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Core-level Auger energy shifts in palladium alloys
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kurface science
Surface Science 287/288 (1993) 794-797 North-Holland
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Core-level Auger energy shifts in palladium alloys G.G. Kleiman, Institute
R. Landers,
de Fisica, Universidade
S.G.C. de Castro
Estadual de Campinas,
13081 Campinas,
SP, Brazil
and P.A.P. Nascente Departamento
de Engenharia
de Materiais,
Universidade
Federal de Go
Carlos. 13560 ScTo Carlos, SP, Brazil
Received 4 September 1992; accepted for publication 19 November 1992
We have measured PdL,M,,sM,,s Auger shifts in Pd-Cu and Pd-Ag alloys and compared them with the corresponding MsVV shifts. The same valence band effects which distort the PdMW lineshape are reflected in deviations of the shifts of the corresponding tG, peak from the quasi-atomic limit. These deviations are manifested in comparisons with shifts of PdL,M,,,M,,, core level spectra, which are well-defined and are naturally invariant in form. Therefore, in alloy studies true core-level Auger energy shifts should be employed. The relation of these shifts to electronic structure determinations is discussed.
The necessity of extracting complementary electronic structure information from XPS corelevel binding energy shifts has led to the suggestion that Auger kinetic energy shifts be considered [1,2-61. This suggestion arises from the fact that, to a good approximation, the shift of the kinetic energy (relative to the Fermi energy) of the iik Auger core-level transition of atom A, can be put in the form of a core-level binding energy shift [2,3,7]. In this way the core-level Auger energy shift could provide additional, independent information to help in removing the ambiguities in analyzing XPS shifts. The most commonly measured spectra in the transition metals correspond to valence level (i.e., iW) Auger transitions. For metals with full initial state d-bands and for noble metals, there is convincing evidence that the iW spectra are quasi-atomic in form for the 3d [8], the 4d [9,10] and the 5d [11,12] series. The evidence for the iVV spectra of metals with more than one ground state d-band hole is more indirect. In the case of the PdM,,N,,N,, 3 , % 0039-6028/93/$06.00
spectra, for example, lineshape analyses and theoretical calculations [10,13] indicate that the final-state holes responsible for the ‘G, multiplets may be considered as core holes [10,13-171. The interpretation of Pd final-state d-band holes as core-like has been extended to Pd-based alloys. Model calculations applied to dilute Pd alloys seem to indicate that the observed Auger spectra of both Pd-Cu 1181 and Pd-Ag [19,20] can be explained as quasi-atomic in the sense of the theories of Cini [211 and Sawatzky [22]. It is well-known, however, that the forms of the experimental PdMW Auger spectra depend on the valence band density of states (DOS). It is interesting, therefore, to ask to what extent the measured PdMW Auger shifts can be interpreted as arising from core-like final-state d-band holes and to what extent they furnish useful information complementary to XPS shifts. In this paper, we argue that the observed spectral sensitivity to the DOS may introduce a direct effect of the DOS on the measured shift, vitiating its utility as a binding energy shift [23].
0 1993 - Elsevier Science Publishers B.V. All rights reserved
G. G. Kleiman et al. / Core-level Auger energy shifts in palladium albys
In order to verify this argument we present measurements of PdLsM,,,M,, shifts in Pd-Cu alloys and compare them with the corresponding MsW shifts. Explanations of the dilute Pd iW Auger spectra in Pd-Cu [18] and Pd-Ag [19,20], which assume that the local Pd d-band is full [21,22], relate them to the Cini model [21] of the local two-hole spectral function, D(E), for the two-hole binding energy E = Bi - K, (where Bi denotes the initial state XPS binding energy and Ki, the Auger kinetic energy) as in the equation D(E)
I/,I,w12 + wmJ(E)12)~
=D,(E)/(
Eqs. (1) constitute the generic solution of a number of models [24] involving impurity states derived from those of an unperturbed band through a localized potential [23]. The analogy is exact if we interpret D(E) as the one electron DOS at the impurity site and Z as the corresponding Green function component. The physical consequences of eqs. (1) have been discussed generally [24], as well as in the context of Auger spectroscopy [25]. In particular, it is clear that band effects on the energies of the maxima in D(E) are not negligible, in general. Since, however, measurements of Auger energies of spectra which may display this effect have been reported [26], it would be well to discuss this point briefly. In this model, a true bound state exists at energy E, if its energy does not overlap the continuum of band states so that D,(E,) = 0 and E, is determined from the condition Z&E,) l/V, = 0. In the region of the bound state, D(E) = NB S(E - E,J, where Nn = I VzZd(E,) I -’ is the bound state intensity (i.e., Nn IN) and Zd= dZ,,/d E. For large I V, I, E, -C+VoN, and Nn=N, where C is the band centroid and N is the number of states in the band, so that, for deeply
[l -
(14
where the quantity V, denotes the hole-hole Coulomb repulsion and Z is the real part of the Green function. Quantities with subscript zero denote non-interacting holes (V, = 0) so that D, is the self-convolution of the valence band local DOS in the alloy and is related to I, by the dispersion relation Z,(E) =Z’lm dtD,(t)/(E-t), -cc
(lb)
where P denotes the Cauchy principal value.
2447.0
2457.0
795
2467.0
Energy
2477.0
2487.0
(eV)
Fig. 1. Pd L3M4,,M4,, X-ray excited Auger spectrum in Ag0.5Pd0., and pure Pd. The features at energies at and above that of the main peak are attributable to jj-intermediate coupling transitions [28]. The loss structure below the main peak is not observable in In, Sn and Sb [28].
796
G.G. Heirnan et al. / Core-level Auger energy shifts in palladium
bound states, the energy and lineshape are both insensitive to the details of the DOS 1251. This conclusion corresponds to the condition 5 B 2W for quasi-atomic Auger spectra [8,211, where .$ is the Auger parameter, and 2W the bandwidth. In this case, the spectrum is atomic in nature and C corresponds to the binding energy of two uncorrelated holes so that I/,N is the Auger parameter [21-231. As / V, I decreases, the bound state becomes more shallow until it finally becomes resonant [23]. ~n~mitantly, Nat the number of states contained in the bound state decreases [231. From the definition of Nn, this result is just a reflection of the deviation of 1, from the quasi-atomic form, N/(E - C) [23]. This deviation produces, at the same time, a difference between E, and the energy of a quasi-atomic bound state, C + YON. In other words, when Nn deviates from the atomic limit, one expects E, to do the same. Since Na is multiplied by atomic matrix elements in calculating spectra to compare with experiment, it is possible to have spectra which are not quasi-atomic even though the state is bound. The resulting deviations in E, might affect Auger energy shifts in alloys.
alloys
The alloy samples studied were prepared according to standard metallurgical techniques reported elsewhere 1271, and the experimental details are described in other work [23,281. The samples were cleaned by argon-ion sputtering, with subsequent heating for a few minutes at temperatures between 500 and 800°C to remove sputter damage. Contamination was monitored before and after analysis (the L3M4,sM4,s measurements took typically from 12 to 18 hours to achieve adequate signal-to-noise ratios) through the carbon 1s and oxygen 1s lines. In figs. 1 and 2, we display, respectiveiy, the L,M,,,M,,, Auger spectra of Pd and Ag in a alloy and in the corresponding pure Ag0.5Pd0.5 metal. Within experimental error, the lineshapes are invariant in each figure. In addition, theoretical lineshapes calculated within the intermediatecoupling scheme 1281manifest as good agreement as in the case of In, Sn and Sb 1281.In contrast to those metals, the ones treated in figs. 1 and 2 present loss structure below the main, ‘G4, peak. This loss structure tends to move closer to the main peak as the atomic number is decreased, and appears to agree with the notion that spectator holes participate in the emission process
2580
2590
2600
Energy (eV) Fig. 2. AgL3M,,,M4., X-ray excited Auger spectrum in AgO,,PdO,S and pure Ag. The higher energy structure corresponds to jj-intermediate coupling transitions [ZS]. The lower energy structure appears to arise from losses not observable in In. Sn and Sb
1281.
G.G. Kleiman et al. / Core-level Auger energy shifts in palladium alloys
0.5
-!@
I
$
Kleiman, Phys. Rev. B 43 (1991) 4659.
MVV
l21 G.G. Kleiman, R. Landers, S.G.C. de Castro and J.D.
0.0
E
PdiCu-Pd)
t z
0
0
-05
+
q
G 5
-1.0
-
References 111P.A.P. Nascente, S.G.C. de Castro, R. Landers and G.G.
+LMM
$ AqfAg-Pd)
5
ERROR
797
+
-+
I .5
0
q
,
,
•t +LMM 0 MVV
0, 0.0
+
, 0.2
0.4
,
,
,
0.6
ATOM I C CO~ENTRAT
,
0.8
,
I .o
I ON
Fig. 3. Measured MVV and LsM,,sM,,s Auger energy shifts for Pd in Pd-Cu and Ag in Pd-Ag. Band effects produce the illustrated difference in shifts for Pd. The quasi-atomic nature of Ag is manifested by the similarity in shifts.
[13,17]. Elsewhere, we discuss this loss structure and show that, by studying its experimental systematics in the metals from MO to Ag, we can clarify the extent to which the spectator hole interpretation is valid [29]. In fig. 3, we present measured MW and L,M,,M,, Auger energy shifts of Pd in Pd-Cu and of Ag in Ag-Pd. Were the PdMW shifts representative of core-levels, they would equal the core-level LMM shifts, which is clearly not the case here, given our experimental error. For true quasi-atomic MW spectra, those of Ag in Pd-Ag, the MVV and LMM shifts coincide, within experimental error, as expected in this case. In conclusion, we have argued that valence band Auger spectra in alloys, although of quasiatomic origin, can present Auger energy shifts which are not of quasi-atomic nature, because of the direct influence of the valence band DOS. We have compared measured L3M4,5M4,5 and MW Auger shifts of Pd in Pd-Cu alloys. It would appear that, in general, measurement of the required shifts should be made through the LMM core spectra, which are well-defined. We would like to thank R.C.G. Vinhas and R.F. Suarez for technical assistance and CNPq, FAPESP and FINEP of Brasil for partial support.
Rogers, Phys. Rev. B 44 (1991) 8529. [31 G.G. Kleiman, Appl. Surf. Sci. 11/12 (1982) 730. [41 G.G. Kleiman, V.S. Sundaram and J.D. Rogers, Solid State Commun. 39 (1981) 1171. f51 G.G. Kleinran, VS. Sundaram and J.D. Rogers, J. Vat. Sci. Technol. 18 (1981) 585. t61 T.D. Thomas and P. Weightman, Phys. Rev. B 33 (1986) 5406. [71 N.D. Lang and A.R. Williams, Phys. Rev. B 20 (1979) 1369. 181E. Antonides, E.C. Janse and G.A. Sawatzky, Phys Rev. B 15 (1977) 1669. 191A.C. Parry-Jones, P. Weightman and P.T. Andrews, J. Phys. C 12 (1979) 1587. [lOI N. Mirtensson, R. Nyholm and B. Johansson, Phys. Rev. Lett. 45 (1980) 7.54. El11J.F. McGilp, P. Weightman and E.J. McGuire, J. Phys. C 10 (1977) 3445. 1121R. Nyholm, K. Helenelund, B. Jobansson and S. HGrnstrijm, Phys. Rev. B 34 (1986) 67.5. [131 N. Mirtensson and B. Johansson, Phys. Rev. Lett. 45 (1980) 482. 041 G.G. Kleiman, R. Landers, S.G.C. de Castro and P.A.P. Nascente, Phys. Rev. B44 (1991) 3383. I151 G.G. Kleiman, R. Landers, P.A.P. Nascente and S.G.C. de Castro, Phys. Rev. B 46 (1991) 4405. l161G.G. Kleiman, R. Landers, S.G.C. de Castro and P.A.P. Nascente, J. Vat. Sci. Technol. A 10 (1992) 2839. t171 N. Mgrtensson, P. Hedegird and B. Johansson, Phys. Ser. 29 (1984) 154. [181 P. Weightman, H. Wright, SD. Waddington, D. van der Marel, G.A. Sawatzky, G.P. Diakun and D. Norman, Phys. Rev. B 36 (1987) 9098. U91 M. Vos, GA. Sawatzky, M. Davies, P. Weightman and P.T. Andrews, Solid State Commun. 54 (1984) 159. [201 P. Hedegird and B. Johansson, Phys. Rev. Lett. 52 (1984) 2168; Phys. Rev. B 31 (1985) 7749. [211 M. Cini, Solid State Commun. 20 (1986) 605; 24 (1977) 681; Phys. Rev. B 17 (1978) 2728. [22] G.A. Sawatzky, Phys. Rev. Lett. 39 (1977) 504. [23] G.G. Kleiman, R. Landers, S.G.C. de Castro and P.A.P. Nascente, Phys. Rev. B 45 (1992) 13899. [24] Yu.A. Izyumov, Adv. Phys. 14 (1965) 569. [25] G.A. Sawatzky and A. Lenselink, Phys. Rev. B 21 (1980) 1790. 1261N. M~tens~n, R. NyhoIm, H. Cal&, J. Hedman and B. Johansson, Phys. Rev. B 24 (1981) 1725. 1271 V.S. Sundaram, M.B. de Moraes, J.D. Rogers and G.G. Kieiman, J. Phys. F 11 (1981) 1151. [281 G.G. Kleiman, R. Landers, P.A.P. Nascente and S.G.C. Castro, Phys. Rev. B 46 (1992) 1970. [29] G.G. Kleiman, S.G.C. Castro and R. Landers, unpublished.
::
~:“i’::~~~:~::::~.:.: ..c...:.:.:
:.:.:,,.,., ,,
:‘:~::~
. . . . . .:.:.: .,..:.:. ~:.:.: :.:.:.:.:.:::
:.:.::.::
kurface science
Surface Science 287/288 (1993) 794-797 North-Holland
..;.::;: ._.....):.: ::::::~~,~.~::j i~.-.:%~~~~ “”‘.‘..v. ..._,,,,, ..,,.,.,., ..........:.:.:.: .... .‘....,‘......:.:.:~:~,~, ,,,,,_ ,,,,,/,,,, ‘ii’:‘ii..,...:....................: :i,:,.,.;_ “..‘.i.‘.:.“....,...:......: (,,_,,,, ,,, “““...‘(........... .........“.....,.....:.,.,,, ................,,.,.,~.,,,,,,,‘~ ... ,,., :,: ,:,:.:,,:,:,,, “!‘.‘~~.:.:.:....: (......i......
Core-level Auger energy shifts in palladium alloys G.G. Kleiman, Institute
R. Landers,
de Fisica, Universidade
S.G.C. de Castro
Estadual de Campinas,
13081 Campinas,
SP, Brazil
and P.A.P. Nascente Departamento
de Engenharia
de Materiais,
Universidade
Federal de Go
Carlos. 13560 ScTo Carlos, SP, Brazil
Received 4 September 1992; accepted for publication 19 November 1992
We have measured PdL,M,,sM,,s Auger shifts in Pd-Cu and Pd-Ag alloys and compared them with the corresponding MsVV shifts. The same valence band effects which distort the PdMW lineshape are reflected in deviations of the shifts of the corresponding tG, peak from the quasi-atomic limit. These deviations are manifested in comparisons with shifts of PdL,M,,,M,,, core level spectra, which are well-defined and are naturally invariant in form. Therefore, in alloy studies true core-level Auger energy shifts should be employed. The relation of these shifts to electronic structure determinations is discussed.
The necessity of extracting complementary electronic structure information from XPS corelevel binding energy shifts has led to the suggestion that Auger kinetic energy shifts be considered [1,2-61. This suggestion arises from the fact that, to a good approximation, the shift of the kinetic energy (relative to the Fermi energy) of the iik Auger core-level transition of atom A, can be put in the form of a core-level binding energy shift [2,3,7]. In this way the core-level Auger energy shift could provide additional, independent information to help in removing the ambiguities in analyzing XPS shifts. The most commonly measured spectra in the transition metals correspond to valence level (i.e., iW) Auger transitions. For metals with full initial state d-bands and for noble metals, there is convincing evidence that the iW spectra are quasi-atomic in form for the 3d [8], the 4d [9,10] and the 5d [11,12] series. The evidence for the iVV spectra of metals with more than one ground state d-band hole is more indirect. In the case of the PdM,,N,,N,, 3 , % 0039-6028/93/$06.00
spectra, for example, lineshape analyses and theoretical calculations [10,13] indicate that the final-state holes responsible for the ‘G, multiplets may be considered as core holes [10,13-171. The interpretation of Pd final-state d-band holes as core-like has been extended to Pd-based alloys. Model calculations applied to dilute Pd alloys seem to indicate that the observed Auger spectra of both Pd-Cu 1181 and Pd-Ag [19,20] can be explained as quasi-atomic in the sense of the theories of Cini [211 and Sawatzky [22]. It is well-known, however, that the forms of the experimental PdMW Auger spectra depend on the valence band density of states (DOS). It is interesting, therefore, to ask to what extent the measured PdMW Auger shifts can be interpreted as arising from core-like final-state d-band holes and to what extent they furnish useful information complementary to XPS shifts. In this paper, we argue that the observed spectral sensitivity to the DOS may introduce a direct effect of the DOS on the measured shift, vitiating its utility as a binding energy shift [23].
0 1993 - Elsevier Science Publishers B.V. All rights reserved
G. G. Kleiman et al. / Core-level Auger energy shifts in palladium albys
In order to verify this argument we present measurements of PdLsM,,,M,, shifts in Pd-Cu alloys and compare them with the corresponding MsW shifts. Explanations of the dilute Pd iW Auger spectra in Pd-Cu [18] and Pd-Ag [19,20], which assume that the local Pd d-band is full [21,22], relate them to the Cini model [21] of the local two-hole spectral function, D(E), for the two-hole binding energy E = Bi - K, (where Bi denotes the initial state XPS binding energy and Ki, the Auger kinetic energy) as in the equation D(E)
I/,I,w12 + wmJ(E)12)~
=D,(E)/(
Eqs. (1) constitute the generic solution of a number of models [24] involving impurity states derived from those of an unperturbed band through a localized potential [23]. The analogy is exact if we interpret D(E) as the one electron DOS at the impurity site and Z as the corresponding Green function component. The physical consequences of eqs. (1) have been discussed generally [24], as well as in the context of Auger spectroscopy [25]. In particular, it is clear that band effects on the energies of the maxima in D(E) are not negligible, in general. Since, however, measurements of Auger energies of spectra which may display this effect have been reported [26], it would be well to discuss this point briefly. In this model, a true bound state exists at energy E, if its energy does not overlap the continuum of band states so that D,(E,) = 0 and E, is determined from the condition Z&E,) l/V, = 0. In the region of the bound state, D(E) = NB S(E - E,J, where Nn = I VzZd(E,) I -’ is the bound state intensity (i.e., Nn IN) and Zd= dZ,,/d E. For large I V, I, E, -C+VoN, and Nn=N, where C is the band centroid and N is the number of states in the band, so that, for deeply
[l -
(14
where the quantity V, denotes the hole-hole Coulomb repulsion and Z is the real part of the Green function. Quantities with subscript zero denote non-interacting holes (V, = 0) so that D, is the self-convolution of the valence band local DOS in the alloy and is related to I, by the dispersion relation Z,(E) =Z’lm dtD,(t)/(E-t), -cc
(lb)
where P denotes the Cauchy principal value.
2447.0
2457.0
795
2467.0
Energy
2477.0
2487.0
(eV)
Fig. 1. Pd L3M4,,M4,, X-ray excited Auger spectrum in Ag0.5Pd0., and pure Pd. The features at energies at and above that of the main peak are attributable to jj-intermediate coupling transitions [28]. The loss structure below the main peak is not observable in In, Sn and Sb [28].
796
G.G. Heirnan et al. / Core-level Auger energy shifts in palladium
bound states, the energy and lineshape are both insensitive to the details of the DOS 1251. This conclusion corresponds to the condition 5 B 2W for quasi-atomic Auger spectra [8,211, where .$ is the Auger parameter, and 2W the bandwidth. In this case, the spectrum is atomic in nature and C corresponds to the binding energy of two uncorrelated holes so that I/,N is the Auger parameter [21-231. As / V, I decreases, the bound state becomes more shallow until it finally becomes resonant [23]. ~n~mitantly, Nat the number of states contained in the bound state decreases [231. From the definition of Nn, this result is just a reflection of the deviation of 1, from the quasi-atomic form, N/(E - C) [23]. This deviation produces, at the same time, a difference between E, and the energy of a quasi-atomic bound state, C + YON. In other words, when Nn deviates from the atomic limit, one expects E, to do the same. Since Na is multiplied by atomic matrix elements in calculating spectra to compare with experiment, it is possible to have spectra which are not quasi-atomic even though the state is bound. The resulting deviations in E, might affect Auger energy shifts in alloys.
alloys
The alloy samples studied were prepared according to standard metallurgical techniques reported elsewhere 1271, and the experimental details are described in other work [23,281. The samples were cleaned by argon-ion sputtering, with subsequent heating for a few minutes at temperatures between 500 and 800°C to remove sputter damage. Contamination was monitored before and after analysis (the L3M4,sM4,s measurements took typically from 12 to 18 hours to achieve adequate signal-to-noise ratios) through the carbon 1s and oxygen 1s lines. In figs. 1 and 2, we display, respectiveiy, the L,M,,,M,,, Auger spectra of Pd and Ag in a alloy and in the corresponding pure Ag0.5Pd0.5 metal. Within experimental error, the lineshapes are invariant in each figure. In addition, theoretical lineshapes calculated within the intermediatecoupling scheme 1281manifest as good agreement as in the case of In, Sn and Sb 1281.In contrast to those metals, the ones treated in figs. 1 and 2 present loss structure below the main, ‘G4, peak. This loss structure tends to move closer to the main peak as the atomic number is decreased, and appears to agree with the notion that spectator holes participate in the emission process
2580
2590
2600
Energy (eV) Fig. 2. AgL3M,,,M4., X-ray excited Auger spectrum in AgO,,PdO,S and pure Ag. The higher energy structure corresponds to jj-intermediate coupling transitions [ZS]. The lower energy structure appears to arise from losses not observable in In. Sn and Sb
1281.
G.G. Kleiman et al. / Core-level Auger energy shifts in palladium alloys
0.5
-!@
I
$
Kleiman, Phys. Rev. B 43 (1991) 4659.
MVV
l21 G.G. Kleiman, R. Landers, S.G.C. de Castro and J.D.
0.0
E
PdiCu-Pd)
t z
0
0
-05
+
q
G 5
-1.0
-
References 111P.A.P. Nascente, S.G.C. de Castro, R. Landers and G.G.
+LMM
$ AqfAg-Pd)
5
ERROR
797
+
-+
I .5
0
q
,
,
•t +LMM 0 MVV
0, 0.0
+
, 0.2
0.4
,
,
,
0.6
ATOM I C CO~ENTRAT
,
0.8
,
I .o
I ON
Fig. 3. Measured MVV and LsM,,sM,,s Auger energy shifts for Pd in Pd-Cu and Ag in Pd-Ag. Band effects produce the illustrated difference in shifts for Pd. The quasi-atomic nature of Ag is manifested by the similarity in shifts.
[13,17]. Elsewhere, we discuss this loss structure and show that, by studying its experimental systematics in the metals from MO to Ag, we can clarify the extent to which the spectator hole interpretation is valid [29]. In fig. 3, we present measured MW and L,M,,M,, Auger energy shifts of Pd in Pd-Cu and of Ag in Ag-Pd. Were the PdMW shifts representative of core-levels, they would equal the core-level LMM shifts, which is clearly not the case here, given our experimental error. For true quasi-atomic MW spectra, those of Ag in Pd-Ag, the MVV and LMM shifts coincide, within experimental error, as expected in this case. In conclusion, we have argued that valence band Auger spectra in alloys, although of quasiatomic origin, can present Auger energy shifts which are not of quasi-atomic nature, because of the direct influence of the valence band DOS. We have compared measured L3M4,5M4,5 and MW Auger shifts of Pd in Pd-Cu alloys. It would appear that, in general, measurement of the required shifts should be made through the LMM core spectra, which are well-defined. We would like to thank R.C.G. Vinhas and R.F. Suarez for technical assistance and CNPq, FAPESP and FINEP of Brasil for partial support.
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