J. Phys. Chem. 1987, 91, 5535-5537
5535
CO Chemisorption Effects on Cerium Initial and Final States in the Core Level Photoelectron Spectrum of Ce I r2 John M. Lindquist and John C. Hemminger*+ The Institute for Surface and Interface Science and Department of Chemistry, University of California. Irvine, California 9271'7 (Received: July 13, 1987)
Carbon monoxide (CO) adsorption on the mixed-valence compound CeIr2has been studied by X-ray photoelectron spectroscopy. CO adsorption on this surface is predominantly molecular. We show that changes in the cerium 3d spectrum upon CO chemisorption may be separated into initial- and final-state effects. In the initial state, stabilization of the [Xe](Sd6~)~4f' configuration occurs due to an increase in effective nuclear charge on the cerium atom. This leads to a decrease in f intensity in the recorded spectrum. Increased f-electron repulsion in the final state raises the energy of the f2 final-state configuration and causes a 2.2-eV shift to higher binding energy of the 3ds/$ peak. The 0.5-eV binding energy shifts of the 3d5/,fl and 3d3/2f1peaks along with the 2.2-eV shift of the 3d& peak indicate that the f orbitals are quite sensitive indicators of the surface oxidation state in this compound.
Introduction In studies of surface-adsorbate interactions utilizing X-ray photoelectron spectroscopy (XPS), shifts of surface atom core level binding energies are often viewed as evidence of changing oxidation state. However, it is commonly observed that initial-state effects (such as a change in oxidation state) may not extend directly to the photoelectron spectrum. Often, rearrangement of electron energies in the final state (relaxation) obscures energy shifts in the initial state, leading to uncertain conclusions as to the effect of the adsorbate on the oxidation state of the surface atoms.' In following the effects of carbon monoxoide (CO) adsorption on the cerium 3d photoelectron spectrum of the mixed-valent compound CeIr2, we have observed changes in binding energies and peak heights which may be separated according to whether they stem predominantly from alterations to initial or final states of the atom. This separation of initial- and final-state effects shows the sensitivity of the f levels to adsorbates and will add to the present understanding of XPS as a probe of the complex energy levels of mixed-valence compounds. For those unfamiliar with mixed-valent compounds and their core level photoelectron spectra, a brief overview would prove useful. The phenomenon of mixed valence occurs for systems in which two atomic electronic configurations are close in energy. The result is a net ground-state configuration which is a strong mixture of the two nearly isoenergetic states. In the case of cerium mixed-valent compounds, the two electronic configurations [Xe](5d6~)~4f' and [ X e ] ( 5 d 6 ~ ) ~ 4can P be very close in energy. The photoionized atom, however, is not a mixture of states because the two once isoenergetic initial-state configurations have very different final-state relaxation effects which depend on the extent of interaction between the nucleus and remaining electrons in each configuration. In the ion, states with high f density are of lower energy than those with low f density as a result of the small radius of the f orbital relative to the 5d and 6s orbitals. The f electrons feel the increased effective nuclear charge of the photoion more than do the 5d and 6s electrons. Thus, the configuration (5d6~)~4f' In the will be of lower energy in the final state than (5d6~)~4f0. model proposed by Schonhammer and G u n n a r ~ s o n hybrid,~~~ ization of the f orbitals with the 5d6s band allows for a high probability of electron transfer from the 5d6s band into an f orbital as a result of the lower energy of the f orbital accompanying photoionization. This process of final-state electronic rearrangement, called "shakedown", produces a third possible final state with electronic configuration [Xe+](5d6s)*4F (where [Xe'] represents the photoionized xenon core) (see, for example, ref 4-6 and references therein). Thus, from a single initial-state energy, core level photoemission produces three distinct final-state energies and thus three peaks in the photoelectron spectrum. A schematic +Alfred P. Sloan Research Fellow, 1984-1988.
0022-365418712091-5535$01.50/0
TABLE I: Cerium 3d Binding Energies BE,eV clean CeIr, CO-saturated CeIr,
BE shift. eV
3d5/2f 3dspf' 3d~2fZ
896.8 884.4 819.9
896.8 884.9 882.1
0.0 0.5 2.2
3d3/2f 3d3/2f' 3d3/2fZ
914.6 902.9 ?
914.6 903.4
0.0 0.5
900.7
of the energetics of core level photoemission in these types of compounds is shown in Figure 1. Note that both [Xe+](Sd6~)~4f' and [Xe'] ( 5 d 6 ~ ) ~ 4final-state fz configurations may be produced via the shakedown process. Results and Discussion The results described here stem from data obtained with a VG Escalab surface analysis system. The sample used in this study was polycrystalline CeIrz prepared by arc melting Ce and Ir in the correct stoichiometry. A detailed characterization of this sample and surface cleaning procedures have been presented previously.' Recently, we have shown that monolayer saturation exposure of C O on the surface of this compound results in adsorption with approximately 80% of the C O in molecular adsorption states.' The bonding of CO involves withdrawing electron density from the surface into the C O 2 ~ orbital, * not unlike C O ? ~ 2a shows the cerium 3d region adsorption on i r i d i ~ m . ~Figure photoelectron spectrum of a clean CeIr2 surface. The spectrum is dominated by five features of different final-state configuration and includes two separated peaks due to excitation from X-ray satellites. The 3d3# peak is masked by intensity due to 3d5,2p emission. The same region after C O saturation, Figure 2b, shows a number of changes from the clean surface. Relative to the 3ds(2f1 peak, the 3d3/2f0 peak has decreased in intensity while its binding (1) Carlson, Thomas A. Photoelectron and Auger Spectroscopy; Plenum: New York, 1975 and references therein. (2) Schonhammer, K.; Gunnarsson, 0. Solid State Commun. 1977, 23, 691. (3) Fuggle, J. C.; Hillebrecht, F. U.; Zolnierek, Z.; Lasser, R.; Frieburg, Ch.; Gunnarsson, 0.;Schonhammer, K. Phys. Reu. B: Condens. Matter 1983, 27, 7330. (4) Fuggle, J. C. J . Less-Common Met. 1983, 93, 159. ( 5 ) Fuggle, J. C.; Campagna, M.; Zolnierek, Z.; Lasser, R.; Platau, A. Phys. Reu. Lett. 1980, 45, 1597. (6) Lasser, R.; Fuggle, J. C.; Beyss, M.; Campagna, M.; Steglich, F.; Hulliger, F. Physica 1980, 102B, 360. (7) Lindquist, J. M.; Hemminger, J. C.; Lawrence, J. M., Phys. Rev. B: Condens. Matter, in press. ( 8 ) Kuppers, J.; Plagge, A. J. J . Vac. Sci. Technol. A 1976, 13, 259. (9) Zhdan, P. A,; Boreskov, G. K.; Boronin, A. I.; Schepelin, A. P.; Egelhoff, W. F.; Weinberg, W. H. Surf. Sci. 1978, 71, 267.
0 1987 American Chemical Society
Letters
5536 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 GROUND STATE WAVEFUNCTION
A
([Xel ( S d 6 ~ ) ~ 4 f ' )
+
B
([Xe] ( 5 d 6 ~ ) ~ 4 f ' )
I
I
,
[Xe+] ( 5 d 6 d 3 4 f i [Xe']
I
\
G I
PHOTOION F I N A L S T A T E S
[Xes] ( 5 d 6 d 4 f 2
(5d6~.)~4f'
I
\
$w 1
RECORDED SPECTRUM
3d Ionized F i n a l States
\
I
I I
Photoelectron Binding Energy
Figure 1. Energy schematic of cerium core level photoemission for a
mixed-valent cerium compound.
I c
!A
c ._
C
a
=-.
e
c ._
2
v
r
.-c !A
c m c i + c
Figure 2. Cerium 3d photoelectron spectra: (a) clean CeIr2, (b) COsaturated CeIr2. Peaks are labeled by final-state f count.
energy remains unchanged. The binding energy of the 3ds/2f2 peak has increased by approximately 2.2 eV, and the 3d3pf1and 3d5 Zf' binding energies have increased by approximately 0.5 eV. Aiso, the 3d3# peak has emerged well separated from the 3d5$" peak. The spectra shown in Figure 2 were obtained with the axis of the collection lens of the electron energy analyzer perpendicular to the sample surface. Thus, these spectra include intensity from subsurface Ce atoms which should not be substantially affected by the C O adsorption. It is evident in Figure 2b that two features may be assigned to the 3d@ final state. The lower binding energy shoulder (coincident in energy with the 3d& peak from the clean surface) may be attributed to emission from Ce atoms in the subsurface region. Table I lists the binding energies of each peak for the clean and CO-saturated surfaces. The increased binding energies of the 3d3/,f', 3dS ,fl, and 3d5# peaks are consistent with the bonding scheme flor C O on this surface. The constant binding energy of the p peak varies from what one would expect from oxidation of the surface by adsorbed C O if only initial-state effects are considered. However, as shown previously,' the topmost surface layer of CeIr2 is largely void of fo character. Thus, the constant binding energy fo intensity in Figure 2b is due to emission from Ce atoms in the subsurface region which are unaffected by C O adsorption. The change in fo peak intensity along with the different binding energy shifts of the f' and f2 peaks indicates that movement of electron density from the valence orbitals of the surface atom onto C O must have an indirect effect on the more localized f orbitals. Discussion of
I N I T I A L STATES c
f -count C l e a n CeIr2
I
f-count CO S a t u r a t e d CeIrz
Figure 3. Energy schematic of the effects of CO on cerium electronic energy levels before and after photoionization.
this effect must include both initial and final states of the atom as the photoelectron binding energy is dependent on the energies of the final-state photoion as well as the initial-ground-state atom. Figure 3 shows qualitative initial- and final-state energy levels for Ce configurations in both the clean and CO-saturated CeIr2 surfaces. The diagrams in Figure 3 are modeled after those of Fuggle et al.294,'0and are designed to show that the f count in these types of systems need not have purely integral values. Comparison of the two sets of diagrams for the clean and CO-saturated surfaces shows a number of differences which we will correlate with experimental data. Each initial-state diagram in Figure 3 shows the f" and f' levels to be nearly isoenergetic. (The fi level actually lies slightly lower in energy than the f" level as the f count in CeIr, is believed to be between 0.7 and 0.8.") The final-state diagrams show that photoionization yields three final states with different f counts. The diagram of the initial state of the CO-saturated surface shows both fo and f' energies lowered slightly with respect to the clean surface as an indication of surface oxidation by chemisorbed CO. In addition, as we have discussed in detail in an earlier publication,' the energy of the f' configuration has dropped with respect to the f" configuration due to the higher effective nuclear charge on the Ce atom. The stabilization of the [Xe](5d6~)~4f' configuration vs. the [ X e ] ( 5 d 6 ~ ) ~configuration 48 in the initial state accounts for the decrease in f" intensity seen in the X-ray photoelectron spectrum after saturation C O exposure to CeIr2. A decrease in fo intensity in the photoelectron spectrum cannot be quantitatively related to a decreased 'F weighting in the initial state of the atom because of the complex nature of electron wave functions in this type of compound. However, as discussed by Fuggle; a qualitative correlation between photoelectron intensity and initial-state weighting is Feasonable. Inherent in this discussion is the premise that C O interacts primarily with the 5d6s valence band and thus removes little or no electron density from cerium f orbitals upon adsorption. Possibly the most interesting effect of CO adsorption on CeIrz is the variety of binding energy shifts observed for the f' and f2 cerium final-state electronic configurations. As presented by (10) Hillebrecht, F. U.; Fuggle, J. C. Phys. Reu. B: Condens. Matter 1982, 25, 3550. (11) Lawrence, J. M.; den Boer, M. L.; Parks, R. D.; Smith, J. L. Phys. Rev. B: Condens. Matter 1984, 29, 568.
Letters Carlson,’ removal of valence density by a surface adsorbate is standardly viewed as creating a shell of positive charge around the affected atom. This means that the effect of valence electron density removal on all the various core levels of an atom is approximately the same; Le., a 1s core level will change in energy approximately the same amount as a 2s core level. However, as presented above, the binding energies of the 3d5/# and 3d5/,fl configurations accompanying CO adsorption on CeIr, shift by 2.2 and 0.5 eV, respectively. Since these two peaks originate from the same mixed-configuration initial state, the differences in binding energy shift on CO adsorption must arise in the final state of the photoion. The binding energy shifts associated with C O adsorption can be explained by a further discussion of cerium electronic configurations and their final-state energies. It is evident in Figure 2a,b and also in cerium 3d spectra of a number of other cerium mixed-valent compound^^^^'^^'^ that the difference in energy between the fo and f’ final-state configurations is much greater than that between the f’ and fz final states. These data indicate that the stabilizing effect of the presence of one electron in an f orbital vs. an empty f orbital is significantly larger than that of having two electrons in an f orbital instead of one. This relative decrease in stabilization with increasing f count is due to the Coulomb correlation energy (see, for example, ref 2 and 13) which takes into account the repulsive interaction between electrons in the same f orbital. Thus, although in the presence of reduced electron density (e.g., a core hole created in the photoemission process or valence density removed by an adsorbate) configurations of high f count are generally of lower energy than those of low f count, the degree of stabilization of an P+l configuration compared with an fh configuration decreases with the value of n. From the above discussion the different binding energy shifts of the f1 and fz cerium final-state configurations with C O adsorption must stem from a combination of effects which involve f-electron-f-electron and f-electron-nucleus interactions. As we indicate in Figure 3, C O adsorption lowers the energy of all configurations in the Ce initial state due to removal of some valence electron density by adsorbed CO. The increase in effective nuclear charge reduces the radius of each orbital. While this decrease in the radius of the f orbitals has the effect of only decreasing the energy of the f’ configuration, the fz configuration will have both increased f-electron-nucleus interaction (decreasing the overall energy of the configuration) and increased f-electron-f-electron interaction (acting to increase the overall energy of the configuration.) Knowing that the fz configuration is occupied only in the final state, the relatively large binding energy shift of this configuration indicates that the destabilizing effect (12) Kappler, J. P.; Krill, G.; Besnus, M. J.; Ravet, M. F.; Hamdaoui, N.; Meyer, A. J . Appl. Phys. 1982, 53, 2152. (13) Campagna, M.; Bucher, E.; Wertheim, G. K.; Buchanan, D. N. E.; Longinotti, L. D. Phys. Reu. Lett. 1974, 32, 885.
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5537 of the f-electron-f-electron interaction outweighs the increase in the attractive interaction of the f electrons with the nucleus (compared with the same interactions in the final state of the clean surface). This makes sense since the Coulomb correlation of the f electrons in the fz configuration depends on the “volume” of the f orbital while the f-electron-nucleus attractive interaction depends on the orbital radius. If the sum of the seven f orbitals is approximated as a spherical sector, we can expect a decrease in radius to give a greater decrease in volume of the f orbital upon C O adsorption because the volume of a sphere is proportional to the radius cubed. Thus, the large binding energy shift of the 3d5/2fZ peak stems predominantly from the increase in f-electron-felectron interaction in an f orbital which is smaller in the final state of the CO-staturated CeIr2 surface than the clean surface. We are now left with a brief discussion of the fl binding energy shift. There is no Coulomb correlation term in the initial and final states of this configuration. Therefore, the fl peak shows the “standard” effect of an increase in binding energy associated with surface oxidation by an adsorbate. Keeping in mind that C O chemisorbs approximately 80% molecularly on this surface, the extent of the CO-surface interaction is mild in comparison with dissociative chemisorption. Accordingly, the effect of C O adsorption on the Ir4f peaks in CeIr, is a shift of only 0.10 eV.’ Thus, the relatively large binding energy shifts of the Ce f’ and fz peaks appear to give an amplified view of the effect of CO adsorption on this surface. The binding energies of the f’ and f2 peaks act as highly sensitive indicators of the surface oxidation state, a useful tool in the study of the surface chemistry of this type of compound. We have shown that changes in the 3d photoelectron spectrum of Ce due to CO adsorption on the mixed-valent compound CeIr, can be separated according to whether they stem from initial- or final-state effects. The decrease in f“ intensity results from initial-state stabilization of the [Xe]( 5 d 6 ~ ) ~ 4configuration. f’ The large binding energy shift of the 3d~l2fzpeak is a predominantly final-state effect caused by decrease in radius of the f shell which destabilizes this configuration due to increased f-electron-f-electron interaction. In closing, we must remind the reader that in mixed-valence compounds the f orbitals are correlated with the 5d6s band and thus relaxation of the f states is a complex process. We have attempted here to give a qualitative picture of the effects of C O chemisorption on the various electronic energy levels of CeIr,. A complete understanding would involve a detailed theoretical treatment of all states in this compound which is beyond the scope of this report.
Acknowledgment. This work was supported by a grant from the University Research Instrumentation Program of the Office of Naval Research. We thank Jon Lawrence of the Department of Physics, University of California, Irvine, for preparation of the CeIr, sample. J.M.L. acknowledges support from the IBM Corp. in the form of a Graduate Research Fellowship for 1986-1987. Registry No. CeIr2, 12258-84-3; CO, 630-08-0; Ce, 7440-45-1.
