~ently.~ We report here the first Xu-SW calculations for a Ni-ethylene ‘‘ surface complex”.’ They were undertaken in order to provide an additional test for the local bonding approach to chemisorption,6 to gain further information on the electronic structure of a chemisorbed ethylene molecule, and possibly to extract information from the photo-emission spectrum on the geometry of the “ surface complex ”.that ethylene (C,H,) binds to transition-metals through an interaction of its n electrons with the metal d-electrons. However, the geometry of the chemisorbed ethylene (neglecting for the moment possible dehydro- genation etc. after chemisorption) relative to the surface atoms is controversial. One assumption is that ethylene forms a n-complex coordinating symmetrically to a single metal atom, as suggested by Dewar 7* and found in many organometallic complexes. From various surface reactions 9 9 O and from infrared-spectra of ethyl- ene chemisorbed on silica-supported Ni,’ it has been concluded, however, that each of the two carbon atoms forms a o-bond to a different metal atom.As a model for the “surface complex” we choose a Ni-diatom with a bond length of 2.492A (4.709 a.u.) equal to the nearest neighbour distance in bulk Ni and an ethylene molecule with equilibrium geometry. Corresponding to the two possible bonding schemes, two different geometrical arrangements have been studied. In the complex representing the a-diadsorbed ethylene, the Ni-Ni bond has been taken parallel to the plane of the ethylene molecule ; in the n-complex the N i N i bond was * Supported by NSF Grant GH-31909, and by the Advanced Research Projects Agency through the Cornell Materials Science Centre. -f permanent address : Lehrstuhl fur Theoretische Chemie, Technische Universitat, Miinchen.West Germany. It is commonly accepted 7* 28N. ROSCH AND T. N. RHODIN 29 arranged perpendicular to that plane and symmetric to the C-C bond. Both model complexes exhibit Cz, symmetry. The distance of chemisorbed ethylene to the metal surface is unknown. Guided by the sum of covalent radii and by distances found in X-ray (1.9-2.2 A) crystallo- graphic studies of Ni-olefin complexes, a value of 2.0A was chosen for the distance between the two parallel bonds in the di-o-complex and for the distance from the bonding Ni atom to the centre of the C-C bond in the n-complex. (According to previous X-a calculations the results should not be sensitive to small changes in this distance.) The radii of the muffin tin spheres surrounding the various Ni atoms were chosen to be half the Ni-Ni bond length.For ethylene we have used the over- lapping sphere parameterization ' (parameter set D of ref. (12) with rout = 4.828 a.u. ; n-complex: rout = 6.283 a.u.). The atomic exchange parameters a for carbon and FIG. 1.-SCF-Xa-SW electronic energy levels for the model complex Niz-C2H4 : 7r-complex (" r ") against di-o-complex (" di-a "). Also shown are the Xot-SW levels of Niz and CzH4. The levels are filled up to the Fermi level Ef, marked by a dashed line. Other levels (not shown) are not involved in the metal-ethylene binding. hydrogen were taken from spin-polarized atomic calculations,' the U-value for nickel was taken from the tabulation by Schwarz.13 The a-values for the inter- atomic and the extramolecular region were set equal and taken as the weighted average over the atomic a values, where ethylene was given the same weight as one Ni atom.4 The core charges corresponding to the configurations C ls2 and Ni ls22p6 as determined from atomic calculations were included but kept fixed during the SCF cycle^.^ The remaining 48 electrons were fully taken into account in the iterations to self-consistency according to the usual Xa-SW procedure.3p30 BONDING OF ETHYLENE TO DIATOMIC NICKEL The resulting Xa-SW ground-state orbital energies for the two geometries are compared in fig.1. Only low-lying unoccupied and the valence-type orbitals with energies above -0.7 Ry are shown. The levels are labelled according to the irreduc- ible representations of the point group C2v and filled as marked up to the Fermi level, Ef. Also shown in fig.1 are the Xa-SW energy levels of Ni2 l4 and C2H4 l2 which fall in the displayed range; for ethylene these are the n- and +-level bSu and b2g, respectively, and the highest a-level bSg. To discuss and compare the bonding in the two model complexes we make use of the orbital charge distributions as obtained from the SCF-Xu-SW calculations as well as of contour plots of the individual orbitals (some typical ones are indicated in fig. 2 and 3). Thereby a clear assignment of the levels to the two components of each model complex is possible. The levels between -0.3 and -0.4 Ry correspond to the Ni diatom and represent the Ni d-band. They undergo some mixing and a slight upward shift due to the interaction with the ethylene.This parallels a reduction of the Ni work function A+ = -0.9 eV found after chemisorytion of ethy1ene.l Taking the differences of the highest occupied Xcc-SW orbital energies in each model complex and Ni2, we find A& = -0.60 eV in the n-complex and = -0.90 eV in the di-a-complex. The ethylene remains mostly unchanged in both complexes as can be inferred from the charge distributions of the corresponding orbitals. This is consistent with experi- mental informati~n,~. especially with the structure of the photo-emission peaks corresponding to a-1evels.l The only level found to interact strongly is the n-level b3*, again consistent with the photo-emission spectrum. l According to the Dewar- Chatt scheme for a n-complex, electrons of this level are donated to the metal forming a a-bond and donated back from the metal into the empty n*-level of C2H4 giving rise to a n-bond.Both these components of the Ni-ethylene bond may be identified from contour plots (fig. 2, (a) and (c)). As found in Zeise’s anion,4 but to a much lesser degree, the n-level also mixes into other orbitals, especially the a,-level contributing most to the Ni-Ni bond (fig. 2 (b)). The metal-ethylene bond in this n-complex is therefore expected to be weaker than in Zeise’s anion.N . ROSCH AND T . N . RHODIN r - ' I b, e = - 0.377 Ry 31 FIG. 2.-Contour plots for individual bonding orbitals of the n-complex. The contour values increase in absolute magnitude with increasing absolute values of the contour labels. The sign of the labels gives the sign of the orbital lobes.The selected set of the contour values plotted is the same for each of the three orbitals. (a) The ethylene n-orbital ; (b) the al orbital giving the main contribution to the Ni-Ni bond ; (c) the bl orbital showing significant r*-back bonding. For the di-a-complex we find essentially the same two-way interaction mechanism, giving rise, however, to two a-bonds, one from each carbon atom to the corresponding Ni atom. The charge donation out of the ethylene n-level is equally strong as in the n-complex, but divided among the two Ni atoms (fig. 3, (a) and (6)). On ths other hand, the back-donation out of a Ni2 anti-bonding level into the n::-orbital of ethylene32 BONDING OF ETHYLENE TO DIATOMIC NICKEL is approximately 50 % greater than in the n-complex, leading to a weaker C-C bond (fig.3(b)).11 The shift of the ethylene n-level due to n-d bonding has been derived from the photo-emission spectrum of chemisorbed ethylene to be = 0.9kO.1 eV.I In the Xu-SW approach, ionization potentials are normally calculated by invoking Slater's transition-state proced~re,~ and thereby taking relaxation effects into account. For comparable systems this relaxation is found to be nearly equal.12 As the ethylene subunit is preserved to a large degree in both model complexes, we estimate the shift of the ionization potential of the n-level upon chemisorption by taking the difference of the corresponding Xa-SW orbital energies. The accuracy of this procedure should be sufficient to compare with the value derived from measurements with an energy resolution de - 0.1 eV.l.l5 We find from our calculations de, = 0.72 eV for the n-complex and de, = 0.30 eV for the di-a-complex. The results of our calculation for the shift of the n-level and the change in the work function favour, when compared with the experimental values, the n-complex. The di-a-complex may play the role of a reaction intermediate for various surface reactions 9 9 lo due to its weaker C-C bond. The photo-emission spectra alone do not allow a discrimination between the two geometries for the " surface complex ", as n-d interaction is possible in both cases. These conclusions, however, have to be drawn with due caution because the model complexes are rather simple ones. First, one should increase the number of metal atoms to ensure a realistic description of the model '' surface 7 7 .Also, the geometry of the adsorbate should be varied, as it is known, for instance, that the ethylene subunit in Zeise's anion is not planar.16 (The effect on the ionization potentials is expected to be minor). As a next step, the model presented here should be tested for different adsorbate molecules, e.g., acetylene and ethane, and various other metal substrates. Such investigations are under way and will be reported elsewhere. The calculations presented here have shown how a local bonding approach to chemisorp- I I I FIG. 3 (a)N. ROSCH AND T. N . RHODIN 33 FIG. 3.-Contour plots for orbitals of the di-a-complex. The orbitals shown correspond to those of fig. 2. The contour values are the same.tion the bonding of hydrocarbons to a metallic diatom. implemented with the SCF-Xu-SW method can yield unique information on We thank Drs. J. E. Demuth and D. E. Eastman for making their results available before publication. We are particularly grateful to Prof. K. H. Johnson for providing us with the Xa-SW computer programmes and helpful advice in the use of the method. 58-B34 BONDING OF ETHYLENE TO DIATOMIC NICKEL Useful discussions with Dr. R. H. Paulsen are also acknowledged. The continued in- terest and support of Prof. Roald Hoffmann in this work is sincerely appreciated. One of us (N. R.) is also grateful to the Deutsche Forschungsgemeinschaft for a sti- pend. J. E. Demuth and D. E. Eastman, Phys. Rev. Letters, 1974, 32, 1123. E. W. Plummer et al., private communication. J. C. Slater and K. H. Johnson, Phys. Reu. B, 1972,5, 844. N. Rosch, R. P. Messmer and K. H. Johnson, J. Arner. Chem. SOC., 1974,96, 3855. T. B. Grimley, in Molecular Processes on Solid Surfaces, ed. E. Drauglis, R. D. Gretz and R. 1. J d e (McGraw-Hill, New York, 1969), p. 299. K. H. Johnson and R. P. Messmer, J. Vac. Sci. Tech., 1974, to be published. J. Chem. SOC., 1953,2939. G. C. Bond, Disc. Faraday SOC., 1966,41,200. G. C. Bond, CataZysis by Metals (Academic Press, New York, 1962), p. 234. (Academic Press, New York, 1973), vol. 23, p. 91. B. A. Morrow and N. Sheppard, Proc. Roy. SOC. A, 1969,311,391. ' (a) M. J. S . Dewar, Bull. SOC. Chim. France, 1951,18, C79 ; (b) J. Chatt and L. A. Duncanson, lo (a) G. I. Jenkins and E. K. Rideal, J. Chem. Soc., 1955, 2491 ; (6) J. H. Sinfelt, Ado. CatalysiJ I2 N. Rosch, W. G. Klemperer and K. H. Johnson, Chern. Phys. Letters, 1973, 23, 149. l3 K. Schwarz, Phys. Rev. B, 1972, 5, 2466. l4 We thank Dr. R. Paulson for kindly providing us with these results. J. M. Baker and D. E. Eastman, J. Vac. Sci. Tech., 1973, 10, 223. l6 W. C. Hamilton, K. A. Klandermann and R. Spratley, Acta Cryst. A, 1969, 25, S172.

ISSN:0301-7249    

DOI:10.1039/DC9745800028

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Ultra-violet Photoemission Studies of CO, N2 and C Adsorbed on W( 100) BY WILLIAM F. EGELHOFF, JOHN W. LINNETT AND DAVID L. PERRY Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EP Received 12th July, 1974 Ultra-violet photoelectron spectra are reported for C, CO and N2, and mixtures of these, adsorbed on W(100) at room temperature. The results have been compared with simultaneous flash desorption and work function measurements, and with known LEED data for the same adsorption systems. Sequential removal of the j?-desorption states of N2 and CO results in marked changes in the photo- electron spectrum, demonstrating considerable surface rearrangement during the temperature flash. Comparison of the spectra of the c(2 x 2) adsorbate structures of C, CO and N2, and also of higher coverages, including mixtures, suggests that the adsorbate spectra are determined more by the structure of the adsorbed layer than by its chemical nature.Ultra-violet photoelectron spectroscopy (UPS) has been used in a number of studies to determine the orbital energy levels of species adsorbed on metal surfaces. The results show varying degrees of complexity. In general, species adsorbed in a molecular state (e.g. a-CO on W,' methanol on p-CO saturated W,2 CO and hydro- carbons on Ni) have yielded photoelectron spectra showing energy levels which can be correlated with those of the free molecule, whereas dissociative adsorption (e-g. H2 or N, on W 5 * and CO on Mo 7, gives rise to spectra which are more difficult to interpret. This is indicative of the more extensive overlap between the adsorbate and metal orbitals.UPS has also been of use in the evaluation of results obtained by other surface techniques. For example, flash desorption has been widely used in recent years to investigate the presence of different chemisorbed states on a surface. It is important, however, to differentiate between (a) multiple desorption states arising from distinct adsorbed states which coexist at the temperature of adsorption and (b) multiple desorption states from a chemically homogeneous adlayer, caused by coverage dependent lateral interactions or changes in the binding states induced by the temperature flash. UPS can enable these to be distinguished. In our studies of the room temperature adsorption of a number of gases on the (100) crystal face of tungsten.the He1 photoelectron spectra have been compared with the various binding states observed by in situ flash desorption experiments and with the known surface structures as determined by low energy electron diffraction (LEED). Amongst these have been studies of the adsorption of gaseous N2 and CO, and of adsorbed carbon. Both N2 and CO are strongly adsorbed in their p-desorp- tion states (activation energy for desorption, Ed > 200 kJ mol-l) at coverages < 5 x and both form 4 2 x 2) surface structures following heat treatment.". [The W(100) surface contains 10 x I O l 4 surface atoms CM-~.] Less information is available about the adsorbed states of carbon, there being no suitable desorption product, but again heat treatment produces a 4 2 x 2) structure.The c(2 x 2) LEED patterns for C , CO and N2 are reported to have very similar intensity 35 molecule cm-2 8*36 ULTRA-VIOLET PHOTOEMISSION STUDIES against voltage In all cases coverages in excess of the c(2 x 2) can be gen- erated. ; the structures for N2 and C have not been reported. This coverage increase causes an increase of 0.6 eV in the work function for both N2 and Co.ll' l5 The photoelectron spectra of these systems show a number of similarities, partic- ularly when comparison is made between different adsorbates having the same struct- ure. It appears that the principal features of the spectra are governed more by the geometric arcangement of adsorbed species on the surface than by their chemical nature. For CO a (1 x 1) surface structure is produced EXPERIMENTAL The photoelectron spectrometer operates at a base pressure of 2x Torr of active gases (1 Torr = 1.33 x lo2 Pa).Electron energy analysis is by means of a 127" electrostatic analyser of resolution 1 % which receives electrons photoemitted within a cone of apex angle 4". The axis of this cone, the direction of propagation of the light beam and the axis of rotation of the crystal are mutually perpendicular (see fig. 1). The photoelectrons may be accelerated into the analyser by application of a small negative potential to the crystal. This enables a sharp zero energy cut off to be obtained and hence the work function of the crystal to be measured. I t 1 1 1 1 I I I -16 -14 -12 -10 -8 -6 -4 -2 0 = & electron bindiag energy /eV FIG.1.-The He 1 photoelectron spectrum of a W(100) surface exposed to nitrogen at 350 K. - (a) clean surface ; - - - (b) 0.25 L of N2 ; . . . (c) 0.4 L of Nz ; - . - . (d) 0.7 L of N2 ; - (e) 2.0 L Of NZ. (1 L = 1 x Tom s.) The W(100) crystal was cleaned by repeated electron bombardment heating in TOIT oxygen to 1800 K and in vacuo to 2300 K until a reproducible clean surface spectrum was obtained. The crystal could be heated radiatively to 1OOOK by a tungsten filament or resistively to 1400K. Temperature measurement was by means of a W/W-Re thermo- couple. Rates of desorption during electron bombardment heating were monitored with aW. F. EGELHOFF, J . W. LINNETT AND D. L. PERRY 37 quadruple mass spectrometer.The crystal was cleaned before each experiment and allowed to cool to 400 K (for CO) or 350 K (for N3) before gases were introduced into the chamber. In the study of systems which have a known behaviour in flash desorption and work function experiments, the use of these techniques alongside UPS allows one to observe any photon-induced perturbation of the adsorbed layer. No such effects have been observed in any system studied so far. XESULTS AND DISCUSSION Photoemission from single crystal surfaces shows marked anisotropy. To provide a full description of the experimental situation, it is necessary to define both the polar angle and the azimuthal angle with respect to the direction of the exciting radiation and the direction of electron detection. In all the experiments described here, the polar angle (0 of fig.1) was - 12" and the azimuth fixed so that the component of the photoelectron momentum parallel to the surface was in the (1 1) direction. Strong angular dependent effects on the spectrum have been observed for a W(100) surface on which hydrogen was ad~orbed.~ In the case of N2 and CO adsorption, no systematic study of angular effects has been attempted, but those results which have been obtained indicate that the peak positions of the difference spectra are independent of the polar angle, though changes in relative peak intensities are observed. The intensity of photoemission from the surface falls off to zero as the polar angle is increased towards normal photon incidence, owing to reduction of the electric vector of the radiation at the surface.All the spectra reported in this paper were obtained using He1 (21.2 eV) radiation. The results for CO adsorption have been thoroughly cross-checked using NeI (16.8 eV) radiation. The consistency of the results indicate that final state effects are not important. NITROGEN At room temperature nitrogen adsorbs to give a disordered 4 2 x 2) structure which at higher coverages (-2 x 1014 molecule crn-,) can be ordered by heating to 800 K.l0 Exposures of c 5 langmuirs (1 langmuir, L = 1 x Torr s) are required to saturate this Bz state which causes a work function change A$* = -0.6 eV l5 and desorbs with second order kinetics (Ed = 310 W mol-l) at temperatures around 1200 K.8 All the evidence suggests that the 2.5 x 1014 molecules of N2 required to populate the pz state are dissociatively adsorbed, each nitrogen atom occupying four-fold next nearest neighbour sites. Fig.1 shows the changes in the photoelectron spectrum of a W(100) surface with increasing exposure to nitrogen. The surface state peak at -0.5 eV is destroyed and there is a continuous build-up of intensity between -5 and -9 eV. However, between - 1 and - 4 eV the changes are not proportional to exposure. There is a rise in intensity at - 3.2 eV for 0.25 and 0.4 L, while at - 2 eV no increase is produced by 0.4 L though exposures of 0.7 and 2.0 L produce a considerable intensity increase. This is probably due to a reduction of intensity on the low energy side of the surface state peak. At low coverage this would dominate an increasing intensity from an adsorbate induced energy level.Coverage dependent effects for N, on W(100) have been reported by Demuth, Baker and Eastman l6 but not by P1ummer,17 both in experiments which collect photoemitted electrons over a large solid angle. No coverage dependent effects arising from adsorbate-adsorbate interactions are apparent in fig. 2. Heating the surface to 1100 K, the temperature for the onset of &N * All vaIues of A 4 are quoted with respect to the clean surface work function38 ULTRA-VIOLET PHOTOEMISSION STUDIES desorption, produces no change in the spectrum, indicating that UPS is, in this case, insensitive to the increased long range order caused by heat treatment. Extended Huckel molecular orbital calculations for the adsorption of nitrogen on any array of nine tungsten atoms to simulate a W(100) surface have been reported recently by Anders, Hansen and Bartel1.l8 They predict that nitrogen adsorbs in the five-coordinate sites of four-fold symmetry and gives rise to a series of energy levels I I I 1 I I -9 -8 -7 -6 -5 - L -3 -2 - 1 O = € F electron binding energy /eV FIG.2.-Difference spectra for nitrogen adsorbed on W(100) (adsorbed nitrogen spectrum minus the clean spectrum). - (a) 2 L at 350 K, also heated to 1100 K ; - - - (b) 10oO L of N2 at 350 K to adsorb ; a (c) exposure (b) heated to 1100 K to desorb b1-N2. fairly evenly spaced over an 11 eV energy range. Precise comparison between the calculated energy levels and the photoelectron spectrum is not possible due to the inaccuracies of the calculation and the effects of surface induced relaxation shifts, the magnitude of which are not known.The spectrum of the &-N2 covered surface may be conveniently presented as a difference curve as in fig. 2(a) (the N2 covered surface spectrum minus the clean surface spectrum). This is not the spectrum of the adsorb- ate but it does indicate the changes occurring in the spectrum as a result of adsorption. The pronounced dip in the difference spectrum at -4.4 eV has been observed for several different adsorbates on W( 100). Extended Huckel calculations for both N2 * and H2 predict that the lowest d-orbital valence level of tungsten is lowered in energy following adsorption. The minimum at -4.4 eV may be caused by a reduc- tion in the intensity of emission from near the bottom of the d-band due to their involvement in bonding, implying that the true density of states of the electrons involved in chemisorption is a more or less continuous function over a wide energy range as predicted by the calculations.Nitrogen can be further adsorbed, in excess of the 5 x 1014 atom cm-2 required for the c(2 x 2) structure, into the B1 desorption state which causes an increase of the work function (to A ~ x O ) and desorbs with first-order kinetics around lo00 K. However, adsorption into this state occurs with low sticking probability (estimated atW . F. EGELHOFF, J . W. LTNNETT AND D . L . PERRY 39 < 10-3).8 The very high exposures of N2 required to populate this state introduces the possibility of contamination by CO which adsorbs with high sticking probability on a @,-N2 covered surface.Indeed, Adams and Germer claim that adsorption into the Pr-N2 state requires the presence of adsorbed C0.15 No quantitative measure- ments of this effect are reported. However, Hopkins and Usami 2o have examined the desorption of N, and CO from a W(100) surface exposed to lo4 L of N,, at which exposure they record 50 % population of the /I1 state (A+ = -0.3 eV) and find that the ratio of N2 to CO desorbed is > 1000 : 1. In the experiments reported below the mass 12 and mass 14 peaks were monitored during desorption of B1-N2. The CO contamination was less than 15 %. The U.V. photoelectron spectrum obtained by exposure to lo3 L of N, is shown as a difference spectrum in fig. 2(b).The orbital energies fall apparently within the same range as for P2-N2 ; there is an intensity increase between -2 and - 3 eV and the peak at -5.5 eV is enhanced. The changes observed during the build-up of the pl-state are a continuation of those described for the adsorption of B2-N2. It appears that nitrogen is adsorbing into a single binding state and that the two peaks in the desorption spectrum arise from adsorbate-adsorbate interactions within the ad- sorbed layer. Fig. 2(c) shows the spectrum obtained after desorption of B1-N2 at 950 K (A+ = -0.3 eV). The spectrum of the remaining @,-state is quite different from the @,-state of fig. 2(a), there being a large increase in intensity between -2 and -4 eV with peaks at - 3.3 and -2.8 eV. The reason for this may be that completion of the @,-state, which is reported * not to occur at room temperature, may require the presence of excess nitrogen on the surface at the desorption temperature of P1-N2.Alternatively, the changes might reflect the reconstruction of the W surface. Fig. 2(c) shows many similarities with the spectra of 4 2 x 2 ) structures of CO and C, reported later, which are also produced by high temperature removal of high coverage states. Exposure to a further lo3 of N2 regenerates the spectrum of fig. 2(b). CARBON MONOXIDE Before discussing the adsorption of COY it is necessary to establish the coverage of CO in the 8-CO desorption states quantitatively. In most previous studies of CO on W(lOO), it has been assumed that the saturation coverage of CO in the @-states was 10 x 1014 molecule cm-2.This arose because 50 % coverage gave a c(2 x 2) LEED pattern when heated to lo00 K, and by making the reasonable assumption that it is CO molecules which act as equivalent scattering centres for low energy electrons." However, Goymour and King 21 have recently shown that the saturation coverage of @-CO on polycrystalline W foil (mainly 100) is only - 5 x 1014 molecule cm-2. On the basis of this, they suggest that CO is dissociatively adsorbed in the @-states, with C and 0 occupying equivalent adsorption sites, and that the 4 2 x 2 ) LEED pattern arises because C and 0 act as equivalent scattering centres. They reviewed previous results showing that they were not inconsistent with a dissociative adsorption mech- anism.We have examined both the coverage data and the LEED pattern interpret- ation in the following way. The coverage of N2 in the @,-desorption state has been estimated,* though not firmly established, as 2.5 x 1014 molecule cm-2. From measurements of the relative pumping speeds for N2 and CO, and from the areas under the thermal desorption curves from a W(100) surface saturated with B-CO and then with P2-N2, the ratio of the number of CO molecules adsorbed in the /I-states to the number of N2 molecules adsorbed in the f12-state, is 2.0k0.2. This confirms Goymour and King's measure- ment of the saturation fl-CO coverage, as - 5 x 1014 molecule40 ULTRA - VIOLET P I-I 0 TO EM IS S I 0 N S T U D I ES Calculations of low energy electron scattering factors by Fink, Martin and Somor- jai show that for C and 0 atoms, the scattering factors may differ by up to a factor of two.22 Clearly, a perfectly ordered distribution of carbon and oxygen atoms, individually occupying alternate four-fold sites formerly assumed to be occupied by CO molecules, will not generate a c(2 x 2) LEED pattern.However, a kinematical simulation by computer calculation shows that a completely random distribution of carbon and oxygen atoms among the 4 2 x 2) sites, assuming a ratio of 1 : 2 for tbe scattering factors, will give a LEED pattern indistinguishable from a perfect 4 2 x 2) pattern down to the level of 0.01 % of maximum intensity. The degree of ordering of the C and 0 atoms [with respect to each other within the perfect c(2 x 2) array] which can be tolerated before it becomes apparent in a real LEED experiment, has yet to be investigated.However, the dissociative model of the 42x2) surface structure is undoubtedly a possibility. We recognise, however, that a temperature of 1000 K is required to form a c(2 x 2) pattern and that reconstruction of the surface cannot be ruled out. Also, this possible interpretation of the 4 2 x 2) pattern does not necessarily give us any information about the structure of CO adsorbed at room temperature. n 2 c B .C( 8 h U .d a .C( wl electron binding energy/eV FIG. 3.-The He I photoelectron spectrum of a W(100) surface exposed to CO at 400 K. -(a) clean surface; - - - (6) 0.15 L of CO ; . - . . ( ~ 1 0 . 3 L of CO ; - . - . ( d ) 0.8 L of CO ; -(e) 2.0L of CO. The changes in the photoelectron spectrum of a W(100) surface when exposed to CO are shown in fig.3. The adsorption temperature was 400 K, chosen to avoid the simultaneous adsorption of a-CO which desorbs around 350 K. With increasing coverage, the surface state is attenuated and there are intensity increases in the - 1.5 to -4 eV and -5 to -9 eV energy ranges. As for N2 adsorption, there is a larger increase at -3.5 eV for small exposures than at -2 eV, whereas at higher exposures the reverse is true. Also, an exposure of 2.0 L of CO, which is sufficient to saturate the P-desorption states, causes a decrease in the - 1.5 to - 3.5 eV range,W. F. EGELHOPF, J . W. LINNETT AND D. L. PERRY 41 relative to the spectrum following a 0.8 L exposure. Adsorption of a-CO also causes this decrease but an adsorption temperature of 400K precludes this as a possible interpretation.The decrease may be due to the increasing number of nearest- neighbour interactions at higher coverages. The room temperature adsorption of CO causes a work function change of A$ = 0.5 eV, but the only change in the LEED pattern is an increase in background intens- ity. CO is assumed to adsorb in a random manner to give a (1 x 1) structure. Flash desorption from this saturated surface reveals three desorption states. These have been previously observed by Clavenna and Schmidt, who showed that the three states pl, p2 and p3 desorbed with first (Ed = 240 kJ mol-l), first (Ed = 260 kJ mol-l), and second (Ed = 310 kJ mol-l) order kinetics respe~tively.~ The peak temperatures are approximately 1000 K (PI), 1100 K (p2) and 1450 K (p3).Desorp- tion of the fll and p2 states causes a work function reduction (A4 = -0.2 ev) and the c(2 x 2) structure, discussed previously, is generated. The desorption spectrum has been previously observed as two peaks, p1 (first order, now and p2) and f12 (second order, now p3). King and Goymour 23 have shown that this spectrum can be simulated, based on the model of dissociative adsorption at room temperature, assuming nearest-neighbour repulsive interactions within a homogeneous adsorbed layer. -9 -8 -7 -6 -5 - L -3 -2 - I O = €, electron binding energy lev FIG. 4.-Difference spectra for CO adsorbed on W(100). - (a) 3 L of CO at 400 K ; - - - (b) heated to 900 K to desorb /3,-CO ; . . . (c) heated to 1100 K to desorb PZ-CO.Exposing (c) to 2 L CO results in spectrum (a). Fig. 4 shows the changes in the photoelectron spectrum of adsorbed CO as each desorption state is sequentially removed : (a) full coverage at room temperature, A+ = 0.5 eV, (b) after desorption of PI, A$ ~ 0 . 2 5 eV, (c) after desorption of #I2, A+ = -0.2 eV. The final state corresponds to the c(2 x 2) structure. As each state is desorbed, there is a progressive increase between -2 and -4 eV causing a peak at - 3.2 eV. Below -4.5 eV, the spectrum shows an increase for desorption of PI, followed by a decrease for desorption of pz. These striking changes, which are not42 ULTRA-VIOLET PHOTOEMISSTON STUDIES the reverse of the changes occurring during adsorption, indicate considerable changes in the electronic structure at the surface.These results, combined with the work function data, demonstrate that the temperature flash is changing the distribution of electronic states and that the PI, P2 and p3 desorption states do not correspond to the states of CO existing on the surface followiiig adsorption at room temperature. Similar results have been obtained by observing the shifts in the binding energy of the oxygen 1s peak by X-ray photoelectron spectroscopy during desorption of CO from W(100).25 Adsorption of CO on the 42x2) structure [spectrum of fig. 4(c)] gen- erates a (1 x 1 ) LEED pattern and the spectrum reverts to that of fig. 4(a). CARBON In the absence of LEED facilities, it could be difficult to produce carbon overlayers of known coverage and structure. However, studies of the carburisation of single crystal tungsten by high temperature decomposition of hydrocarbons shows that the W( 100) surface possesses properties which enable two surface coverages to be reproduced without difficulty.12 A saturation coverage of carbon was produced in our experi- ments by adsorbing butadiene and heating to 1OOOK.This resulted in a surface which resisted further butadiene decomposition. Heating to 2300 K caused a loss of carbon by diffusion into the bulk to give another stable carbide surface which, by analogy with previous work, has a c(2 x 2) structure.12 The loss of carbon is evi- denced by the following experiments. The 42x2) carbide surface can be cleaned by adsorption of oxygen followed by flash desorption of CO.Subsequent heating at 2300 K regenerates a partial c(2 x 2) carbide which must arise by diffusion of carbon from the bulk. Similar effects have been observed in Auger studies of polycrystalline W The coverage of each surface structure could be probed chemically by adsorption of CO and H2. CO adsorbs on the c(2 x 2) carbide but not on the satur- ated carbide. H2 does not adsorb on the complete c(2 x 2). (Similarly, H2 will not adsorb on c(2 x 2) coverages of N, or CO at room temperature.26. 2 7 ) I I 1 1 I -9 -8 -7 -6 -5 -4 -3 -2 -1 O z f ~ electron binding energy lev (b) - - - a 42 x 2) carbide surface ; . (c) a 42 x 2) carbide exposed to 2 L of CO at 320 K. FIG. 5.-Difference spectra for carbon adsorbed on W(100). - (a) a saturated carbide surface ;W.F. EGELHOFF, J . W. LTNNETT AND D. L . PERRY 43 The difference curves for the saturated and the c(2 x 2) carbides are shown in fig. 5(u) and (b). The spectra show the same general features as seen previously for CO and N 2 . Again, creation of the 4 2 x 2 ) structure causes a marked increase in the - 2 to - 4 eV range, giving a peak at - 2.2 eV in addition to the customary " 4 2 x 2 ) peak " at - 3.2 eV. Adsorption of CO on the c(2 x 2) carbide gives a spectrum, fig. 5(c), which resembles that of other saturated surfaces, previously described. The increased intensity below -7 eV is caused by the simultaneous adsorption of a-CO at 320 K. COADSORPTTON OF Nz AND co The adsorption of CO on a c(2 x 2) nitrogen structure causes a big change in work function (from A+ = -0.6 to A 4 = +0.4 eV) and the LEED pattern indicates a (1 x 1) structure.28 The difference curve for c(2 x 2)-N is shown in fig.2(u). Adsorption of CO produces a spectrum almost exactly superimposable upon fig. 2(b) (the B1-N2 spectrum), except for a larger work function change. Adsorption of CO on the surface generated by desorption of P1-N2 [fig. 2(c) and fig. 6(a)] causes a reduc- tion of intensity between - 2 and -4 eV and an increase below - 5 eV [fig. 6(b)]. Desorption of CO reverses this change. -9 -8 -7 -6 -5 -L -3 -2 - I O = € F electron binding energy/eV --- (6) 2 L CO exposed to surface (a). FIG. 6.-Difference spectra for Nz and CO coadsorbed on W(lo0). - (a) &-N2 as in (c) of fig. 2 ; GENERAL DISCUSSION The spectra of the various adsorbate covered surfaces, discussed in the preceding sections, show a remarkable similarity, both in their overall form and particularly in the changes which occur on producing a c(2 x 2) adsorbate structure. An intense band between - 2 and - 4 eV, with a peak at - 3.2 eV, appears to be characteristic of the 4 2 x 2) structure.For nitrogen this band is more intense after desorption of44 ULTRA-VIOLET PHOTOEMISSION STUDIES B1-N2, compared with the 4 2 x 2 ) structure formed at room temperature, and is typical of the half-coverage states formed by desorption of higher coverages at high temperatures. Comparison between the 4 2 x 2) coverages and higher coverages always shows a decreased intensity between - 2 and - 4 eV. This is true whether the coverage of a 4 2 x 2 ) structure is increased by adsorption or a 4 2 x 2 ) structure is generated by decreasing the coverage of a saturated later.It is also true for a gas coadsorbing with c(2 x 2) coverages of a different adsorbate. These changes in the spectrum are coincident with the occupation of nearest-neighbour sites within the c(2 x 2) adlayer and indicate a modification of the adsorbate-surface interaction with increased crowding of the surface. A cornon feature of all the spectra, except the saturated carbide, and of the spectra observed elsewhere,l' is the minimum in the difference curves at around -4.5 eV. The possible explanation advanced in the discussion of nitrogen adsorption, that it is caused by a lowering of the energies of electrons near the bottom of the d-band, is applicable generally.Examination of the coverage dependence of the energy of this minimum shows that it moves towards EF as the coverage is increased. This is consistent with the gradual removal of electrons from the lower part of the d-band to take part in bonding. A general view of the difference graphs can be summarised as follows. The minimum at -4.5 eV is probably not a reflection of the density of states associated with the adsorbates which is probably a dome-shaped curve extending from near EF to around - 10 eV. On occupation of the nearest-neighbour sites, the distribution seems to shift to lower energy. This may signify that the electrons involved in bonding become more localised on the adsorbate atoms, all of which are more electronegative and have higher atomic ionisation potential than tungsten.An alternative explanation for this could be a decrease in the electronic reorganisation energy of the ionised system when nearest-neighbour interactions occur, or a syste- matic change in the matrix elements for optical excitation. The general similarity of the spectra presented here, combined with the reported similarity of the LEED intensity against voltage curves for the same systems, suggests that all the adsorbates exist in similar states on the surface. This implies a dissociative adsorption mechanism for both N2 and CO. The spectrometer was constructed with the financial assistance of the Paul Instru- ment Fund of the Royal Society. Support from the S.R.C. (to D. L. P.) is gratefully acknowledged. J. M.Baker and D. E. Eastman, J. Vac. Sci. Tech., 1973, 10,223. W. F. Egelhoff, D. L. Perry and J. W. Linnett, Proc. Int. Conf. Electron Spectroscopy,Namur Belgium, 1974, to be published. G. E. Becker and H. D. Hagstrum, J. Vuc. Sci. Tech., 1973, 10, 31. J. E. Demuth and D. E. Eastman, Phys. Reu. Letters, 1974, 32, 1123. W. F. Egelhoff and D. L Perry, to be published B. Feuerbacher and B. Fitton, Phys. Rev. B, 1973, 8,4890. S . J. Atkinson, C. R. Brundle and M. W. Roberts, Chem. Phys. Letters, 1974, 24. 175. * L. R. Clavenna and L. D. Schmidt, Surface Sci., 1970, 22, 365. L. R. Clavenna and L. D. Schmidt, Surface Sci., 1972, 33, 11. lo D. L. Adams and L. H. Germer, Surface Sci., 1971, 26, 109. l1 J. Anderson and P. J. Estrup, J . Chem. Phy.~., 1967, 46, 563. l2 D. F. Ollis and M. Boudart, Surface Sci., 1970,23, 320. l3 P. J. Estrup and J. Anderson, Satrface Sci., 1967, 8, 101. l4 T. E. Fischer, J. Vac. Sci. Tech., 1972, 9, 860. l6 J. E. Demuth, J. M. Baker and D. E. Eastman, Bull. Amer. Phys. Soc., 1973, 18, 392. D. L. Adams and L. H. Germer, Surface Sci., 1971,27, 21. E. W. Plummer, personal communication.W. F. EGELHOFF, J . W. LINNETT AND D . L . PERRY 45 l8 L. W. Anders, R. S. Hansen and L. S. Bartell, to be published. l9 L. W. Anders, R. S. Hansen and L. S. Bartell, J. Chem. Phys., 1973, 59, 5277. 2o B. J. Hopkins and S. Usami, in The Structure and Chemistry of SolidSurfaces, ed. G. A. Somorjai, (Wiley, New York, 1969), p. 67.1. C. G. Goymour and D. A. King, J.C.S. Faraday I, 1973, 69, 736. 22 M. Fink, M. R. Martin and G. A. Somorjai, Surface Sci., 1972, 29, 303. 23 C. G. Goymow and D. A. King, J.C.S. Faraday I, 1973,69,749. 24 R. W. Joyner, J. Rickman and M. W. Roberts, Surfice Sci., 1973, 39,445. 25 J. T. Yates, personal communication. 26 J. T. Yam and T. E. Madey, J. Vac. Sci. Tech., 1971, 8, 63. 27 J. T. Yates and T. E. Madey, J. Chem. Phys., 1972, 54,4969. 28 P. J. Estrup and J. Anderson, J. Chem. Phys., 1967,46, 567.

ISSN:0301-7249    

DOI:10.1039/DC9745800035

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Ultra-violet Photoelectron Spectroscopy of Adsorbed Oxygen BY A. M. BRADSHAW, D. MENZEL AND M. STEINKILBERG Technische Universitat Munchen, 8046 Garching, West Germany Receiued 17th May, 1974 The adsorption of oxygen on W (loo), W (110) and Ag (110) has been studied with ultra-violet photoelectron spectroscopy. The experimental difference spectra are interpreted phenomenologically, using data obtained from the application of other surface-sensitive techniques. The possible mech- anisms for changes in emission from metal states, as well as the bearing of the results on the concept of the oxygen chemisorption bond are discussed. The recent application of ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) to chemisorption has resulted in photo- ionization becoming one of the more widely studied photo-effects associated with the adsorbed state.The relatively high surface sensitivity of UPS is mainly due to the small average mean escape depth of photo-emitted electrons in the energy range 5-50 eV. The feasibility of utilizing this high surface sensitivity in chemisorption studies was first demonstrated by Eastman and Cashion in 1971. That considerable interest has since developed in this field is shown by the contents of the present Discussion. (For a review of the literature up to September 1973, see ref. (2).) The technique measures essentially the changes in the electron energy distribution of photo- emission from a metal surface caused by the adsorption of gas. In principle it is then possible to obtain information on the position and width of the bonding and non- bonding orbitals associated with the presence of adsorbate on the surface.Penn has recently attempted to link the experimentally determined difference spectra with the adsorbate spectral function obtained from many-body treatments of chemisorption. In the present paper we have chosen to report UPS results from the chemisorption of oxygen on tungsten (loo), tungsten (1 10) and silver (1 10). These three systems have been studied extensively by LEED, XPS, surface potentials and various other techniques, thus assisting in a phenomenological interpretation of the UPS data. Qxygen chemisorption on W (100) is fast,4* on W (110) initially fast followed by slow effects,6* The three faces thus form an interesting cross-section of the wide range of oxygen chemisorption systems.and on Ag (1 10) slow EXPERIMENTAL The spectra were taken in a combination LEED-UPS-XPS electron spectrometer con- structed for the authors by Vacuum Generators Ltd. A description of the instrument is published elsewhere, together with a discussion on the application of photo-electron spectro- scopy to chemisorption studies.1° We note simply here that it is possible with this spectro- meter to clean and characterise single crystal surfaces, to perform photo-electron energy analyses and to apply other surface techniques in situ. The reasonably good system base pressure of 3 x Torr) is required to maintain freedom from contamination during the long scan times often required for UP-spectra.The position of the crystal at the UPS measuring level is shown schematically in fig. 1. The angle 4 is defined as the angle between the crystal and the " x-axis " of the spectrometer.1° The crystal must also be Pa (2-3 x 46A. M. BRADSHAW, D. MENZEL AND M. STEINKILBERG 47 rotated slightly about the " z-axis ", so that the radiation from the source falls on the crystal surface at near to glancing angles of incidence. This relative geometry and the fixed analyser position unfortunately does not permit the measurement of the angular dependence of photo- emission except in a qualitative way. In addition, the 10-mm slit length (normal to the page in fig. 1) results in a broad range of angles of emission being accepted by the analyser, but it is nonetheless possible to determine approximately from the LEED pattern to which range of crystallographic directions this corresponds.O single crystal disc \J:;r:f analyser slit assembly FIG 1.-Schematic of the crystal in the UPS position of the spectrometer ; see also ref. (10). The tungsten samples were 10-mm discs of approximately 1-mm thickness and had been spark erosion cut from oriented bulk single crystals supplied by Metals Research Ltd. Standard oxygen cleaning procedures were used to eliminate carbon contamination, which could be monitored using the X P S facility. Details of the preparation, polishing and in situ cleaning of the Ag (110) sample are to be found in a previous publication.8 During the exposure of both the W (110) surface 6 v and the Ag (110) surface to oxygen it has been found that CO in the gas phase can lead to irreproducible adsorption behaviour, probably due to co-adsorption effects.In the present work the CO partial pressure was kept well below 1 % of that of oxygen by frequent renewal of the titanium film in the liquid-nitrogen- cooled sublimation pump. The methods used to obtain sequences of experimental difference spectra, such as those in fig. 2-7, are described fully in ref. (10). It suffices here to remark that, even at pressures as low as 3 x Pa, one must be mindful of the build-up of contamination during long scan times. RESULTS Clean surface spectra and difference spectra for the three systems at different exposures are shown in fig. 2-7. Exposures are given in Langmuir (L), where 1 Langmuir = 1 x Pa s, which is equivalent to 3.6 x loL4 molecule cm-2 s-l for oxygen at a temperature of 300 K.The angle 4 was approxi- mately 45" for all six series of spectra. By examination of the LEED pattern it was ascertained that for both W (100) and W (110) there would be a strong contribution from emission parallel to the [ 1 1 11 crystallographic direction. The clean surface spectra of fig. 2 and 4 are indeed very similar to the W (1 11) spectrum of Feuerbacher and Christiansen l1 at 21.2 eV recorded with a small solid angle of acceptance about the surface normal. Similarly we expect a strong contribution from the [loo] crystallographic direction in the spectra from the Ag (1 10) surface in fig. 6 and 7. In the clean surface He1 spectrum of fig. 6, the d band edge lies 4.3 eV under E,, similar to the results obtained from polycrystalline surfaces.12* A heavily facetted Torr s = 1.3 x48 ADSORBED OXYGEN Ag (1 10) surface gives a spectrum closely resembling these polycrystalline spectra in the literature.Fig. 2 shows the effect of oxygen on the He1 spectrum from the W (100) surface. The two prominent features are the growth of a positive peak between - 5 and - 7 eV and the loss of the surface state 14-16 peak at -0.5 eV. The attenuation of the Ex p o S U I - Y ~ooocps (Langrnuir, L' 1 2 3 4 5 I Vl IL 12.0 9.0 6.0 3 . 0 E f energy/eV below & FIG. 2.-Photoemission spectra for the adsorption of oxygen on tungsten (100); hw, = 21.2 eV. latter proceeds more slowly as a function of oxygen coverage than is the case with hydrogen or CO adsorption.The major positive peak occurs at - 6.0 eV at low coverages and splits into two peaks at higher coverages. Strong positive features of this nature we refer to as adsorbate resonance^.^ After an exposure of lOL, which according to the data of Madey corresponds to a monolayer on the surface, these two peaks lie at - 6.5 and - 5.2 eV (designated as 1 and 2 respectively in fig. 2). The positive peaks 3-5 lie at - 3.7, - 2.6 and 1.5 eV respectively ; the first two men- tioned lie in a region where there has been a broad overall loss in intensity. The minima each side of peak 4 in this region of intensity loss correspond to peaks in the clean surface spectrum and peak 4 itself to the minimum between them. Changes in the spectrum below - 10 eV are generally not reproducible ; an exception to this may be a peak at approximately - 12 eV, which was observed in several series ofA. M.BRADSHAW, D. MENZEL AND M. STEINKILBERG 49 difference spectra. The corresponding He11 spectrum is shown in fig. 3. At low coverages the major adsorbate resonance lies at -6.2 eV and at higher coverages splits in the same way into the two peaks 1 and 2 at - 6.5 eV and - 5.2 eV respectively. Exposure ( La n g m ui r, L) 3 I000 c p s I L 1 12.0 9 . 0 6.0 3.0 Ef energy/eV below E f FIG. 3.-Photoemission spectra (as in fig. 2) for the adsorption of oxygen on tungsten (100) ; hwo = 40.8 eV. Just as in the He1 spectra there is a small positive peak 3 at-3.7 eV. Further structure is difficult to elucidate. An extended negative feature also occurs on the low-binding-energy side of the main resonance. The surface-state peak in the clean surface spectrum is weak as expected from the dependence on source energy measured by Waclawski and P1ummer.l Accordingly we observed only a small negative peak in the difference spectra.We can already conclude at this stage that there are no differences between the He1 and He11 difference spectra for this adsorption system, except for a small shift of the major peak at low coverages.50 ADSORBED OXYGEN Fig. 4 shows the spectra from the W (1 10) surface using the He1 line. Much higher oxygen doses are required to produce a monolayer : our XPS and LEED data indicates that exposures in the range 102-103 L give a coverage of approximately 0.8-0.9. Certain differences are noticed between these spectra and those of fig.2. ~ ~- Exposure (Langmuir, L: t I I I clean surface J !a jO! 10 I 5L 2L 3.5 15.0 12.0 9.0 6.0 3 . 0 €-f energy/eV below Ef FIG. 4.---Photoemission spectra (as in fig. 2) for oxygen adsorbed on tungsten (110) ; tzw, = 21.2 eV. The major resonance is narrower and lies initially at -6.7 eV, shifting to -6.2 eV at higher coverages, in reasonable agreement with Baker and Eastman. Secondly, there is no extended negative region on the low binding energy side of the resonance, but rather a broad structured positive peak. The sharp peak 2 at - 3.9 eV appears to correspond to peaks in the clean surface spectrum, whilst the very sharp negative peak 5 corresponds to a sharp (positive) peak in the clean surface spectrum.The corresponding He11 spectra of fig. 5 show an extra feature not seen with the He1 line, namely, a shoulder 1 at - 7.6 eV on the main resonance 2 at - 6.2 eV. At lowA . M. BRADSHAW, D. MENZEL A N D M. STEINKILBERG 51 coverages the latter lies at -7.1 eV, thus 0.4 eV lower in energy than in the corres- ponding He1 spectrum. A similar shift in energy of the adsorbate resonance on going from He1 to He11 has also been observed in the CO/Ni(poly.) l 8 and CO/W (1 10) l9 systems. The small peak 3 of fig. 5 lies at - 3.9 eV and thus seems to be a j I - I Exposu rz (La ng m uir, L7 [I000 cps 2 3 c.r 2L I *Q5 clean L 5.0 12.0 9.0 6.0 3.0 Ef energy/eV below E f FIG. 5.-Photoemission spectra (as in fig. 2) for oxygen adsorbed on tungsten (110) ; ho = 40.8 eV.feature common to both surfaces and both excitation energies. The remaining structure in the He11 spectrum above -3.5 eV seems to reflect (in an opposite sense) the clean surface spectrum, although not too much weight may be placed on this conclusion because of the poor signal-to-noise ratio. The silver spectra of fig. 6 and 7 are quite different in appearance to the tungsten spectra, in that the d band lies several eV under E,, as we have already mentioned, and in that the adsorbate resonance(s) are weak and lie at lower binding energies. The maximum coverage after lo4 L is suspected to be only 0.5 monolayers.8* The d band peaks are quite strongly attenuated by the adsorption : in the He1 spectra of fig. 6 the negative peaks 1 and 2 correspond in energy to the maxima in emission from52 ADSORBED OXYGEN the d band.We also noted that small changes in the lamp pressure can produce quite large changes in the line intensity, in particular in the case of HeII. Such an effect could also give rise to structure in the difference spectrum. Particular care was taken here to keep the lamp pressure constant and the authors are convinced that the re- ported attenuation effects are genuine. (Also, in fig. 4 peaks 3 and 4 appear to be due to an increased intensity of clean surface peaks and peak 5 to a decrease, so that a lamp effect is unlikely to be responsible.) The positive features 3 and 4 in fig. 6 lie Exposure (Langmu ir, L) I 2 3 4 5 18000 c p s I IIJJ LO ‘1 !03 L 3001 SOL 5 0 L - - 15.0 12.0 9.0 6.0 3.0 Ef energy/eV below Ef FIG.6.-Photoemission spectra (as in fig. 2) for oxygen adsorbed on silver (110); Ptwo = 21.2 eV. at - 3.9 and -2.9 eV respectively ; at the highest coverage, peak 5 at -2.1 eV also appears. This positive peak structure exhibits a “ tail ” reaching up to the Fermi edge. The corresponding HeII spectra of fig. 7 show similar features, but the positions of positive peaks are markedly different. Peaks 3 and 4 lie at -3.3 and -2.0 eV respectively, and their relative intensities change during the exposure. Several authors have already observed the expected angular anisotropies in photo-A . M. BRADSHAW, D. MENZEL AND M. STEINKILBERG 53 emission from clean single crystal surfaces 20-23 or from polycrystalline films with fibre textures.24 Whilst it is not possible with our present instrumentation to record angular distributions quantitatively, it is nonetheless interesting to explore the effect of changing the relative geometry of source, sample and analyser by varying the E X ~ osure ( Lan g m ui r, 1 T I 2 3 4 L c lea n I I I I 9.0 6.0 3.0 I energy/eV below E f FIG.7.-Photoemission spectra (as in fig. 2) for oxygen adsorbed on silver (110); tiw, = 40.8 eV. angle 4. Quite pronounced changes in the clean surface spectra from Ag (1 10) and W (1 10) have been found, as is shown in fig. 8 and 9. In both systems the changes in the He11 spectrum were very slight compared to those in the He1 spectrum (see, e.g., fig. 8 for Ag (1 10)). Of greater importance for the present work is the question as to whether the strong angular anisotropy in emission from the clean surface is carried over into the difference spectra.Fig. 9 shows on the left-hand side three radically differing He1 spectra from a clean W (1 10) surface, produced merely by changing the angle 4. On the right-hand side are the corresponding difference spectra after an exposure of 5 L. The position and width of the major adsorbate resonance at54 ADSORBED OXYGEN - 6.4 eV is virtually unaltered, and the - 3.9 eV peak is also always present. Notice- able differences are observed, however, in the region between Ef and -2.0 eV. The strong, narrow negative peak at - 1.5 eV in the 4 = 60" spectrum corresponds to a sharp (positive) peak in the clean surface spectrum, as in fig. 4. The stronger this peak in the clean surface spectrum, the stronger is its corresponding negative peak in the difference spectrum.Both the clean surface and difference spectra at 60" show more structure below -8 eV than at the other two angles. The He1 W (100) clean surface spectrum was not greatly altered by varying the angle 4, and the differ- ence spectra were virtually identical. This lack of response to angle may have been purely coincidental : an alternative mounting of the crystal involving a rotation about the surface normal could well result in a larger anisotropy. The corresponding difference spectra for the Ag (I 10) clean surface series in fig. 8 have not yet been measured. n 3 ," ." Y B U .I 1 I I I I I I I I 1 15.0 12.0 9.0 6.0 3.0 Ef 12.0 9.0 6.0 3.0 ff energy/eV below Ef FIG. 8.-Clean surface photoemission spectra from a silver (110) surface at various angles 4 (defined in fig.1) ; bo = 21.2 and 40.8 eV. DISCUSSION There are two general approaches to the interpretation of UPS from adsorbate layers. The first may be conveniently described as " fingerprinting " : the spectra are compared at different coverages and the appearance or disappearance of features may perhaps be correlated with different adsorption states on the surface. The latter are identified by comparison with other techniques, preferably available in the sameA . M. BRADSHAW, D. MENZEL AND M . STEINKILBERG 55 system. A successful example of this kind of approach is provided by the authors' UPS and XPS work on the CO/W (1 10) adsorption sy~tern.'~ Further consideration of the position of peaks and correlations between different adsorption systems can be continued in terms of qualitative (e.g., ref.(25)) or semi-quantitative (e.g., ref. (26)) models for chemisorption. At the second level of sophistication one attempts to 12500 C.P.S. I I 1 I I I I 1 1 120 9 0 6 0 3 0 Ef !2.C 9.0 6.0 3.0 Ef energylev below I$ FIG. 9.-Clean surface (1.h.s.) photoemission spectra from a tungsten (1 10) surface at three angles 4. The corresponding difference spectra after an exposure of 5 L oxygen are shown on the r.h.s. ; tioo = 21.2. eV. link the experimental spectra to the calculated adsorbate spectral function (see, e.g., Penn 3), obtained from a theory of chemisorption where many-body effects 27 are taken into account. However, the present state of the art here does not allow a complete description of oxygen chemisorption, and we restrict ourselves to the first line of approach, citing only the Penn theory where relevant.The correlation of the UPS results with XPS, LEED and electron impact desorp- tion (EID) for the oxygen/W (100) system has been published el~ewhere.~ The splitting of the main resonance after approximately 2 L corresponds to the appearance of a shoulder on the high-binding-energy side of the oxygen K peak in XPS, and can be linked to a profound change in the nature of the adsorbate layer in this coverage range.4 Our LEED results confirm this pi~ture.~ Although a single chemisorption state involving only one adsorbate orbital can certainly give rise to a quite complicated adsorbate spectral function consisting of several peaks (see, e.g., Brenig and Schon- hammer's " strong coupling " limit for the hydrogen/Ni system 27), this does not seem to be the case here : the splitting is only apparent at higher coverages. Another possible reason for the effect could be the strong adsorbate interactions expected at high coverages.This might also be the explanation for the shoulder observed in the He11 spectrum from the oxygen/W (110) system. On the W (110) surface, LEED and XPS give little evidence for more than one chemisorption state.For the tungsten results in general we can conclude that the main adsorbate resonance(s) are broad and lie 5 to 7 eV below E,, and are accompanied by a small peak 3.7 to 3.9 eV below Ef. This part of the difference spectrum is relatively simple and were this the whole story we would conclude that there were no inajor differences in behaviour between the two faces.Changes in the spectra above - 3.5 eV, however, seem to depend on the structure in the ct band, and thus on the crystal orientation and its position relative to the analyser slit. These positive or negative features can arise for several reasons. There is first of all the possibility of an overall attenuation of emission from the unperturbed metal states because of a diminished effective source depth after adsorption. This is seemingly the most convincing explanation for the strong negative peaks in the Ag (1 10) difference spectra. Secondly, there could be a change in the reflection properties for electrons at the metal/vacuum interface, caused by an adsorption-induced change in the shape of the potential barrier at the surface.It is difficult to predict whether this effect would lead to enhanced or reduced emission from metal states, i.e., to positive or negative peaks in the difference spectrum. These two effects taken together are normally described by a transmission function or escape probability which relates the energy distribution of the joint band density of states to the observed photoemission current.28 In future theoretical analyses of difference spectra, the change in the transmission function due to adsorption must obviously be considered. Neither effect is likely to show strong energy-dependent structure and thus account for the W (1 10) results, where one peak froin the clean surface spectrum is attenuated and two are enhanced.A third, and possibly more germane explanation for the changes in the d band of tungsten is the change in the density of states as a result of chemisorption. The interaction of metal and adsorbate orbitals can obviously result in changes in the density of higher-lying metal states, consistent with the formation of the chemisorption bond. The " anti-resonance " in the Penn theory already includes a special case of this effect, but the approximations in the treatment preclude its ready application. Within the framework of the Anderson Penn derives an expression for the change in the " no-loss " photoelectron current for low adsorbate coverages (eqn (4) of ref. (3)). When the adsorbate resonance lies within the conduction band, it will be shifted to lower kinetic energy in the spectrum and accompanied by a negative peak on the high kinetic energy side (the " anti-resonance ").The theory seems to fit the data of Eastmaii and Cashion for CO/nickel ; only for W (100) is a similar effect observed in the present results, which can also be explained by attenuation of the d-band emission. An improvement of the Penn model which takes into account overlap between adsorbate and metal orbitals as well as correlation effects in the metal could conceivably lead to the derivation of the complete adsorbate spectral function from the UPS difference spectra, thereby interpreting the changes in the d band region of tungsten and perhaps also accounting for attenuation effects.The difference spectra from the oxygen/Ag (1 10) system are radically different from the corresponding tungsten spectra. The adsorbate resonances occur in the region of the .T band ; however, small positive features below - 4 eV would be masked by the strong attenuation of emission from the d bands. This would indicate that the interaction takes place primarily between adsorbate orbital(s) and s electrons from the metal. The differences between the He1 and He11 spectra remain at this stage unexplained. The position of the resonances indicates a weaker chemisorption bond : the heat of adsorption for oxygen on Ag (1 10) has been estimated 30 at - 120 kJ niol-1 compared with 385 kJ mol-' for W (1 for example. Similarly, the adsorption of CO on the noble metals is weaker than on the transition metals, which has also been interpreted in terms of the low lying d band in silver.32A .M . BRADSHAW, D. MENZEL AND M. STEINKILBERG 57 It does not seem particularly useful at this stage to make comparison with photo- ionization processes in the free oxygen atom. We simply note that the first ionization potential is 13.5 eV 33 in the gas phase. Depending on the value chosen for the work function (also a subject for debate), the unperturbed oxygen 2p level would have an energy somewhere between -6 and -9 eV relative to Ef. We are still in doubt as to the number of electrons which interact strongly with the metal and those which remain essentially non-bonding. The latter, of course, will also give rise to adsorbate resonances, which perhaps will not be so strongly broadened as bonding resonances of similar energies.In the experimental spectrum, however, there is little evidence enabling us to distinguish between bonding and non-bonding levels. CONCLUSIONS Four major conclusions emerge from the present work. 1. The adsorption of oxygen on both tungsten and silver leads to changes in the U.V. photoelectron spectrum, consistent with the picture of well-defined adsorbate resonances. 2. Apart from the complexity of effects in the difference spectra associated with changes in the d band region, the major features of the tungsten spectra are relatively simple and common to both crystal faces investigated. 3. Whereas in the case of tungsten the interaction appears to take place with the d band, for silver s band states are mainly involved. 4.Further detailed interpretation of results awaits a sound theoretical treatment of oxygen chemisorption as well as a theory linking the calculated adsorbate spectral function with the experimental difference spectra. We acknowledge valuable discussions with W. Brenig and the members of his group. The work has been supported financially by the Deutsche Forschungsge- meinschaft . l D. E. Eastman and J. K. Cashion, Phys. Rev. Letters, 1971 27, 1520. D. Menzel, Electrbn Fis.Apli. (Madrid), 1974, 17, 1 13. D. R. Penn, Phys. Rev. Letters, 1972, 28, 1041. T. E. Madey, Surface Sci., 1972, 33, 355. A. M. Bradshaw, D. Menzel and M. Steinkilberg, Jap. J. Appl. Phys., 1974, suppl. 2, part 2,841. L. H. Germer and J. W.May, Surface Sci., 1966, 4, 452. A. M. Bradshaw and D. Menzel, Ber. Bunsenges. phys. Chem., 1974,78,1140. A. Engelhardt, A. M. Bradshaw and D. Menzel, Surface Sci., 1973, 40,410. * A. M. Bradshaw, A. Engelhardt and D. Menzel, Ber. Bunsenges. phys. Chem., 1972,76, 501. lo A. M. Bradshaw and D. Menzel, Vakuum-Technik, 1975, 24, 15. l1 B. Feuerbacher and N. E. Christensen, Phys. Rev. B, 1974, 10, 2373. l2 D. E. Eastman and J. K. Cashion, Phys. Rev. Letters, 1970, 24, 310. l 3 I. Lindau and L. WalldCn, Physica Scripta, 1971, 3, 77. 14 F. Forstmann and J. B. Pendry, Z. Phys., 1970, 235, 75. l5 B. J. Waclawski and E. W. Plummer, PJzys. Rev. Letters, 1972, 29, 783. l6 B. Feuerbacher and B. Fitton, Phys. Rev. Letters, 1972, 29, 786. l7 J. M. Baker and D. E. Eastman, J. Vac. Sci. Tech., 1973, 10, 223. ‘9 A. M. Bradshaw, D. Menzel and M. Steinkilberg, Chern. Phys. Letters, 1974, 28, 516. 2o U. Gerhardt and E. Dietz, Phys. Rev. Letters, 1971, 24, 1477. 21 T. Gustafsson, P. 0. Nilson and L. Wallden, Phys. Letters, 1971, 37A, 121. 22 B. Feuerbacher and B. Fitton, Phys. Rev. Letters, 1973, 30, 923. 23 P. 0. Nilsson and D. E. Eastman, Physica Scripta, 1973, 8, 113. 24 R. Y. Koyama and L. R. Hughey, Phys. Rev. Letters, 1972, 29, 1518. 2 5 G. Blyholder, J. Phys. Chem., 1964, 68, 2772. P. J. Page, D L. Trimm, and P. M. Williams, J.C.S. Faraday I, 1974, 70, 1769.58 ADSORBED OXYGEN 26 G. Doyen and G. Ertl, Surface Sci., 1974, 43, 197. 27 W. Brenig and K. Schonhammer, 2. Phys., 1974,267,201. 28 C. N. Berglund and W. E. Spicer, Phys. Reo. A, 1964, 136, 1030 and 1044. 29 P. W. Anderson, Phys. Reu., 1971, 124,41. 30 A. Engelhardt and D. Menzel, unpublished results. 31 C. Kohrt and R. Gomer, J. Chem. Phys., 1970,52,3283. 32 A. M. Bradshaw and J. Pritchard, Proc. Roy. SOC. A , 1970,316, 169. 33 P. M. Delimer, J. Berkowitz and W. A. Chupka, J. Cltem. Phys., 1973,59,5777.

ISSN:0301-7249    

DOI:10.1039/DC9745800046

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

DISCUSSION REMARKS Papers 1, 2, 3,4 Dr. T. B. Grimley (Liverpool) said: I would ask Rosch whether these nickel- ethylene complexes are stable. Have they made any calculations of the total energy? There must always be some doubts about the usefulness of a discussion of chemical bonding unless the theory also leads to a stable complex. Dr. C. R. Brundle (Bradford) said: It is suggested in the paper of Rosch and Rhodin that the Xa calculations of Ni2-C2H4 favour a n-complex assignment for adsorbed C2H4 on Ni. One of the supporting pieces of evidence given is that the n-complex calculation produces a shift of 0.72 eV in the n level on chemisorption, the di-o-complex 0.30 eV, whereas the experimental value of Eastman and Demuth is 0.9 eV. The validity of the experimental value is open to question.The He1 photo- electron spectra of C2H4 condensed (at 77 K) and chemisorbed (at 200 K) on a nickel film are shown in the figure. It is clear that experimentally the position of the n level does not alter but the G levels move uniformly by 0.9 eV. Demuth and Eastman consider that this uniform o shift can only be explained as a difference in the relaxation effect in the final ionized states, between condensed and chemisorbed C2H4 the amount being the same for all ionized o levels. They then make the questionable assumption that the n ionized state suffers the same change in relaxation energy (theoretically this would not be expected, especially if it is the orbital involved in the chemisorption bonding to the metal surface) and therefore shift the whole of the chemisorbed -J E, 2 4 6 8 I0 I 2 binding energy /eV FIG.1.-He I Photoelectron spectrum of the Ni/C2H4 system. Top : schematic spectrum of gaseous ethylene. -, clean Ni film at 77 K ; - - -, Ni + multilayers of condensed ethylene (77 K) ; . . . ., Ni + chemisorbed ethylene (200 K). The gas phase spectrum has arbitrarily been lined up with the condensed spectrum at the 2nd ionisation potential. 5960 GENERAL DISCUSSION spectrum to higher binding energy to remove the supposed uniform relaxation change, thus ending up with a 7t shift of 0.9 eV. There is no way, however, of experimentally knowing whether the relaxation change is the same for 7t and 0 levels and therefore the 0.9 eV value is made up of two inseparable terms, a “ real ” initial state 7t shift, and a component due to the difference between and 0 of the change in relaxation on going from the condensed to chemisorbed states.Dr. H. D. Hagstrum (Bell Laboratories) said: I would ask Bradshaw whether in his work with oxygen adsorption he has observed any behaviour that would lead him to suspect that adsorbed oxygen was, in fact, being absorbed into the near-surface bulk or selvedge. In some earlier unpublished work with oxygen on nickel, Becker and I have observed that heating a c(2 x 2) oxygen layer on Ni(100) can cause the ion- neutralization (INS) spectrum to revert towards the clean surface spectrum whereas the UPS spectrum remains essentially unchanged. We interpreted this to mean that oxygen was being absorbed, that is, was leaving the sites outside the surface for positions inside the crystal.INS, being sensitive essentially to the surface monolayer only, would detect this change but UPS would not if the total amount of oxygen in surface and selvedge sites in the range of UPS sensitivity remained constant and if the orbital spectrum in the two positions did not change appreciably. We have more recently observed that Hg sorbed on Ni(100) is observed both by INS and UPS but that Hg sorbed on Si(ll1) is not observed by INS. UPS would still “see” the Hg if it had gone a layer or two into the rather open Si crystal. I would also comment on the problem of final state dependence in UPS when one measures photoemission as a function of exit angle. My comment is to point out that methods which collect all ejected electrons, such as the hemispherical grid system we have been using, essentially sidesteps this final state problem.It then enables one to concentrate on the equally interesting variation with incidence angle of the light. This is productive even for unpolarized light. Here we have observed that surface orbitals due to chemisorption which are strongly visible at 60” incidence angle are invisible at normal incidence.l This phenomenon is the result of the ogra&V term in the photoexcitation matrix element. Here E is the electric vector of the incidence light and grad,V is the gradient vector in the direction of the maximum gradient of the unperturbed electrostatic potential. This is the surface photoelectric effect for surface orbitals and indicates that the orbital projects into the surface potential gradient.The point is that this observation could be confused with, and thus be inseparable from, final state variations if exit angle is simultaneously restricted to a narrow range. Dr. A. M. Bradshaw (Munich) : said In reply to Hagstrum, it is difficult with the present problems of interpretation to make any definite pronouncements as to whether the room temperature adsorption of oxygen on tungsten involves simple chemisorption or chemisorption with reconstruction. Certainly, heating the (100) surface covered with an oxygen monolayer brings about distinct changes in the UPS difference which can be correlated with changes in the LEED pattern and in the O(1s) peak from the XP-spectrum. It has long been thought that the high- temperature LEED patterns for this adsorption system are indicative of a reconstructed s~rface.~ Our own work on the interaction of oxygen with a tungsten (110) surface H.D. Hagstrum and G. E. Becker, Proc. Roy. SOC. A . 1972,331, 395. A. M. Bradshaw, D. Menzel and M. F. Steinkilberg, Japan. J. Appl. Phys., in press ; J. Fuggle and D. Menzel, unpublished results. P. J. Estrup and J. Anderson, Proc. 27th Physical Electronics Conference, 1967, p. 47.GENERAL DISCUSSION 61 at high temperatures also gives a series of distinctly different UP-spectra corresponding to each of the rather complicated LEED structures reported by Germer and May.l As far as the double-peaked structure on the main resonance in the oxygen/W( 100) system is concerned, we have correlated this result with Madey’s electron-induced desorption data,3 and conclude that is it due to the appearance of a second adsorption state on the surface after an exposure of approximatley 1 L.In the light of Liebsch and Plummer’s paper at this Discussion it cannot, however, be ruled out that a final- state effect is responsible, which alters the spectrum at a particular coverage. A change in LEED pattern is also observed at this point in the exposure. Dr. T. B. Grimley (Liverpool) said: I am surprised to hear Perry say that for the ordered surface structures formed by CO and N2 on (lOO)W, the difference photo- emission shows little angular dependence. Are there some experimental results available ? Dr. W. F. Egelhoff, Prof. J. W. Linnett and Dr. D.L. Perry (Cambridge) (com- municated) : The question posed by Grimley is of interest since the observations of angular effects reported in our paper are apparently not consistent with the predictions of either final state scattering effects or the expected effects of chemisorption band geometry 5 * The features observed in the difference graphs of N2, CO and C adsorbed on W(100) are very broad and no systematic changes of peak position with angle are evident, whereas the much sharper features of both the clean and of the H covered W(100) surface ’ show strong angular effects. However, this does not rule out the possibility that the changes of relative intensity within these broad bands, which are observed as the angle is varied, may be interpreted as arising from final state scattering effects although different relative changes in the optical transition possibility due to orbital symmetries also have to be considered. In general, however, the broad distribution of adsorbate induced energy levels shown in the difference spectra, show the same dependence on the surface structure over the entire range of angles. on the photoemission angular distribution. ’ L. H. Germer and J. W. May, Surface Sci., 1966,4,452. A. M. Bradshaw, D. Menzel and M. F. Steinkiiberg, Japan. J. Appl. Phys., in press ; J. Fuggle and D. Menzel, unpublished results. T. E. Madey, Surface Sci., 1972,33, 355. A. Liebsch and E. W. Plummer, this Discussion, p. 19. T. B. Grimley, this Discussion, p. 7. J. W. Gadzuk, to be published. ’ W. F. Egelhoff and D. L. Perry, Phys. Reo. Letters, 1975, 34,93.

ISSN:0301-7249    

DOI:10.1039/DC9745800059

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Ultra-violet and X-ray Photoelectron Spectroscopy (UPS and XPS) of CO, COz, 0, and H,O on Molybdenum and Gold Films BY S . J. ATKINSON, C. R. BRUNDLE AND M. W. ROBERTS* School of Chemistry, University of Bradford Received 12 June, 1974 Electron spectroscopic (XPS and UPS) studies of the interaction of CO, COz, O2 and H 2 0 with Mo, and H20 and C 0 2 with Au are reported. The object was first to explore how far one could distinguish between physical and chemisorption and between differmt states of chemisorption ; whether there was any relation between bond strength and electron binding energy and whether electron spectroscopy, particularly UPS, could provide unambiguous information on the moleculal- nature of surface species. PhysicaI adsorption data for COz and H20 on ALI at 77 K provide reference binding-energy data.Arguments are given foi believing that, with molybdenum, CO can exist in both molecular and dissociated forms, that C02 dissociates into C and 0 and that water exists in both molecular and dissociated forms. The role of the substrate temperature in determining the relative stability of the diffeient states of the molecules is emphasized. Implicit is the assumption that identical electron- binding data by XPS reflect identical surface-bonding situations. In this context the role of com- plementary UPS data is important. Whze information was available on the relative strengths of surface bonding then for any given adsorbate the electron binding energy was greatest when bonding to the surface was weakest. Electron spectroscopy is an important experimental approach for the study of adsorption at solid surfaces.There is now clear evidence for both good analytical sensitivity and the ability to obtain chemical bonding inf~rmation.”~ The question of the magnitude of electron escape-depths is now largely resolved 8a* although neither the exact dependence on electron energy nor the theoretical foundation of this dependence is completely clear.sc X-ray induced photoelectron spectroscopy (XPS) provides unambiguous informa- tion on the atomic nature of the surface, but less unambiguous information on chemical bonding since we are dependent on a change in the electron configuration of the valence electrons being reflected by a change in the binding energy of the core electron, as has been established by Siegbahn and his co-workers for gas-phase molecules.With vacuum u.-v. radiation (UPS) more direct information is obtained on the valence electrons. By combining both fmms of electron spectroscopy distinct advantages accrue over and above each separately and this is the approach adopted. We are not concerned here with electron-induced Auger spectroscopy although brief mention is made of X-ray induced Auger spectra in one or two specific instances. The systems studied are inherently simple but chosen so that they cover a range of possible surface phenomena (e.g., physical and chemisorption, multilayer formation and oxidation) and for which in many cases there is a reasonably sound background information. We believe this to be essential at this stage of development of electron spectroscopy in surface chemistry.For the carbon monoxide-molybdenum system there is clear evidence for a number 62S. J. ATKINSON, C. R . BRUNDLE A N D M. W. ROBERTS 63 of distinguishable regimes of adsorption.fo* l1 By comparison with the W + CO system,12 these “ regimes of adsorption ” may be analogous to the “ states ” invoked to explain the tungsten + CO data. The term “ states ” originates largely from flash- filament studies and any one state has implied a specific adsorption energy. On the other hand, previous work lo suggested a range of metal-CO bond energies for the Mo+CO system at temperatures below 295 K, the heat of adsorption decreasing linearly with increasing coverage. Similar behaviour has been observed for H2 desorption from dissociatively chemisorbed H2S on Fe These conclusions were based on both isotherm and desorption data for evaporated films.Flash filament studies were carried out with single crystals, ribbons, or polycrystalline wires. We use the terminology generally accepted l 2 to describe the adsorption of CQ on W but do not necessarily imply a direct similarity of surface bonding. For example “ virgin layer ” follows Gomer’s terminology for low temperature adsorption, 77 K or less ; P-state refers to the initial adsorption at room temperature (e.g., at lo-* Torr) ; the a-state is of lower heat of adsorption than the jl and occurs at higher CO pressure (e.g., Torr); y-state refers to the reversible adsorption which occurs lo. l 1 at 77 K subsequent to adsorption at higher temperature (295 K).We have reported briefly electron spectroscopy data for the Mo+CO system 4 9 ’ as also have Madey et aL5 on the W+CO system. In this paper we describe more detailed experimental work and compare the results with the Mo + 02, Mo + CO, and Mo+H20 systems. In the case of XPS we refer to three important parameters : (a) the binding energy of a core electron (b) peak width at half-maximum height (FWHM) and (c) peak intensities. Together with the information obtained from UPS (He I and He 11) studies they form the main basis of the discussion. XPS and UPS studies on the physical and multilayer adsorption (condensation) of CQ2 and H,O at 77 K on Au are also included. They provide “ standard data ” for XPS electron binding energies, and intensities, and for valence levels (UPS) in the undissociated molecules.The role of the substrate in influencing the binding energies of core electrons of the physically adsorbed layer is also explored. and N2 from W. EXPERIMENTAL The electron spectrometer incorporating both XPS and UPS facilities and the experi- mental procedures used for the preparation of Mo and Au substrates in the form of evaporated films have been described.2* l6 The Mo films were never obtained completely free of contamination, the relative peak intensities of O(1s) and C(1s) before and after an adsorption experiment indicating an initial contamination of < 15 % coverage. Specpure gases were admitted directly from bottles through the appropriate temperature traps. Calibration of the spectra was made against the Mo 3d peak of clean M o , ~ which has previously been calibrated against Au 4f,/2, at 83.7k0.1 eV below &.I6 RESULTS ADSORPTION OF co ON MOLYBDENUM ADSORPTION AT 298 K ( b STATE) It is first essential to refer to our previous results for CQ adsorbed at 298 K ; these are given in fig.1. The O( 1 s) peak height as a function of exposure is shown ; the FWHM value was constant at 2.2 eV during the formation of the ad-layer. The O(1s) peak position remained invariant during formation of the ad-layer at the same binding energy, 530eV, as the small oxygen contaminant initially present on the molybdenum film. The Mo(3q) ratio is 1 : 0.9 & 0.06 for CO exposures of cu. 10 L64 ELECTRON SPECTROSCOPIC STUDIES (fig. 1) but is about 10 % greater, 1 : 1.07 k 0.05 at much higher CO exposures ( - 400 L).As mentioned previously,’ monitoring the C( 1s) signal is not particularly satisfactory since it is superimposed on a broad characteristic loss feature (Mo 3d) which appears to be sensitive to adsorption. 1 1 1 1 1 1 1 l l I I 5 3 0 536 524 5 3 0 536 542 eV 2 4 6 8 I0 exposure/L FIG. 1.-(a) O(1s) peak as a function of exposure to CO at 298 K. (6) Mo(3s3) : O(1s) ratio as a function of exposure to CO at 298 K. (c) Formation of the yCO state at 77 K. (1) 1 L exposure ; (2) 3 L exposure ; (3) and (4) 6 and 12 L exposure. After completion of the monolayer, raising the CO pressure from -lo-’ to 1 x Torr resulted in no change in the O(ls) peak either in intensity or width. We estimate that the auerage sticking probability for CO is 0.2 at 298 K since the monolayer is virtually complete after an exposure of 5 L (fig.lb). Table 1 summarises the principal binding-energy, peak-width and peak-height data for this and other systems studied. ADSORPTION AT 77 K (a) SUBSEQUENT TO ADSORPTION AT 298 K.-We are particularly concerned with how do(es) the y state(s) form during exposure to CO and have therefore studied the development of the envelope of the binding energies which make up the y state (fig. lc). The y state is desorbed on warming to 298 K and reformed on cooling in CO to 77 K. We have monitored the O(1s) peak after CO exposures of 1, 3, 6 and 12 L ; above 6 L we observed no further change. The ratio of the areas of the B to the y envelopes was 1.9 :1, 1.8 :1 and 1.76 :1 in three separate experiments.The CO pressure was less than 1 x Torr.TABLE 1.-O(1s) AND C(1s) B.E’s, INTENSITIES AND HALF-WIDTHS FOR ADSORPTION ON Au AND Mo B.E.’s (eV) relative intensities FWHM (eV) adsorption state c (1s) 0 (1s) 0 (ls)/C (1s) 0 (ls)/Mo ( 3 ~ 4 ) c (Is) Mo (298 K)---clean CO/Mo (298 K)-P-state CO/Mo (298 K)+ CO/Mo (77 K) CO/Mo (77 K)-virgin state virgin state warmed to 298 K (P+y states) 02/M0 (298 K) heated to 420 K in Torr O2 heated to 520 K in 10-l Torr O2 Au (77 K)-clean C02/Au (77 K)-physical ads. C02/Au (77 K)-multilayers H20/Au (77 K)-multilayers C02/Mo (298 K) C02/Mo (298 K) cooled to 77 K+ COZ CO;!/Mo (77 K) then warmed to 298 K H20/M0(298 K) then cooled to 77 Kffurther H20 then heated to 523 K in vacuo H20lM0 (298 K)+Oi 284.2( @) 282.7,284.2 282.7, 287.2 f 284.6 282.7, 284.2 284.2( 0 ) 285.1 ( 0 ) 290.9 290.9(s), 292.3 285.1 ( 0 ) 284.0(0), 283.0 284.0(0), 283.0, 291.6 284.5, 291.6 283.5 530.2 530.0 530.0, 534.5 f 53 1.2 530.0 530.0 530.0 530.0 none 534.3 534.3(s), 535.6 532.7 530.0 530, 534.2 53 1 .O, 534.1 530.0 530.0, 532.3(s) 530.0(s), 532.5 530.0, 532.3(s) 530.0 2.0 2.2 ca.2.5 2.2 - large large large - 4.8 4.6 large 2.2 -, 4.4 -, 4.4 3.3 - - - - < 0.2 0.9 0.9 - - 2.5 4.8 6.0 - - - I 0.9 0.9, large - , large 1 1.64, - - , large 1.8 2.06 - 4.5 5e, 5 2.9 g 4.5 e9g - - - 2.1 2.1 2.2 - not taken not taken 3.5 3.0 - - - I 0 (1s) 2.2 2.2 2.2, >5 3.8 3.0 2.4 2.4 2.4 - 2.2 2.2 2.3 2.4 2.4, 2.4 2.4, 2.4 2.7 overlapped overlapped 2.5 overlapped 0 estimated accuracy i 0.3 eV ; b approximate ; C taken at 50 eV resolution unless otherwise stated ; d taken at a few monolayers exposure ; e FWHM of oveLIapped p s k s ;fenties of very broad flat peaks (see spectra) ; g taken at 100 eV analyzing energy ; .contaminant peak ; s shoulder.66 ELECTRON SPECTROSCOPIC STUDIES (b) ADSORPTION AT 77 K (VIRGIN STATE).-Fig.2 (a and b) shows the O(1s) and C( 1s) spectra resulting from adsorption of CO during exposure at a pressure of about 2 x The O(1s) and C(1s) spectra at zero exposure correspond to adsorp- tion on the clean film during and after formation and prior to commencing CO exposure. With increasing CO exposure both the O( 1s) and C( 1s) binding energies Torr. . . . 524 530 536 ' 5 4 0 20L 14L FIG. 2.-(u) O(1s) signal at 77 K as a function of exposure of Mo to CO.(6) C(1s) signal at 77 K as a function of exposure of Mo to CO. (c) O(1s) signal from CO(ads) layer at 77 K and during warming to 295 K. (d) C(1s) signal from CO(ads) layer at 77 K and during warming to 298 K. iiiove gradually to higher values, O( 1s) shifting about 1.2 eV and C( 1s) about 0.4 eV compared to the initial contaminant positions. The O( 1s) peak height increases with exposure until a constant height is reached after about 14 L; the FWHM value at 3.0 L exposure is about 3.0 eV but gradually broadens to a value of about 3.8 eV after 20 L. The C(ls) envelope behaves analogously, the FWHM value increasing from 1.8 to 2.9 eV after 20 L exposure. Adsorption states with much higher O( 1s) binding energies than that correspond- ing to the major adsorption also develop at high exposure (shaded area) ; this behaviour is associated with the formation of the y state(s) which are reversible l 1 and charac- terised by a low heat of adsorption.(c) THERMALLY INDUCED CONVERSION OF THE VIRGIN (77 K) STATE TO THE p (298 K) STATE.-Fig. 2(b and d ) shows the O( 1 s) and C( 1 s) spectra during the gradual warming of the ad-layer formed at 77 to 298 K. The middle O(ls) spectrum corresponds to aS . J . ATKINSON, C . R . BRUNDLE AND M. W. ROBERTS 67 temperature of about 170 K ; at this stage the adsorbed species giving rise to the high binding energy O(1s) states (cp. 7) have desorbed. The total CO coverage at 77 K is some 35 % greater than at 295 K.l0* l 1 The O(1s) peak position after warming has shifted to a lower binding energy, and its FWHM decreased (table 1).The changes observed with the C(1s) spectra are more complex (fig. 2d) in that a gradual broadening of the C(ls) envelope (in contrast to the narrowing of the O(1s) envelope) occurs with emergence of two C(1s) peaks at 282.7 and 284.2 eV. The peak at 284.2 eV is at the same value as the small C( 1s) peak associated with contami- nation present on the “ clean ” film prior to CO adsorption (fig. 2), and the 282 7 V peak is at the same position as for CO in the /I-state. UPS STUDIES OF co INTERACTION WITH M O FILMS We have reported results of UPS studies of the adsorption of CO on molybdenum films at 295 and 77 K and the changes observed on warming the adlayer from 77 to 295 K. The spectra for these states, and also the y-state, are shown in fig.3a. The I I I I I I I I l l 8 E 4 8 1 2 e v E 3 6 9 FIG. 3.-(u) UPS (Hel) of CO adsorbed on Mo. (1) Mo ; (2) CO(ads) 298 K ; (3) CO(ads) 298+ CO (ads) 77 K ; (4) CO(ads) 77 K. (6) XPS of Mo film. (1) Mo film ; (2) Mo+ CO (lo00 L) at 298 K. positions of the observed bands with respect to the Fermi level are summarised in table 2. During the UPS studies, XPS spectra were occasionally taken both to monitor the coverage and to relate unambiguously the two types of spectra. The XPS studies of the valence levels were not particularly informative. Fig. 3b shows the molybdenum valence band before and after adsorption of CO at 295 K ; the CO exposure was about 1000 L. The only notable point was a decrease in the intensity, there being a lack of structure both for the “clean” and CO covered surfaces.The CO features are presumably not observable because of high escape depths and low cross-section for ionization. INTERACTION OF OXYGEN WITH MOLYBDENUM Fig. 4(a) shows the Mo(3s+) and O(ls) peaks for the as-prepared Mo film, after exposure (20 L) at 295 K to oxygen at a pressure of -2 x Torr, at which point the sticking probability had fallen to a low value (< 1/100 of this initial value), and after increasing the oxygen pressure to lo-’ Torr. Both C and 0 were present as surface impurities in concentrations of < 15 % of a monolayer. Both the O( 1s) peak68 ELECTRON SPECTROSCOPIC STUDIES position and FWHM remained constant at 530.0+0.3 and 2.4 eV respectively, during exposure to oxygen.The Mo(3s+) :O(ls) ratio is 1 : 2.5 & 0.05 for saturation exposures at pressures up to about Torr. On increasing the pressure to 10-1 Torr the ratio decreased to 1 : 3.1. Slight broadening of the Mo(3d4,+) peaks (from 1.1 to 1.3 eV) was observed during adsorption. Repitition of the experiment at 77 K produced identical results, and no spectral changes were observed on warming to 295K. We recall that no changes in work function were observed in a similar experiment reported earlier. TABLE 2.-UPS VALENCE LEVELS a FOR C 0 2 AND H20 ADSORBED ON Au (77 K) and CO (p, y, VIRGIN STATES) ADSORBED ON Mo orbital energies/eV b CO~/AU (77 K) H e I { ~ ~ ~ ~ ~ d C02(d He1 orbital assignment 6.9; 9.6; 11.0; 13.1 8.3; 11.6; 12.3; 13.6(?) 6.8; 9.3; 11.0; 12.5 8.2; 11.4; 12.4; 13.7 13.7; 17.6; 18.0; 19.4 ( 7 d 4 ; ; (ad2 ; (d2 H20/Au (77 K) He1 6.3; 10.2; 12.6 H20(8) He1 orbital assignment CO/Mo He1 p Y virgin CO(d He1 orbital assignment 4-7 7.8; 11.2 6.7; 10.2 14.0; 16.8; 19.7 (a,)"; (nu)"; (aJ2 a All the adsorbed levels are broad.The quoted values represent the centre of the bands. b Ad- sorption system energies are ieferenced to EF of the substrate. For position relative to the vacuum level an appropriate work function term (see discussion) should be added. C ref. (31). Although from the known reactivity of molybdenum to oxygen, lattice penetration and oxidation would be unlikely at room temperat~re,~ flash filament-mass spectro- metric data with tungsten have been interpreted l8 in terms of some oxide formation, but the results are open to question.We therefore explored the possibility of " oxidation " of molybdenum at higher temperatures and pressures. First, on heating the chemisorbed layer to 523 K in a background pressure of Torr (" in uacuo ") no change occurred in either the O(1s) or Mo(3d+,+) peaks (fig. 4b). At a higher oxygen pressure (10-l Torr) and after 5 min heating at 420 K, the Mo(3d3) peak was unchanged but the Mo(3d+) peak had broadened from 1.8 to 2.0 eV (FWHM). The MO(%+) : O(1s) ratio decreased to 1 : 4.8t0.2 (cp. 1 : 3.1 at 295 K) but no change occurred in either binding energy or the width (FWHM) of the O(1s) envelope (fig. 4c). On increasing the temperature to 520 K (oxygen pressure 10-1 Torr), a further decrease (fig. 4b) in the intensity of the main Mo peaks occurred with a corresponding increase in the O( 1s) intensity, the Mo(3s3) :0( 1s) ratio approaching 1 :6.0.The Mo(3d3) was broadened even further to 2.55 eV (cp. 1.6 eV, 30 L, O2 at 235 K ; 1.8 eV 0.1 Torr 0, at 295 K) and a distinct broad feature ( N 3 eV wide) appeared at higher binding energy centred around 234 eV (fig. 4b, (iii) shaded area).S . J . ATKINSON, C. R. BRIJNDLE AND M. W. ROBERTS 69 FJG. 4 . 4 3 ) (i) Mo(3q) and O(1s) signals after 20 L exposure to O2 at 298 K. (ii) Signals after further exposure to O2 (0.1 Torr). (b) Mo(3d) peaks after (i) 520 K ‘‘in vacua" ; (ii) 420 K at 0.1 torr O2 : (iii) 520 K at 0.1 torr 02. (c) (i) O(1s) for b(i) above; (ii) O(1s) for b(ii) above ; (iii) high resolution O(ls) for b(iii) above. eV 240 200 160 eV (4 (6) FIG.5.- (a) Mo Auger signals during the interaction of Mo with O2 (1) Mo ; (2) Mo+monolayer of adsorbed oxygen ; (3) Mo+O.l Torr O2 at 573 K. (6) Mo(3d) XPS peaks for (l), (2) and (3) above.70 ELECTRON SPECTROSCOPIC STUDIES The O(ls) binding energy remained at 530 eV throughout these experiments (fig. 4c) though there was a slight broadening to high B.E. at all exposures. No additional oxygen features which might be associated with the broad additional Mo band (fig. 4b(iii)) were observed even when the O(1s) peak was examined under high resolution (fig. 4c(iii)). The X-ray induced Auger signals are shown in fig. 5 with the main Mo XPS peaks for comparative purposes. Clearly, the X-ray induced Mo(3d) peaks are more sensitive to the surface oxygen than are the X-ray induced Auger peaks in that they clearly reveal what we believe are electrons on the high energy side of the 3d peak (shaded in (3), fig.5) associated with oxidation of the metal. There are no shifts in the Auger peaks at this stage, though they become apparent at much later stages of oxidation.8b INTERACTION OF C 0 2 AND H 2 0 WITH Au AT 77 K XPS data for physically adsorbed CO, have been reported briefly.2 If, however, multilayers are formed by exceeding the vapour pressure of COz at 77 K, distinct differences in the O(1s) and C(1s) binding energies are observed (fig. 6). Since the ' ' ' ' ' ' ' 5 5 2 e v 528 5 4 0 a FIG. 6.-XPS for adsorbed COz on Au at 77 K. (a) at equilibrium at 2 x lo-" Torr ; (b) at equilibrium at - lo-' Torr ; (c) 100 s, O(1s) and Au(4s) peaks after- exposure to C 0 2 Torr ; (d) 50 min, 4 x 10-6Torr.change in binding energy is sharp, we conch e that the changes are genuine and do not reflect charging. Charging, resulting in a broadening and shifting of adsorbate levels, does occur at much higher exposures. The binding energies, O(1s) and C(ls), are given in table 1. The UPS data (He I and He 11) for both monolayer and multilayers of C02 are shown in fig. 7a and the " orbital energies " observed given in table 2, together with the free molecule values. The positions of the lowest at 6.9 eV (physical adsorption) and 8.3 eV (multilayer adsorption) are clear but there is less certainty about higher levels. Valence-level data by XPS were not easy to obtain due to the long escape-depthS .J . ATKINSON, C . R . BRUNDLE AND M . W. ROBERTS 71 of the photoelectrons ejected by A1 Ka radiation and also the rather low cross- sections of these levels with 1486 eV energy photons. Data were therefore only obtained from thick multilayers (fig. 7b). We estimate from the shift and broadening , €F 4 0 12 r, 6 12 eV i3 " fF " 2 0 ' 40 ' m e v FIG. 7.-(4 He1 and He11 spectra for C 0 2 on Au (77 K) (1) clean Au ; (2) physisorbed COz (equili- brium at - Torr) ; (3) condensed multilayers COz. (b) XPS for multilayers of C 0 2 on Au (feature at 5 eV represents the X-ray satellite of the 15 eV peak). of the O(1s) peak that there is about a 1.5 eV charging effect present so that the two lower energy peaks observed by XPS (10 and 15 eV) agree fairly well with the lowest level (8.3 ev) recorded by U P S for C 0 2 multilayers and the centre of gravity (ca.13 eV) of the three higher energy orbitals. The orbital at 33.5 eV (XPS) is not observable by UPS. For H20, only one O(1s) peak (fig. 8) and only one set of orbitals are observed in the UPS (see also ref. (19)), the positions of which are given in tables 1 and 2. Fig. 8 also shows an O(1s) spectrum of co-adsorbed C 0 2 and H20 illustrating that a chemical shift exists in the solid-state spectra whereas in the gas phase the O(1s) values reported are virtually identical (540 eV). The valence level and O(2s)XPS data are also reported for H20 multilayers (fig. 8b). A charge-induced shift of about 1 eV is apparently present in this spectrum. ADSORPTION OF CARBON DIOXIDE ON MOLYBDENUM ADSORPTION AT 295 K FOLLOWED BY COOLING TO 77 K The development of the O(1s) and C(1s) signals during the exposure of the M o surface at 295 K to CO,(g), and also the O(ls) signal as a function of exposure, is72 ELECTRON SPECTROSCOPIC STUDIES shown in fig.9. The maximum C 0 2 pressure was about Torr. There is little change in intensity after an exposure of about 2 L which suggests an average sticking probability of 0.5 ; there is no change in the O(ls) binding energy which occurs at the same value as the original oxygen contaminant throughout the adsorption. The O(1s) peak height has been corrected for the O(ls) contamination peak which is I I I I I I zv 20 40 t l l l l l EF 6 12 eV FIG. &-(a) Coadsorbed H20 and C02 on Au at 77 K, O(1s) positions ; (b) XPS band structure of H20 multilayers on Au at 77 K ; (c) UPS He1 of Au and H20 multilayer on Au at 77 K.approximately 10 % of the intensity at CO saturation. At saturation, the Mo(3s+) : O(1s) ratio is 1 : 0.9 and the O(1s)FWHM is 2.4 eV (table 1). The C(ls) peak increased in intensity but remained fixed at an energy of about 283 eV (fig. 9). No change occurred in either the C( 1s) or O(1s) intensities on increasing the C 0 2 pressure to On cooling to 77 K at this pressure, new oxygen and carbon peaks appeared at higher binding energy, O*(ls), (534.2 eV) C*(ls), (291.6 eV) correspond- ing to multilayers of condensed C02 (table 1). Torr. ADSORPTION AT 77 K Adsorbing C 0 2 at 77 K resulted in the appearance (fig. 10) of two O(1s) peaks, one, O(ls), at 531 eV and the other O(ls)* at 534.1 eV.On removing CO,(g) and warming in uacuo to 298 K the 0(1s)* peak disappeared and there was a gradual shiftS . J . ATKINSON, C . R . BRUNDLE A N D M . W. ROBERTS 73 of the O(1s) peak to lower binding energy; at 298 K the value was 530.0 eV, a shift from the initial value at 77 K of about 1 eV. This behaviour is analogous to that observed with CO adsorbed at 77 K and subsequently warmed to 298 K when the I 1 1 I I I I 2 4 6 exposure/L L, 5 i O ' 540 FIG. 9.-(a) Ratio of intensity l o f O(ls) signal to O(1s)signal at monolayer as a function of exposure to C02(g) at 295 K. (b) Development of O(1s) and C(1s) signals as a function of exposure of Mo film to C02 at 295 K. observed O(1s) was virtually identical to the O(ls) value for CO adsorption at 298 K. It is tempting to suggest that the molecular events occurring with CO,(ads) are also analogous, namely, that at 77 K, CO, dissociates into CO(ads) and O(ads) followed by further dissociation of Cotads), which may be considered to be at 77 K in the virgin CO state, to C(aas) and O(ads) at 298 K.INTERACTION OF WATER WITH MOLYBDENUM The O(1s) peak was monitored as a function of exposure to water at 295 K and a nominal pressure of 2 x The height of the O(1s) peak (530.3 eV) became constant at an exposure of 6 L, the Mo(3s3) :O(ls) ratio then being 1 : 1.64 (table 1). This suggests an average sticking probability of 0.16 for the chemisorbed layer. A high-resolution spectrum of the O(1s) peak is shown in fig. 11. With continued exposure (i.e., beyond 6 L) the O(1s) peak broadens and at about 100 L a second signal centred at 532.5 eV has developed (fig. 11).The Mo(3d~) peak behaviour during adsorption was similar to that observed with CO and O2 in that it broadened slightly (1.1 to 1.3 eV) while the intensity of the main Mo peaks decreased. On cooling the surface which had interacted with water at 295K (H,O(g) had been removed by evacuation) to 77 K an incipient second O(1s) peak was more evident. This presumably arises from the enhanced physical adsorption at 77 K Torr (fig. 11).74 ELECTRON SPECTROSCOPIC STUDIES Increasing T I 5 2 7 . 533 ' 539 eV FIG. lO.-(a) Mo(3s) and O(ls+) signals for : (1) Mo film with small oxygen contamination at 77 K. (2) Mo+C02 at 77 K. (b) O(1s) spectra during heating of COz layer at 77 K to 298 K.(compared with 298 K) of the small quantity of water vapour still present (pressure - Torr) ; this contention is supported by the rapid increase in the peak at 532.5 eV on exposure to H20 vapour at 77 K (fig. 11). Continued exposure to water vapour resulted in the 532.5 eV O(1s) peak dominating the O(1s) peak at 530 eV. On warming to 298 K the 532.5 eV peak decreased to become only a shoulder on the O(ls) peak at 530 eV, this being a characteristic of interaction at 298 K (table 1). Heating to 523 K in vacuo removed this shoulder with a consequent sharpening of the O(ls) peak. After further heating in water vapour (523 K, 0.1 Torr, 15 min) and cooling in water vapour, initially at a pressure of lo-' Torr and finally -4 x 10-l' Torr), the Mo(3s3) : O(1s) ratio decreased to 1 : 2.2 and the high binding energy O( 1 s) reappeared.When a surface which had interacted with H,O(g) at 295 K to the stage where the O(ls) peak height was constant (exposure of -6 L) was exposed to O,(g) (1 x Torr for 5 min), the O(1s) signal at 530 eV increased immediately by about 17 % and the Mo(3s3) : O(ls) ratio decreased to 1 : 2.06. There was no apparent change in the O(ls) shoulder. DISCUSSION There are now available experimental data enabling XPS peak heights to be used to provide relative concentrations of surface species present. For example, for COz condensed on Au the C(ls) : O(1s) ratio is 1 : 4.6 which establishes a relativeS . J . ATKINSON, C. R . BRUNDLE A N D M . W . ROBERTS 75 effective ionization cross-section of 1 :2.3 for the C( 1s) and O( 1s) electrons, and which may be used in other systems involving surface carbon and oxygen.I 2 4 6 8 10 L 1 1 1 1 1 1 ' 1 ~ ~ 1 ~ FIG. 1 1 . 4 ~ ) Ratio of the observed intensity Z of O(ls) to the O(1s) intensity at the monolayer during exposure of Mo to H20(g) at 295 K ; (b) O(1s) spectra after 100 L exposure to H20(g) at 295 K ; (c) O(1s) after cooling (b) to 77 K and further exposure to H20(g). 524 530 536 524 530 536 eV CHEMICAL SHIFTS Distinct energies of binding can occur for different state of adsorption. Analogies with gas phase studies, where clear correlations exist between electron density (or charge) and binding energy, suggest that, e.g., for the y-state(Mo + CO system) the charge on the oxygen atom is more positive than the charge on the oxygen atom in the B-state.A complete interpretation of these shifts is not possible and, in particular, the problem of relaxation effects has to be unravelled. Theoretical work on relaxation contributions for gaseous molecules is in progress but the relevance to surface species is uncertain. Demuth and Eastman 2o have assumed that any shift in the energy of non-bonding valence electrons during adsorption is due entirely to relaxation effects ; consequently, if this correction is then applied to the observed shifts in other valence levels the remaining shift will be due to adsorption. In the present work we are less concerned about absolute binding energy values since both XPS and UPS data have mainly been used to distinguish between different states of adsorption.If, however, we compare, e.g., the binding energy of the valence electrons of adsorbed CO with those in CO(g), then since the experimental adsorption values are referred to the Fermi level of the substrate then, in addition to relaxation effects, an appropriate work function correction is necessary. It is not clear what the appropriate correction should be but we have added the clean substrate76 ELECTRON SPECTROSCOPIC STUDIES work function in order to compare our experimental UPS data with gas phase studies (cp. also Yates, Madey and Erickson 9. No attempt is made to account for relaxation effects. PHYSICAL AND MULTILAYER ADSORPTION ON AU There is a shift of - 1.4 eV in the O(1s) binding energy between monolayer and multilayer adsorption of C02 on Au; no similar shift occurred with H20. These facts suggest that for the Au + C02 system the first layer is influenced strongly by the Au substrate but less significantly for multilayer formation. We would expect to observe differences in heat of adsorption during monolayer and multilayer formation and these are probably reflected in the O(1s) binding energy.The apparent absence of a similar effect with H20 is puzzling. The valence levels of physically adsorbed C02 are important for elucidating the adsorbed state in the Mo + C02 system. At present, no UPS data for the Mo + C 0 2 systems are available. The lowest binding-energy feature for the Au + C02 system is clearly the oxygen lone pair orbital (n,), (table 2).The next three orbitals of C02(g), (nu),, (a,)2 and two of which overlap, are probably all present in the region 9-13 eV for C 0 2 (phys. ads.) and 10-14.5 for C02 (multilayer). Both He I and He I1 spectra show structure in these regions and in the He I1 spectra for C02 (phys. ads.), three distinct features are present (fig. 8). The X P S valence and O(2s) levels of “ solid” H20 have been reported by Siegbahn 21 ; with allowance for the 1 eV charging estimated to be present in our spectra, the agreement is reasonable. Presumably all the valence levels (lbl, 2a and 1b2) observed clearly in the UPS spectrum of H20 (ads) are present in the broad band from 6 to 13 eV (fig. 8). Again, the valence level data for H,O/Au (77 K) will prove useful for interpreting UPS data in the H,O/Mo system, when available.CHEMISORPTION ON MOLYBDENUM By observing O( Is), C( 1s) and valence level spectra for the Mo + CO system, three regimes of adsorption have been identified; adsorption at 77 K (the virgin state), adsorption at 295 K (D state) and adsorption at 77 K following adsorption at 295 K (y state). The different “ states ” reflect the difference between the thermal energy available at 295 compared with 77 K and the significance of surface coverage in determining the nature of the adsorption bond. It is therefore the interplay between available sites (O), thermal energy, surface diffusion and the activation energy necessary to acquire a particular bonding configuration that determine the observed spectra. Studies in the temperature range 77-295 K offer a number of possibilities where one or more of the above factors are of more or less significance. Briefly, the 8-state was judged 4 9 ’ to be dissociated in that the O(ls) spectra are identical to O(1s) features during oxygen interaction (table 1 and fig.1) and that UPS did not reveal CO-like orbitals (fig. 3) but features more like atomic C and 0 orbitals were present. In contrast, both the virgin and y states possess CO-like features ; furthermore, when virgin CO was warmed to 295 K these features dis- appeared and were replaced by /3-type spectra. The O(ls) : C(ls) ratios (1 : 2.5, table 1) indicate that in the adsorbed state the C : 0 ratio is 1 : 1 as would be expected. We have no evidence for a-CO state(s) on Mo which contrasts with data reported for the W + CO system.sa Since this work was completed, Viswanath and Schmidt 22 have studied the desorption of adsorbed oxygen and carbon from Mo(100) and W(100) and reported that it is identical to that of adsorbed CO.There is therefore substantial evidenceS. J . ATKINSON, C. R. BRUNDLE AND M. W. ROBERTS 77 that indicates that CO is dissociated in the P-state and this is compatible with the absence of infra-red activity when CO is strongly chemisorbed. 32* 33 However, electron spectroscopy can only show whether there is appreciable bonding between the carbon and oxygen atoms in adsorbed carbon monoxide; this would appear not to be the case. Whether the surface carbon and oxygen exist as independent entities is uncertain. As to the y-state, we have clear evidence for a range of O(1s) states (fig.lc) which correlate with the range of M-CO bond energies corresponding with the known decrease in the heat of chemisorption with increasing coverage at low temperature. Unfortunately, the signal is too weak and overlaps two greatly with the contribution from the O(1s) P-state to be certain that the y states fill up starting with the low binding energy (higher heat of adsorption) surface species, which would be consistent with a heat of adsorption that decreases with increasing coverage.1° The O(1s) features are completely reversible in that they are removed on warming to 295 K and reappear on cooling to 77 K in CO(g) at - Torr pressure. Bonding to the surface is therefore weak. The UPS data for the y-state (fig.3) suggest the presence of molecular CO but in view of the much smaller coverage of y-state than the virgin state the evidence is less convincing. The O(1s) peak associated with the dissociative chemisorption of oxygen on molybdenum remains unchanged in energy and width during the interaction in the temperature range 77-520 K (table 1). This behaviour is similar to that recently observed for the W + 0 2 system and is in keeping with work function l7 and photo- emission studies 23 of the Mo+02 system where no evidence for interaction beyond the chemisorption stage was obtained at 298 K. We conclude from the present work that the electronic configuration of chemisorbed oxygen is essentially invariant with coverage and note that the main O(1s) peak occurs at exactly the same energy as the p-CO state and both the COz chemisorption and the major component of the H20 chemisorption at 298 K.The small tailing (cp. also W+02) of the O(1s) peak to higher binding energy may possibly be attributed to the presence of a small proportion of molecular oxygen of low heat of adsorption. The O(1s) : Mo(3s+) ratio of 2.5 : 1 (table 1) implies that the oxygen adatom coverage is some 1.25 times greater than for CO at the monolayer. This is in general agreement with comparative adsorption studies of O2 and CO on Mo and W surfaces. The emergence of the broad feature centred at 234 eV (fig. 4) in the Mo(3d) spectrum is clearly related to bulk oxidation and compares with the observations of Fraser et ~ 1 . ~ ~ on exposing a clean Mo surface to ambient atmosphere for several hours when a feature also developed at 234 eV.The major O(1s) component arising from the Mo+H,O interaction at 298 K is at 530 eV. The higher binding energy component (532.3 eV) is close to the O(1s) position for H20 adsorption on Au at 77 K (532.7 eV, table 1). We suggest therefore that the minor component (about 25 % of the total signal at 295 K) is due to molecu- larly adsorbed water whereas the major component arises from dissociatively chemi- sorbed water. Unpublished work and also comparison with H2S studies would suggest the presence of OH(ads) (cp. SH with HzS on Ni and Fe13), so that OH species are likely to contribute to the O(1s) spectrum. Kinetic 2 5 and electrical resistance studies 26 suggest " compound formation " at 295 K.At 77 K, Suhrmann et aZ.26 interpreted their electrical resistance data in terms of molecular adsorption, with dissociation occurring in the temperature range 77-298 K. The Mo : main O(1s) ratio of 1 : 1.6 by height indicates about 60 % as many oxygen adatoms present as in the Mo+O, system at 298 K. The minor O(1s) peak accounts for a further 20 "/o. Clearly, the specificity of the Mo surface for dissociative78 ELECTRON SPECTROSCOPIC STUDIES chemisorption of oxygen is appreciably less than for the dissociative chemisorption of water and probably arises from the competition between molecular dissociation and molecular adsorption in the case of H20. Molecular adsorption of the highly polar H 2 0 molecule is much more likely than that of molecular oxygen so that the surface phase after interaction with water vapour probably contains an appreciable fraction (20 %, according to the present data) of molecularly adsorbed water.The 02(g) + H,O(ads) data, although of a preliminary nature, support this viewpoint since they suggest that dissociative chemisorption of 0, occurs (reflected in the increase of the intensity of the 530 eV peak) but without desorption of molecular water (no change in the shoulder at 533.2 eV). Replacement of H,O(ads) by O(ads) with further readsorp- tion of molecular water probably occurs. There is considerable support for the view that CO, is dissociatively chemisorbed on Mo and W at or just above room temperature. It includes isotopic exchange experiments infra-red 28 and field-emission arguments based on thermo- chemical data 30 and the detection of CO(g) arising from dissociative chemisorption of CO,.The present data also suggest dissociative chemisorption but the final chemisorbed state at 298 K is C(ads) and O(ads) and not CO(ads)+O(ads). The basis of this is the identical O(1s) and C(1s) binding energies to flCO(ads). The heavy tailing to higher binding energies is compatible with a small coverage of either CO(ads) or CO,(ads). Some reversible uptake of CO, by Mo, W and Ta has been reported. 30 On cooling the chemisorbed layer formed at 298 K to 77 K, further adsorption occurred but the C(1s) and O(1s) signals are at higher binding energy, comparable with those observed in the Au + CO, system at 77 K where only molecular adsorption occurs (table 1).The area ratios (C(1s) : O(1s)) are consistent with molecular adsorption (table 1). Adsorption of C 0 2 on a " clean " Mo film at 77 K resulted in the immediate growth of the low binding energy O( 1s) and C( 1s) signals at energies observed for the interaction of CO at 77 K (virgin state). On warming to 298 K the peaks moved to energies typical of the p-CO state. We therefore propose a step-wise dissociation model for CO, interaction at 77 K : 71 K C02(g) --+ CO(ads) + O(ads) C(ads) + O(ads). 5- 298K Quantitative deductions based on a comparison of Mo(3.s) : O(1s) ratios for CO and CO, adsorption are not attempted since the ratios depend on the respective surface coverages attained in each case. If we assume, however, that in our experi- ments the CO coverage is 2.3 times the C02 coverage, a ratio based on previous studies,30 then the Mo(3s) :O(ls) is about what is expected.However, at 298 K the C(1s) : O(1s) ratio is only about half that expected (table 1) for a surface containing surface carbon and oxygen in the ratio 1 : 2 .Clearly, our two sets of data (Mo : 0 and C : 0 ratios) are incompatible. We would not expect dissociative chemisorption with concurrent oxygen desorption and favour the Mo(3s) : O(1s) data in view of the rela- tively poor accuracy of measuring C(1s) : O(1.s) ratios. We are grateful to the Science Research Council for support of this work and to Mr. A. Carley for experimental assistance.S . J . ATKINSON, C. R . BRUNDLE AND M. W. ROBERTS 79 W. T. Bordass and J. W. Linnett, Nature, 1969,222,660 ; D.Eastman and J. K. Cashion, Phys. Rev. Letters, 1971, 27, 1520. C. R. Brundle and M. W. Roberts, Proc. Roy. SOC. A, 1972,331, 383. C. R. Brundle arid M. W. Roberts, Chem. Phys. Letters, 1973, 18, 380. S. J. Atkinson, C. R. Brundle and M. W. Roberts, Chem. Phys. Letters, 1974, 24, 175. (a) T. E. Madey, J. T. Yates and N. E. Erickson, Chem. Phys. Letters, 1973, 19,487. (b) J. T. Yates and N. E. Erickson, Surface Sci., to be published M. Barber, E. L. Evans and J. M. Thomas, Chem. Phys. Letters, 1973, 18,423. S . J. Atkirtson, C . R. Brundle and M. W. Roberts, J. Eiectron Spectr., 1973, 2, 105. (a) e.g., C. R. Brundle and M. W. Roberts, Chem. Phys. Letters, 1973, 18, 380; M. P. Seah, Surface Sci., 1972, 32, 703. (b) C. R. Brundle, J. Vuc. Sci. Tech., 1974, 11, 212. (c) C. J. Powell, Surface Sci., to be published. K. Siegbahn et al., ESCA Applied to Free Molecules (North Holland, Amsterdam, 1969). M. W. Roberts, Trans. Faraciuy Soc., 1963, 59,698. lo J. G. Little, C. M. Quinn and M. W. Roberts, J. Catalysis, 1964, 3, 57. l2 R. R. Ford, Adv. Catalysis, 1970, 21, 51. l3 J. R. H. Ross and M. W . Roberts, Trans. Faraday SOC., 1966, 62,2301. l4 D. 0. Hayward, D. A. King and F. C. Tompkins, Proc. Roy. SOC. A, 1967,297, 305. Is R. Gomer and A. A. Bell, J. Chem. Phys., 1966,44,1065 ; D. Menzel and R. Gomer, J. Chem. Phys., 1964,40,1164. l6 C. R. Brundle, M. W. Roberts, K. Yates and D. Latham, J. Electroiz Spectr., 1974, 3, 241 ; G. Johansson, J. Hedman, A. Berndtsson, M. Klasson and R. Nilsson, J. Electron Spectr., 1973,2, 295. D. A. King, T. E. Madey and J. T. Yates, J. Chem. Phys., 1971,55,3236 ; 1971,553247. l7 C. M. Quinn and M. W. Roberts, Trans. Farday SOC., 1964, 60,899. l9 C. R. Brundle and M. W. Roberts, Surface Sci., 1973, 38, 234. 2o J. E. Demuth and D. E. Eastman, Phys. Rev. Letters, 1974, 32, 1123. 21 K. Siegbahn et al, ref. (9) p. 82. zZ Y. Viswanath and L. D. Schmidt, J. Chem. Phys., 1973,59,4184. 23 C. M. Quinn and M. W. Roberts, unpublished data. 24 W. A. Fraser, J. V. Florio, W. N. Delgass and W. D. Robertson, Reu. Sci. Instr., 1973,44,1490. 25 H. Imai and C. Kemball, Proc. Roy. SOC. A, 1968,302,399. 26 R. Suhrmann, J. M. Heras, L. V. De Heras and G. Wedler, 2. Elektrochem., 1964,68,511. 27 D. Brennan, E. Greenhalgh and B. M. W. Trapnell, unpublished data. 28 R. P. Eischens and W. A. Pliskin, Ado. Catalysis, 1957,9,662. 29 D. 0. Hayward and R. Gomer, J. Chem. Phys., 1959,30,1617. 30 D. Brennan and D. 0. Hayward, Phil. Trans., 1965,258, 375. 31 D. W. Turner, A. D. Baker, C. Baker and C. R. Brundle, Molecular Photoelectron Spectroscopy 32 A. M. Bradshaw and J. Pritchard, Proc. Roy. SOC. A, 1970,316,169. 33 D. A. King, C. G. Goymour and J. T. Yates, Proc. Roy. SOC. A, 1972,331, 361. (Academic Press, London, 1970).

ISSN:0301-7249    

DOI:10.1039/DC9745800062

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Ultra-violet Photoelectron Spectroscopy Studies of Gas Adsorption on Transition Metals BY P. J. PAGE AND P. M. WILLIAMS* Dept. of Chemical Engineering and Chemical Technology, Imperial College Received 3rd June, 1974 The adsorption of CO and CzH4 on oriented single crystals of nickel has been studied by means of ultra-violet photoelectron spectroscopy (UPS). At He1 energies (21.2 eV), distinctly different d band densities of states are observed for the clean (1 1 l), (1 lo), and (100) faces which are interpreted in terms of bulk rather than surface band calculations. Following CO adsorption, states derived from the 772p antibonding level of CO may be distinguished in the structure close to E f in addition to the deeper 02p and r2p bonding states. The cracking of ethylene at high temperature over Ni(l10) is shown to produce graphitic overlayers. Preliminary studies of the oxidation of chromium are also reported.The study of adsorption phenomena on surfaces by means of ultra-violet photo- electron spectroscopy (UPS) has been the subject of considerable interest in recent years following the early work of Eastman.'. With ultra-high vacuum hemi- spherical analyser systems fitted with windowless discharge lamps, photons at energies of 21.2, 40.8, and 48.4 eV from helium excitation and 16.8, 26.9 eV from neon are now available. These energies permit the examination of adsorbate-induced valence levels over an energy window of 10-1 5 eV from the uppermost layers of a surface, so that direct changes in the bonding states of both adsorbate and host may be observed.Thus the adsorption of various gases on polycrystalline 3 9 and single crystal n i ~ k e l , ~ copper,6 molybdenum,' and tungsten have all been studied using UPS. In many of these investigations, an interpretation of the energy levels induced by the adsorbate has been attempted on the basis of a direct comparison with the molecular orbitals of the free molecule measured in the gas phase.9 This approach has been particularly successful, for example, in explaining the behaviour of CO on adsorption on a range of metals with relatively little adjustment in energy to the free molecular orbitals. The situation becomes more complex, however, in situations involving dissociative ad~orption,~. large hydrocarbon molecule^,^ or the possibility of more than one adsorbed * Under such circumstances, large substrate- adsorbate interactions, which may involve considerable charge transfer, may invali- date simple extrapolation from the free molecular case.A direct knowledge of the behaviour of the d electrons of the metal itself is neces- sary for a complete understanding of the nature of these interactions. In the present paper we therefore report changes within the d electron density of states of nickel between different crystal faces and following the adsorption of CO and C2H4. A preliminary extension of the work to the oxidation of chromium is also described. EXPERIMENTAL Single crystal discs of nickel ((lll), (110), and (100) orientations) and chromium ((111) orientation) were cut to within 1-2" and mechanically polished to a mirror finish on a vibratory polisher with 5000A alumina and Syton.These were welded directly to tungsten rods for 80P . J . PAGE AND P . M . WILLIAMS 81 support in the electron spectrometer (Vacuum Generators Ltd., ESCA3). Following bake- out, pressures -2x 10-lo Torr were routinely obtained, and crystals were cleaned in the specimen chamber of the instrument by repeated cycles of argon-ion bdbardrnent, oxidation, annealing and finally flashing to -1000°C. Surface impurities could be monitored by AlKa or MgKa excited XPS, although the precise form of the UPS spectrum was found to be the most sensitive test for surface contaminants. Spectroscopically pure gases (CO and C2H4) were admitted only to the specimen preparation chamber of the instrument via an all metal leak valve while the analyser remained at 2x Torr.Pressures in the analyser chamber increased to 7-8 x 10-lo Torr during running of the windowless He discharge lamp. Spectra were recorded digitally using a PDP/8f based data system,14 and all data are presented in " raw ", unsmoothed form. RESULTS CLEAN NICKEL SINGLE CRYSTALS Previous UPS work on polycrystalline nickel evaporated in ultra-high vacuum 2-4* l5 revealed considerable changes in the density of states within the d band on adsorption, in addition to the appearance of deeper lying levels. Interpretation of such be- haviour in terms of surface states has been suggested 3 9 l 5 but relatively few data are available on the density of states of pure nickel with which to compare these pre- dictions.With tungsten, the comparison of UPS spectra from different crystal faces has facilitated the identification of surface states lo* in addition to characterising features in the bulk band structure. Angular effects are important, however, for single crystal substrates when only electrons in specific directions of emission relative to the crystal surface are admitted to the analyser.12* 16. l7 In the present investiga- tion, therefore, single crystals of (lll), (110), and (100) orientation nickel were so oriented as to allow electrons emitted as closely as possible to the normal to the surface to enter the analyser. These directions were defined by a slit system rather than by a circular aperture, but the spectra do indicate a sufficiently high degree of angular selectivity despite this non-ideality.Fig. 1 compares He1 spectra recorded from the three crystal faces of Ni following cleaning, with the corresponding spectrum from a polycrystalline evaporated film for comparison. The (111) spectrum is dominated by a single state, fwhm 0.5 eV immediately below E,, with a weaker level 2.5 eV below Ef. A doublet feature with a splitting of just over 0.5 eV is seen in the (100) spectrum, whilst the (1 10) face shows three less distinct levels. The bandwidth and structure in the polycrystalline sample are consistent with an appropriately weighted sum over the single crystal densities of states. ADSORPTION OF co ON NICKEL Fig. 2-6 show the changes produced in the densities of states of the clean nickel on adsorption of CO to saturation coverage at room temperature. Thus, fig.2,3 and 4 show the He1 spectra over the d band region for (1 1 l), (1 lo), and (100) faces before and after exposure to CO. The He1 spectra from the CO saturated faces are com- pared in fig. 5. In all cases, particularly the (1 11) face (fig. 2) the effect of adsorption is a broadening in the d band together with the appearance of a lower energy broad state, 1.2 eV below Ef for (1 10) and (loo), and 1.5 eV for (1 11). He11 spectra from all three faces are similar (fig. 6) and show the now familiar 0- and n-bonded CO states at 8.0 and 11.0 eV below Ef. HIGH TEMPERATURE ADSORPTION OF C2H4 ON Ni (110) The interaction of hydrocarbons with Ni at elevated temperatures may be expected to produce complete dehydrogenation, and fig.7 shows the effect of exposing a clean82 GAS ADSORPTION ON TRANSITION METALS Ni(ll0) surface to C2H, at 600°C for 13 min at Torr. This spectrum is compared with that from vacuum-cleaved well-oriented pyrolytic graphite,' * also recorded using Hen photons. 1 I I I 1 L-f - 1 -2 -3 -4 binding energy /eV FIG. 1.-He1 UPS (21.2 eV) from (a) Ni(lll), (b) Ni(100), (c) Ni(ll0) and (d) polycrystalline Ni. Single crystals oriented so that near-normally emitted electrons enter analyser. Inset is calculated density of states from ref. (20).P. J . PAGE AND P. M. WILLIAMS 83 I 1 1 I i € f - I - 2 - 3 binding energy/eV FIG. 2.-He1 UPS from (a) clean Ni(l1 l), (6) Nit 11 1) saturated with CO (= 100 langmuirs) at 295 K. Shoulder mowed in (b) corresponds to peak in clean density of states in (a)..r-., . . . * *-.. -... . . - ... ..I. - ...--. __“C. 1 I I E f - I -2 -3 binding energy/eV FIG. 3.-He1 U P S from (a) clean Ni(1 lo), (b) Ni(ll0) saturated CO.I / 1 1 1 € f - 1 - 2 - 3 binding energy/eV FIG. 4.-He1 UPS from (a) clean Ni(lOO), (b) Ni(100) saturated CO. . .*.- . ...... ........ - 4::. .- .. - ..p*. .. .-~.%..-...*.:.I .. .- -***. (= 1 - .... . -- ,......'"-* "* (I 00) : __..I ..-,a . . . . *.n . . (110) :: ........... ..-.-.."--* . ............ -. I 8 t ET - I -2 -3 binding energyleV FIG. 5.-Comparison of CO-saturated surfaces: He1 UPS from (a) Ni(lll), (b) Ni(l10), and (c) Ni(100). The extra state near 1.2 eV below fi is seen for all faces ; feature near 0.6 eV only for the (1 11) face.P .3 . PAGE AND P . M. WILLIAMS k I I I €f -- 5 -10 -15 binding energy/eV 85 FIG. 6.-He11 UPS (40.8 eV) from CO-saturated surfaces : (a) Ni(l11), (6) Ni(lOO), and (c) Ni(l10). The 0 and n CO levels at 8.0 and 11 .O eV below E f are clearly visible.86 GAS ADSORPTION ON TRANSITION METALS OXIDATION OF CHROMIUM (111) Eastman has shown l9 that in moving from nickel (f.c.c., 3d84s2) to chromium b.c.c., 3d54s1) there is a considerable change in the density of d states as determined by UPS. It is therefore of interest to compare the effects of adsorption on these two metals. Little or no interaction could be detected with CO, but the effects of oxygen on the clean Cr (1 1 I ) surface are seen in fig. 8, where spectra excited with He1 and I I I ..... ..... .. . . a . . . ........... (b) ... (c) ......... ............... ......... ........... I I I Ef - I - 2 -3 binding energy/eV FIG. 8 . 4 ~ ) He1 UPS from Cr(ll1) following oxidation (30langmuir.s at room temperature); (b) HeII UPS from Cr following oxidation ; (c) HeII UPS from clean Cr(ll1) ; (d) He1 UPS from clean Cr. Note that a remnant of the sharp level immediately below E f persists in the He1 spectrum after oxidation. He11 photons for both clean and oxidised Cr (1 11) are shown. For the clean surface, a sharp state (fwhm -0.3 eV) is observed immediately below E,, particularly at HeI. This state is removed on exposure to O2 and a new level develops 2.0 eV below Ef. DISCUSSION CLEAN SURFACES A detailed discussion of the features in the densities of states for the clean Ni surfaces is beyond the scope of the present paper.Nevertheless, it is necessary to consider briefly the nature of the predominant d states in the spectra in the light of previous adsorption measurements on polycrystalline films and the present results on single crystals. Polycrystalline spectra (e.g., fig. Id) show a sharp peak immedi- ately below Ef followed by a broader d band and the attenuation of this sharp peak onP . J . PAGE AND P . M. WILLIAMS 87 chemisorption led to its possible interpretation in terms of a surface state.3* l 5 Spectra for the single crystal faces of Ni show considerable variation in structure with orientation, and the photo-emission from the (1 11) face in particular is dominated by a similar sharp peak at Ef. This high intensity for the (1 11) face (implying a lack of bulk contribution to the spectrum) does not in itself preclude an interpretation in terms of surface states as a similarly dominant peak on ( 1 0 ) tungsten has been so interpreted.1° However, whereas in the tungsten band structure, there is a favour- able hybridization gap immediately below Ef in which an evanescent surface mode may propagate, the only likely gap for such a state in Ni between the A; and A$ bands 2o occurs some 4-5 eV below Ef and so cannot provide a simple explanation of the sharp state.Furthermore, while the W( 100) surface state vanishes on exposure to less than a quarter of a monolayer of hydrogen, the sharp (1 11) state on Ni (fig. 2) in fact persists at saturation coverage of CO (arrowed).Its apparent attenuation relative to the second peak in the polycrystalline observations should be thought of for the CO chemisorption in terms of a relative increase in density of states structure 1-2 eV below Ef due to the appearance of the antibonding n2p level, rather than the quenching of an evanescent surface mode. The redistribution of d electrons due to population of, and admixture with the antibonding n2p level does not in itself neces- sarily imply the existence of a surface state. This interpretation removes the apparent anomaly in the behaviour of the sharp d state in the comparison of C 0 2 adsorption where, for a comparable heat of adsorption to CO, little attenuation is observed; in this case, however, there is no counterpart to the antibonding level which disturbs the d electron density of states in the CO adsorption.With H2S and 02, the drastic attenuation of the sharp d state is indicative of strong charge transfer in an ionic sense to form Ni-S or Ni-0 bonds ; the effective sampling depth of only 2-3 atomic layers ensures that the underlying metallic Ni contributes insignificantly to the observed intensity. We therefore discount the evanescent surface mode as a possible explanation of the sharp d state and consider the effect of the surface only in terms of narrowing of the bulk energy bands 21* 22 and possible s to d charge transfer.23 Such effects may be regarded as perturbations of the bulk band structure so that we may attempt an initial interpretation of the densities of states in fig. 1 in terms of the bulk density of states. Comparison with Zornberg’s 2o calculations suggests that for spectra in which only normally emitted electrons contribute, the peak in the (111) spectrum corresponds to the sharp L32 density of states maximum, and the doublet in (100) to the X2-X5 maxima ; the relative lack of sharply-defined structure on the (1 10) face is expected in view of the band dispersion along K.Inset in fig. 1 for comparison is Zornberg’s paramagnetic density of states, showing clearly the L32 maximum. This represents only a preliminary attempt at a band structure interpretation, and takes no account of exchange interaction splitting in the ferromagnetic case (which may be as large as 0.5-0.6 eV). A fuller treatment of the single crystal measurements is to be submitted elsewhere.13 ADSORPTION OF co ON NICKEL The immediately characteristic feature in the spectrum from all three crystal faces following adsorption is the appearance of levels 8.0 and 11.0 eV below Ef.These have previously been interpreted as deriving from the a2p and n2p bonding states of the CO molec~le,~’~ and their precise energies have been discussed by Eastman in terms of the relative strengths of a-d and n-d interactions between the adsorbed molecule and the surface d band. We make no further mention of them88 GAS ADSORPTION ON TRANSITION METALS here, except to note that there appears to be a slight shift of 0.2 eV in the 7c state to higher energy on the (1 10) face relative to the other two. Closer to the Fermi level, further adsorbate-induced effects are observed on all three faces.On both (100) and (1 lo), an extra level 1.2 eV below Ef masks the structure in the clean spectra; the corresponding state is observed 1.5 eV below Ef on the (111) face. This apparent shift for the (1 11) spectrum relative to the other two is not thought to indicate a genuine difference as the new level appears on a very low background density of states on (1 11) but overlaps underlying Ni structure, shifting the mean peak position to lower binding energy, on the other two faces. However, in the (1 11) case, between the shoulder in the CO-saturated spectrum, which corresponds to the peak in the clean density of states, and the level at 1.5 eV, a new band 0.6 eV below Ef appears. These observations cannot be interpreted, as was suggested in the polycrystalline case, simply in terms of attenuation of the sharp d state immediately below E,, although some decrease in intensity of the leading d band did occur following adsorption and the spectra are approximately normalized in all cases.Additional density-of-states structure does appear following adsorption as is conclusively shown on the (111) face. It is tempting, therefore, to assign the state 1.2 eV below Ef to the antibonding n2p state of the CO, previously unobserved in polycrystalline data because of over- lapping nickel d band emission. This state is populated by back donation from the Ni d bands following formation of the primary o-d bond on adsorption. Doyen and Ertl 24 have recently calculated the energy of the antibonding state as 0.63 eV below E,, and find an occupation number of 0.023 per molecule.Penn has pointed to the possibility of asymmetric shifts in energy of adsorbate levels due to degeneracy with metallic states, so that the agreement in energy for the antibonding level between theory and experiment is quite good. The intensity in the measured density of states peak, however, would seem to indicate a greater occupation number than calculated. Considerable admixing with d states should occur as a result of the above degeneracy, and the structure close to 0.6 eV following adsorption on the (1 11) face may merely reflect broadening of the d states due to such mixing. DEHYDROGENATION OF C2H4 Previous results on polycrystalline Ni films indicated that at high coverages of C2H4 at room temperature, a new set of adsorbate induced UPS levels develops, suggesting some decomposition of the adsorbate, possibly leaving a graphitic over- layer.3 Eastman has also shown using (1 11) orientation single crystals of nickel that, whereas at 100 K ethylene is non-dissociatively adsorbed, at 230 K dehydrogena- tion occurs producing an adsorbate spectrum identical to that from acetylene.At elevated temperatures (600°C in the present case), complete dehydrogenation would therefore be expected to occur, and the comparison between the ethylene-induced adsorbate spectrum in fig. 7 and that from well-oriented pyrolytic graphite supports this view. The predominant peak common to both spectra corresponds to the uppermost Q band at r whilst the other peak in the graphite spectrum corresponds to the flat n bands along the KM direction in the Brillouin zone.18 This latter peak would be partly obscured by the Ni d band and is not resolved as a separate band on the ethylene treated Ni surface.OXIDATION OF CHROMIUM (1 11) The clean spectra for Cr (1 11) again show a sharp state (fwhm 0.3 eV) immediately below E,, followed by a broader d band at both He1 and He11 energies (fig. 8). Once more, the sharpness of the structure is initially suggestive of a surface, rather than aP. J. PAGE A N D P . M. WILLIAMS 89 bulk state, and it is possible that the antiferromagnetic interactions in Cr at these temperatures may open up a gap close to the Fermi favourable for surface state propagation. No change could be detected in the sharp level following adsorp- tion of CO at room temperature, however, and further work will be required to identify its origins.Adsorption of O2 at room temperature proceeded even more readily than for Ni to produce a surface oxide (of, as yet, undetermined stoichio- metry). There is a clear withdrawal of electrons from the uppermost d band leaving a very low density of states at E,, and possibly opening up a band gap of 0.5 eV in the d manifold. A single broad d band (fwhm 1.5 eV at 2.5 below E,) remains following oxidation, which may correspond to chromium in a 3+(d3) charge state. Further work on this system is currently in progress. CONCLUSIONS UPS spectra for ( l l l ) , (110), and (100) faces of clean single crystal nickel show distinct differences which are interpreted in terms of a bulk, rather than a surface density of states.Following the adsorption of CO, a new level 1.2 eV below Ef is observed in addition to the 02p and n2p states of CO. This shallow level is tenta- tively assigned to the antibonding 712p state of CO, populated by back donation from the Ni, although effects of degeneracy and admixture with the Ni d bands may contribute. The cracking of C2H4 over Ni (110) at 600°C produces a graphitic overlayer by comparison with UPS data for pyrolytic graphite. NOTE (added after preparation of manuscript). Recent observations on clean polycrystalline foils of platinum show a sharp d band immediately below E,, and following adsorption of CO, the c and n levels of CO appear 9.2 and 11.6 eV below Ef.This represents not only a shift of both states to higher binding energy compared to CO on Ni, but also a decrease in the a-n separation from 3.0 eV for Ni to 2.4 eV for Pt ; further results on this system will be reported. D. E. Eastman in Electron Spectroscopy, ed. D. A. Shirley (North Holland, 1972), p. 487. D. E. Eastman and J. K. Cashion, Phys. Rev. Letters, 1971, 27, 1520. P. J. Page, D. L. Trimm and P. M. Williams, J.C.S. Faraday I, 1974, 70, 1769. R. Joyner and M. W. Roberts, J.C.S. Faraday I, 1974, 70, 1819. D. E. Eastman and J. E. Demuth, Phys. Rev. Letters, 1974, 32, 1123. E. L. Evans, J. M. Thomas, M. Barber and R. J. M. Griffiths, Surface Sci., 1973, 38, 245. ' S . J. Atkinson, C. R. Brundle and M. W. Roberts, Chem. Phys. Lett., 1974, 24, 175. A. M. Bradshaw, D. Menzel and M. Steinkilberg, Jup. J. Appl. Phys., 1974, to be published. D. W. Turner et al., Molecular Photoelectron Spectroscopy (Wiley, New York, 1970). B. J. Waclawski and E. W. Plummer, Phys. Rev. Letters, 1972, 29, 783. l o B. Feuerbacher and B. Fitton, Phys. Rev. Letters, 1972, 29, 786. l 2 B. Feuerbacher and B. Fitton, 1974, to be published. l3 P. J. Page and P. M. Williams, 1974, to be published. l4 P. J. Page and B. H. Blott, 1974, to be published. l 5 J. W. Linnett, Clzem. SOC. Autumn Meeting (University of East Anglia, 1973). l6 N. V. Smith, Phys. Rev. Letters, 1974, to be published. N. V. Smith, Phys. Rev. Letfers, 1974, to be published. l8 F. R. Shepherd and P. M. Williams, 1974, to be published. l 9 D. E. Eastman, J. Appl. Phys., 1969, 40, 1387. 2o E. I. Zornberg, Phys. Rev. B, 1970, 1, 244. 21 R. Haydock and M. J. Kelly, Surface Sci., 1973, 38, 139. 22 R. Haydock, V. Heine, M. J. Kelly and J. B. Pendry, Phys. Rev. Letters, 1972, 29, 868, 23 P. Fulde, A. Luther and R. E. Watson, Phys. Rev. B., 1973, 8,440. 24 G. Doyen and G. Ertl, Surface Sci., 1974, 43, 197. 2 5 D. R. Penn, Phys. Rev. Letters, 1972, 28, 1041. 26 S, Asano and J, Yamashita, J. Phys. SOC. Japan, 1967, 23, 714,

ISSN:0301-7249    

DOI:10.1039/DC9745800080

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

GENERAL DISCUSSION Papers 5, 6 Dr. C. M. Quinn (Birmingham) said: Has Roberts any comment to make about the substantial invariance of the O(h) binding energy for adsorbed oxygen? The major influence on core levels for large separations of adatoms is likely to be the residual screened Coulomb potential due to neighbouring atoms, with little change in the local charge density. It is likely that the lack of change in the O(1s) level reflects the rather large distances between chemisorption sites and the probably equally large, but unknown distance of the adatoms from the substrate surface? Dr. C. R. Brundle (Bradford) said: More than one O(1s) peak is often observed during the chemisorption of oxygen on metals (even at 8 < 1). One of these always seems to be near 530 eV (tentatively+ 0.5 eV, confidence in an accuracy being limited by the varied calibration techniques used).The other peak, or peaks, if present, come from 0 to 4 eV higher binding energy. Many bulk metal oxides (transition and non- transition, including alkali metals) also have O( 1s) peaks close to 530 eV.2 These data suggest a rather simple interpretation. The oxygen atom is strongly electro- negative and all metals have a component of free or nearly free electrons. Con- sequently in bulk oxides involving only free metal electrons (e.g., CuO, NiO, but not Cu20 or Ni203) the oxygen atom is able to capture easily the electron density it requires to satisfy its electronegativity and become close to a formal " 02- ". Hence the O(1s) binding energies in the different metal oxides have similar values.This in turn suggests that the 530 eV O(1s) peak in chemisorption also represents an oxygen atom that has come close to " 02- ", i.e., the oxygen atom " thinks " it is in an oxide- like environment. This is perhaps evidence that one has in fact at least a reconstructed surface (deeper penetration may be occurring) and that in the cases where higher binding energy O(1s) peaks are also present these represent adsorbed oxygen in a non-reconstructed (and non " 02- " -like) environment. The important point is that this seems to be a rather general phenomenon (even for W and Mo) occurring even during the early stages of adsorption. Dr. R. W. Joyner (Brudford) (in part communicated) : Brundle suggests that re- construction is the cause of the almost identical oxygen Is binding energies observed in oxygen chemisorption on copper, nickel, silver, molybdenum, tungsten and platin~m.~ In drawing this conclusion from XPS data alone he ignores evidence from other techniques which suggests that the initial stage of oxygen interaction with metals is often chemisorption on top, without rearrangement of the metal atoms.For example, the work function of a nickel surface increases on oxygen exposure even at 440 K and then falls slowly, suggesting that chemisorption precedes incorporation, the latter being a slower processS4 LEED intensity against voltage analysis also supports the view that the initial structures formed by oxygen on Ni (100) and W(110) are non- R. W. Joyner and M. W. Roberts, Chem. Phys.Letters, 1974, 28,246. C. R. Brundle, to be published. R. W. Joyner and M. W. Roberts, Cheni. Phys. Letters, 1974, 28, 246. M, W. Roberts and B. R. Wells, Trans. Faraday. Sac., 1966,62, 1608. 90GENERAL DISCUSSION 91 reconstructed. , 2 For silver and platinum the totality of evidence weighs heavily against surface reconstruction, and lattice penetration is certainly not occurring. 3-5 Thus, as we have said previ~usly,~ the demand of the oxygen atom for an electronic environment associated with a binding energy of 530eV appears to be a unifying feature in oxygen chemisorption. It cannot, however, be accepted as demonstrating that reconstruction is the initial stage of oxygen chemisorption. The above discussion has assumed that it is apposite to compare binding energies measured with respect to the Fernii level, B.E.(,+ In photoelectron spectroscopy it has been more common to use : where 4 is the sample work function and B.E.(,, was assumed to be the absolute binding energy.For B.E.,,, to have thermodynamic validity it should be defined as the energy necessary to remove an electron to a state of rest in vacuum at infinity, the same definition as is used in gas phase photoelectron spectroscopy. It has recently been shown that for a conducting solid in electrical contact with earth (the usual situation in electron spectrometers) equation (1) is not valid and that the absolute binding energy is related to B.E.(F) by a fixed although unknown difference.’ Thus in the comparison discussed above it is appropriate to use the binding energy referred to the Fermi level, and not l3.EacF)+4, where q5 varies from system to system.Dr. D. Menzel (Mzmich) said: I agree with Joyner that the question of reference levels in photoelectron spectroscopy of adsorbates is a difficult one, and that it is most probably not appropriate just to add the average work function in order to relate binding energies measured on a surface (relative to EF) to binding energies measured in the gas phase. I do not agree with his conclusion, however, that the ‘‘ absolute ” binding energy of an electron in an atom on a surface is independent of the work function, although this is probably correct for a particle embedded in the solid. In order to relate these energies, comparison should be made of the energies required to take an electron to a point in space in which the chemically induced potential changes (including image force contributions for the metal) have decayed.The long- range dipole or multipole forces, which develop between different faces of the same solid, should not be of importance, on the other hand. It appears to me that the correction to be added to B.E.(F, should then be equal to the difference between EF and the potential in the vacuum above the surface atom. This will be equal to the macroscopic work function only for layers which can be considered as uniform dipole sheets. This may be a good model for a layer of alkali metal atoms or of a close-packed layer of physisorbed or chemisorbed identical particles. It will be a bad one for all layers with “ graininess ”, whether this may be caused by the adsorbates not covering all metal atoms even at maximum coverage (e.g., hydrogen on metals), or by the side-by-side existence of species with different dipole moments (e.g., p-CO and a-CO on W).It would be concluded, then, that B.E.(F)-ValUeS for the same particles on different faces of a metal should differ. This seems to be so for a-CO on W.8 In other cases, J. E. Demuth, D. W. Jepsen and P. M. Marcus, Phys. Rev. Letters, 1973, 31, 540. J. C. Bucholz, G. C. Wand and M. B. Lagally, Surface Sci., in press. H. A. Engelhardt, A. M. Bradshaw and D. Menzel, Ber. Bunsenges. Phys. Chem., 1972,76,500. H. A. Engelhardt, A. M. Bradshaw and D. Menzel, Surface Sci., 1972,40,410. A. E. Morgan and G. A. Somorjai, Surfuce Sci., 1968,12,405 ; J.Chem. Phys., 1969,51,3309. R. W. Joyner and M. W. Roberts, Chem. Phys. Letiers, 1974, 28, 246. ’ A. F. Carley, R. W. Joyner and M. W. Roberts, Chem. Phys. Letters, 1974, 27, 580. * A. M. Bradshaw, this Discussion p. 132.92 GENERAL DISCUSSION a compensation between the values of 4 (clean) and A$ may take place and make the B.E.,,,-values virtually identical. The fact that there is often little or no change of B.E.(F, for changing coverage and therefore work function can be understood by the notion that the local potential above the first adsorbed atom is (almost) the same as above an adatom in the full layer, if the adatom dipole moment is independent of coverage.' It should be realized, however, that core level binding energies often do show shifts with coverage (e.g., O(1s) of oxygen on W(l It is concluded that the proper value which would have to be added to B.E.(,) to compare to gas phase binding energy or even to compare values for different surfaces, is quite unclear at present.Dr. R. W. Joyner (Bradford) (communicated) : I agree with Menzel's concluding paragraph. It is still not clear what should be added to B.E.(,) for comparison with gas-phase binding energies, although studies of condensed materials rather that chemisorbed layers suggests that the correct value might be about 6 eV.3 Menzel concludes that the reference level should be the potential energy of an electron at rest in vacuum above the surface atom. This choice appears to me to violate the essential criterion that the reference level should be constant and indepen- dent of the system under study. Only if this condition is satisfied can valid com- parisons of binding energy be made and thermodynamic integrity be maintained.I therefore reiterate that the only appropriate absolute reference level is the potential energy of an electron at rest in vacuum at infinity with all that this implies concerning the binding energy referred to the Fermi Also, if we accept, with Menzel, that " it is probably not appropriate just to add the average work function ", the Fermi level becomes the only accessible and therefore the only practicable reference level. Dr. S. Evans and Dr. M. J. Tricker (Aberystwyth) (in part communicated) : We consider that it is most unwise to associate a constancy of binding energy with con- stancy of charge, formal or otherwise.It is already well established for bulk systems that to account adequately for an observed chemical shift it is necessary to conisder both the atomic charge and the so-called " Madelung '' potential arising from all the other charges surrounding the atom in questi~n.~ The contribution of this second term to the observed chemical shift may rival or even exceed that of the charge (valence) term : this is well demonstrated in Pb304, for example, where the two oxi- dation states of lead are not distinguished by XPS because the roughly doubled partial charge between Pb" and Pb'" is entirely offset by the difference in '' Madelung " terms between the Pb" snd PbrV sites.6 The fact that PbO, has a lower Pb 4f binding energy than PbO can similarly be explained by taking into account this " Madelung " potential.The approximate constancy (k 1 eV) of 0 (1s) binding energy in many metal-oxygen systems may well have an analogous explanation, especially for bulk oxides and where reorganisation of the surface occurs following chemisorption. This constancy of anion binding energies is also found in other systems; for in- stance, the three crystallographically inequivalent F atoms of LaF3 cannot be resolved G. E. Becker and H. D. Hagstrum, J. Vac. Sci. Tech., 1973, 10. 31. A. M. Bradshaw and D. Menzel, Vakuum-Technik, in press. R. W. Joyner and M. W. Roberts, unpublished results. this Discussion, and A. F. Carley, R. W. Joyner and M. W. Roberts, Chem. Phys. Letters, 1974, K. Siegbahn et al., ESCA (Almqvist and Wiksell, Uppsala, 1967).J. M. Thomas and M. J. Tricker, J.C.S. Faradar 11, 1975, 71, 317; D. E. Parry, J.C.S. Faraday IZ, 1975, 71, 344. 27, 580.GENERAL DISCUSSION 93 by XPS.l As we outlined above, the experiment tells us nothing in a direct way about the charge on 0 or F because of the " Madelung " term. What it does indicate, at this stage, is that the total potentials (i.e., Madelung+valence) at the anion sites are similar, for a given anion, in many environments. The approximate constancy of anion binding energy may thus originate in a tendency for the total potential at the different sites to become the same,2 rather than in a tendency of (for example) oxygen atoms in oxide systems to come " close to a formal 02- ". Furthermore, adsorbate binding energy data referenced to the Fermi level of different substrates are not, strictly, directly comparable, either with each other or with gas-phase data.They must first be corrected so that the zeros of the various energy scales coincide, since the energy required to excite an electron from the Fermi level into the lowest unbound state in the vacuum (the reference level of gas-phase UPS data) differs in each case. The best procedure available for this correction is probably to add the work function after adsorption to the observed Fermi level- referenced binding energy, although even this is only really valid for a complete (infinite) uniform layer of a monatomic adsorbate on a uniform, single crystal, sub- strate. The observed constancy of 0 (1s) binding energy thus also depends on the fortuitous constancy of this correction term, or, perhaps, on variations in it offsetting differences in partial charge and/or Madelung potential.Dr. R. W. Joyner (Bradford) (communicated): There is as yet no evidence for the importance of Madelung potential terms in considering binding energies of surface species. Our observations refer to oxygen 1s binding energies of the chemisorbed species, and that for chemisorbed carbon contamination on metal films, the carbon 1s level is usually found close to B.E.,,, = 285.0 eV. It is therefore plausible that atomic charge and " Madelung " terms can be equal on different metals, there being no obvious reason to invoke a compensation of these two terms. Evans and Tricker neglect the requirement of an absolute energy zero.The addition of a work function as they suggest, leads to unacceptable inconsistencies ; for example, an electron excited inside a crystal would therefore have a different binding energy depending on whether it leaves through a (100) or (1 10) layer, which have different work functions. Dr. C. M. Quinn (Birmingham) said: Brundle appears to be suggesting that an 02- surface species is a better defined " surface molecule " than a less charged adatom. Such suggestions tend to prolong the divisions between localized bonding concepts which encompass such adsorbate fragments as 02-, and band models in which adatoms are viewed more as extensions of the bulk solid. The surface molecule concept, currently being developed by several groups, 4-7 seems to provide a framework for re- conciling these two extremes.It appears * that the definition of a clearly localized surface molecule, e,g., a fragment like 02- or a discrete 0"-, depends largely on a difference term involving the first and second energy moments of the surface density of states at the adsorption site divided by the second energy moment of the bulk density states. These terms are determined by the substrate geometry and current R. G. Hayes and N. Edelstein, in Electron Spectroscopy, ed. D. A. Shirley (North Holland, Amsterdam, 1972), p. 771. M. J. Tricker, Inorg. Chem., 1974, 13, 742. R. W. Joyner and M. W. Roberts, Chem. Phys. Letters, 1974, 28, 246. T. B. Grimley, J. Vac. Sci. Tech., 1971, 7, 31. R. Haydock, V. Heine and M.J. Kelly, J. Phys. C, 1972, 5, 2845. J. R. Schrieffer, J. Vac. Sci. Tech., 1972, 9, 561. J. W. Gadzuk, Surface Sci., 1974, 43, 44. * M. J. Kelly, J. Pliys. C, 1974, 7, LT57.94 GENERAL DISCUSSION results, although somewhat preliminary, do not support such high localization as would be required for the formation of an 02- species at any surface. Dr. C. R. Brundle (Bradford) (communicated) : It seems that the use of ' formal "02-" ' may be misleading to those not well acquainted with inorganic chemistry conventions. Just as no inorganic chemist would assume that the use of V5+ to signify V205 implies five positive charges on the vanadium atom, " 02- " does not imply two complete negative charges on the oxygen. One could write the species as 0-", but this would be more uncommon.What is implied in my comment is that oxides having similar O(1s) binding energies have a similar (and optimum) net potential at the oxygen atom. For a comparison of chemisorbed 0 and the oxide of the same metal only, the conclusion that if the (Is) binding energies are similar in the two cases the chemisorbed oxygen is likely to be in an oxide-like environment is quite indepen- dent of the value for the absolute charge on " 02- " that might be assigned. Dr. C. M. Quinn (Birmingham) said: The binding energy N 530 eV observed by Roberts et al. does not appear to depend to any great extent on the nature of the substrate or the coverage. This behaviour contrasts with results for gas-phase species containing oxygen and also with the changes in binding energy observed for O(1s) and C(1s) in adsorbed carbon monoxide species.Core levels are influenced by the residual screened coulombic potential due to neighbours, here, both substrate and adsorbate, together with any appropriate changes in charge density. Richardson and I have recently initiated a programme of MSXa calculations on model clusters chosen to be representative of various conditions of adsorbed species. We find in preliminary non-SCF calculations"' that the superimposed coulombic potential at an oxygen adatom does not vary to any great low or high coverage on a lithium used the models 0 Li Li / \ (LiSO) Li extent for oxygen in an oxygen molecule, nor at (100) surface. For the adsorbed states we have 0 0 0 \ / 0 ' I \o with an 0-Li distance of about 3.0A.The non-SCF O(ls) energies are -515.32 eV in gas phase 02, -514.86 eV in Li,O and -514.69 eV in Li05 (centraI 0) and - 514.4 1 eV (peripheral 0). The lack of change in the O( 1s) values after superim- position of the potentials in the cluster reflects the rapid decay of the atomic potentials with radial distance. It seems that similar changes in charge density may be occurring in all the cases of oxygen adsorption where oxygen adatoms are the only surface species. A similar situation can be expected for the energy levels resulting from super- imposition of potentials due to adsorbed carbon monoxide and possible fragments. However, charge density changes for different configurations, viz., 0 C C co 0 may now substantially alter the O(ls) and C(1s) binding energies.* C. M. Quinn and N V. Richardson, to be published.GENERAL DISCUSSION 95 Prof. M. W. Roberts (Bradford) said: It may well be incorrect, at least with oxygen, to think in terms of discrete chemisorbed adatoms with a localized electron density, but instead to take an approach which is more akin to the collective electron outlook involving considerable delocalization. In this respect Quinn's results are interesting. On the other hand with CO (when in the molecularly adsorbed state), the electronic distribution as reflected by the O( 1s) binding energy parallels the heat of chemisorption' (fig. l), and we are dealing with a discrete surface molecule. If we accept that in both cases the O(1s) binding energy reflects the electron density (or electrostatic potential) in the neighbourhood of the oxygen then in oxygen chemisorption it is substantially invariant of the metal and not particularly dependent on the oxygen coverage.With CO it is sensitive to the metal-CO bond strength (fig. 1). We suggest also that there is a critical heat (- 300 kJ mol-l) above which CO dissociates at room temperature when the experimentally observed O(ls) value is about 530 eV. The criterion we have used for dissociation is the presence of atomic-like " orbitals " and the absence of " orbitals " akin to those in molecular CO. 537'0t 535.0 \ \ I I 1 I 100 2 00 300 - A Hab/kJ mol-' FIG. l.-Q, Platinum; a, nickel; 0, al state on tungsten; A, at state on tungsten; 0, copper; V, y state(s) on molybdenum. Prof. J. M. Thomas and Dr.D. E. Parry (Aberystwyth) said: Fig. 7 of the paper by Page and Williams contains some information which is of considerable general interest in regard to the reliability of photoelectron spectroscopy in monitoring valence bands of clean solids, particularly of systems such as graphite. Whilst we do not doubt that curve (b) in the figure is attributable to the graphitic solid formed by dehydrogenation, we would like to know more about the precise shape, and especially the total width, as determined by He I1 radiation, that Williams finds for his graphite valence band. In the first reported 3*4 XPS studies of the valence band of graphite, the total (What happens at lower energies, e.g., at - 30 eV in fig. 7 ?). R. W. Joyner. and M. W. Roberts, Cheni. Phys. Letters, 1974, 29, 447. A.F. Carley, R. W. Joyner and M. W. Roberts, Chetn. P ~ J L S . Letters, 1974, 27, 580. K. Hamrin, S. Johansson, U. Gelius, C. Nordling and K. Siegbahn, Physica Scriptlr, 1972, 2, 277. J. M. Thomas, E. L. Evans, M. Barber and P. Swift, Trcirrs. Furdaj, SOC., 1971, 67, 1875.96 GENERAL DISCUSSION width appeared to be 31 f2eV, disturbingly in excess of the figure of 19.5 eV obtained by the best theoretical computation of Painter and Ellis,l whose ab initio LCAO procedure was judged by Coulson to be within an electron volt or so of the correct value for graphite. have carefully reinvestigated the valence band structure of graphite, using high resolution XPS, and they conclude that the experi- mental width is not less than 24eV. (Examination of their fig.7, in which they '' eliminate artificial tailing " leads to the view that their XPS determined width could well be larger by another 4 eV). Kieser has recently quoted a band width of ca. 21 eV, but an examination of his data also shows uncertainty as regards the precise width. The implication, clearly, is that photoelectron spectroscopy can, under certain circumstances which are not yet fully understood, yield unreliable information at values of the energy far below the Fermi level. With UPS in place of XPS the difficulties could be more severe for another reason : final-state effects are not likely to be eliminated when such relatively low energy (40.8 or 48.4 eV) of stimulating radiation is employed. McFeely et al. Dr. P . M . Williams (Imperial CoZZege) said: The question raised by Thomas concerning the band-widths determined for solids by UPS and XPS is an impor- tant one.Both scattering and effects due to the coupling between photoelectron and hole states can broaden the experimentally observed structure. 5-7 With reference to fig. 7 of our paper, we reproduce there only the uppermost valence bands of graphite, as indicated in the text, to show the agreement between the ethylene-cracking product and pyrolytic graphite. The full He I1 (40.8 eV photon) spectrum for the latter shows a further prominent state, FWHM 3 eV centred at 18 eV below Ef (together with a weaker feature at 13.5 eV), so that the total bandwidth is close to 21 eV, in agreement with the data of Kieser * and the calculations of Painter and E1lis.l The XPS value for the bandwidth of 23 eV determined by McFeely et aL3 may, as these authors point out, be a slight overestimate due to the inaccuracy in fixing Ef and to scattering at the high binding energy side of the band ; similar considerations should apply to the XPS value of 31 eV measured by Thomas. The most important differ- ence between the high energy UPS (i.e. hw 2 30 eV) and XPS data appears to lie not in the appearance of final state effects in UPS (although these are undoubtedly of importance, particularly at He I energies) but in the different cross sections for excitation from the " s " and " p '' like parts of the band. In other respects, we believe our data (to be published elsewhere) to be consistent with the findings of McFeely et al. G. S. Painter and D. E. Ellis, Phys. Rev. B, 1970, 1, 4747. C. A. Coulson, 1971, personal communication to J. M. Thomas. F. R. McFeely, S. P. Kowalzyk, L. Ley, R. G. Cavell, R. A. Pollack and D. A. Shirley, Phys. Rev. B., 1974, 9, 5268. J. Kieser, Physica Fennica, 1974, 9, suppl. Sly 173. B. 1. Lundquist, Phys. Kondens. Mater., 1969, 9, 236. S. Doniach and M. Sunjic, J. Phys. C., 1970, 285. J. Kieser, Phys. Fennica, 1974, 9, suppl. S1, 173. ' See for example, G. K. Wertheim and S. Hufner, Phys. Rev. Letters, 1972, 28, 1028.

ISSN:0301-7249    

DOI:10.1039/DC9745800090

出版商:RSC    

年代:1974

数据来源: RSC