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Relaxation effects during core ionization in metal—metal-bonded complexes
Journal of Electron Spectroscopy and Related Phenomena, 27 (1982) 63-67 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Short communication
RELAXATION EFFECTS DURING CORE IONIZATION IN METAL-METAL-BONDED COMPLEXES*
PATRICK BRANT* * Chemistry Division, Naval Research Laboratory,
Washington, DC 20375
(U.S.A.)
(First received 28 August 1981; in final form 12 February 1982)
ABSTRACT Using the ligand group shift model to calculate metal binding energies (BE’s), comparisons of BE(calc) and BE(obs) values reveal a correlation between relaxation energy and the extent of metal-metal bonding. No significant difference in relaxation energy is detected between monometal complexes and the dipalladium complexes which contain a single M-M bond. However, substantially larger relaxation energies (l-2 eV) relative to monometal complexes are found for multiply M-M-bonded complexes. The magnitude of the additional relaxation energy is in the range expected on the basis of comparisons with other systems.
INTRODUCTION
Solid-state metal core binding energies (BE’s) in tertiary phosphine or arsine halide monometal complexes can be predicted accurately using the ligand group shift model [l]. Binding energies are calculated using the equation BE(metal)
=
f
A.&f; +iV+M
(1)
i=l
where AML is the BE shift due to ligand i, N is the contribution from the metal oxidation state (+l.O eV/formal unit charge), and M is the “bare
* Contribution from the Chemistry Division, Code 6130 Naval Research Laboratory, Washington, DC 20375, U.S.A. ** Present address: P.O. Box 5200, Chemical Technology Center, Exxon Chemical Co., Baytown, TX 77522, U.S.A.
0368-2048/82/0000-0000/$02.75
01982
Elsevler Scientific Publishing Company
64
metal” (0) BE. Values of AML and N are deduced from metal BE comparisons between synthesized complexes, while M is derived by extrapolation to the condition where all ligands are removed from the central metal and it is in its zero oxidation state. The excellent accordance between BE(calc) and BE (obs) values for these monometal complexes implies that the relaxation energy contribution per ligand is constant and is independent of the complex for this class of complexes. Success in the reproduction of solid-state metal BE’s for monometal complexes has led to the question of whether the model might be applicable to dimetal complexes containing metal-metal bonds [2] . Reliable solid-state metal BE data are available for a number of metal-metal-bonded systems: having M-M bond orders of 1, 3, 3.5 and 4 [ 3-71. These systems offer an opportunity to evaluate the effect of the M-M bond(s) on observed metal BE’s. DISCUSSION All metal BE data for more than 20 tertiary phosphine or amine halide M-M bonded dimetal complexes have been examined. A representative list of these complexes, having M-M bond orders of 1,3, 3.5 and 4, is presented in Table 1. Also included in the Table are the observed and calculated metal BE’s. It can be seen from these comparisons that the difference (BE(calc) BE(obs)} is consistently between + 1 and +2 eV, except in the case of Pdz(dppm), Cl:, and Pd,(dam)* CIZ where agreement between the observed and calculated Pd 3ds12 BE’s is excellent (to within 0.3 eV). These results are in sharp contrast with the excellent agreement (93% of I BE (talc) - BE (obs) I G 0.4 eV) between BE (obs) and BE (talc) found using eqn. (1) for more than 40 monometal tertiary phosphine or arsine halide complexes [ 11. The large positive errors in (BE (talc) - BE(obs)} do not appear to be related to the fact that the complexes considered are of molybdenum and rhenium, since the subset of molybdenum and rhenium tertiary phosphine or amine halide monometal complexes generally gives the same good agreement between BE(calc) and BE(obs) as do the complexes of other transition metals. It is clear, however, that one or more of the empirical parameters developed for monometal complexes must be changed and/or a new term added to eqn. (1) in order to eliminate the variance between BE (talc) and BE (obs). The parameters AM= and N can be eliminated as the source(s) of error since, starting with any complex in the series, the observed metal BE of any other like dimetal complex in the series can be derived to within 0.2 eV using the AM, and N values derived for monometal complexes. Thus, the source of the error must be due either to the choice of “bare metal” BE’s or to the contribution of a new term. Within the constraints of the empirical ligand group shift model there is no basis on which a choice can be made between the two possibilities. In
65 TABLE 1 COMPARISONS OF CALCULATED AND EXPERIMENTAL GIES FOR COMPLEXES WITH METAL-METAL BONDS Compound
PdzCIz(dppm)zb b Pdz Cl2 (d=n)z Mo2CLU’Jhh
MozC14 [P(r~+‘r)~]~ Mo2 Cl4 (PMePhz )4 Rel Cl4 (dppm$, Rel Cl4 (dppe)z Rel CL (PEts )4 Rel Br4 (PEt3 )4 [ Rel Cl4 (PEts )4 1PF6 Rez Cl6 (PMePhl )2 Rel Br6 (PEW2
METAL BINDING ENER-
Formal metal oxidation state
Core level
Metal BE (eV) BE(obs)’
BE ( calc)g
+1 +1 +2 +2 +2 +2 +2 +2 +2 +2.5 -l-3 +3
3dw 3&/z 3dw 3dst2 3dstz 4flll
331.3c 337.4c 228.gd 229.0d 228.gd 41.7e 41.6e 41.5e 41.3e 42.3f 43.2e 43.1e
331.2 337.7 230.5 230.5 230.5 42.8 42.8 42.1 42.5 43.2 44.4 44.1
4flll
4fll2
4fll2
45112 Jflll 4flll
BE(ca.lc) BE (obs)
M-M Bond order
-0.1 +0.3 +1.6 +1.5 +1.6 +1.1 +1.2 +1.2 +1.2 +0.9 +1.2 +1.0
1 1 4 4 4 3 3 3 3 3.5 4 4
a Ail BE’s are referenced to C 1s (285.0 eV). b dppm is bis(diphenylphosphino)methane; dam, bis(diphenylarsino)methane; dppe, 1.2 bis(diphenylphosphino)ethane. ’ Ref. 7. d Ref. 4. e Ref. 3. f Ref. 5. ’ Parameters for derivation of BE(caIc) (eqn. (l)), taken from ref. 1, are: AM,, -0.5eV (Cl-), -0.6 eV (Br-), -1.2eV (PRs), -2.3 eV (dppe and dppm), -1.8 eV (dam); N, -i-1.0 eV/unit metal charge; M, 339.0 eV (Pd), 231.9 eV (MO), 44.1 eV (Re).
order to decide, it is necessary to take into consideration the nature of the M-M bond(s) and their contribution to the energies of the initial and final (core-hole) states. The effect of the adjacent metal atom on the initial state can be reasonably assumed to be zero or positive in the context of the ligand group shift model, for the following reasons. First, as required by the symmetry of the dimetal complexes considered, the electron density in the covalent M-M bond(s) is shared equally, so the effective charge contribution of the adjacent metal atom to the atom from which the core photoelectron is emitted must be zero. Second, the Madelung-like potential of the adjacent positively charged metal atom will be positive and so will act to increase the BE’s of the metal from which the photoelectron is emitted. While the contribution of the Madelung-like potential cannot be quantified at present, it is clearly antagonistic to the observed decrease in metal BE’s relative to the calculated values and so cannot be the source of the discrepancy between metal BE(obs) and metal BE (talc). At this point the iV and ML values could be arbitrarily subtracted from the observed metal BE’s to obtain new “bare metal” (0) BE’s for the dimetal complexes. This approach will of course guarantee reproduction of the
66
observed metal BE’s for the dimetal complexes, since for such a limited data base it amounts to nothing more than a tautology. Furthermore, the above approach ignores the possible contribution of the M-M bond(s) to the finalstate energies in these complexes. Briefly considering the final-state contribution, it might reasonably be postulated that the M-M bonding electrons can aid in more effective screening of the core hole, thus providing for more efficient relaxation [8] during photoemission in M-M-bonded complexes than is found in monometal complexes. As a consequence, BE(obs) will be lower than BE(calc) (eqn. (1)) and the quantity (BE(calc) - BE (obs)} will be positive, as is found in Table 1 for those complexes having M-M bond orders 23.0. These arguments are consistent with the nature of the M-M bond(s), which place greater amounts of metal d electron density near the metal core [9, lo] and which are more polarizable than Lewis-base ligands attached to the metal center. On the basis of these arguments, the magnitude of relaxation might be expected to increase with the number of electrons shared between the two metal centers, as reflected by the M-M bond order (Table 1). A cursory examination of the data in Table 1 shows that the results agree with this expectation. The measured Pd 3& BE’s for the dipalladium complexes having M-M bond orders of 1 are not significantly different from BE’s calculated according to eqn. (1). Thus, the additional relaxation energy due to the Pd-Pd bond is effectively zero (or the relaxation energy is just cancelled by the Madelung-like term due to the adjacent metal atom). The higher M-M bond orders (3-4) for the dimolybdenum and dirhenium complexes result in observed metal BE’s which are lower by l-2 eV than those calculated by eqn. (1). Given a possible antagonistic Madelung-like contribution from the adjacent metal atom in these complexes, the additional relaxation energy due to M-M bonding in these complexes is at least of the order of l-2 eV. The data in Table 1 provide some insight concerning the magnitude of the Madelung-like contribution due to the adjacent metal center. Among the dimolybdenum and dirhenium complexes having M-M bond orders of 3-4 there are complexes having formal metal oxidation states of +2, +2.5 and +3. Increasing the charge of the adjacent metal atom, as reflected in the metal oxidation state, will increase its (positive) Madelung-like potential contribution and so decrease the difference between BE (talc) and BE(obs). The {BE(ca.lc) - BE(obs)) values from Table 1 are: for + 2 complexes, +1.4(*0.1)eV; for the +2.5 complex, +0.9eV; for +3 complexes, +l.l (+ 0.2) eV. The trend observed is of the correct sign but is so small as to be significant only marginally. The magnitude of the difference (BE(calc) - BE(obs)} (0 to -t-2 eV) for the M-M bonded complexes also appears to be reasonable on the basis of comparisons available in the literature. First, the additional relaxation (l-2 eV) for M-M bonded systems (bond order >3) compared with mono-
67
metal complexes is, as might be expected, smaller than that found (+2.9+4.5 eV) for bulk metals compared with metal atoms [ll-161. Second, the additional O-2 eV relaxation energy attending photoemission in the dimetal complexes relative to the monometal complexes is of the same magnitude as that (1.0 eV) observed [17] in an XPS study of a molecular beam containing Bi atoms and Biz molecules.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
R. D. Feltham and P. Brant, J. Am. Chem. Sot., 104 (1982) 641. F. A. Cotton, Chem. Sot. Rev., 4 (1975) 27; F. A. Cotton, Act. Chem. Res., 11 (1978) 225. J. R. Ebner and R. A. Walton, Inorg. Chem., 14 (1975) 2289; J. R. Ebner, D. R. Tyler and R. A. Walton, Inorg. Chem., 15 (1976) 833. R. A. Walton, J. Less Common Met., 54 (1977) 71. P. Brant, D. J. Salmon and R. A. Walton, J. Am. Chem. Sot., 100 (1978) 3203. P. Brant, H. D. Glicksman, D. J. SaImon and R. A. Walton, Inorg. Chem., 17 (1978) 3203. P. Brant, L. S. Benner and A. L. Balch, Inorg. Chem., 18 (1979) 3422. A. W. Coleman, J. C. Green, A. J. Hayes, E. A. Seddon, D. R. Woyd and Y. Niwa, J. Chem. Sot., Dalton Trans., (1979) 1057. J. G. Norman, Jr., H. J. Kolari, H. B. Gray and W. C. TrogIer, Inorg. Chem., 16 (1977) 987. J. G. Norman and H. J. Kolari, J. Am. Chem. Sot., 97 (1975) 33. M. S. Banna, D. C. Frost, C. A. McDowell and B. WaIlbank, J. Chem. Phys., 68 (1978) 696. M. S. Banna, B. Wailbank, D. C. Frost, C. A. McDowell and J. S. H. Q. Perera, J. Chem. Phys., 68 (1978) 5459. S. Svensson, N. Martensson, E. Basilier, P. A. MaImquist, U. Gelius and K. Siegbahn, J. Electron Spectrosc. Relat. Phenom., 9 (1976) 51. L. Ley, S. P. Kowalczyk, F. R. McFeely, R. A. Pollak and D. A. Shirley, Phys. Rev. B, 8 (1973) 2392. R. E. Watson, M. L. Perlman and J. F. Herb&., Phys. Rev., B, 13 (1976) 2358. L. Ley, F. R. McFeely, S. P. Kowalczyk, J. G. Jenkin and D. A. Shirley, Phys. Rev. B, 11 (1975) 600. Y. S. Khodeyev, H. Siegbahn, K. Hamrin and K. Siegbahn, Chem. Phys. Lett., 19 (1973) 16.
Short communication
RELAXATION EFFECTS DURING CORE IONIZATION IN METAL-METAL-BONDED COMPLEXES*
PATRICK BRANT* * Chemistry Division, Naval Research Laboratory,
Washington, DC 20375
(U.S.A.)
(First received 28 August 1981; in final form 12 February 1982)
ABSTRACT Using the ligand group shift model to calculate metal binding energies (BE’s), comparisons of BE(calc) and BE(obs) values reveal a correlation between relaxation energy and the extent of metal-metal bonding. No significant difference in relaxation energy is detected between monometal complexes and the dipalladium complexes which contain a single M-M bond. However, substantially larger relaxation energies (l-2 eV) relative to monometal complexes are found for multiply M-M-bonded complexes. The magnitude of the additional relaxation energy is in the range expected on the basis of comparisons with other systems.
INTRODUCTION
Solid-state metal core binding energies (BE’s) in tertiary phosphine or arsine halide monometal complexes can be predicted accurately using the ligand group shift model [l]. Binding energies are calculated using the equation BE(metal)
=
f
A.&f; +iV+M
(1)
i=l
where AML is the BE shift due to ligand i, N is the contribution from the metal oxidation state (+l.O eV/formal unit charge), and M is the “bare
* Contribution from the Chemistry Division, Code 6130 Naval Research Laboratory, Washington, DC 20375, U.S.A. ** Present address: P.O. Box 5200, Chemical Technology Center, Exxon Chemical Co., Baytown, TX 77522, U.S.A.
0368-2048/82/0000-0000/$02.75
01982
Elsevler Scientific Publishing Company
64
metal” (0) BE. Values of AML and N are deduced from metal BE comparisons between synthesized complexes, while M is derived by extrapolation to the condition where all ligands are removed from the central metal and it is in its zero oxidation state. The excellent accordance between BE(calc) and BE (obs) values for these monometal complexes implies that the relaxation energy contribution per ligand is constant and is independent of the complex for this class of complexes. Success in the reproduction of solid-state metal BE’s for monometal complexes has led to the question of whether the model might be applicable to dimetal complexes containing metal-metal bonds [2] . Reliable solid-state metal BE data are available for a number of metal-metal-bonded systems: having M-M bond orders of 1, 3, 3.5 and 4 [ 3-71. These systems offer an opportunity to evaluate the effect of the M-M bond(s) on observed metal BE’s. DISCUSSION All metal BE data for more than 20 tertiary phosphine or amine halide M-M bonded dimetal complexes have been examined. A representative list of these complexes, having M-M bond orders of 1,3, 3.5 and 4, is presented in Table 1. Also included in the Table are the observed and calculated metal BE’s. It can be seen from these comparisons that the difference (BE(calc) BE(obs)} is consistently between + 1 and +2 eV, except in the case of Pdz(dppm), Cl:, and Pd,(dam)* CIZ where agreement between the observed and calculated Pd 3ds12 BE’s is excellent (to within 0.3 eV). These results are in sharp contrast with the excellent agreement (93% of I BE (talc) - BE (obs) I G 0.4 eV) between BE (obs) and BE (talc) found using eqn. (1) for more than 40 monometal tertiary phosphine or arsine halide complexes [ 11. The large positive errors in (BE (talc) - BE(obs)} do not appear to be related to the fact that the complexes considered are of molybdenum and rhenium, since the subset of molybdenum and rhenium tertiary phosphine or amine halide monometal complexes generally gives the same good agreement between BE(calc) and BE(obs) as do the complexes of other transition metals. It is clear, however, that one or more of the empirical parameters developed for monometal complexes must be changed and/or a new term added to eqn. (1) in order to eliminate the variance between BE (talc) and BE (obs). The parameters AM= and N can be eliminated as the source(s) of error since, starting with any complex in the series, the observed metal BE of any other like dimetal complex in the series can be derived to within 0.2 eV using the AM, and N values derived for monometal complexes. Thus, the source of the error must be due either to the choice of “bare metal” BE’s or to the contribution of a new term. Within the constraints of the empirical ligand group shift model there is no basis on which a choice can be made between the two possibilities. In
65 TABLE 1 COMPARISONS OF CALCULATED AND EXPERIMENTAL GIES FOR COMPLEXES WITH METAL-METAL BONDS Compound
PdzCIz(dppm)zb b Pdz Cl2 (d=n)z Mo2CLU’Jhh
MozC14 [P(r~+‘r)~]~ Mo2 Cl4 (PMePhz )4 Rel Cl4 (dppm$, Rel Cl4 (dppe)z Rel CL (PEts )4 Rel Br4 (PEt3 )4 [ Rel Cl4 (PEts )4 1PF6 Rez Cl6 (PMePhl )2 Rel Br6 (PEW2
METAL BINDING ENER-
Formal metal oxidation state
Core level
Metal BE (eV) BE(obs)’
BE ( calc)g
+1 +1 +2 +2 +2 +2 +2 +2 +2 +2.5 -l-3 +3
3dw 3&/z 3dw 3dst2 3dstz 4flll
331.3c 337.4c 228.gd 229.0d 228.gd 41.7e 41.6e 41.5e 41.3e 42.3f 43.2e 43.1e
331.2 337.7 230.5 230.5 230.5 42.8 42.8 42.1 42.5 43.2 44.4 44.1
4flll
4fll2
4fll2
45112 Jflll 4flll
BE(ca.lc) BE (obs)
M-M Bond order
-0.1 +0.3 +1.6 +1.5 +1.6 +1.1 +1.2 +1.2 +1.2 +0.9 +1.2 +1.0
1 1 4 4 4 3 3 3 3 3.5 4 4
a Ail BE’s are referenced to C 1s (285.0 eV). b dppm is bis(diphenylphosphino)methane; dam, bis(diphenylarsino)methane; dppe, 1.2 bis(diphenylphosphino)ethane. ’ Ref. 7. d Ref. 4. e Ref. 3. f Ref. 5. ’ Parameters for derivation of BE(caIc) (eqn. (l)), taken from ref. 1, are: AM,, -0.5eV (Cl-), -0.6 eV (Br-), -1.2eV (PRs), -2.3 eV (dppe and dppm), -1.8 eV (dam); N, -i-1.0 eV/unit metal charge; M, 339.0 eV (Pd), 231.9 eV (MO), 44.1 eV (Re).
order to decide, it is necessary to take into consideration the nature of the M-M bond(s) and their contribution to the energies of the initial and final (core-hole) states. The effect of the adjacent metal atom on the initial state can be reasonably assumed to be zero or positive in the context of the ligand group shift model, for the following reasons. First, as required by the symmetry of the dimetal complexes considered, the electron density in the covalent M-M bond(s) is shared equally, so the effective charge contribution of the adjacent metal atom to the atom from which the core photoelectron is emitted must be zero. Second, the Madelung-like potential of the adjacent positively charged metal atom will be positive and so will act to increase the BE’s of the metal from which the photoelectron is emitted. While the contribution of the Madelung-like potential cannot be quantified at present, it is clearly antagonistic to the observed decrease in metal BE’s relative to the calculated values and so cannot be the source of the discrepancy between metal BE(obs) and metal BE (talc). At this point the iV and ML values could be arbitrarily subtracted from the observed metal BE’s to obtain new “bare metal” (0) BE’s for the dimetal complexes. This approach will of course guarantee reproduction of the
66
observed metal BE’s for the dimetal complexes, since for such a limited data base it amounts to nothing more than a tautology. Furthermore, the above approach ignores the possible contribution of the M-M bond(s) to the finalstate energies in these complexes. Briefly considering the final-state contribution, it might reasonably be postulated that the M-M bonding electrons can aid in more effective screening of the core hole, thus providing for more efficient relaxation [8] during photoemission in M-M-bonded complexes than is found in monometal complexes. As a consequence, BE(obs) will be lower than BE(calc) (eqn. (1)) and the quantity (BE(calc) - BE (obs)} will be positive, as is found in Table 1 for those complexes having M-M bond orders 23.0. These arguments are consistent with the nature of the M-M bond(s), which place greater amounts of metal d electron density near the metal core [9, lo] and which are more polarizable than Lewis-base ligands attached to the metal center. On the basis of these arguments, the magnitude of relaxation might be expected to increase with the number of electrons shared between the two metal centers, as reflected by the M-M bond order (Table 1). A cursory examination of the data in Table 1 shows that the results agree with this expectation. The measured Pd 3& BE’s for the dipalladium complexes having M-M bond orders of 1 are not significantly different from BE’s calculated according to eqn. (1). Thus, the additional relaxation energy due to the Pd-Pd bond is effectively zero (or the relaxation energy is just cancelled by the Madelung-like term due to the adjacent metal atom). The higher M-M bond orders (3-4) for the dimolybdenum and dirhenium complexes result in observed metal BE’s which are lower by l-2 eV than those calculated by eqn. (1). Given a possible antagonistic Madelung-like contribution from the adjacent metal atom in these complexes, the additional relaxation energy due to M-M bonding in these complexes is at least of the order of l-2 eV. The data in Table 1 provide some insight concerning the magnitude of the Madelung-like contribution due to the adjacent metal center. Among the dimolybdenum and dirhenium complexes having M-M bond orders of 3-4 there are complexes having formal metal oxidation states of +2, +2.5 and +3. Increasing the charge of the adjacent metal atom, as reflected in the metal oxidation state, will increase its (positive) Madelung-like potential contribution and so decrease the difference between BE (talc) and BE(obs). The {BE(ca.lc) - BE(obs)) values from Table 1 are: for + 2 complexes, +1.4(*0.1)eV; for the +2.5 complex, +0.9eV; for +3 complexes, +l.l (+ 0.2) eV. The trend observed is of the correct sign but is so small as to be significant only marginally. The magnitude of the difference (BE(calc) - BE(obs)} (0 to -t-2 eV) for the M-M bonded complexes also appears to be reasonable on the basis of comparisons available in the literature. First, the additional relaxation (l-2 eV) for M-M bonded systems (bond order >3) compared with mono-
67
metal complexes is, as might be expected, smaller than that found (+2.9+4.5 eV) for bulk metals compared with metal atoms [ll-161. Second, the additional O-2 eV relaxation energy attending photoemission in the dimetal complexes relative to the monometal complexes is of the same magnitude as that (1.0 eV) observed [17] in an XPS study of a molecular beam containing Bi atoms and Biz molecules.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
R. D. Feltham and P. Brant, J. Am. Chem. Sot., 104 (1982) 641. F. A. Cotton, Chem. Sot. Rev., 4 (1975) 27; F. A. Cotton, Act. Chem. Res., 11 (1978) 225. J. R. Ebner and R. A. Walton, Inorg. Chem., 14 (1975) 2289; J. R. Ebner, D. R. Tyler and R. A. Walton, Inorg. Chem., 15 (1976) 833. R. A. Walton, J. Less Common Met., 54 (1977) 71. P. Brant, D. J. Salmon and R. A. Walton, J. Am. Chem. Sot., 100 (1978) 3203. P. Brant, H. D. Glicksman, D. J. SaImon and R. A. Walton, Inorg. Chem., 17 (1978) 3203. P. Brant, L. S. Benner and A. L. Balch, Inorg. Chem., 18 (1979) 3422. A. W. Coleman, J. C. Green, A. J. Hayes, E. A. Seddon, D. R. Woyd and Y. Niwa, J. Chem. Sot., Dalton Trans., (1979) 1057. J. G. Norman, Jr., H. J. Kolari, H. B. Gray and W. C. TrogIer, Inorg. Chem., 16 (1977) 987. J. G. Norman and H. J. Kolari, J. Am. Chem. Sot., 97 (1975) 33. M. S. Banna, D. C. Frost, C. A. McDowell and B. WaIlbank, J. Chem. Phys., 68 (1978) 696. M. S. Banna, B. Wailbank, D. C. Frost, C. A. McDowell and J. S. H. Q. Perera, J. Chem. Phys., 68 (1978) 5459. S. Svensson, N. Martensson, E. Basilier, P. A. MaImquist, U. Gelius and K. Siegbahn, J. Electron Spectrosc. Relat. Phenom., 9 (1976) 51. L. Ley, S. P. Kowalczyk, F. R. McFeely, R. A. Pollak and D. A. Shirley, Phys. Rev. B, 8 (1973) 2392. R. E. Watson, M. L. Perlman and J. F. Herb&., Phys. Rev., B, 13 (1976) 2358. L. Ley, F. R. McFeely, S. P. Kowalczyk, J. G. Jenkin and D. A. Shirley, Phys. Rev. B, 11 (1975) 600. Y. S. Khodeyev, H. Siegbahn, K. Hamrin and K. Siegbahn, Chem. Phys. Lett., 19 (1973) 16.