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CrLVV Auger emission and photoreduction of hexavalent Cr oxides
139
Surface Science 250 (1991) 139-146 North-Holland
Cr LW
Auger emission and photoreduction of hexavalent Cr oxides
A.G. Schrott a, G.S. Frankel a, A.J. Davenport b, H.S. Isaacs b, C.V. Jahnes a and M.A. Russak a e IBM Research Division, T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, WSh ’ Departmen! of Applied Science, Brookhaven National Laboratory,
Upton, NY 1 I973, USA
Received 25 September 1990; accepted for publication 18 January 1991
The Cr LW Auger emission has been used to investigate the local chemical environment of Cr(VI) in connection with the photoreduction produced by a non-monochromatic X-ray source. We compared Cr(VI) in oxides on anodized Al exposed to a chromate solution, with Cr(V1) in oxides on Al/Cr alloys polarized in a borate solution. The former were found to be much more sensitive to photoreduction than the latter. This is correlated with the intensity of the Cr LW Auger emission, attributed to a charge transfer transition in the final state of the photoemission process
I. IiWoduction
The presence of Cr(V1) has been previously observed in oxides formed on Al and Al/Cr alloys which had been anodized, and it has been suggested that hexavalent Cr plays a role in determining the resistance of these materials to pitting attack [l-3]. However, a detailed ~derst~ding of the phenomena has not been attained yet. In order to better understand the basic mechanisms for passivation, we have been investigating the properties of oxide films formed by polarization of Al/Cr alloys of various concentrations, using X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES). XANES is particularly well suited to the study of Cr(V1) due to the presence of a distinct pre-edge peak in the K edge absorption spectrum, related to the symmetry of the tetrahedral crystal field, which is absent for other valencies of Cr. XPS is a standard analytical tool for assessing the chemical state and composition of oxide films. However, due to the complexity of the photoemission process, the local chemical environment of the sample’s constituents is generally not unequivocally determined by the energy position of the XPS peaks. In addition, decomposition effects have 0039-6028/91/$03.50
been observed in the case of hexavalent Cr [4,5]. Therefore, special care must be taken to assess these effects in order to produce an accurate picture of the chemistry involved. While the XANES results will be presented in detail elsewhere, this work focuses on the Cr LW Auger emission which results from one of the competing relaxation mechanisms after phot~~ssion, and on the connection between this process and photoreduction of the oxides.
2. Experimental Al and Al/Cr alloys samples were prepared by sputter deposition onto quartz substrates. A composite target, consisting of Cr pins in a standard Al target, was used to deposit alloys with 4 to 46% Cr. The alloys were polarized in a solution of OSM boric acid +O.O5M sodium borate with pH 7.4 for 5 min at a fixed potential of 5 V versus a mercurous sulphate reference electrode (MSE). The Al films were also polarized at 5 V MSE for 5 min but were then exposed at open circuit to O.lM K,CrO, adjusted to pH 7. The samples were transferred through air to the analysis chamber within 5 min after polarization. The XPS spectra
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
140
A. G. Schrott et al. / Cr L VV Auger emission of hexavalent
Cr oxides
of these oxides were compared to those of various Cr oxide standards. Monochromatized Al Ka radiation was utilized for the analyses. Since the flux of the monochromatic beam is only about 3% of that from the non-monochromatized AlKa radiation, photodecomposition during setup is reduced. Furthermore, photodecomposition during data acquisition is also reduced because of the narrower interval of the photon energy distribution. Nevertheless, to further minimize photodecomposition, we opted to limit the counting time at the expense of a good signal-to-noise ratio. Due to sample charging, the peak energies were determined through a comparison with the Cls line at 284.6, which arises from adventitious carbon.
3. Results and discussion For the Al/Cr alloys polarized to a high potential, the presence of Cr(V1) in the oxides was clearly evident from both the pre-edge peak in the XANES spectra as well as a high binding energy peak in the Cr2p XPS spectra. The XANES data and a discussion of the kinetics of formation and dissolution of the oxides on Al/Cr films will be published elsewhere [6]. Hereafter, this work will focus on the connection between local chemistry, as inferred from the Cr LW emission, and photoreduction. Auger lines in the photoemission spectra result from the filling of the core hole created by the photoemission process. LW Auger transitions provide information of the local chemical environment, in contrast with valence band photoemission which probes both the local and the extended character of the electronic structure. In the case of Cr oxides, the Cr Auger emission has not been routinely analyzed because most of the lines overlap with the 0 Auger lines. However, the Cr LW line does not overlap with any other lines and is a revealing analytical tool. Before analyzing the LW Auger spectra, it is useful to describe the general features of the valence band photoemission for Cr and Cr-oxides. Fig. 1 shows valence band photoemission spectra corresponding to various Cr compounds. The spectra were aligned with that of metallic Cr using
ENERGY (eV RELATIVE
TO EF)
Fig. 1. X-ray photoemision spectra of the valence region for: Curve 1, in situ evaporated Cr; curve 2, Cr,03 powder; curve 3, CrO, scales; curve 4, Al-43% Cr polarized at 5 V in borate.
the energy position of the Cls line from hydrocarbon contamination. Curve 1, which corresponds to an in situ evaporated Cr film, shows a narrow peak of d character followed by a tail due mainly to a 4s-derived band, which is in agreement with previous results [7]. The spectrum corresponding to Cr,O, is shown in curve 2. It consists of a narrow peak near the Fermi energy which arises from the localized d band and a broader structure at higher binding-energies representing the 02p band [8]. Cr in CrO, has no d electrons so the corresponding valence band photoemission spectrum (curve 3) exhibits only a peak corresponding to the 0 2p band. The oxide formed by polarizing a Al-43% Cr alloy to 5 V generated curve 4. This sample contains a large amount of Cr(VI), as determined by XANES and by the Crap XPS line. In this case, because of the O-Al hybridization, the local contributions to the density of states are not straightforward. A more direct description of the changes in the local chemical environment of Cr as a result of the alloying with Al can be obtained from the Cr LW Auger spectra.
A.G. Schrott et al. / Cr LVV Auger emission of hexavalent
j
556
566
ENERGY kV
576
566
RELATIVE TO EF)
Fig. 2. Cr LW Auger transition for: Curve 1, in situ evaporated Cr; curve 2, Cr,Os powder; curve 3, Cr(OH), powder; curve 4; CrO, powder; curve 5, CrO, scales; curve 6, K,Cr,O, powder
Fig. 2 shows Cr LW Auger spectra for Cr metal (curve 1) and for various Cr-oxides. The spectra corresponding to the octahedrally coordinated Cr-oxides (Cr,O,, Cr(OH), and CrO,, curves 2 to 4 respectively) exhibit two excursions. The one at higher kinetic energy corresponds to transitions from the localized Cr3d band (intraatomic transition), while the lower kinetic energy excursion derives from the 02p bonding orbitals [8] as is suggested by fig. 1, and has extra-atomic origin [9,10]. A third distinct peak representing a final state with a hole in each band is not observed, perhaps because of the poor overlap between the corresponding orbitals [9]. The spectra corresponding to the Cr(V1) oxides (curves 5 and 6) in which Cr is tetrahedrally coordinated to 0, exhibit only one excursion at lower kinetic energies. Since the localized d band is nominally empty in the initial state of photoemission, intraatomic LW transitions are not possible. Thus, the
141
Cr oxides
LW Auger signal from the Cr(V1) compounds has only extra-atomic character (interatomic transition), and reflects the degree of overlap of the bonding 02p-derived orbitals as seen in the final state of photoemission (initial state for Auger) [9,10]. The lower kinetic energy of the Auger peaks in curves 5 and 6 compared to that of the excursion of extra-atomic character in curves 2 to 4 is contrary to what should be expected from the higher binding energy of the Cr2p level in the Cr(V1) oxides, and probably reflects a large value of the hole-hole interaction energy, U, a correlation term that decreases the potentially available Auger energy [9,11]. The area under the LW Auger peak for the tetrahedrally coordinated (Cr(VI)), normalized by the area under the Cr2p peak, is typically twice as large as for the octahedrally coordinated oxides. These ratios are listed in table 1. The higher intensity of the Auger emission for the Cr(V1) oxides as well as the lower kinetic energy (in contrast with the similar 02pderived bands appearing in photoemission) seem to indicate that the two holes in the final state are located in the same bonding orbital. From fig. 2, which shows relative changes in the two excursions of the Auger spectra, and from table 1, we deduce that the intensity of the interatomic Auger emission increases with increasing nominal valency. This correlates well with the decreasing Cr-0 distances [12] which in turn may induce a larger overlap in the initial state, and may also produce larger amount of charge transfer and thus
Table 1 LW Auger/CrZp intensity different configuration
ratios
Compound
AES/Cr
CraOs Cr(OH) 3
0.095 0.080 0.075 0.144 0.134 0.143 0.140 - 0.1 negligible
C&2 Cd3
K 2Cr0, KaCra% Ads. chromate Ads. chromate irrad. Pol. Al-Cr alloy
for various
2p
Cr-oxides
of
Configuration octahedral octahedral octahedral tetrahedral tetrahedral tetrahedral tetrahedral ? tetrahedral
‘) ‘)
‘) The coordination the Cr ion is inferred by the appearance the pre-edge peak in the XANES spectra.
of
142
A.G. Schrott
IO
-600
ENERGY
-590 kV
-580
RELATIVE
et al. / CrLVV
Auger
-570 TO
EF)
Fig. 3. Cr2p spectra for: curve 1, Cr,O, powder; curve 2, Cr(OH), powder; curve 3, CrO, powder; curve 4, CrO, scales; curve 5, K,Cr,O, powder; curve 6, Al-43% Cr polarized at 5 V in borate.
a higher degree of screening in the photoemission final state. The difference in final state screening also affects the intensity and position of the satellites in the Cr2p photoemission spectra. Fig. 3 shows XPS spectra corresponding to the Cr2p emission for various Cr oxides. Curves 1 to 3 correspond to the octahedrally coordinated Cr,O,, Cr(OH),, and CrO,, respectively, while curves 4 to 6 correspond and the oxide formed by to CrO,, K&t-,0,, polarizing a Al-43% Cr alloy to 5 V MSE, respectively. The peak energies for the reference compounds agree fairly well (within 0.3 eV) with previously reported values, [13] and references therein. However, we agree with other authors [14] on the intrinsic difficulties associated with assigning binding energies in oxides through the use of internal standards, and therefore we will focus our discussion on the appearance of satellites rather than on binding energy variations. As it has been pointed out before [13], the spectra from Cr(II1) compounds exhibit higher satellite intensity than
emission
ofhexavalent
Cr oxides
those from Cr(V1) compounds. Feature A in fig. 3 is a satellite associated with the 2p,,, peak. The satellite associated with the 2~,,~ overlaps with the Cr2p,,, peak making the relative intensity of the latter higher than what it is expected from the population of the J = : state. Among the octahedrally coordinated oxides of fig, 3, the intensity of the satellite is lower for CrO, (curve 3). According to results for Cu and Ni-halides [11,15], the intensity of the satellites has been related to a difference in hybridization between initial and final states in the photoemission process. For these compounds, the satellites has been ascribed to a nonscreened final state. Therefore, within this interpretation, the lower satellite intensity for CrO, indicates a higher average screening charge transfer from the ligand in the final state, and this is in agreement with the correspondingly higher interatomic Auger component in fig. 2. This seems to apply also to the Cr(V1) oxides, since the low satellite intensity in this case is consistent with the higher Auger emission shown in table 1. According to this argument, curve 6, which unlike curves 4 and 5 shows a shoulder (feature B) in the Cr2p,,, peak, may indicate a decreased screening in the final state of photoemission for the oxide formed by polarization of the alloy. Having described the main features of the spectra for the Cr(II1) and Cr(V1) compounds, it is now possible to gain insight on the distinct features of the oxides on the Al/Cr alloys. Fig. 4a shows the Cr2p photoemission spectra for Al which had been anodized in the borate solution and then subsequently exposed at open circuit to the chromate solution (curve l), and for the Al43% Cr polarized in the borate solution (curve 2). Aside from the shoulder in curve 1, which is produced by photoreduction as will be discussed later, the two spectra are similar in energy position and intensity to each other and to those of the Cr(V1) oxides shown in fig. 3. In contrast, the corresponding Cr LW Auger spectra shown in fig. 4b are very different. Curve 1, which corresponds to the same adsorbed sample as curve 1 in fig. 4a, exhibits lineshape and energy position similar (except for a small shoulder also due to photoreduction) to the Cr(V1) spectra in fig. 2. However, the polarized alloy exhibits insignificant LW Auger
143
A. G. Schrott et al. f Cr LVV Auger emission of hexavalent Cr oxides
emission (curve 2), even if we take into account the lower amount of Cr present in the alloy (see table 1). This difference results from the influence of alloying on the local electronic structure of Cr in the corresponding oxide. The low Auger intensity observed in the Cr(VI) oxides on the alloy may be related to the oxygenaluminum hybridization. XPS data for Al,O, [16,17] indicate that the 0 2p derived valence bands [18] are wider than those corresponding to the Cr oxides [8,19], in agreement with our spectra of fig. 1. In particular, fig. 1 curve d shows an overlap of the Cr d-Op bonding band with the Als,p-0 p nonbonding band. This would make partial transfer of the screening charge to 0 nonbonding orbitals likely to occur, thus reducing the electronic density at the Cr site and therefore the Auger transition rate. More importantly, a broader valence band may allow the potential energy U to be converted into the kinetic necessary for a onehole hop, reducing the lifetime of the two-hole final state 19,201. Thus, the change in screening charge suggested by the appearance of the satellite in the Cr2p spectrum corresponding to the oxide on the alloy may not be the only cause for the negligible Auger intensity. A gross estimate of the connection between the interatomic Auger intenI
,
I
,
I
,
t
,
sity and that of the XPS satellite, could be obtained from a comparison of curves 1 and 3 in fig. 3. A substantial reduction in the satellite intensity for CrO, is observed, compared to Cr,O,. Therefore, the screening charge transfer should increase and the same should happen to the extra-atomic portion of the Auger peak for CrO,. Table 1 indicates that for CrO, the total LW Auger intensity decreases by only - 20% compared to Cr,O,, while it should have decreased by - 33% due to the disappearance of one electron from the localized d band. Therefore the change in satellite intensity shown in fig. 3 for the octahedrally coordinated oxides seems to be related to about a 10% increase in the intensity of the interatomic transition. Since feature B in fig. 3, curve 6 seems of the same order of magnitude as the changes in feature A, it may not fully account for the loss of Auger intensity shown in fig. 4b for the polarized alloy. The large breadth of the valence band, on the other hand, may drastically decrease the lifetime of the Auger final state, and may lead to a very broad Auger lineshape [9], which could be difficult to detect. The striking differences in the LW Auger emission may be used to understand the effects of X-ray irradiation on these Cr(V1) oxides, which will be presently discussed.
-7
a
p&razed
alloy
.r ,..,
-.
..
_,
.:.
-600 ENERGY (eV RELATIVE
546 TO EF)
556
566
576
586
ENERGY (eV RELATIVE TO E, 1
Fig. 4. (a) Cr2p spectra for: Curve 1, adsorbed chromate on anodized Al (the arrows denote the presence of Cr3+ due to fast photoreduction); curve 2, Al-43% Cr polarized at 5 V in borate. (b) Cr LW Auger spectra for: Curve 1, adsorbed chromate on anodii AI; curve 2, Al-43% Cr polarized at 5 V in borate. These spectra were taken with the mon~hromatic source.
144
A.G. Schrott et al. / Cr LVVAuger
The hexavalent oxides formed on the Al/Cr alloys by polarization in borate solution did not suffer from photodecom~sition during the XANES experiments or the XPS measurements using monochromatic radiation. In contrast, a minor degree of reduction was observed in the XPS spectra corresponding to the adsorbed chromate films (see curves 1 in figs. 4a and 4b). Due to its higher photon flux, we used a standard AlKa source to purposely produce radiation damage, and compared the photoreduction that occurred upon a fixed irradiation dose on the oxide films formed on the Al/Cr alloys films with that of the adsorbed chromate films on anodized Al. Spectra taken after various exposures indicated that the reduction of Cr(V1) for the Al samples polarized in borate and then exposed to chromate occurred after few minutes of exposure to the standard XPS source. In contrast, for the oxide on the alloy the reduction of Cr(VI) occurred at a much slower rate. A comparison of the photore-
a
I
’
I
’
I,’
emission of hexavalent Cr oxides
duction produced by exposing the samples to the standard X-ray source is shown in figs. 5a and 5b. Curves such as 1 can be obtained after only IO min of irradiation, and remain constant thereafter. It appears that all the Cr(V1) from the Al sample that had been immersed in chromate following polarization in borate had reduced and converted mainly to to Cr(III), as will be discussed in the next paragraph. The spectra corresponding to the irradiated oxide formed on Al/Cr alloy films indicated a higher resistance to photoreduction. Curve 2 in fig. 5a, was obtained after 3 h of irradiation, when further evolution of the spectra with exposure to X-ray i~adiation was no longer detected. This curve indicates a mixture of Cr(V1) and Cr(III), with more Cr(V1) than Cr(II1). Curve 3 is a peak synthesis of curve 2. The fit was made using peaks with energies and relative intensities taken from curve 2 in fig. 4a for 6 + , and from curve 1 in fig. 5a for the 3 + . Curve 2 was obtained at zero take-off angle. Spectra taken at
I
after 3 hrs
-610
-600
ENERGY CeV RELATIVE TO EF)
546
556
576
ENERGY (eV RELATIVE TO E,)
Fig. 5. The samples in fig. 4 after AlKa radiation. (a) Cr2p XPS spectra: curve 1, the adsorbed chromate after 10 mm and further exposure; curve 2, the polarized alloy after 3 h or more; curve 3, peak synthesis of 2, using peak positions for 6 + as in fig. 4a, 1, and for 3 + as in curve 1 above; (b) LW Auger: curve 1, the adsorbed chromate after 10 min and further exposure (arrows indicate the position of the Auger peak for chromate, and for the inter and intra atomic features for Cr,Os, respectively from fig. 2); curve 2, the polarized alloy after 3 h or more. Since the resolution for Auger is independent of the line-width of the excitation source, these nonevolving Auger spectra were taken with the nonmon~~omatic source, to improve the count rate.
A.G. Schrott et al. / Cr L W Auger emission of hexadent
higher take-off angle with respect to the sample’s normal indicated that the reduction occurred at the surface. Fig. 5b shows the Auger emission from the samples following the irradiation with non-monochromatized X-rays described above. Curve 1 arises from the reduced form of Cr for the adsorbed chromate. We believe the reduced form to be Cr(II1) because of (a) the similarity of the Auger lineshape to that of Cr,O, (curve 2 in fig. 2) which has an interatomic feature lower in intensity than the intra-atomic one, and (b) The similarity of the LW/2p intensity ratios (- 0.1) shown in table 1. This identification is made despite the fact that the Auger energies are offset from the Cr,O, standard (see arrows in fig. 5b, curve 1 for a comparison), and that the binding energy of the corresponding Cr 2p,,, photoemission peak in fig. 5a, curve 1, falls at - 0.8 eV higher than that of the Cr,O, standard in fig. 3. These differences could be explained by a lower extra-atomic relaxation expected from the presence of defects and from the different coordination of the Cr(II1) in the surface oxide. However, the presence of some Cr(IV) or Cr(I1) cannot be ruled out completely. Cr4+ ions (not in a CrO, phase [12]) could also lead to the binding energy observed in fig. 5a, curve 1. On the other hand, due to the lower amount of available 3d electrons, the Cr(IV) contribution to the intra-atomic portion of the Auger lineshape is expected to be lower than the interatomic one, as in CrO, (curve 4 in fig. 2), and this is not reflected in the Auger spectrum of the reduced adsorbed sample (fig. 5b, curve 1). By extrapolation, we could expect Cr(I1) to generate a LW Auger spectrum more consistent with that of curve 1 in fig. 5b. We have not measured yet the Auger emission of any Cr(I1) standard. However, the binding energy of Cr,O, thin films (identified by TEM) was found to be only 0.4 eV higher than that of Cr,O,, using the signal from the underlying Cr metal as reference [21]. Therefore, from the data in fig. 5 and the above discussion, we believe that Cr(II1) is the major component of the reduced form of Cr in our films. Since the Cr(V1) in the polarized alloy produced negligible LW Auger emission prior to irradiation (fig. 4b, curve 2), curve 2 in fig. 5b
Cr oxides
145
results only from the Cr that has been reduced to the 3 + oxidation state. This spectrum however, is different from that of the adsorbed sample (curve 1) which also results from Cr reduced to the 3 + oxidation state. The intra-atomic peak at higher kinetic energy is similar but there is only a shoulder in the low energy portion of the spectrum which has extra-atomic character. The absence of a strong peak indicates that the interatomic Auger transition for Cr(III) in the oxide on the polarized alloy sample is inhibited similarly to Cr(V1) (curve 2 in fig. 4b). Thus, regardless of the oxidation state of Cr in the oxide on the alloy, the screening charge transfer from the ligand is reduced due to the presence of Al. For completeness, we also examined the photoreduction of oxide films formed by anodization of Al in the chromate solution, and found a behavior similar to that of the Cr(V1) in oxides on the alloy. This similarity is consistent with the chromate ions being incorporated into the thickening alumina film [nl. as opposed to the chromate film adsorbed on the anodized Al. Judging from the XPS and XANES spectra only, the different response of these Cr(V1) oxides to irradiation would have been difficult to explain. Because it is a two-electron process, the LW Auger transition is very sensitive to changes in the final state, and provides a qualitative information on the correlation between chars transfer from the ligand and photoreduction. The electron or photon-stimulated desorption of O+ ions from the surfaces of maximum valency ionic systems, above a threshold incident energy, has been explained by the Knotek-Feibelman model to proceed by Auger decay following a core-level ionization (231. After the Auger process has occurred, two or more holes can reside on an anion, which can be desorbed due to the Madelung potential. More recent results seem to indicate that maximum valency is not necessarily required [24]. This mechanism may also apply to covalent systems when the Coulomb correlation energy U is greater than the width of the band generated by the bonding orbitals (20). Our results of figs. 4 and 3 indicate that there is a direct connection betthe sensitivity to photoreduction of Cr(VI) and the intensity of the Cr LW
146
A.G. Schrott et al. / Cr L VV Auger emission of hexaualent
Auger emission. The 0 p-Al s,p hybridization seems to reduce the effectiveness of the Auger decay as a mechanism for the desorption of 0 ions, either because that channel is closed due to a lower Cr-0 overlap, or because the two-hole state is short-lived. In the latter case, the inhibition may occur because the local inversion of the charge in the anion site should last an interval comparable to a period for nuclear motion for the ion to be expelled [20].
4. Conclusions The intensity of the Cr LW Auger transition in Cr(V1) oxides has been shown to be connected to the tendency for surface photoreduction, which may be qualitatively explained by the KnotekFeibelman model. It has also been proposed, based on valence band photoemission, that O-Al rehybridization may lead to a decreased charge transfer and hole-hole interaction which may inhibit the role of the Auger decay as a channel for photoreduction of Cr(V1) in the oxides formed by polarization of the Al-Cr alloys. Finally, the Cr LW Auger lineshape constitutes a useful fingerprint for chemical identification, considering that for the Cr-oxides there is not a direct correlation between valency and binding energy [13], which is very sensitive in these oxides to final state relaxation effects.
References 111W.C. Moshier, G.D. Davis, J.S. Ahearn and H.F. Hough, J. Electrochem.
Sot. 134 (1987) 2677. G.D. Davis and G.O. Cote, J. Electrothem. Sot. 136 (1989) 356. H.S. Isaacs, S.M. Heald, J. Tranquada. [31 J.K. Hawkins, G.E. Thompson and G.C. Wood, Corrosion Sci. 27 (1987) 391. [41 B.A. DeAngelis, J. Electron Spectrosc. Relat. Phenom. 9 (1976) 81. [51 G.P. Halada, C.R. Clayton and D.H. Lindsley, Mater. Sci. Eng. A 103 (1988) L5, and references therein. H.S. Isaacs, A.G. Schrott. [61 G.S. Frankel, A.J. Davenport, C.V. Jahnes and M.A. Russak, to be published. S.P. Kowalczyk, F.R. McFeely 171 L. Ley. O.B. Dabbousi, and D.A. Shirley, Phys. Rev. B 16 (1977) 5372. H.J. Guggenheim and S. Hiifner, Phys. PI G.K. Wertheim, Rev. Lett. 21 (1973) 1050. in: Chemistry and Physics of Solid [91 D.E. Ramaker, Surfaces, Vol. 4, Eds. R. Vanselow and R. Howe (Springer. Berlin, 1982) p. 19. DOI J.A. Tossell, J. Electron Spectrosc. Relat. Phenom. 8 (1976) 1.
121W.C. Moshier,
[Ill [=I P31
u41 I151 WI [I71
Acknowledgements The authors wish to thank P. Avouris, D.M. Newns, and J.D. Tersoff for useful discussions. The XANES experiments were carried out at the X-11 beam line of the National Synchrotron Light Source at Brookhaven National Laboratory and partially supported by the US Department of Energy under Contract Nos. DE-AC0276CH00016 and DE-AS0580-ER10742.
Cr oxldes
WV v91 PO1 WI PI 1231 [24]
G. van der Laan, C. Westra, C. Haas and G.A. Sawatzky, Phys. Rev. B 23 (1981) 4369. R.W.G. Wyckoff, in: Crystal Structures, Vols. 1 and 2 (Wiley, New York 1964) pp. 251, 7, 91. I. Ikemoto, K. Ishii, S. Kinoshita, H. Kuroda, M.A. Alario Franc0 and J.M. Thomas, J. Solid State Chem. 17 (1976) 425. D.A. Stephenson and N.J. Binkowski. J. Non-Cryst. Solids 22 (1976) 399. J. Zaanen, C. Westra and G.A. Sawatzky, Phys. Rev. B 33 (1986) 8060. A. Balzarotti and A. Bianconi, Phys. Status Solidi 76 (1976) 689. W.J. Gignac, R.S. Williams and S.P. Kowalczyk. Phys. Rev. B 32 (1985) 1237. I.P. Batra, J. Phys. C 15 (1982) 5399. F. Werfel and 0. Briimmer, Phys. Ser. 28 (1983) 92. D.E. Ramaker, CT. White and J.S. Murday, Phys. Lett. A 89 (1982) 211. R.M. Matz, G.W. Rubloff and P.S. Ho, unpublished. S.W.M. Chung, J. Robinson, G.E. Thompson, G.C. Wood and H.S. Isaacs, Philos. Msg., in press. M.L. Knotek and P.J. Feibelman, Phys. Rev. Lett. 40 (1978) 964. E. Bertel, R. Stockbauer, R.L. Kurtz, D.E. Ramaker and T.E. Madey, Phys. Rev. B 26 (1985) 1885.
Surface Science 250 (1991) 139-146 North-Holland
Cr LW
Auger emission and photoreduction of hexavalent Cr oxides
A.G. Schrott a, G.S. Frankel a, A.J. Davenport b, H.S. Isaacs b, C.V. Jahnes a and M.A. Russak a e IBM Research Division, T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, WSh ’ Departmen! of Applied Science, Brookhaven National Laboratory,
Upton, NY 1 I973, USA
Received 25 September 1990; accepted for publication 18 January 1991
The Cr LW Auger emission has been used to investigate the local chemical environment of Cr(VI) in connection with the photoreduction produced by a non-monochromatic X-ray source. We compared Cr(VI) in oxides on anodized Al exposed to a chromate solution, with Cr(V1) in oxides on Al/Cr alloys polarized in a borate solution. The former were found to be much more sensitive to photoreduction than the latter. This is correlated with the intensity of the Cr LW Auger emission, attributed to a charge transfer transition in the final state of the photoemission process
I. IiWoduction
The presence of Cr(V1) has been previously observed in oxides formed on Al and Al/Cr alloys which had been anodized, and it has been suggested that hexavalent Cr plays a role in determining the resistance of these materials to pitting attack [l-3]. However, a detailed ~derst~ding of the phenomena has not been attained yet. In order to better understand the basic mechanisms for passivation, we have been investigating the properties of oxide films formed by polarization of Al/Cr alloys of various concentrations, using X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES). XANES is particularly well suited to the study of Cr(V1) due to the presence of a distinct pre-edge peak in the K edge absorption spectrum, related to the symmetry of the tetrahedral crystal field, which is absent for other valencies of Cr. XPS is a standard analytical tool for assessing the chemical state and composition of oxide films. However, due to the complexity of the photoemission process, the local chemical environment of the sample’s constituents is generally not unequivocally determined by the energy position of the XPS peaks. In addition, decomposition effects have 0039-6028/91/$03.50
been observed in the case of hexavalent Cr [4,5]. Therefore, special care must be taken to assess these effects in order to produce an accurate picture of the chemistry involved. While the XANES results will be presented in detail elsewhere, this work focuses on the Cr LW Auger emission which results from one of the competing relaxation mechanisms after phot~~ssion, and on the connection between this process and photoreduction of the oxides.
2. Experimental Al and Al/Cr alloys samples were prepared by sputter deposition onto quartz substrates. A composite target, consisting of Cr pins in a standard Al target, was used to deposit alloys with 4 to 46% Cr. The alloys were polarized in a solution of OSM boric acid +O.O5M sodium borate with pH 7.4 for 5 min at a fixed potential of 5 V versus a mercurous sulphate reference electrode (MSE). The Al films were also polarized at 5 V MSE for 5 min but were then exposed at open circuit to O.lM K,CrO, adjusted to pH 7. The samples were transferred through air to the analysis chamber within 5 min after polarization. The XPS spectra
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
140
A. G. Schrott et al. / Cr L VV Auger emission of hexavalent
Cr oxides
of these oxides were compared to those of various Cr oxide standards. Monochromatized Al Ka radiation was utilized for the analyses. Since the flux of the monochromatic beam is only about 3% of that from the non-monochromatized AlKa radiation, photodecomposition during setup is reduced. Furthermore, photodecomposition during data acquisition is also reduced because of the narrower interval of the photon energy distribution. Nevertheless, to further minimize photodecomposition, we opted to limit the counting time at the expense of a good signal-to-noise ratio. Due to sample charging, the peak energies were determined through a comparison with the Cls line at 284.6, which arises from adventitious carbon.
3. Results and discussion For the Al/Cr alloys polarized to a high potential, the presence of Cr(V1) in the oxides was clearly evident from both the pre-edge peak in the XANES spectra as well as a high binding energy peak in the Cr2p XPS spectra. The XANES data and a discussion of the kinetics of formation and dissolution of the oxides on Al/Cr films will be published elsewhere [6]. Hereafter, this work will focus on the connection between local chemistry, as inferred from the Cr LW emission, and photoreduction. Auger lines in the photoemission spectra result from the filling of the core hole created by the photoemission process. LW Auger transitions provide information of the local chemical environment, in contrast with valence band photoemission which probes both the local and the extended character of the electronic structure. In the case of Cr oxides, the Cr Auger emission has not been routinely analyzed because most of the lines overlap with the 0 Auger lines. However, the Cr LW line does not overlap with any other lines and is a revealing analytical tool. Before analyzing the LW Auger spectra, it is useful to describe the general features of the valence band photoemission for Cr and Cr-oxides. Fig. 1 shows valence band photoemission spectra corresponding to various Cr compounds. The spectra were aligned with that of metallic Cr using
ENERGY (eV RELATIVE
TO EF)
Fig. 1. X-ray photoemision spectra of the valence region for: Curve 1, in situ evaporated Cr; curve 2, Cr,03 powder; curve 3, CrO, scales; curve 4, Al-43% Cr polarized at 5 V in borate.
the energy position of the Cls line from hydrocarbon contamination. Curve 1, which corresponds to an in situ evaporated Cr film, shows a narrow peak of d character followed by a tail due mainly to a 4s-derived band, which is in agreement with previous results [7]. The spectrum corresponding to Cr,O, is shown in curve 2. It consists of a narrow peak near the Fermi energy which arises from the localized d band and a broader structure at higher binding-energies representing the 02p band [8]. Cr in CrO, has no d electrons so the corresponding valence band photoemission spectrum (curve 3) exhibits only a peak corresponding to the 0 2p band. The oxide formed by polarizing a Al-43% Cr alloy to 5 V generated curve 4. This sample contains a large amount of Cr(VI), as determined by XANES and by the Crap XPS line. In this case, because of the O-Al hybridization, the local contributions to the density of states are not straightforward. A more direct description of the changes in the local chemical environment of Cr as a result of the alloying with Al can be obtained from the Cr LW Auger spectra.
A.G. Schrott et al. / Cr LVV Auger emission of hexavalent
j
556
566
ENERGY kV
576
566
RELATIVE TO EF)
Fig. 2. Cr LW Auger transition for: Curve 1, in situ evaporated Cr; curve 2, Cr,Os powder; curve 3, Cr(OH), powder; curve 4; CrO, powder; curve 5, CrO, scales; curve 6, K,Cr,O, powder
Fig. 2 shows Cr LW Auger spectra for Cr metal (curve 1) and for various Cr-oxides. The spectra corresponding to the octahedrally coordinated Cr-oxides (Cr,O,, Cr(OH), and CrO,, curves 2 to 4 respectively) exhibit two excursions. The one at higher kinetic energy corresponds to transitions from the localized Cr3d band (intraatomic transition), while the lower kinetic energy excursion derives from the 02p bonding orbitals [8] as is suggested by fig. 1, and has extra-atomic origin [9,10]. A third distinct peak representing a final state with a hole in each band is not observed, perhaps because of the poor overlap between the corresponding orbitals [9]. The spectra corresponding to the Cr(V1) oxides (curves 5 and 6) in which Cr is tetrahedrally coordinated to 0, exhibit only one excursion at lower kinetic energies. Since the localized d band is nominally empty in the initial state of photoemission, intraatomic LW transitions are not possible. Thus, the
141
Cr oxides
LW Auger signal from the Cr(V1) compounds has only extra-atomic character (interatomic transition), and reflects the degree of overlap of the bonding 02p-derived orbitals as seen in the final state of photoemission (initial state for Auger) [9,10]. The lower kinetic energy of the Auger peaks in curves 5 and 6 compared to that of the excursion of extra-atomic character in curves 2 to 4 is contrary to what should be expected from the higher binding energy of the Cr2p level in the Cr(V1) oxides, and probably reflects a large value of the hole-hole interaction energy, U, a correlation term that decreases the potentially available Auger energy [9,11]. The area under the LW Auger peak for the tetrahedrally coordinated (Cr(VI)), normalized by the area under the Cr2p peak, is typically twice as large as for the octahedrally coordinated oxides. These ratios are listed in table 1. The higher intensity of the Auger emission for the Cr(V1) oxides as well as the lower kinetic energy (in contrast with the similar 02pderived bands appearing in photoemission) seem to indicate that the two holes in the final state are located in the same bonding orbital. From fig. 2, which shows relative changes in the two excursions of the Auger spectra, and from table 1, we deduce that the intensity of the interatomic Auger emission increases with increasing nominal valency. This correlates well with the decreasing Cr-0 distances [12] which in turn may induce a larger overlap in the initial state, and may also produce larger amount of charge transfer and thus
Table 1 LW Auger/CrZp intensity different configuration
ratios
Compound
AES/Cr
CraOs Cr(OH) 3
0.095 0.080 0.075 0.144 0.134 0.143 0.140 - 0.1 negligible
C&2 Cd3
K 2Cr0, KaCra% Ads. chromate Ads. chromate irrad. Pol. Al-Cr alloy
for various
2p
Cr-oxides
of
Configuration octahedral octahedral octahedral tetrahedral tetrahedral tetrahedral tetrahedral ? tetrahedral
‘) ‘)
‘) The coordination the Cr ion is inferred by the appearance the pre-edge peak in the XANES spectra.
of
142
A.G. Schrott
IO
-600
ENERGY
-590 kV
-580
RELATIVE
et al. / CrLVV
Auger
-570 TO
EF)
Fig. 3. Cr2p spectra for: curve 1, Cr,O, powder; curve 2, Cr(OH), powder; curve 3, CrO, powder; curve 4, CrO, scales; curve 5, K,Cr,O, powder; curve 6, Al-43% Cr polarized at 5 V in borate.
a higher degree of screening in the photoemission final state. The difference in final state screening also affects the intensity and position of the satellites in the Cr2p photoemission spectra. Fig. 3 shows XPS spectra corresponding to the Cr2p emission for various Cr oxides. Curves 1 to 3 correspond to the octahedrally coordinated Cr,O,, Cr(OH),, and CrO,, respectively, while curves 4 to 6 correspond and the oxide formed by to CrO,, K&t-,0,, polarizing a Al-43% Cr alloy to 5 V MSE, respectively. The peak energies for the reference compounds agree fairly well (within 0.3 eV) with previously reported values, [13] and references therein. However, we agree with other authors [14] on the intrinsic difficulties associated with assigning binding energies in oxides through the use of internal standards, and therefore we will focus our discussion on the appearance of satellites rather than on binding energy variations. As it has been pointed out before [13], the spectra from Cr(II1) compounds exhibit higher satellite intensity than
emission
ofhexavalent
Cr oxides
those from Cr(V1) compounds. Feature A in fig. 3 is a satellite associated with the 2p,,, peak. The satellite associated with the 2~,,~ overlaps with the Cr2p,,, peak making the relative intensity of the latter higher than what it is expected from the population of the J = : state. Among the octahedrally coordinated oxides of fig, 3, the intensity of the satellite is lower for CrO, (curve 3). According to results for Cu and Ni-halides [11,15], the intensity of the satellites has been related to a difference in hybridization between initial and final states in the photoemission process. For these compounds, the satellites has been ascribed to a nonscreened final state. Therefore, within this interpretation, the lower satellite intensity for CrO, indicates a higher average screening charge transfer from the ligand in the final state, and this is in agreement with the correspondingly higher interatomic Auger component in fig. 2. This seems to apply also to the Cr(V1) oxides, since the low satellite intensity in this case is consistent with the higher Auger emission shown in table 1. According to this argument, curve 6, which unlike curves 4 and 5 shows a shoulder (feature B) in the Cr2p,,, peak, may indicate a decreased screening in the final state of photoemission for the oxide formed by polarization of the alloy. Having described the main features of the spectra for the Cr(II1) and Cr(V1) compounds, it is now possible to gain insight on the distinct features of the oxides on the Al/Cr alloys. Fig. 4a shows the Cr2p photoemission spectra for Al which had been anodized in the borate solution and then subsequently exposed at open circuit to the chromate solution (curve l), and for the Al43% Cr polarized in the borate solution (curve 2). Aside from the shoulder in curve 1, which is produced by photoreduction as will be discussed later, the two spectra are similar in energy position and intensity to each other and to those of the Cr(V1) oxides shown in fig. 3. In contrast, the corresponding Cr LW Auger spectra shown in fig. 4b are very different. Curve 1, which corresponds to the same adsorbed sample as curve 1 in fig. 4a, exhibits lineshape and energy position similar (except for a small shoulder also due to photoreduction) to the Cr(V1) spectra in fig. 2. However, the polarized alloy exhibits insignificant LW Auger
143
A. G. Schrott et al. f Cr LVV Auger emission of hexavalent Cr oxides
emission (curve 2), even if we take into account the lower amount of Cr present in the alloy (see table 1). This difference results from the influence of alloying on the local electronic structure of Cr in the corresponding oxide. The low Auger intensity observed in the Cr(VI) oxides on the alloy may be related to the oxygenaluminum hybridization. XPS data for Al,O, [16,17] indicate that the 0 2p derived valence bands [18] are wider than those corresponding to the Cr oxides [8,19], in agreement with our spectra of fig. 1. In particular, fig. 1 curve d shows an overlap of the Cr d-Op bonding band with the Als,p-0 p nonbonding band. This would make partial transfer of the screening charge to 0 nonbonding orbitals likely to occur, thus reducing the electronic density at the Cr site and therefore the Auger transition rate. More importantly, a broader valence band may allow the potential energy U to be converted into the kinetic necessary for a onehole hop, reducing the lifetime of the two-hole final state 19,201. Thus, the change in screening charge suggested by the appearance of the satellite in the Cr2p spectrum corresponding to the oxide on the alloy may not be the only cause for the negligible Auger intensity. A gross estimate of the connection between the interatomic Auger intenI
,
I
,
I
,
t
,
sity and that of the XPS satellite, could be obtained from a comparison of curves 1 and 3 in fig. 3. A substantial reduction in the satellite intensity for CrO, is observed, compared to Cr,O,. Therefore, the screening charge transfer should increase and the same should happen to the extra-atomic portion of the Auger peak for CrO,. Table 1 indicates that for CrO, the total LW Auger intensity decreases by only - 20% compared to Cr,O,, while it should have decreased by - 33% due to the disappearance of one electron from the localized d band. Therefore the change in satellite intensity shown in fig. 3 for the octahedrally coordinated oxides seems to be related to about a 10% increase in the intensity of the interatomic transition. Since feature B in fig. 3, curve 6 seems of the same order of magnitude as the changes in feature A, it may not fully account for the loss of Auger intensity shown in fig. 4b for the polarized alloy. The large breadth of the valence band, on the other hand, may drastically decrease the lifetime of the Auger final state, and may lead to a very broad Auger lineshape [9], which could be difficult to detect. The striking differences in the LW Auger emission may be used to understand the effects of X-ray irradiation on these Cr(V1) oxides, which will be presently discussed.
-7
a
p&razed
alloy
.r ,..,
-.
..
_,
.:.
-600 ENERGY (eV RELATIVE
546 TO EF)
556
566
576
586
ENERGY (eV RELATIVE TO E, 1
Fig. 4. (a) Cr2p spectra for: Curve 1, adsorbed chromate on anodized Al (the arrows denote the presence of Cr3+ due to fast photoreduction); curve 2, Al-43% Cr polarized at 5 V in borate. (b) Cr LW Auger spectra for: Curve 1, adsorbed chromate on anodii AI; curve 2, Al-43% Cr polarized at 5 V in borate. These spectra were taken with the mon~hromatic source.
144
A.G. Schrott et al. / Cr LVVAuger
The hexavalent oxides formed on the Al/Cr alloys by polarization in borate solution did not suffer from photodecom~sition during the XANES experiments or the XPS measurements using monochromatic radiation. In contrast, a minor degree of reduction was observed in the XPS spectra corresponding to the adsorbed chromate films (see curves 1 in figs. 4a and 4b). Due to its higher photon flux, we used a standard AlKa source to purposely produce radiation damage, and compared the photoreduction that occurred upon a fixed irradiation dose on the oxide films formed on the Al/Cr alloys films with that of the adsorbed chromate films on anodized Al. Spectra taken after various exposures indicated that the reduction of Cr(V1) for the Al samples polarized in borate and then exposed to chromate occurred after few minutes of exposure to the standard XPS source. In contrast, for the oxide on the alloy the reduction of Cr(VI) occurred at a much slower rate. A comparison of the photore-
a
I
’
I
’
I,’
emission of hexavalent Cr oxides
duction produced by exposing the samples to the standard X-ray source is shown in figs. 5a and 5b. Curves such as 1 can be obtained after only IO min of irradiation, and remain constant thereafter. It appears that all the Cr(V1) from the Al sample that had been immersed in chromate following polarization in borate had reduced and converted mainly to to Cr(III), as will be discussed in the next paragraph. The spectra corresponding to the irradiated oxide formed on Al/Cr alloy films indicated a higher resistance to photoreduction. Curve 2 in fig. 5a, was obtained after 3 h of irradiation, when further evolution of the spectra with exposure to X-ray i~adiation was no longer detected. This curve indicates a mixture of Cr(V1) and Cr(III), with more Cr(V1) than Cr(II1). Curve 3 is a peak synthesis of curve 2. The fit was made using peaks with energies and relative intensities taken from curve 2 in fig. 4a for 6 + , and from curve 1 in fig. 5a for the 3 + . Curve 2 was obtained at zero take-off angle. Spectra taken at
I
after 3 hrs
-610
-600
ENERGY CeV RELATIVE TO EF)
546
556
576
ENERGY (eV RELATIVE TO E,)
Fig. 5. The samples in fig. 4 after AlKa radiation. (a) Cr2p XPS spectra: curve 1, the adsorbed chromate after 10 mm and further exposure; curve 2, the polarized alloy after 3 h or more; curve 3, peak synthesis of 2, using peak positions for 6 + as in fig. 4a, 1, and for 3 + as in curve 1 above; (b) LW Auger: curve 1, the adsorbed chromate after 10 min and further exposure (arrows indicate the position of the Auger peak for chromate, and for the inter and intra atomic features for Cr,Os, respectively from fig. 2); curve 2, the polarized alloy after 3 h or more. Since the resolution for Auger is independent of the line-width of the excitation source, these nonevolving Auger spectra were taken with the nonmon~~omatic source, to improve the count rate.
A.G. Schrott et al. / Cr L W Auger emission of hexadent
higher take-off angle with respect to the sample’s normal indicated that the reduction occurred at the surface. Fig. 5b shows the Auger emission from the samples following the irradiation with non-monochromatized X-rays described above. Curve 1 arises from the reduced form of Cr for the adsorbed chromate. We believe the reduced form to be Cr(II1) because of (a) the similarity of the Auger lineshape to that of Cr,O, (curve 2 in fig. 2) which has an interatomic feature lower in intensity than the intra-atomic one, and (b) The similarity of the LW/2p intensity ratios (- 0.1) shown in table 1. This identification is made despite the fact that the Auger energies are offset from the Cr,O, standard (see arrows in fig. 5b, curve 1 for a comparison), and that the binding energy of the corresponding Cr 2p,,, photoemission peak in fig. 5a, curve 1, falls at - 0.8 eV higher than that of the Cr,O, standard in fig. 3. These differences could be explained by a lower extra-atomic relaxation expected from the presence of defects and from the different coordination of the Cr(II1) in the surface oxide. However, the presence of some Cr(IV) or Cr(I1) cannot be ruled out completely. Cr4+ ions (not in a CrO, phase [12]) could also lead to the binding energy observed in fig. 5a, curve 1. On the other hand, due to the lower amount of available 3d electrons, the Cr(IV) contribution to the intra-atomic portion of the Auger lineshape is expected to be lower than the interatomic one, as in CrO, (curve 4 in fig. 2), and this is not reflected in the Auger spectrum of the reduced adsorbed sample (fig. 5b, curve 1). By extrapolation, we could expect Cr(I1) to generate a LW Auger spectrum more consistent with that of curve 1 in fig. 5b. We have not measured yet the Auger emission of any Cr(I1) standard. However, the binding energy of Cr,O, thin films (identified by TEM) was found to be only 0.4 eV higher than that of Cr,O,, using the signal from the underlying Cr metal as reference [21]. Therefore, from the data in fig. 5 and the above discussion, we believe that Cr(II1) is the major component of the reduced form of Cr in our films. Since the Cr(V1) in the polarized alloy produced negligible LW Auger emission prior to irradiation (fig. 4b, curve 2), curve 2 in fig. 5b
Cr oxides
145
results only from the Cr that has been reduced to the 3 + oxidation state. This spectrum however, is different from that of the adsorbed sample (curve 1) which also results from Cr reduced to the 3 + oxidation state. The intra-atomic peak at higher kinetic energy is similar but there is only a shoulder in the low energy portion of the spectrum which has extra-atomic character. The absence of a strong peak indicates that the interatomic Auger transition for Cr(III) in the oxide on the polarized alloy sample is inhibited similarly to Cr(V1) (curve 2 in fig. 4b). Thus, regardless of the oxidation state of Cr in the oxide on the alloy, the screening charge transfer from the ligand is reduced due to the presence of Al. For completeness, we also examined the photoreduction of oxide films formed by anodization of Al in the chromate solution, and found a behavior similar to that of the Cr(V1) in oxides on the alloy. This similarity is consistent with the chromate ions being incorporated into the thickening alumina film [nl. as opposed to the chromate film adsorbed on the anodized Al. Judging from the XPS and XANES spectra only, the different response of these Cr(V1) oxides to irradiation would have been difficult to explain. Because it is a two-electron process, the LW Auger transition is very sensitive to changes in the final state, and provides a qualitative information on the correlation between chars transfer from the ligand and photoreduction. The electron or photon-stimulated desorption of O+ ions from the surfaces of maximum valency ionic systems, above a threshold incident energy, has been explained by the Knotek-Feibelman model to proceed by Auger decay following a core-level ionization (231. After the Auger process has occurred, two or more holes can reside on an anion, which can be desorbed due to the Madelung potential. More recent results seem to indicate that maximum valency is not necessarily required [24]. This mechanism may also apply to covalent systems when the Coulomb correlation energy U is greater than the width of the band generated by the bonding orbitals (20). Our results of figs. 4 and 3 indicate that there is a direct connection betthe sensitivity to photoreduction of Cr(VI) and the intensity of the Cr LW
146
A.G. Schrott et al. / Cr L VV Auger emission of hexaualent
Auger emission. The 0 p-Al s,p hybridization seems to reduce the effectiveness of the Auger decay as a mechanism for the desorption of 0 ions, either because that channel is closed due to a lower Cr-0 overlap, or because the two-hole state is short-lived. In the latter case, the inhibition may occur because the local inversion of the charge in the anion site should last an interval comparable to a period for nuclear motion for the ion to be expelled [20].
4. Conclusions The intensity of the Cr LW Auger transition in Cr(V1) oxides has been shown to be connected to the tendency for surface photoreduction, which may be qualitatively explained by the KnotekFeibelman model. It has also been proposed, based on valence band photoemission, that O-Al rehybridization may lead to a decreased charge transfer and hole-hole interaction which may inhibit the role of the Auger decay as a channel for photoreduction of Cr(V1) in the oxides formed by polarization of the Al-Cr alloys. Finally, the Cr LW Auger lineshape constitutes a useful fingerprint for chemical identification, considering that for the Cr-oxides there is not a direct correlation between valency and binding energy [13], which is very sensitive in these oxides to final state relaxation effects.
References 111W.C. Moshier, G.D. Davis, J.S. Ahearn and H.F. Hough, J. Electrochem.
Sot. 134 (1987) 2677. G.D. Davis and G.O. Cote, J. Electrothem. Sot. 136 (1989) 356. H.S. Isaacs, S.M. Heald, J. Tranquada. [31 J.K. Hawkins, G.E. Thompson and G.C. Wood, Corrosion Sci. 27 (1987) 391. [41 B.A. DeAngelis, J. Electron Spectrosc. Relat. Phenom. 9 (1976) 81. [51 G.P. Halada, C.R. Clayton and D.H. Lindsley, Mater. Sci. Eng. A 103 (1988) L5, and references therein. H.S. Isaacs, A.G. Schrott. [61 G.S. Frankel, A.J. Davenport, C.V. Jahnes and M.A. Russak, to be published. S.P. Kowalczyk, F.R. McFeely 171 L. Ley. O.B. Dabbousi, and D.A. Shirley, Phys. Rev. B 16 (1977) 5372. H.J. Guggenheim and S. Hiifner, Phys. PI G.K. Wertheim, Rev. Lett. 21 (1973) 1050. in: Chemistry and Physics of Solid [91 D.E. Ramaker, Surfaces, Vol. 4, Eds. R. Vanselow and R. Howe (Springer. Berlin, 1982) p. 19. DOI J.A. Tossell, J. Electron Spectrosc. Relat. Phenom. 8 (1976) 1.
121W.C. Moshier,
[Ill [=I P31
u41 I151 WI [I71
Acknowledgements The authors wish to thank P. Avouris, D.M. Newns, and J.D. Tersoff for useful discussions. The XANES experiments were carried out at the X-11 beam line of the National Synchrotron Light Source at Brookhaven National Laboratory and partially supported by the US Department of Energy under Contract Nos. DE-AC0276CH00016 and DE-AS0580-ER10742.
Cr oxldes
WV v91 PO1 WI PI 1231 [24]
G. van der Laan, C. Westra, C. Haas and G.A. Sawatzky, Phys. Rev. B 23 (1981) 4369. R.W.G. Wyckoff, in: Crystal Structures, Vols. 1 and 2 (Wiley, New York 1964) pp. 251, 7, 91. I. Ikemoto, K. Ishii, S. Kinoshita, H. Kuroda, M.A. Alario Franc0 and J.M. Thomas, J. Solid State Chem. 17 (1976) 425. D.A. Stephenson and N.J. Binkowski. J. Non-Cryst. Solids 22 (1976) 399. J. Zaanen, C. Westra and G.A. Sawatzky, Phys. Rev. B 33 (1986) 8060. A. Balzarotti and A. Bianconi, Phys. Status Solidi 76 (1976) 689. W.J. Gignac, R.S. Williams and S.P. Kowalczyk. Phys. Rev. B 32 (1985) 1237. I.P. Batra, J. Phys. C 15 (1982) 5399. F. Werfel and 0. Briimmer, Phys. Ser. 28 (1983) 92. D.E. Ramaker, CT. White and J.S. Murday, Phys. Lett. A 89 (1982) 211. R.M. Matz, G.W. Rubloff and P.S. Ho, unpublished. S.W.M. Chung, J. Robinson, G.E. Thompson, G.C. Wood and H.S. Isaacs, Philos. Msg., in press. M.L. Knotek and P.J. Feibelman, Phys. Rev. Lett. 40 (1978) 964. E. Bertel, R. Stockbauer, R.L. Kurtz, D.E. Ramaker and T.E. Madey, Phys. Rev. B 26 (1985) 1885.