- Email: [email protected]
Interatomic Auger processes and the density of states
Surface Science 0 North-Holland
53 (1975) 689-697 Publishing Company
INTERATOMIC
AUGER PROCESSES AND THE DENSITY OF STATES
M.SALMER6N Centro Coordinado CSIC-hive&dad Autdnoma Fundamental, Canto Blanco, Madrid, Spain
de Madrid, Departamento
de Fi’sica
and
A.M. BAR6 and J.M. ROJO Departamento de Fisica Fundamental, Madrid, Spain
Universidad Autdnoma
de Madn’d, Canto Blanco,
The correlation between the line shape of Auger peaks and the density of states near the surface has been the subject of recent controversy. In certain cases, it has been possible to obtain the density of states by numerical deconvolution of a KW peak (Amelio, 1970) or directly using a KLV peak (Cardona et al., 1973). However, the extension of this technique to transition metals (Cu, Zn) has encountered serious difficulties, related to the perturbation created by the presence of localized charges either in the initial or in the final state, although it is not yet clear why this perturbation is strong only in certain cases. The purpose of the present communication is to show a series of results that can throw some light on the abovementioned problem. The main point is that Auger processes of interatomic type, as those occurring in the INS technique of Hagstrum, are free of these perturbations. Recently, the authors have studied the line shape of the Auger peaks of 0, C, N and S adsorbed on Cu, Ni and Fe. These results show that only that part of the Auger structure originated by interatomic transitions between substrate and adsorbate atoms can be related to the local density of states (LDOS). The rest of the structure, due to normal intraatomic processes, is dominated by the spectral terms in the final configuration of the ion. This new interpretation allows a separation of perturbation effects and clarifies the contribution of the LDOS to the peak line shape. In this communication, we present the line shape analysis of the L2 3W and KW Auger peaks of Mg and 0 in MgO. Due to the strong ionic character of t’his compound, the L2,3VV peak of Mg u is mainly due to interatomic processes between Mg++ and O= ions, whereas the KVV peak of 0 is mainly due to interatomic processes. This analysis shows that good agreement exists between the L2,3VV Mg++ Auger peak and the selfconvolution of MgO density of states, whereas the KW Auger peak of O= is dominated by the spectral terms of the final configuration. Only a small peak in the high energy side of the latter peak can be related to the density of states and could be interpreted as an interatomic transition between two neighboring oxygen ions, in agreement with the interpretation given by others.
1. Introduction
Auger electron spectroscopy (AES) is being extensively used as a tool for the detection of surface impurities. Although the use of Auger transitions of the WXV or 689
690
M. Sdmer&
et al. /Interatomic
Auger processes
WW type, where V represents a level in the valence band, has been proposed as a possible way for the obtention of the density of states (DOS), few attempts have been carried out successfully [l-.4] . In particular, it could be of considerable interest in connection with the study of the local DOS in chemisorption [5,6]. If this analysis could be done successfully, it would provide an information complementary, in a certain way, to that furnished by UPS and also INS. The reason is that the major contribution to the Auger peak comes from electrons which are located in the neighborhood of the parent ion at the time the transition occurs [2]. In UPS and INS, the measured DOS, ~though localized in the z-direction (normal to the surface), is averaged over the xy plane. On the other hand, the comparison of WVV Auger peaks, corresponding to A and B atoms in an AB compound crystal, i.e., peaks associated to ionization of a W inner level in either constituent, shows significant differences in their shapes [7] . This suggests that the interpretation of Auger peak structure based on an average DOS is clearly inadequate and that the effective DOS, which locally varies around each constituent A or B, is to be taken into account. In particular, valence electrons arising from s, p or d-like bands tend to have markedly different densities around each constituent. Unfortunately, in trying to interpret the Auger spectra of several adsorbates on Cu and Ni, no correlation with the DOS obtained by INS is found [8] except in the higher energy features of the Auger peaks, which have been interpreted as arising from interatomic processes between the adsorbate and metal substrate, in a similar way to that in the INS Auger process. This result, along with those of INS, suggests that the perturbation in the DOS, due to the presence of localized charges in the Auger process, is very small for interatomic Auger processes. To further check this idea, we have studied the Auger peaks of Mg and 0 in MgO. Due to the strong ionic character of this compound, the Auger L~,JW of Mg’+ is of interatomic type, the V levels in the band being fairly localized on the O= ions. On the other hand, the KVV peak of O= is mostly of intraatomic type. So, the comparison of both Auger peaks, and also with the self-convolution of the DOS in MgO, can give us a very useful information on the questions above.
2. Experimental The experiments were performed in a UHV system with a base pressure in the lo-lo torr range, Magnesium samples were prepared in situ by evaporation of a Mg ribbon over Cu and Si substrates. The magnesium oxide was formed by oxidation of the thin Mg layer through controlled exposure to, an 02 atmosphere. Auger detection was performed either by the retarding field method or by the use of an electrostatic cylindrical analyzer.
M. Salmerdn et al. /Interatomic
Auger processes
691
3. Results and comments 3.1. Position and shape of the Auger peaks In fig. 1 we show the L~,J VV Auger spectrum of Mg and MgO at different stages of the oxidation process. The main characteristics of that process are a linear increase of the 34 and 25 eV peaks from Bpproximately 1 to 5 L exposure. Further exposure results in a bending of the curves of intensity versus exposure for these two peaks, indicating that monolayer coverage has been reached. The striking result is that further exposure results in a decrease of the intensity of the 25 eV peak and a slower increase of the 34 eV peak. These results are in agreement with those published by other authors [9]. For the purposes of the present work, we shall not try to interpret these results. They will be the object of a forthcoming publication.
Fig. 1. L2,3VV Auger spectrum of a magnesium film corresponding to three stages of the oxidation process. The decrease in intensity of the peak at 44 eV characteristic of pure Mg is followed by an increase of that of peaks at 34 and 25 eV of MgO.
692
M. &lmer&
et al. /Interatomic
Auger processes
In fig. 2a we show the L2,sW Auger peak of Mg++ in MgO and in fig. 3 the KW Auger peak of O= in the same compound. Also, a scheme of the (100) plane of MgO is shown, with circles representing the O= and Mg++ ions. The radii are proportional to the ionic radii. The peak with minimum derivative at 34 eV is assigned to an Auger deexcitation from the hole in the L,,, shell of Mg++ by electrons of the valence band, which is located in the L,,,(p) shell of Oz. The shift of approximately 10 eV between the positions of the W peaks of Mg in the pure metal and the oxide is consistent with the -9 eV gap of MgO. We must have effectively such a shift if the Fermi level is located approximately midway in the gap because of the doubling of energies in the Auger process. More quantitatively, if we assume, as suggested by some experimental results [lo], the Fermi level to be located 6.5 eV above the top of the valence band, and the L,,, binding energy shifted 2-3 eV towards higher binding energies in going from the metal to the oxide, we get a difference in Auger energies between
Fig. 2. (a) L~,JVV Auger peak of Mg u Kp-emission band of Mg++ in MgO.
in MgO, and (b) derivative
of the self-convolution
of
AK Salmerdn et al. /Interatomic
Auger processes
693
+jo
15'
2s'
2p
Mg
IS'
25'
2p
ihJ
Fig. 3. KVV Auger peak of O= in MgO with the most intense the position of the K level of oxygen.
spectral
terms indicated.
K(O=) is
9 and 10 eV, in good agreement with the experimental results. The structure of this Auger peak consists, in the dN/dE form, of a minimum at 34 eV and a shoulder at 3 1.5 eV. Possible structure in the low-energy side is obscured by the presence of the peak with minimum at 25 eV to which it partially overlaps. The KW Auger peak in fig. 3 shows a shape which is commonly observed in other oxygen compounds. The main features are due to spectral terms of the electrostatic and magnetic interactions between the two final holes._Our result is then in agreement with previously reported studies. Also, a high-energy shoulder at 5 16 _+2 eV is apparent in fig. 3. This peak lies outside the region of the possible spectral terms and has been interpreted as an interatomic transition between two oxygen ionsinMg0 [Ill. From the comparison of the figs. 2 and 3, it becomes quite clear that the two Auger transitions, although involving a common valence band, have very different shapes. That of oxygen is dominated by the spectral terms which account for most of the structure, except the 5 16 eV peak. Also, the energy position of the main (lD> peak is shifted towards lower energies with respect to the position expected E, - @Analyzer, if no interaction between the two holes were to occur, by ap% 3 proximately 4 eV. On the other hand, the small peak at 5 16 eV would correspond to an interatomic transition between two O=, as indicated in fig. 4. Their expected position with no shift is approximately 5 15 eV.
694
M. Saline&
etal. /Interatomic
K
-532 0’
Auger processes
U-532 0=
Mg++
Fig. 4. Schematic representation of the Auger transitions occurring in MgO. Two neighboring 0’ ions have been drawn in the same line of Mg’+ ion, for convenience only,
3.2. DOS and Auger line-shape In fig. 2b we show the derivative of the self-convolution of the K&X-ray emission band of MgO obtained by Cardona et al. [ 131, which corresponds to a similar process, i.e., the initial hole is located in the Mg ion whereas the neutralizing electron comes from the valence band in MgO. As seen in the figure, the minimum at 34 eV and the shoulder at 31.5 eV of curve a agree with similar structure in curve b. The peak at 26 eV lies outside the region of curve b. So, the 34 and 26 eV peaks have a ~fferent origin as expected by their different behavior as mentioned earlier. This result shows that the DOS of MgO is responsible for the structure in the Lz,~W peak of Mg++. In order to confirm this connection, the experimental curve has been deconvoluted by a digital Fast Fourier Transform method. The result is shown in fig. 5 along with the K-0 emission band. Also, the dN/dE experimental curve is plotted to show that no artifacts due to the deconvolution procedure are present [14]. It must be mentioned that the deconvoluted curve in fig. 5 depends markedly on the extrapolation procedure to remove the background in the experimental curve. This dependence does not affect the appearance of the two peaks of the figure, but does change their relative intensities, so that the result shown here must be regarded as indicative only of the shape of the DOS which is contained in the Auger peak.
M. Sidmerch et al. /Interatomic Auger processes
695
Fig. 5. Comparison of the Kp emission band of Mgff (MgO) (ref. [ 131) with the -dN/dE half spectrum (L2 3VV) and the deconvolution of this experimental curve. The two arrows indicate the energies 01 the peaks in the Kp band with respect to the top of valence band.
4. Discussion
In order to understand the results presented in the last section, it is useful to introduce the conceit of interatomic Auger transitions. In these, one or two of the final holes are located in an atom different from that in which the inner hole is created. In fig.4 we have schematically represented some of these transitions between O= and Mg++ ions in MgO. Of course, this concept is only meaningful if the electronic structure around each atom is of localized type, as occurs in MgO, whose strong ionic character is confirmed by X-ray intensity measurements [ 121. In this situation, a hole in the L2,3 shell of Mg++ is to be neutralized by electrons of the neighboring O= whose 2p-shell originates the MgO valence band. With this concept in mind, we are able to connect the processes of deexcitation of L2,3 shell in Mg2+ to the Auger processes occurrring in INS, where the incoming He+ ion is neutralized through Auger processes by electrons in the conduction band of the metal substrate. Although the occurrence of interatomic Auger processes has been invoked to explain high-energy features of certain Auger peaks [8,15], the relation between these processes and the perturbations in the DOS have only been partially established. The reason was that these interatomic peaks appear as the result of competitive processes of the normal intraatomic ones, which have a much larger probability and to
696
M. Salmero% et al. /Interatomic
Auger processes
which they partially overlap in energy. In MgO, however, only interatomic processes can contribute to the L, 3(Mg++)W transition which can then be carefully analyzed. The results presented in section 3, allow us to conclude that, as in INS, in the interatomic processes there is no appreciable perturbation of the DOS. This perturbation plays a very important role indeed for intraatomic transitions in atoms with localized bands. That is confirmed by the results of section 3 concerning the KW peak of oxygen in MgO, where the same valence band contributes the up and down electrons. This result can be related to the M,,JVV and L,,,W peaks of Cu and Zn, where, as a result of the localized character of the d-bands of these metals, the Auger peaks are of quasiatomic type, as reported in several papers [ 16,171. The appearance of the spectral terms of the final configuration of the ion (L-S coupling) is indicative of the quasiatomic modification of the band due to the presence of the initial inner hole. Also, for more loosely-bound electrons, as those of free-electron-like metals and for covalent compounds, where the interatomic concept loses its sense or where by an extension of language all processes can be thought of as interatomic type, the band character of the WVV or WXV peaks (W, X inner levels), has been clearly established [l-4] . The use of.the KJ emission band of Mg in MgO to represent the DOS seen by Mg++ ions is justified on the grounds that it has a similar interatomic character, with only one final hole, however, in the band in the final state. Also, it must be considered that the DOS probed by the Auger transition has a local character in the sense that it is sensitive to the electronic density of band electrons around the parent ion. This point has been emphasized by Tejeda et al. [2] studying the Auger peaks in several Mg compounds.
5. Conclusions In conclusion, we can say that our results show that the perturbations in the DOS due to the presence of localized charges in the Auger process are only important for intraatomic processes in which the outer electrons are fairly concentrated around the parent ion. This result is also consistent with the quasiatomic shape of the L and MW peaks of Cu and Zn. On the other hand, for processes where the initial hole is located away from the atom in which the up and/or down electrons originate, the DOS accounts for the line-shape of the Auger peak.
Acknowledgements We are indebted to V. Martinez SaCz of the IBM Research Center, Universidad Autonoma de Madrid, for kindly providing us with the deconvolution method which he has developed.
M. Salmet&
et al. /Interatomic
Auger processes
697
References [l] [2] [3] [4] [5] [6] [ 71 [S] [9]
[lo] [ 111 [12]
[ 131 [ 141 [15] [16] [17]
G.F. Amelio, Surface Sci. 22 (1970) 301. J. Tejeda, M. Cardona, N.J. Shevchik, D.W. Langer and E. Schonherr, Phys. Status Solidi (b) 58 (1973) 189. D.R. Arnott and D. Haneman, Surface Sci. 45 (1974) 128. S. Ferrer, A.M. Bar6 and M. Salmerbn, Solid. State Commun. 16 (1975) 65 1. J.P. Coad and J.C. Riviere, Proc. Roy. Sot. London A331 (1972) 403. E.N. Sickafus, Phys. Rev. B7 (1973) 5100. For example in the L2 3W Auger peaks of Si and Mg in Mg2Si (M. Salmcron and A.M. Barb, unpublished results). M. Salmeron and A.M. Barb, Surface Sci. 49 (1975) 356. A.P, Janssen, R. Schoonmaker, J.A.D. Matthew and A. Chambers, Solid State Commun. 14 (1974) 1263. J.R. Stevenson and E.B. Hensley, J. Appl. Phys. 32 (1961) 166. P.J. Bassett, T.E. Gallon, M. Prutton and J.A.D. Matthew, Surface Sci. 33 (1972) 213. P.L. Sanger, Acta Cryst. A25 (1969) 694. M. Cardona, J. Tejeda, N.J. Shevchik and D.W. Langer, Phys. Status Solidi (b) 58 (1973) 483. H.D. Hagstmm and G.E. Becker, J. Chem. Phys. 54 (1971) 1015. A.M. Barb, M. Salmeron and J.M. Rojo, J. Phys. F (Metal Phys.) 5 (1975) 826. L. Yin, 1. Adler, T. Tsang, M.H. Chen and B. Crasemann, Phys. Letters 46A (1973) 113. S.P. Kowalczyk, R.A. Pollak, F.R. McFeely, 1. Ley and D.A. Shirley, Phys. Rev. B8 (1973) 2387.
53 (1975) 689-697 Publishing Company
INTERATOMIC
AUGER PROCESSES AND THE DENSITY OF STATES
M.SALMER6N Centro Coordinado CSIC-hive&dad Autdnoma Fundamental, Canto Blanco, Madrid, Spain
de Madrid, Departamento
de Fi’sica
and
A.M. BAR6 and J.M. ROJO Departamento de Fisica Fundamental, Madrid, Spain
Universidad Autdnoma
de Madn’d, Canto Blanco,
The correlation between the line shape of Auger peaks and the density of states near the surface has been the subject of recent controversy. In certain cases, it has been possible to obtain the density of states by numerical deconvolution of a KW peak (Amelio, 1970) or directly using a KLV peak (Cardona et al., 1973). However, the extension of this technique to transition metals (Cu, Zn) has encountered serious difficulties, related to the perturbation created by the presence of localized charges either in the initial or in the final state, although it is not yet clear why this perturbation is strong only in certain cases. The purpose of the present communication is to show a series of results that can throw some light on the abovementioned problem. The main point is that Auger processes of interatomic type, as those occurring in the INS technique of Hagstrum, are free of these perturbations. Recently, the authors have studied the line shape of the Auger peaks of 0, C, N and S adsorbed on Cu, Ni and Fe. These results show that only that part of the Auger structure originated by interatomic transitions between substrate and adsorbate atoms can be related to the local density of states (LDOS). The rest of the structure, due to normal intraatomic processes, is dominated by the spectral terms in the final configuration of the ion. This new interpretation allows a separation of perturbation effects and clarifies the contribution of the LDOS to the peak line shape. In this communication, we present the line shape analysis of the L2 3W and KW Auger peaks of Mg and 0 in MgO. Due to the strong ionic character of t’his compound, the L2,3VV peak of Mg u is mainly due to interatomic processes between Mg++ and O= ions, whereas the KVV peak of 0 is mainly due to interatomic processes. This analysis shows that good agreement exists between the L2,3VV Mg++ Auger peak and the selfconvolution of MgO density of states, whereas the KW Auger peak of O= is dominated by the spectral terms of the final configuration. Only a small peak in the high energy side of the latter peak can be related to the density of states and could be interpreted as an interatomic transition between two neighboring oxygen ions, in agreement with the interpretation given by others.
1. Introduction
Auger electron spectroscopy (AES) is being extensively used as a tool for the detection of surface impurities. Although the use of Auger transitions of the WXV or 689
690
M. Sdmer&
et al. /Interatomic
Auger processes
WW type, where V represents a level in the valence band, has been proposed as a possible way for the obtention of the density of states (DOS), few attempts have been carried out successfully [l-.4] . In particular, it could be of considerable interest in connection with the study of the local DOS in chemisorption [5,6]. If this analysis could be done successfully, it would provide an information complementary, in a certain way, to that furnished by UPS and also INS. The reason is that the major contribution to the Auger peak comes from electrons which are located in the neighborhood of the parent ion at the time the transition occurs [2]. In UPS and INS, the measured DOS, ~though localized in the z-direction (normal to the surface), is averaged over the xy plane. On the other hand, the comparison of WVV Auger peaks, corresponding to A and B atoms in an AB compound crystal, i.e., peaks associated to ionization of a W inner level in either constituent, shows significant differences in their shapes [7] . This suggests that the interpretation of Auger peak structure based on an average DOS is clearly inadequate and that the effective DOS, which locally varies around each constituent A or B, is to be taken into account. In particular, valence electrons arising from s, p or d-like bands tend to have markedly different densities around each constituent. Unfortunately, in trying to interpret the Auger spectra of several adsorbates on Cu and Ni, no correlation with the DOS obtained by INS is found [8] except in the higher energy features of the Auger peaks, which have been interpreted as arising from interatomic processes between the adsorbate and metal substrate, in a similar way to that in the INS Auger process. This result, along with those of INS, suggests that the perturbation in the DOS, due to the presence of localized charges in the Auger process, is very small for interatomic Auger processes. To further check this idea, we have studied the Auger peaks of Mg and 0 in MgO. Due to the strong ionic character of this compound, the Auger L~,JW of Mg’+ is of interatomic type, the V levels in the band being fairly localized on the O= ions. On the other hand, the KVV peak of O= is mostly of intraatomic type. So, the comparison of both Auger peaks, and also with the self-convolution of the DOS in MgO, can give us a very useful information on the questions above.
2. Experimental The experiments were performed in a UHV system with a base pressure in the lo-lo torr range, Magnesium samples were prepared in situ by evaporation of a Mg ribbon over Cu and Si substrates. The magnesium oxide was formed by oxidation of the thin Mg layer through controlled exposure to, an 02 atmosphere. Auger detection was performed either by the retarding field method or by the use of an electrostatic cylindrical analyzer.
M. Salmerdn et al. /Interatomic
Auger processes
691
3. Results and comments 3.1. Position and shape of the Auger peaks In fig. 1 we show the L~,J VV Auger spectrum of Mg and MgO at different stages of the oxidation process. The main characteristics of that process are a linear increase of the 34 and 25 eV peaks from Bpproximately 1 to 5 L exposure. Further exposure results in a bending of the curves of intensity versus exposure for these two peaks, indicating that monolayer coverage has been reached. The striking result is that further exposure results in a decrease of the intensity of the 25 eV peak and a slower increase of the 34 eV peak. These results are in agreement with those published by other authors [9]. For the purposes of the present work, we shall not try to interpret these results. They will be the object of a forthcoming publication.
Fig. 1. L2,3VV Auger spectrum of a magnesium film corresponding to three stages of the oxidation process. The decrease in intensity of the peak at 44 eV characteristic of pure Mg is followed by an increase of that of peaks at 34 and 25 eV of MgO.
692
M. &lmer&
et al. /Interatomic
Auger processes
In fig. 2a we show the L2,sW Auger peak of Mg++ in MgO and in fig. 3 the KW Auger peak of O= in the same compound. Also, a scheme of the (100) plane of MgO is shown, with circles representing the O= and Mg++ ions. The radii are proportional to the ionic radii. The peak with minimum derivative at 34 eV is assigned to an Auger deexcitation from the hole in the L,,, shell of Mg++ by electrons of the valence band, which is located in the L,,,(p) shell of Oz. The shift of approximately 10 eV between the positions of the W peaks of Mg in the pure metal and the oxide is consistent with the -9 eV gap of MgO. We must have effectively such a shift if the Fermi level is located approximately midway in the gap because of the doubling of energies in the Auger process. More quantitatively, if we assume, as suggested by some experimental results [lo], the Fermi level to be located 6.5 eV above the top of the valence band, and the L,,, binding energy shifted 2-3 eV towards higher binding energies in going from the metal to the oxide, we get a difference in Auger energies between
Fig. 2. (a) L~,JVV Auger peak of Mg u Kp-emission band of Mg++ in MgO.
in MgO, and (b) derivative
of the self-convolution
of
AK Salmerdn et al. /Interatomic
Auger processes
693
+jo
15'
2s'
2p
Mg
IS'
25'
2p
ihJ
Fig. 3. KVV Auger peak of O= in MgO with the most intense the position of the K level of oxygen.
spectral
terms indicated.
K(O=) is
9 and 10 eV, in good agreement with the experimental results. The structure of this Auger peak consists, in the dN/dE form, of a minimum at 34 eV and a shoulder at 3 1.5 eV. Possible structure in the low-energy side is obscured by the presence of the peak with minimum at 25 eV to which it partially overlaps. The KW Auger peak in fig. 3 shows a shape which is commonly observed in other oxygen compounds. The main features are due to spectral terms of the electrostatic and magnetic interactions between the two final holes._Our result is then in agreement with previously reported studies. Also, a high-energy shoulder at 5 16 _+2 eV is apparent in fig. 3. This peak lies outside the region of the possible spectral terms and has been interpreted as an interatomic transition between two oxygen ionsinMg0 [Ill. From the comparison of the figs. 2 and 3, it becomes quite clear that the two Auger transitions, although involving a common valence band, have very different shapes. That of oxygen is dominated by the spectral terms which account for most of the structure, except the 5 16 eV peak. Also, the energy position of the main (lD> peak is shifted towards lower energies with respect to the position expected E, - @Analyzer, if no interaction between the two holes were to occur, by ap% 3 proximately 4 eV. On the other hand, the small peak at 5 16 eV would correspond to an interatomic transition between two O=, as indicated in fig. 4. Their expected position with no shift is approximately 5 15 eV.
694
M. Saline&
etal. /Interatomic
K
-532 0’
Auger processes
U-532 0=
Mg++
Fig. 4. Schematic representation of the Auger transitions occurring in MgO. Two neighboring 0’ ions have been drawn in the same line of Mg’+ ion, for convenience only,
3.2. DOS and Auger line-shape In fig. 2b we show the derivative of the self-convolution of the K&X-ray emission band of MgO obtained by Cardona et al. [ 131, which corresponds to a similar process, i.e., the initial hole is located in the Mg ion whereas the neutralizing electron comes from the valence band in MgO. As seen in the figure, the minimum at 34 eV and the shoulder at 31.5 eV of curve a agree with similar structure in curve b. The peak at 26 eV lies outside the region of curve b. So, the 34 and 26 eV peaks have a ~fferent origin as expected by their different behavior as mentioned earlier. This result shows that the DOS of MgO is responsible for the structure in the Lz,~W peak of Mg++. In order to confirm this connection, the experimental curve has been deconvoluted by a digital Fast Fourier Transform method. The result is shown in fig. 5 along with the K-0 emission band. Also, the dN/dE experimental curve is plotted to show that no artifacts due to the deconvolution procedure are present [14]. It must be mentioned that the deconvoluted curve in fig. 5 depends markedly on the extrapolation procedure to remove the background in the experimental curve. This dependence does not affect the appearance of the two peaks of the figure, but does change their relative intensities, so that the result shown here must be regarded as indicative only of the shape of the DOS which is contained in the Auger peak.
M. Sidmerch et al. /Interatomic Auger processes
695
Fig. 5. Comparison of the Kp emission band of Mgff (MgO) (ref. [ 131) with the -dN/dE half spectrum (L2 3VV) and the deconvolution of this experimental curve. The two arrows indicate the energies 01 the peaks in the Kp band with respect to the top of valence band.
4. Discussion
In order to understand the results presented in the last section, it is useful to introduce the conceit of interatomic Auger transitions. In these, one or two of the final holes are located in an atom different from that in which the inner hole is created. In fig.4 we have schematically represented some of these transitions between O= and Mg++ ions in MgO. Of course, this concept is only meaningful if the electronic structure around each atom is of localized type, as occurs in MgO, whose strong ionic character is confirmed by X-ray intensity measurements [ 121. In this situation, a hole in the L2,3 shell of Mg++ is to be neutralized by electrons of the neighboring O= whose 2p-shell originates the MgO valence band. With this concept in mind, we are able to connect the processes of deexcitation of L2,3 shell in Mg2+ to the Auger processes occurrring in INS, where the incoming He+ ion is neutralized through Auger processes by electrons in the conduction band of the metal substrate. Although the occurrence of interatomic Auger processes has been invoked to explain high-energy features of certain Auger peaks [8,15], the relation between these processes and the perturbations in the DOS have only been partially established. The reason was that these interatomic peaks appear as the result of competitive processes of the normal intraatomic ones, which have a much larger probability and to
696
M. Salmero% et al. /Interatomic
Auger processes
which they partially overlap in energy. In MgO, however, only interatomic processes can contribute to the L, 3(Mg++)W transition which can then be carefully analyzed. The results presented in section 3, allow us to conclude that, as in INS, in the interatomic processes there is no appreciable perturbation of the DOS. This perturbation plays a very important role indeed for intraatomic transitions in atoms with localized bands. That is confirmed by the results of section 3 concerning the KW peak of oxygen in MgO, where the same valence band contributes the up and down electrons. This result can be related to the M,,JVV and L,,,W peaks of Cu and Zn, where, as a result of the localized character of the d-bands of these metals, the Auger peaks are of quasiatomic type, as reported in several papers [ 16,171. The appearance of the spectral terms of the final configuration of the ion (L-S coupling) is indicative of the quasiatomic modification of the band due to the presence of the initial inner hole. Also, for more loosely-bound electrons, as those of free-electron-like metals and for covalent compounds, where the interatomic concept loses its sense or where by an extension of language all processes can be thought of as interatomic type, the band character of the WVV or WXV peaks (W, X inner levels), has been clearly established [l-4] . The use of.the KJ emission band of Mg in MgO to represent the DOS seen by Mg++ ions is justified on the grounds that it has a similar interatomic character, with only one final hole, however, in the band in the final state. Also, it must be considered that the DOS probed by the Auger transition has a local character in the sense that it is sensitive to the electronic density of band electrons around the parent ion. This point has been emphasized by Tejeda et al. [2] studying the Auger peaks in several Mg compounds.
5. Conclusions In conclusion, we can say that our results show that the perturbations in the DOS due to the presence of localized charges in the Auger process are only important for intraatomic processes in which the outer electrons are fairly concentrated around the parent ion. This result is also consistent with the quasiatomic shape of the L and MW peaks of Cu and Zn. On the other hand, for processes where the initial hole is located away from the atom in which the up and/or down electrons originate, the DOS accounts for the line-shape of the Auger peak.
Acknowledgements We are indebted to V. Martinez SaCz of the IBM Research Center, Universidad Autonoma de Madrid, for kindly providing us with the deconvolution method which he has developed.
M. Salmet&
et al. /Interatomic
Auger processes
697
References [l] [2] [3] [4] [5] [6] [ 71 [S] [9]
[lo] [ 111 [12]
[ 131 [ 141 [15] [16] [17]
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