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Vanadium CVV Auger transition
Solid State Communications, Vol. 99, No. 6, pp. 393-397, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 00381098/96 $12.00 + .OO
Pergamon
PI1 SOO38-1098(96)00277-3 Vanadium CVV Auger Transition P. Pervan, T. Valla and M. Milun Institute of Physics, P.O.Box 304, IO 000 Zagreb, Croatia (Received
15 January 1996; accepted 1 May 1996 by C. Calandra)
We report an experimental study of the CVV Auger transition of vanadium. A comparison between the experimental spectrum and the self-convolution of the calculated density of states (DOS) failed to show any significant shift between the spectra as reported for Ti and SC. We have also found good agreement between our experimental and Contini’s et al/l/ calculated CW spectrum of vanadium which may suggest that the screening of the core hole is not as important for the vanadium CW Auger spectrum as might be expected. Copyright
0 1996 Elsevier Science Ltd
1, INTRODUCTION Apart from being a main technique in surface science for the determination of the chemical structure of materials, Auger electron spectroscopy (AES) is also a valuable technique which can provide information on the electron correlation effects. The sensitivity of AES to the electronelectron correlation is consequence of the fact that the final state of the Auger process has two holes and, therefore, the corresponding Auger spectrum represents two-hole density of states which generally reflects the electron-electron Coulomb interaction /2/. Although it might be tempting to use a simple one-electron picture to extract electron Coulomb interactions from Auger spectra (see e.g. ref. 3) the correlation effects can be correctly taken into account only by applying many-body formalism /4/. It is widely accepted that, in the case of core-valencevalence (CVV) Auger transitions, the Coulomb interaction effects may be neglected if the Coulomb interaction within the valence band (U) is much smaller than the width of the valence band itself (W) with the effect that the CW spectrum is well described by the self-convolution of the corresponding one electron DOS. On the other hand, in the limit of U>>W, the two-hole density of states is very much different from the self-convolution of the one particle density of states /5/. It had been realised by Cini/6,7/ and Sawatzky/l/ that CVV Auger spectra of late transition metals are virtually entirely determined by the valence electron-electron correlation effects. They showed that the atomic-like spectrum of the late transition metals is a bound-state part of the two-hole density of states which is shifted to the lower kinetic energies with respect to the self-convolution part of the spectrum by the amount of the Coulomb interaction. The understanding of the CW Auger spectra of early transition metals is still far from being complete. As the Coulomb interaction of valence electrons in scandium, titanium and vanadium is considerably smaller than the width of the corresponding 3d hand/9/ it is to expect that the correlation effects should not play a significant role and that CW Auger spectra will appear as a self-convolution of the one-electron DOS /IO/. However, a comparison of CW Auger spectra of scandium or titanium with the self-
convolution of DOS apparently does not support this assumption. Namely, it has been found that Ti and SC CW Auger spectra are shifted to higher kinetic energies with respect to the corresponding self-convolutions/l 1,12,13/. This shift was initially attributed, by de Boer et al./1 l/, to the negative effective Coulomb interaction which, as suggested, occurs as a consequence of the overscreening of the valence band holes. HedegLd and Hillebrecht/l2/ took the view that the shift of the Auger spectrum to higher kinetic energies with respect to the self-convolution of the valence band is due to the polarisation of the valence band in the presence of the potential of the core-hole. In the case of partially filled valence bands the influence of the potential created by the core-hole (e.g. screening of the core-hole by the valence band electrons) probably should not be neglected as in the case of the late transition metals, However, due to the complexity of the problem there have been only a few attempts to include both, CV and W correlation/ ref. 14 and references therein/. A somewhat different solution to the problem has been proposed by Sarma et al./13/ who neglected entirely the the W correlation but included the possibility that screening cloud (which appears as a response to the corehole) participates in the Auger process. The mechanism which involves the screening charge in the Auger process appears to be a “one-hole process” and the corresponding contribution to the spectrum is proportional to the oneelectron DOS instead of self-convolution of the DOS characteristic for the normal, “two-hole process”. By increasing the probability of the “one-hole” process they increase the spectrum intensity at higher kinetic energy. In this way they successfully fitted the titanium CW Auger spectrum assuming 90% contribution from the “one-hole” like DOS and 10% contribution from the “two-hole” DOS. Recently, Drchal and Cini /15/ formulated a theory which takes into account dynamics of the core-hole screening and correlation of the valence electrons. If the perfectly screened core-hole is assumed (tYly relaxed limit) the theory gives the Auger CVV spectrum as a sum of three contributions:(a) three-particle density of states which is a result of the usual two-hole process in addition convoluted with the one-electron propagator, (b) the one-particle 393
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CVV AUGER T~S~ION
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density of states co~~ponding to the Sarma’s et al.1131 “one-hole” process, and (c) the three-particle density of states associated with the “excitonic-like” state of an electron-hole pair convoluted with the one-hole propagator. It has been also suggested that for the early transition metals a fully relaxed limit (complete screening of the core hole) is much better approximation than neglecting the screening. However, the core-hole screening is much less complete as one approaches the end of the 3d series / 151. In this contribution we present the experimental Auger CW spectrum of v~dium and the analysis of the tr~sition in the context of the recent theories. It is also a particular‘ aim of this paper to point out that some controversies associated with the CW transition of the early transition metals may be a consequence of unreliable experimental data. There are several possible sources of the problems which can affect the conclusions related to the CW Auger transition: (i) effects of surface contamination on the CW Auger spectrum itself (ii) 1 effects of surface contamination on the valence band spectrum which is (when taken with high photon energy, e.g. x-rays), usually used as a spectrum of the total DOS, which in turn is used to calculate CW spectrum {iii) absolute energy c~ibration n~es~~ for the comparison of Auger spectrum and the ~~es~nding seIf-convolution. of the problems in the We believe that some understanding of CW Auger transition of the early transition metals may at least partly be affected by above mentioned effects which will be discussed in detail later in the text. 2. RESULTS AND DISCUSSION All experimental details including the experimental set-up and sample cleaning procedure are given in Ref./I6/ Oxygen is one of the main vanadium surface contaminants and its effect on the v~adium CW spectrum is summarised in Fig.1 The CW spectrum of the clean vanadium is character&d by the peak at kinetic energy of 505.2 eV, associated with the L&r&r transition, and by a broad shoulder at the high kinetic energy side corresponding to the L&S&~ transition. (A very small intensity of the L~m5m5 transition is due to the fact that the 2~1,~ hole decays dominantly through the KosterCronig process.) Oxygen contamination induces an increase of the intensity at around 509 eV and for higher oxygen concentrations an additional peak, associated with the 0-KLILB Auger process, appears at 488 eV. A well developed signal associated with 0-KL&3 transition (Fig.1, IL 02 spect~rn~ is overlapped with the V-CVV signal producing a single, rather broad, and structureless peak which had been occasionally mistaken 131 for the spectrum of clean vanadium. However, it is more often that a very small oxygen contamination is a real problem; an oxygen contamination equivalent to the exposure of around 0.1 L 02 can induce a significant broadening of the vanadium L$%1v145spectrum. This spectrum of vanadium contaminated by small amount of oxygen can be easily mistaken for the CW spectrum of the clean vanadium /see ref. l/. ( nota bene that is the main reason why Contini et
I
1
480
490
Figure 1 :
I
I
500 510 Kinetic Energy (eV)
Vanadium CW different oxygen exposures,
1
520
Auger spectrum taken for
al/l/ were not able to produce a theoretical spectrum as broad as their expe~mental sperm.) As pointed above, in the case of early transition metals the valence band XPS spectra have been usually used as the total DOS in order to construct the CVV spectrum. It is therefore essential that the valence band spectra are not affected by a surface contamination, A typical width of the occupied part of the valence band of early transition metals is in the range of 5 to 7 eV (sp band included)/l7/. Unfortunately, photoemission of the 2p electron of oxygen and carbon adsorbed on solid surfaces gives rise to signals typically in the energy range between 3 and 7 eV below the Fermi level/ill/. In order to illustrate this problem we show in Fig.2 He1 excited normal mission photoemission spectrum of the clean V( 100) surface and the same surface contaminated by oxygen and carbon atoms, segregated to the surface by annealing the vanadium crystal at 1200 K, compared to the XPS spectrum of vanadium by Lay et al./l9/. It is clear that a small concentration of oxygen or carbon contaminants can easily be mistaken for the features of the valence band /see e.g. ref. 20,211. There are two reasons why we use the XPS spectrum of Lay et al./19/ : (i) the fact that their work, even after two decades, still presents probably the most comprehensive investigation of the occupied part of the valence bands of the 3d metals and (ii) these XPS spectra are still frequently used as reliable density of states of the co~~ponding metals. For example, the XPS spectrum of tita~u~l9/ has been recently used by Sarma et al.1131who compared the self-convolution of the valence band spectrum with the titanium CW. It is our impression that the observed shit? of the self-convolution to lower kinetic energies with respect to the CVV spectrum (2.4 eV) is greatly due to the titanium XPS valence band spectrum affected by the oxygen and carbon contamination. Hedeg&rd and Hillebrecht/l2/ compared the titanium CW and the selfconvolution of their own Ti XPS valence band spectrum
Vol. 99, No. 6
VANADIUM CVV AUGER TRANSITION
0 -2p
~
c -2p i,
/ re.
ARUPS
‘(I 00) annealed
at 1200 K
-6 -4 -2 Binding Energy (eV)
395
rather different results regarding the shape and the energy position of the self-convolution. At this point we would like also to comment on the importance of the reliable kinetic energy calibration which enables one to compare measured Auger spectrum with a calculated one. Quite often CVV Auger spectra are shown at the relative kinetic energy scale where the zero energy corresponds to the 2~s~ binding energy reduced by the work function of the spectrometer. This energy is sometimes referred to as the Fermi levei for it corresponds to the kinetic energy of an Auger electron when both electrons participating in CW transition come from the Fermi level. As the position of the Fermi level with respect to the peak position of the TiCVV spectrum measured by Hedegird and Hillebrecht /12/ (-1.3 eV) and Sarma et al/13/ (= 1.8 eV) differs by 0.5 eV it is obvious that the absolute energy calibration still presents a problem which can affect conclusions related to the physics of the CW Auger process. On the example of vanadium we would like to show that the discrepancy between the CVV Auger spectra and the corresponding self-convolution might not be as big as it has been expected/l 3f As we are not aware of any high resolution XPS spectra of vanadium which can be safely regarded unafRected by any kind of contamination we decided to use a calculated DOS /22/. Although the list of potential problems in using calculated DOS may be long (e.g. total neglect of any dynamically effects, surface contributions, etc.) we feel that even this approximation can give us a fair estimate of the position and the line-shape of the self-convolution of the DOS with respect to the experimental CVV spectrum. The shaded area in Fig.3 shows a spectrum of the vanadium DOS calculated by Papaconstantopoulos et al/22/. Our recent ARUPS measurements on the V(100)/23,24/ give
2 d .2
a p
Figure 2: Normal emission ARUPS spectra of clean and
contaminated V( 100) surface (solid line) and XPS spectrum of vanadium film taken from Ref./l9/(filled circles). Parts of the XPS spectrum which are probably affected by oxygen and carbon contamination are indicated by arrows.
I’
-6
.
I
I
*
4
-2
0
Binding Energy (eV)
and obt~~ the shift of 1.4 eV which is by 1 eV smaller than the shift obtained by Sanna et al./i3/. We can not comment on the quality of the valence band spectrum used by Hedegsrd and Hillebrecht/lZ/ as it has not been published. However, we would like to point out that already very small surface contamination as well as different background subtraction procedures can give
Figure 3: Density of states spe~~m of vanadium as calculated by Papa~on~t~topou~os et al./221 (shaded area). The critical points, indicated by broken lines, were obtained by ARUPS measurements on V(100) f23f. Calculated DOS broadened by Gaussian function is shown by solid line. A dotted curve shows he corresponding selfconvolution.
396
VANADIUM
CVV AUGER TRANSITION
critical points Ni and Hi2 (which determine the bottom of the 3d band) in good agreement with the calculated energies/22/. In order to account for the experimental resolution and all other effects which can contribute to the broadening of the spectral lines we convoluted the calculated DOS with a Gaussian function using FWHM=0.5 eV. The resulting convolution (Fig.3 - solid line) shows peaks at 0.5 eV and 2.2 eV. The selfconvolution of the broadened DOS is shown by the broken line in Fig.3. A comparison between this self-convolution and the experimental CW spectrum is shown in Fig.4. A kinetic energy of the Fermi level is calculated to be 507.2 eV. From Fig.4 it is clear that the peak position, the width of the peak and even the overall line-shape of the CVV spectrum is reproduced by the self-convolution reasonably good taking into account simplicity of the model. A discrepancy between experimental and calculated spectrum on the high kinetic energy side would have been probably smaller if the intensity related to the LJ&&s transition had been taken into account. This result strongly suggests that the “one-hole” processes in L&I&& transition may not be as important, if at all, as it has been argued by Sarma et al./13/ and Drchal and Cini/l5/. Namely, Sarma et a1./13/ suggested that the relative contribution of the “one-hole” density of states for vanadium should be around 75% while Cini and DrchaV151 estimated this contribution to be much smaller but still significant (24%). However, in their model the threeparticle density of states related to the “excitonic like “ bound state (which also increases intensity closer to the Fermi level) is estimated to be 20% so that the “selfconvolution” contribution accounts only for 56% of the total intensity. Our result strongly suggests that the “ordinary” two-hole process most probably dominates the C W transition of vanadium. We have also compared our experimental CW spectrum with calculated spectrum for vanadium, reported by Contini et al/l/. As pointed above, Contini et al./]/ were not successful in fitting their own experimental data for the obvious reason: the CW spectrum was affected by a small oxygen contamination which was enough to broaden the spectrum by 0.2 - 0.5 eV to the high kinetic energy side. The agreement between our CW and Contini’s et al/l/. calculated spectrum is, however, much better (see Fig. 3 broken line). Ironically, the calculated spectrum is slightly too broad at the high kinetic energy side. This is not surprising as they probably adjusted free parameters in
their calculations to fit their broad experimental spectrum. It is important to recall that Contini’s et al/l/ model for the CW spectrum is actually an extended Cini-Sawatzky model/25/ based on the solution of a many-band Hubbard Hamiltonian, in order to account for the contributions occurring from the transitions from different bands (d and s-p). This model, therefore, takes into account valencecorrelation but neglects the core-valence valence correlation, screening and polarisation effects. Hence, this mode1 implicitly assumes that the valence band remains unrelaxed after the creation of a core-hole. While this approximation is undoubtedly acceptable for the late transition metals with completely filled d-band it is
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504
506
508
510
512
514
Kinetic energy (eV)
Figure 4: Experimental vanadium L&i&& spectrum (linear background subtracted) compared with the selfconvolution of DOS (solid line- see Fig.3) and the theoretical spectrum calculated by Contini et al/l/ (broken line). The energy position of this spectrum is adjusted so that the peaks of experimental and calculated spectra coincide.
expected that the core-hole screening plays some role for metals with open d-bands, as the early transition metals are. The fact that the CW Auger spectrum is fairly well described by the model which assumes the Auger decay in an unrelaxed ground state shows that the screening of the core-hole in the CVV Auger transition process for the early transition metals may be far from to be complete. In conclusion, we would like to point out that at least a part of the problems in interpreting the experimental CW Auger spectra of the early transition metals is in the use of valence band spectra of metals affected by surface contamination. We strongly believe that the observed shift between the CVV spectrum and the corresponding selfconvolution of DOS is mainly due to the valence band spectra which appear to be contaminated, usually by oxygen and carbon. We have shown that in the case of vanadium, the self-convolution of the calculated DOS reproduces reasonably well the peak position, the width and the overah shape of the C W spectrum. The agreement CW spectrum and the between the experimental spectrum calculated by Contini et al/l/ using the multiband Cini-Sawatzky model suggests an uncomplete relaxation of the valence band upon the core-hole creation. All this findings imply that the contribution of the two-hole process, with nearly independent final state holes, is probably dominant in the CW Auger transition of vanadium.
VANADIUM
CVV AUGER TRANSITION
AcknowledgmentThis work was supported by the Ministry of science of the Republic of Croatia through the project l-03-056. REFERENCES:
1. V.Contini, C.Presilla and FSacchetti, Solid State Comm. 69,439 (1989) 2. D.A.Shirley, Phys.Rev.A’I, 1520 (1973) 3. G.A.Sawatzky and D.Post, Phis.Rev.B 20, 1546 (1979) 4. G.A.Sawatzky and ALenselnik, Phis.Rev.B 21, 1790 (1980) 5. J.Hubbard, Proc.R.Soc.London,Ser. A 276,238 (1963) 6. M.Cini, SolidState Comm. 20. 605 (1976) 7. M.Cini, Solid State Comm. 24,681 (1977) 8. G.A.Sawatzky, Phys.Rev.Letters, 39,504 (1977) 9. T.Bandyopadbyay and D.D.Sanna, Phys. Rev.B39,35 17 (1989) 10. J.J.Lauder, PhyxRev, 91, 1382(1953) 11. D.K.G.deBoer,C.HassaudG.A.Sawatz.ky,~Phys.F14, 2769 (1978) 12. P.HedegHrd and F.UHillebrecht, Phys.Rev.B 34, 3045 (1986) 13. D.D.Sarma,S.R.Bannau,S.NimkarandH.R.Krisbnamurthy, Physica Scripta, T41, 184-186 (1992)
14. M.Pottboff, J.Braun, W.Nolting, G.Bortsel, Surface Sci. D.P.Woodruff, J.E.Rowe, R.H.Gaylord, Phys.Rev.B 39, 12 604 (1989), CLeygrafand W.Erley and HIbach, Solid State Commun. 37, 937 (I 98 I) 307-309, 942 (1994) 15. V.Drcbal and M.Cini, J.Phys. C 6,8549 (1994) 16. T.Valla, P.Pervau and M.Miluu, Surface Sci, 307-309, 843 (1993) 17. O.KAnderson, O.Jepsen, D.Glotzel, page 59 iu Highlights of Condensed Matter Theory ed. F.Bassaui, F.Fumi and M.P.Tosi, Nort-Holland, Amsterdam, 1985 18. A.Liebsch. Phys.Rev.B 17. 1653 (1978) A.L.D.Kilcoyne, 19. LLey, O.B.Dabbousi, S.P.KowaIczyk, F.R.McFeelyand D.A.Shirley, Phys.Rev.B 16, 5372 (1977) 20. D.Chanderesis, JLecante and Y.Petroff, Phys.Rev.B 27, 2630 (1983) 21. SRaaen and V.Murgai, Phys.Rev. B 36, 887 (1987) 22. D.A.Papacontantopoulos, J.R.Anderson and J.W.McCatfrey, Phys.Rev.B 5, 1214 (1972) 23. P.Pervau, T.Valla and M.Milun, Solid State Commun.89, 917 (1994) 24. B.PeriC, T.Valla, M.Miluu and P.Pervan, Vacuum 46, 118 I (1995) 25. C.Presilla and FSacchetti, J.Phys.F17,779 (1987)
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Pergamon
PI1 SOO38-1098(96)00277-3 Vanadium CVV Auger Transition P. Pervan, T. Valla and M. Milun Institute of Physics, P.O.Box 304, IO 000 Zagreb, Croatia (Received
15 January 1996; accepted 1 May 1996 by C. Calandra)
We report an experimental study of the CVV Auger transition of vanadium. A comparison between the experimental spectrum and the self-convolution of the calculated density of states (DOS) failed to show any significant shift between the spectra as reported for Ti and SC. We have also found good agreement between our experimental and Contini’s et al/l/ calculated CW spectrum of vanadium which may suggest that the screening of the core hole is not as important for the vanadium CW Auger spectrum as might be expected. Copyright
0 1996 Elsevier Science Ltd
1, INTRODUCTION Apart from being a main technique in surface science for the determination of the chemical structure of materials, Auger electron spectroscopy (AES) is also a valuable technique which can provide information on the electron correlation effects. The sensitivity of AES to the electronelectron correlation is consequence of the fact that the final state of the Auger process has two holes and, therefore, the corresponding Auger spectrum represents two-hole density of states which generally reflects the electron-electron Coulomb interaction /2/. Although it might be tempting to use a simple one-electron picture to extract electron Coulomb interactions from Auger spectra (see e.g. ref. 3) the correlation effects can be correctly taken into account only by applying many-body formalism /4/. It is widely accepted that, in the case of core-valencevalence (CVV) Auger transitions, the Coulomb interaction effects may be neglected if the Coulomb interaction within the valence band (U) is much smaller than the width of the valence band itself (W) with the effect that the CW spectrum is well described by the self-convolution of the corresponding one electron DOS. On the other hand, in the limit of U>>W, the two-hole density of states is very much different from the self-convolution of the one particle density of states /5/. It had been realised by Cini/6,7/ and Sawatzky/l/ that CVV Auger spectra of late transition metals are virtually entirely determined by the valence electron-electron correlation effects. They showed that the atomic-like spectrum of the late transition metals is a bound-state part of the two-hole density of states which is shifted to the lower kinetic energies with respect to the self-convolution part of the spectrum by the amount of the Coulomb interaction. The understanding of the CW Auger spectra of early transition metals is still far from being complete. As the Coulomb interaction of valence electrons in scandium, titanium and vanadium is considerably smaller than the width of the corresponding 3d hand/9/ it is to expect that the correlation effects should not play a significant role and that CW Auger spectra will appear as a self-convolution of the one-electron DOS /IO/. However, a comparison of CW Auger spectra of scandium or titanium with the self-
convolution of DOS apparently does not support this assumption. Namely, it has been found that Ti and SC CW Auger spectra are shifted to higher kinetic energies with respect to the corresponding self-convolutions/l 1,12,13/. This shift was initially attributed, by de Boer et al./1 l/, to the negative effective Coulomb interaction which, as suggested, occurs as a consequence of the overscreening of the valence band holes. HedegLd and Hillebrecht/l2/ took the view that the shift of the Auger spectrum to higher kinetic energies with respect to the self-convolution of the valence band is due to the polarisation of the valence band in the presence of the potential of the core-hole. In the case of partially filled valence bands the influence of the potential created by the core-hole (e.g. screening of the core-hole by the valence band electrons) probably should not be neglected as in the case of the late transition metals, However, due to the complexity of the problem there have been only a few attempts to include both, CV and W correlation/ ref. 14 and references therein/. A somewhat different solution to the problem has been proposed by Sarma et al./13/ who neglected entirely the the W correlation but included the possibility that screening cloud (which appears as a response to the corehole) participates in the Auger process. The mechanism which involves the screening charge in the Auger process appears to be a “one-hole process” and the corresponding contribution to the spectrum is proportional to the oneelectron DOS instead of self-convolution of the DOS characteristic for the normal, “two-hole process”. By increasing the probability of the “one-hole” process they increase the spectrum intensity at higher kinetic energy. In this way they successfully fitted the titanium CW Auger spectrum assuming 90% contribution from the “one-hole” like DOS and 10% contribution from the “two-hole” DOS. Recently, Drchal and Cini /15/ formulated a theory which takes into account dynamics of the core-hole screening and correlation of the valence electrons. If the perfectly screened core-hole is assumed (tYly relaxed limit) the theory gives the Auger CVV spectrum as a sum of three contributions:(a) three-particle density of states which is a result of the usual two-hole process in addition convoluted with the one-electron propagator, (b) the one-particle 393
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density of states co~~ponding to the Sarma’s et al.1131 “one-hole” process, and (c) the three-particle density of states associated with the “excitonic-like” state of an electron-hole pair convoluted with the one-hole propagator. It has been also suggested that for the early transition metals a fully relaxed limit (complete screening of the core hole) is much better approximation than neglecting the screening. However, the core-hole screening is much less complete as one approaches the end of the 3d series / 151. In this contribution we present the experimental Auger CW spectrum of v~dium and the analysis of the tr~sition in the context of the recent theories. It is also a particular‘ aim of this paper to point out that some controversies associated with the CW transition of the early transition metals may be a consequence of unreliable experimental data. There are several possible sources of the problems which can affect the conclusions related to the CW Auger transition: (i) effects of surface contamination on the CW Auger spectrum itself (ii) 1 effects of surface contamination on the valence band spectrum which is (when taken with high photon energy, e.g. x-rays), usually used as a spectrum of the total DOS, which in turn is used to calculate CW spectrum {iii) absolute energy c~ibration n~es~~ for the comparison of Auger spectrum and the ~~es~nding seIf-convolution. of the problems in the We believe that some understanding of CW Auger transition of the early transition metals may at least partly be affected by above mentioned effects which will be discussed in detail later in the text. 2. RESULTS AND DISCUSSION All experimental details including the experimental set-up and sample cleaning procedure are given in Ref./I6/ Oxygen is one of the main vanadium surface contaminants and its effect on the v~adium CW spectrum is summarised in Fig.1 The CW spectrum of the clean vanadium is character&d by the peak at kinetic energy of 505.2 eV, associated with the L&r&r transition, and by a broad shoulder at the high kinetic energy side corresponding to the L&S&~ transition. (A very small intensity of the L~m5m5 transition is due to the fact that the 2~1,~ hole decays dominantly through the KosterCronig process.) Oxygen contamination induces an increase of the intensity at around 509 eV and for higher oxygen concentrations an additional peak, associated with the 0-KLILB Auger process, appears at 488 eV. A well developed signal associated with 0-KL&3 transition (Fig.1, IL 02 spect~rn~ is overlapped with the V-CVV signal producing a single, rather broad, and structureless peak which had been occasionally mistaken 131 for the spectrum of clean vanadium. However, it is more often that a very small oxygen contamination is a real problem; an oxygen contamination equivalent to the exposure of around 0.1 L 02 can induce a significant broadening of the vanadium L$%1v145spectrum. This spectrum of vanadium contaminated by small amount of oxygen can be easily mistaken for the CW spectrum of the clean vanadium /see ref. l/. ( nota bene that is the main reason why Contini et
I
1
480
490
Figure 1 :
I
I
500 510 Kinetic Energy (eV)
Vanadium CW different oxygen exposures,
1
520
Auger spectrum taken for
al/l/ were not able to produce a theoretical spectrum as broad as their expe~mental sperm.) As pointed above, in the case of early transition metals the valence band XPS spectra have been usually used as the total DOS in order to construct the CVV spectrum. It is therefore essential that the valence band spectra are not affected by a surface contamination, A typical width of the occupied part of the valence band of early transition metals is in the range of 5 to 7 eV (sp band included)/l7/. Unfortunately, photoemission of the 2p electron of oxygen and carbon adsorbed on solid surfaces gives rise to signals typically in the energy range between 3 and 7 eV below the Fermi level/ill/. In order to illustrate this problem we show in Fig.2 He1 excited normal mission photoemission spectrum of the clean V( 100) surface and the same surface contaminated by oxygen and carbon atoms, segregated to the surface by annealing the vanadium crystal at 1200 K, compared to the XPS spectrum of vanadium by Lay et al./l9/. It is clear that a small concentration of oxygen or carbon contaminants can easily be mistaken for the features of the valence band /see e.g. ref. 20,211. There are two reasons why we use the XPS spectrum of Lay et al./19/ : (i) the fact that their work, even after two decades, still presents probably the most comprehensive investigation of the occupied part of the valence bands of the 3d metals and (ii) these XPS spectra are still frequently used as reliable density of states of the co~~ponding metals. For example, the XPS spectrum of tita~u~l9/ has been recently used by Sarma et al.1131who compared the self-convolution of the valence band spectrum with the titanium CW. It is our impression that the observed shit? of the self-convolution to lower kinetic energies with respect to the CVV spectrum (2.4 eV) is greatly due to the titanium XPS valence band spectrum affected by the oxygen and carbon contamination. Hedeg&rd and Hillebrecht/l2/ compared the titanium CW and the selfconvolution of their own Ti XPS valence band spectrum
Vol. 99, No. 6
VANADIUM CVV AUGER TRANSITION
0 -2p
~
c -2p i,
/ re.
ARUPS
‘(I 00) annealed
at 1200 K
-6 -4 -2 Binding Energy (eV)
395
rather different results regarding the shape and the energy position of the self-convolution. At this point we would like also to comment on the importance of the reliable kinetic energy calibration which enables one to compare measured Auger spectrum with a calculated one. Quite often CVV Auger spectra are shown at the relative kinetic energy scale where the zero energy corresponds to the 2~s~ binding energy reduced by the work function of the spectrometer. This energy is sometimes referred to as the Fermi levei for it corresponds to the kinetic energy of an Auger electron when both electrons participating in CW transition come from the Fermi level. As the position of the Fermi level with respect to the peak position of the TiCVV spectrum measured by Hedegird and Hillebrecht /12/ (-1.3 eV) and Sarma et al/13/ (= 1.8 eV) differs by 0.5 eV it is obvious that the absolute energy calibration still presents a problem which can affect conclusions related to the physics of the CW Auger process. On the example of vanadium we would like to show that the discrepancy between the CVV Auger spectra and the corresponding self-convolution might not be as big as it has been expected/l 3f As we are not aware of any high resolution XPS spectra of vanadium which can be safely regarded unafRected by any kind of contamination we decided to use a calculated DOS /22/. Although the list of potential problems in using calculated DOS may be long (e.g. total neglect of any dynamically effects, surface contributions, etc.) we feel that even this approximation can give us a fair estimate of the position and the line-shape of the self-convolution of the DOS with respect to the experimental CVV spectrum. The shaded area in Fig.3 shows a spectrum of the vanadium DOS calculated by Papaconstantopoulos et al/22/. Our recent ARUPS measurements on the V(100)/23,24/ give
2 d .2
a p
Figure 2: Normal emission ARUPS spectra of clean and
contaminated V( 100) surface (solid line) and XPS spectrum of vanadium film taken from Ref./l9/(filled circles). Parts of the XPS spectrum which are probably affected by oxygen and carbon contamination are indicated by arrows.
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and obt~~ the shift of 1.4 eV which is by 1 eV smaller than the shift obtained by Sanna et al./i3/. We can not comment on the quality of the valence band spectrum used by Hedegsrd and Hillebrecht/lZ/ as it has not been published. However, we would like to point out that already very small surface contamination as well as different background subtraction procedures can give
Figure 3: Density of states spe~~m of vanadium as calculated by Papa~on~t~topou~os et al./221 (shaded area). The critical points, indicated by broken lines, were obtained by ARUPS measurements on V(100) f23f. Calculated DOS broadened by Gaussian function is shown by solid line. A dotted curve shows he corresponding selfconvolution.
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critical points Ni and Hi2 (which determine the bottom of the 3d band) in good agreement with the calculated energies/22/. In order to account for the experimental resolution and all other effects which can contribute to the broadening of the spectral lines we convoluted the calculated DOS with a Gaussian function using FWHM=0.5 eV. The resulting convolution (Fig.3 - solid line) shows peaks at 0.5 eV and 2.2 eV. The selfconvolution of the broadened DOS is shown by the broken line in Fig.3. A comparison between this self-convolution and the experimental CW spectrum is shown in Fig.4. A kinetic energy of the Fermi level is calculated to be 507.2 eV. From Fig.4 it is clear that the peak position, the width of the peak and even the overall line-shape of the CVV spectrum is reproduced by the self-convolution reasonably good taking into account simplicity of the model. A discrepancy between experimental and calculated spectrum on the high kinetic energy side would have been probably smaller if the intensity related to the LJ&&s transition had been taken into account. This result strongly suggests that the “one-hole” processes in L&I&& transition may not be as important, if at all, as it has been argued by Sarma et al./13/ and Drchal and Cini/l5/. Namely, Sarma et a1./13/ suggested that the relative contribution of the “one-hole” density of states for vanadium should be around 75% while Cini and DrchaV151 estimated this contribution to be much smaller but still significant (24%). However, in their model the threeparticle density of states related to the “excitonic like “ bound state (which also increases intensity closer to the Fermi level) is estimated to be 20% so that the “selfconvolution” contribution accounts only for 56% of the total intensity. Our result strongly suggests that the “ordinary” two-hole process most probably dominates the C W transition of vanadium. We have also compared our experimental CW spectrum with calculated spectrum for vanadium, reported by Contini et al/l/. As pointed above, Contini et al./]/ were not successful in fitting their own experimental data for the obvious reason: the CW spectrum was affected by a small oxygen contamination which was enough to broaden the spectrum by 0.2 - 0.5 eV to the high kinetic energy side. The agreement between our CW and Contini’s et al/l/. calculated spectrum is, however, much better (see Fig. 3 broken line). Ironically, the calculated spectrum is slightly too broad at the high kinetic energy side. This is not surprising as they probably adjusted free parameters in
their calculations to fit their broad experimental spectrum. It is important to recall that Contini’s et al/l/ model for the CW spectrum is actually an extended Cini-Sawatzky model/25/ based on the solution of a many-band Hubbard Hamiltonian, in order to account for the contributions occurring from the transitions from different bands (d and s-p). This model, therefore, takes into account valencecorrelation but neglects the core-valence valence correlation, screening and polarisation effects. Hence, this mode1 implicitly assumes that the valence band remains unrelaxed after the creation of a core-hole. While this approximation is undoubtedly acceptable for the late transition metals with completely filled d-band it is
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Figure 4: Experimental vanadium L&i&& spectrum (linear background subtracted) compared with the selfconvolution of DOS (solid line- see Fig.3) and the theoretical spectrum calculated by Contini et al/l/ (broken line). The energy position of this spectrum is adjusted so that the peaks of experimental and calculated spectra coincide.
expected that the core-hole screening plays some role for metals with open d-bands, as the early transition metals are. The fact that the CW Auger spectrum is fairly well described by the model which assumes the Auger decay in an unrelaxed ground state shows that the screening of the core-hole in the CVV Auger transition process for the early transition metals may be far from to be complete. In conclusion, we would like to point out that at least a part of the problems in interpreting the experimental CW Auger spectra of the early transition metals is in the use of valence band spectra of metals affected by surface contamination. We strongly believe that the observed shift between the CVV spectrum and the corresponding selfconvolution of DOS is mainly due to the valence band spectra which appear to be contaminated, usually by oxygen and carbon. We have shown that in the case of vanadium, the self-convolution of the calculated DOS reproduces reasonably well the peak position, the width and the overah shape of the C W spectrum. The agreement CW spectrum and the between the experimental spectrum calculated by Contini et al/l/ using the multiband Cini-Sawatzky model suggests an uncomplete relaxation of the valence band upon the core-hole creation. All this findings imply that the contribution of the two-hole process, with nearly independent final state holes, is probably dominant in the CW Auger transition of vanadium.
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AcknowledgmentThis work was supported by the Ministry of science of the Republic of Croatia through the project l-03-056. REFERENCES:
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