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Non-dipole excitation of core holes by electron impact. Multiplet splitting in CO and N2
NON-DIPOLE MULTIPLET Leon UNGIER
1 April 1983
CHEMICAL PHYSICS LETTERS
Volume 96, number 2
EXCITATION SPLITTING
OF CORE HOLES BY ELECTRON
IMPACT.
IN CO AND N2
and T. Darrah THOMAS
Departmentof CRemiitry.Oregon State University. Corrallis. Oregvn 97331. USA
Received 30 December 1982
in on the carbon. The splittins affects
Lowenergy electron impact leads to nondipole excitation of core electrons to triplet states. The multiplet splitting CO is in agreement with theory and shows that the 2~ orbital is localized predominantly both energyqoss and Auger spectra of CO and N2 _
Electron-impact excitation of atoms and molecules at high incident energies is dominated by dipole selection rules because the incident electrons are predominantly scattered forward with small energy loss and hence small momentum transfer_ At lower energies the Born approximation breaks down and spin-forbidden processes become possible. Such non-dipole excitation of valence electrons is well known [ 11. Recently Shaw et al. [3] have reported the first case of non-dipole excitation of a core electron. At low impact energies they have observed excitation of both the singlet and triplet states that are possible when a nitrogen 1s electron in N2 is excited to the vacant lng state. In their experiments they were able to resolve the vibrational structure of the transitions_ We report here results on similar excitations in CO (at lower resolution) and on the effect of such excitation on the Auger spectrum of N2 and CO. The measured singlet-triplet splittings for CO are in agreement with those calculated by Bagus and See1 [3]. The much larger splitting observed for carbon core excitation than for oxygen verifies that the 2n orbital of CO is predominantly localized on the carbon. The multiplet splitting is also seen in subsequent Auger decay of these autoionizing states. That multiplet splitting is not evident in the carbon 1s photoemission spectrum for CO adsorbed on nickel suggests that a reexamination of theoretical interpretations of such photoemission is necessary. In the experiments reported here an electron gun 0 009-2614/S3/0000-0000/S
03-00 0
1983
North-Holland
(Cliftronics CE 3k/5u. Cliftronics, Clifton, New Jersey) is mounted on the axis of the Oregon State University cylindrical mirror analyzer (CMA) [4]_ Electrons with kinetic energies of up to 5 keV are focused on a gas sample cell at the object point of the CMA. Electrons scattering elastically and inelastically through an angle of either 120° or 60” (depending on the orientation of the electron gun) as well as electrons resulting from Auger de-excitation, autoionization, or other processes are analyzed in the CMA. Fig. la shows a typical Auger spectrum for N2 escited with electrons of 3,500 eV. The lower-energy peaks originate from core-ionized N2 ions (N2 [Is]+) while those at higher energies arise from neutral N2 molecules (N2 [Is] 14) in which a core electron has been excited to a vacant In, orbital [5, p_ 67: 6]_ Depending on one’s point of v:ew these transitions can be considered as de-excitation by autoionization, as de-excitation by Auger processes, or as resonant ionization of valence hole states. The highest-energy peak -a doublet unresolved in this experiment - arises from transitions from the N2 [Is] 14 state to the N2[3ug]+ and N,[llr,]+ states, or, equivalently, from resonant ionization of the least bound electrons in N2_ The resonance region is seen more clearly in fig_ lb, which shows a spectrum obtained using the (e, 2e) coincidence technique [7] to observe the decay of the NZ [Is] 14 state. The unresolved highenergy doublet, at 384 eV, arises from transitions in which the 247
\‘olumc
96. number 2
I
CHEMICAL
I
c
-a
a
PHYSICS
by inelastic scattering, we have subtracted a smooth background in order to emphasize the points of interest. The resulting spectrum reveals two features. First, the autoionizing region is strongly enhanced rel-
I
ative
3
KINETIC ENERGY
1. tl~ghhmsnc energy p3rt of KLL Xuger spectrum of b) 1.5 hrv rkctron impact.(b) Excited by 2.5 hc\’ clwtron impact and m coincidence with inelastically \c~!fzrcd rlectronr of401 e\‘encrgy loss.(c) Excited by 500 r\’ clcctr~ mrp~cr. The mset shops zn exp.mded view of the hl&
S:_
(J) ~\LIIL'~
ITT?,clecrron Io\rercnrrgy
p.rrriclp~trs
fe.mre
in the Auger decay: the between 367 and 380 eV arises
fiam rhose in which the 17r%electron remsins as a spec’~.r~or [S. p. 67; 61. III riw sprctr.r shown m t?gs_ I;1 and 1b. only dipole excitation is involved. At lower electron-impact energyhowever. non-dipole processes become important. Fig. Ic shows rhe spectrum taken with electron-impact energy of 500 eV. Since rhe spectrum is dominated 24s
1 April1983
LETTERS
to the normal
Auger
region;
at energies
close
to
the Ii edge, the normal Auger processes disappear completely. Second, the highest-energy peak (shown in detail in the inset of fig. 3c) has become two closely spaced peaks, with the new peak being lower in energy than the old by = 0.8 eV. Close comparison shows that the low-energy feature between 367 and 380 eV is also shifted slightly to lower energies in fig. lc compared to fig. lb. The estra peaks arise from non-dipole excitation of the N, [ Is] 14 state. Under dipole selection rules only the 1 II state is excited. At low impact energies, however. the dipole selection rules break down and exchange processes that allow population of the lower-energy 3Tl state are possible- The splitting between these two is the difference between the ener@es of the singlet and triplet coupling of the core hole with the single 17~~electron. The same splitting can also be seen in the melastically scattered electrons as has been reported by Shaw et al. [2]. Chemically more interesting than N, is CO, in which two different core states can be excited. Furthermore, the multiplet splitting in CO is an important parameter for the interpretation of core emission spectra of CO adsorbed on metals. Calculations by Bagus and See1 [3] and by Goddard and Allison [S] indicate that a single, unpaired electron is transferred from the metal to the vacant 2a orbital of CO during core ionization of CO adsorbed on copper and nickel. If this is the case, the photoemission spectrum should have a multiple1 splitting similar to that found for core-ionized.
free CO with an electron in the 2n
orbital. The inelastic spectra for CO in the region of the carbon and oxygen K edges taken at a scattering angle of 120” and with impact energies of 400 eV for carbon and 650 eV for oxygen are shown in figs. 2a and Zb, respectively_ With these impact energies the carbon and oxygen peaks have nearly the same kinetic energy and are, therefore, measured with the same resolution, = 0.5 eV (full width at half maximum). The multiplet splitting for carbon is obvious_ The oxygen peak, with a width of 1.4 eV is considerably broader than the individual carbon peaks. We have measured
Volume 96, number 2
CHEhUCAL
PHYSICS
1 April 1983
LmERS
.
. .
ai
.
i f
-.
.-
Z:
.
:
-
.
‘Z -.-
.
-
+-_\-
.
- . 0 in CO
.
-
.
. t
-
i .
.
-
-
__
-
:: -.
- .\ _.
-
i
. \=’
.
y._&--
X ..
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.
. .
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.
.-
. -
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-
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-_
-_m
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1
.
. .
l . .
.
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. .
.
\.....-...-. -.-.._ .-
-=
.-
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LOSS
. l* . . .-
l:. l l-.==
- .
.
-
I
!
-*__. _ -..
._
l --
I
50
A_.
. :
-_.
._
-
..
5.
.:._
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1
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\- . .?
.
. ..- \-
- _ .-_ l
C
(eV)
Fig_ 2. (a) Carbon K-shell energy-loss spectrum of CO by 400 eV electron impact. @) Oxygen K-shell energy-loss spectrum of CO by 650 eV electron impact-
the oxygen spectrum over the range of incident electron energies that produce comparable populations of singlet and triplet states in N-) and for carbon in CO, but are unable to see any evidence for a splitting in oxygen. The splitting must be significantly smaller than in carbon This difference arises because the 2n orbital is localized mostly on the carbon 191 (see also p_ 56 of ref. [S] for discussion of multiplet splitting in diatomic molecules); the exchange integral, which determines the multiplet splitting, is therefore larger for a carbon 1s hole than for an oxygen 1s hole.
The multiplet splitting is particularly striking in the carbon Auger spectrum of carbon monoxide. In fig. 3 we show the autoionizing region of this spectrum excited in three different ways: by high-energy electron impact (fig_ 3a), by the (e, 2e) coincidence technique [7] to observe the decay of the CO[20]2rr state (fig. 3b), and excited by 400 eV electrons (fig_ 3~). In the last of these we see that each of the peaks
. .:= . - ._ ._. . . _ - ;. -?A ;t
.
.
i
260
--
.
: .T ._.
S.-_-:
. ._. . r: :
- _ :;
- -\
--
-.-
..‘-
l
1
265
_-_.’ .
!
270
.
_
:e:[ 275
KINETIC ENERGY (eV1 Fig_ 3_ Hi& kinetic energy part of KLL Awer spectrum of CO: (a) Excited by 25 keV electron impact. (b) Excited by 2.5 keV electron impact and in coincidence with inelastically scattered electrons of 287 eV loss. The three peaks on the right correspond to transitions in which carbon monoxide is left with a vacancy in the indicated orbitais. Peaks X and Y represent transitions to fmal states involving an electron in the 2n orbital and two vacancies in the valence orbitals. (c) Exited by 400 eV electron impact. The labels S and T show the splitting of the lines of fig_ 3b that arises because of population of both ‘n and 3n intermediate states at low impact energies_
249
volume
96. number :!
in fig. 3b, which
CHEMICAL
PHYSICS LETTERS
arise only from the ‘II state, has acquired a sarellite of comparable intensity at 1.5 eV lower energy due to decay of the 3iI state. (An additional feature at 262 eV that is found in the spectrum at 111g.hmlpacr energy appears to be strongly enhanced at low tmpact energy. This feature does not appear in the coincidence spectrum, fig. 3b, and. therefore, does not originate from the CO [%]2n state.) Averages of several measurements of the multiplet splitting. usmg both Auger and inelastic scattering spectra. give multiplet splittings of 0.8 eV for N, and 1.45 eV for carbon on CO. The unresolved spectrum for oxygen on CO has a full width at half maximum of 1.-I eV_ slightly broader than the value of 1.3 eV observed by Hitchcock and Brion [lo] with similar instrumental resolution but under conditions where only dipole excitations is possible_ Comparison of the shape of the oxygen curve. fig. 2b, with the shapes ofcomposite curves given by Siegbahn et al. [5, p_ 1701 indicates that the splitting for oxygen is between 0.4 and 0.6 eV. The result for carbon is in excellent agreement with the theoretical value of 1.4 eV calculated by Bagus and See1 [3]. That for oxygen is reasonably close to their calculated value of 0.3 eV_ If the calculations of Bagus and See1 [3] and Goddard and Allison [S] arc correcr. rhe carbon Is XPS spectra for CO adsorbed on transition metals should show a multiplet splitting comparable to what \\\c have measured . or == 1.5 eV. The data shown by f3rundle et al. [ 1 I] for CO adsorbed on nickel does 1101.however. reveal any evidence for such a splitting. The mitt pesk shown in their spectrum is symmetric
250
1 April 1983
and is inconsistent with a doublet split by 1.45 eV with a 2 : 1 intensity ratio. The experimental results, therefore, do not support the theoretical conclusion. We are indebted to Rolf Marme for giving us guidat a key point. This work was supported in part by the U.S. National Science Foundation. ance
References [l]
[2] [3] [4 J
IS]
[6] [7 ] [8] [9] [lo] [ 111
R.A. Bonham, in: Electron spectroscopy, theory, techniques, and applications, Vol. 3, eds. CR. Brundle and A-D. Baker (Academic Press, New York, 1979) p_ 177. D.A. Shaw, G.C. King, F.H. Read and D. Cvejanovic, 1. Phys. B15 (1982) 1785. P.S. Bagus and Jl_ Seel, Phys. Rev. B23 (1981) 2065. P-H. Citrin, R.W. Shaw Jr. and T.D. Thomas, in: Electron spectroscopy, ed. D.A. Shirley (North-Holland, Amsterdam, 1972) p. 105. I;. Siegbahn, C. Nordling, G. Johansson,J. Hedman, P-F. Hed&. K. Hamrin, U. Gelius,T. Bergmark, L-0. Werme, R. Manne and Y. Baer, ESCA applied to free molecules (North-Holland. Amsterdam, 1969). W-E. Moddeman, T-A. Carlson, MO. Krause and B.P. Pullen, J. Chem. Phys. 5.5 (1971) 2317. L. Ungier, J .K. Gimzewski and T.D. Thomas, to be published. \%‘.A.Goddard and J. Allison, unpublished. L.C. Snyder and H. Basch. Molecular wave functions and properties (Wiley, New York, 1972). A-P. Hitchcock and C-E. Brion. J. Electron Spectry_ 18 (1980) 1. C-R. Brundle, P.S. Bagus, D. Mensel and K. Herman, Phys. Rev. B24 (1981) 7041_
1 April 1983
CHEMICAL PHYSICS LETTERS
Volume 96, number 2
EXCITATION SPLITTING
OF CORE HOLES BY ELECTRON
IMPACT.
IN CO AND N2
and T. Darrah THOMAS
Departmentof CRemiitry.Oregon State University. Corrallis. Oregvn 97331. USA
Received 30 December 1982
in on the carbon. The splittins affects
Lowenergy electron impact leads to nondipole excitation of core electrons to triplet states. The multiplet splitting CO is in agreement with theory and shows that the 2~ orbital is localized predominantly both energyqoss and Auger spectra of CO and N2 _
Electron-impact excitation of atoms and molecules at high incident energies is dominated by dipole selection rules because the incident electrons are predominantly scattered forward with small energy loss and hence small momentum transfer_ At lower energies the Born approximation breaks down and spin-forbidden processes become possible. Such non-dipole excitation of valence electrons is well known [ 11. Recently Shaw et al. [3] have reported the first case of non-dipole excitation of a core electron. At low impact energies they have observed excitation of both the singlet and triplet states that are possible when a nitrogen 1s electron in N2 is excited to the vacant lng state. In their experiments they were able to resolve the vibrational structure of the transitions_ We report here results on similar excitations in CO (at lower resolution) and on the effect of such excitation on the Auger spectrum of N2 and CO. The measured singlet-triplet splittings for CO are in agreement with those calculated by Bagus and See1 [3]. The much larger splitting observed for carbon core excitation than for oxygen verifies that the 2n orbital of CO is predominantly localized on the carbon. The multiplet splitting is also seen in subsequent Auger decay of these autoionizing states. That multiplet splitting is not evident in the carbon 1s photoemission spectrum for CO adsorbed on nickel suggests that a reexamination of theoretical interpretations of such photoemission is necessary. In the experiments reported here an electron gun 0 009-2614/S3/0000-0000/S
03-00 0
1983
North-Holland
(Cliftronics CE 3k/5u. Cliftronics, Clifton, New Jersey) is mounted on the axis of the Oregon State University cylindrical mirror analyzer (CMA) [4]_ Electrons with kinetic energies of up to 5 keV are focused on a gas sample cell at the object point of the CMA. Electrons scattering elastically and inelastically through an angle of either 120° or 60” (depending on the orientation of the electron gun) as well as electrons resulting from Auger de-excitation, autoionization, or other processes are analyzed in the CMA. Fig. la shows a typical Auger spectrum for N2 escited with electrons of 3,500 eV. The lower-energy peaks originate from core-ionized N2 ions (N2 [Is]+) while those at higher energies arise from neutral N2 molecules (N2 [Is] 14) in which a core electron has been excited to a vacant In, orbital [5, p_ 67: 6]_ Depending on one’s point of v:ew these transitions can be considered as de-excitation by autoionization, as de-excitation by Auger processes, or as resonant ionization of valence hole states. The highest-energy peak -a doublet unresolved in this experiment - arises from transitions from the N2 [Is] 14 state to the N2[3ug]+ and N,[llr,]+ states, or, equivalently, from resonant ionization of the least bound electrons in N2_ The resonance region is seen more clearly in fig_ lb, which shows a spectrum obtained using the (e, 2e) coincidence technique [7] to observe the decay of the NZ [Is] 14 state. The unresolved highenergy doublet, at 384 eV, arises from transitions in which the 247
\‘olumc
96. number 2
I
CHEMICAL
I
c
-a
a
PHYSICS
by inelastic scattering, we have subtracted a smooth background in order to emphasize the points of interest. The resulting spectrum reveals two features. First, the autoionizing region is strongly enhanced rel-
I
ative
3
KINETIC ENERGY
1. tl~ghhmsnc energy p3rt of KLL Xuger spectrum of b) 1.5 hrv rkctron impact.(b) Excited by 2.5 hc\’ clwtron impact and m coincidence with inelastically \c~!fzrcd rlectronr of401 e\‘encrgy loss.(c) Excited by 500 r\’ clcctr~ mrp~cr. The mset shops zn exp.mded view of the hl&
S:_
(J) ~\LIIL'~
ITT?,clecrron Io\rercnrrgy
p.rrriclp~trs
fe.mre
in the Auger decay: the between 367 and 380 eV arises
fiam rhose in which the 17r%electron remsins as a spec’~.r~or [S. p. 67; 61. III riw sprctr.r shown m t?gs_ I;1 and 1b. only dipole excitation is involved. At lower electron-impact energyhowever. non-dipole processes become important. Fig. Ic shows rhe spectrum taken with electron-impact energy of 500 eV. Since rhe spectrum is dominated 24s
1 April1983
LETTERS
to the normal
Auger
region;
at energies
close
to
the Ii edge, the normal Auger processes disappear completely. Second, the highest-energy peak (shown in detail in the inset of fig. 3c) has become two closely spaced peaks, with the new peak being lower in energy than the old by = 0.8 eV. Close comparison shows that the low-energy feature between 367 and 380 eV is also shifted slightly to lower energies in fig. lc compared to fig. lb. The estra peaks arise from non-dipole excitation of the N, [ Is] 14 state. Under dipole selection rules only the 1 II state is excited. At low impact energies, however. the dipole selection rules break down and exchange processes that allow population of the lower-energy 3Tl state are possible- The splitting between these two is the difference between the ener@es of the singlet and triplet coupling of the core hole with the single 17~~electron. The same splitting can also be seen in the melastically scattered electrons as has been reported by Shaw et al. [2]. Chemically more interesting than N, is CO, in which two different core states can be excited. Furthermore, the multiplet splitting in CO is an important parameter for the interpretation of core emission spectra of CO adsorbed on metals. Calculations by Bagus and See1 [3] and by Goddard and Allison [S] indicate that a single, unpaired electron is transferred from the metal to the vacant 2a orbital of CO during core ionization of CO adsorbed on copper and nickel. If this is the case, the photoemission spectrum should have a multiple1 splitting similar to that found for core-ionized.
free CO with an electron in the 2n
orbital. The inelastic spectra for CO in the region of the carbon and oxygen K edges taken at a scattering angle of 120” and with impact energies of 400 eV for carbon and 650 eV for oxygen are shown in figs. 2a and Zb, respectively_ With these impact energies the carbon and oxygen peaks have nearly the same kinetic energy and are, therefore, measured with the same resolution, = 0.5 eV (full width at half maximum). The multiplet splitting for carbon is obvious_ The oxygen peak, with a width of 1.4 eV is considerably broader than the individual carbon peaks. We have measured
Volume 96, number 2
CHEhUCAL
PHYSICS
1 April 1983
LmERS
.
. .
ai
.
i f
-.
.-
Z:
.
:
-
.
‘Z -.-
.
-
+-_\-
.
- . 0 in CO
.
-
.
. t
-
i .
.
-
-
__
-
:: -.
- .\ _.
-
i
. \=’
.
y._&--
X ..
I I
.
. .
I
.
.
.-
. -
:
-
-..w=
-_
-_m
bj
Y
Eoa650eV
1
.
. .
l . .
.
i
. .
.
\.....-...-. -.-.._ .-
-=
.-
.
i i
l
-:-_Ia_L ENERGY
LOSS
. l* . . .-
l:. l l-.==
- .
.
-
I
!
-*__. _ -..
._
l --
I
50
A_.
. :
-_.
._
-
..
5.
.:._
I
!
1
I
l
\- . .?
.
. ..- \-
- _ .-_ l
C
(eV)
Fig_ 2. (a) Carbon K-shell energy-loss spectrum of CO by 400 eV electron impact. @) Oxygen K-shell energy-loss spectrum of CO by 650 eV electron impact-
the oxygen spectrum over the range of incident electron energies that produce comparable populations of singlet and triplet states in N-) and for carbon in CO, but are unable to see any evidence for a splitting in oxygen. The splitting must be significantly smaller than in carbon This difference arises because the 2n orbital is localized mostly on the carbon 191 (see also p_ 56 of ref. [S] for discussion of multiplet splitting in diatomic molecules); the exchange integral, which determines the multiplet splitting, is therefore larger for a carbon 1s hole than for an oxygen 1s hole.
The multiplet splitting is particularly striking in the carbon Auger spectrum of carbon monoxide. In fig. 3 we show the autoionizing region of this spectrum excited in three different ways: by high-energy electron impact (fig_ 3a), by the (e, 2e) coincidence technique [7] to observe the decay of the CO[20]2rr state (fig. 3b), and excited by 400 eV electrons (fig_ 3~). In the last of these we see that each of the peaks
. .:= . - ._ ._. . . _ - ;. -?A ;t
.
.
i
260
--
.
: .T ._.
S.-_-:
. ._. . r: :
- _ :;
- -\
--
-.-
..‘-
l
1
265
_-_.’ .
!
270
.
_
:e:[ 275
KINETIC ENERGY (eV1 Fig_ 3_ Hi& kinetic energy part of KLL Awer spectrum of CO: (a) Excited by 25 keV electron impact. (b) Excited by 2.5 keV electron impact and in coincidence with inelastically scattered electrons of 287 eV loss. The three peaks on the right correspond to transitions in which carbon monoxide is left with a vacancy in the indicated orbitais. Peaks X and Y represent transitions to fmal states involving an electron in the 2n orbital and two vacancies in the valence orbitals. (c) Exited by 400 eV electron impact. The labels S and T show the splitting of the lines of fig_ 3b that arises because of population of both ‘n and 3n intermediate states at low impact energies_
249
volume
96. number :!
in fig. 3b, which
CHEMICAL
PHYSICS LETTERS
arise only from the ‘II state, has acquired a sarellite of comparable intensity at 1.5 eV lower energy due to decay of the 3iI state. (An additional feature at 262 eV that is found in the spectrum at 111g.hmlpacr energy appears to be strongly enhanced at low tmpact energy. This feature does not appear in the coincidence spectrum, fig. 3b, and. therefore, does not originate from the CO [%]2n state.) Averages of several measurements of the multiplet splitting. usmg both Auger and inelastic scattering spectra. give multiplet splittings of 0.8 eV for N, and 1.45 eV for carbon on CO. The unresolved spectrum for oxygen on CO has a full width at half maximum of 1.-I eV_ slightly broader than the value of 1.3 eV observed by Hitchcock and Brion [lo] with similar instrumental resolution but under conditions where only dipole excitations is possible_ Comparison of the shape of the oxygen curve. fig. 2b, with the shapes ofcomposite curves given by Siegbahn et al. [5, p_ 1701 indicates that the splitting for oxygen is between 0.4 and 0.6 eV. The result for carbon is in excellent agreement with the theoretical value of 1.4 eV calculated by Bagus and See1 [3]. That for oxygen is reasonably close to their calculated value of 0.3 eV_ If the calculations of Bagus and See1 [3] and Goddard and Allison [S] arc correcr. rhe carbon Is XPS spectra for CO adsorbed on transition metals should show a multiplet splitting comparable to what \\\c have measured . or == 1.5 eV. The data shown by f3rundle et al. [ 1 I] for CO adsorbed on nickel does 1101.however. reveal any evidence for such a splitting. The mitt pesk shown in their spectrum is symmetric
250
1 April 1983
and is inconsistent with a doublet split by 1.45 eV with a 2 : 1 intensity ratio. The experimental results, therefore, do not support the theoretical conclusion. We are indebted to Rolf Marme for giving us guidat a key point. This work was supported in part by the U.S. National Science Foundation. ance
References [l]
[2] [3] [4 J
IS]
[6] [7 ] [8] [9] [lo] [ 111
R.A. Bonham, in: Electron spectroscopy, theory, techniques, and applications, Vol. 3, eds. CR. Brundle and A-D. Baker (Academic Press, New York, 1979) p_ 177. D.A. Shaw, G.C. King, F.H. Read and D. Cvejanovic, 1. Phys. B15 (1982) 1785. P.S. Bagus and Jl_ Seel, Phys. Rev. B23 (1981) 2065. P-H. Citrin, R.W. Shaw Jr. and T.D. Thomas, in: Electron spectroscopy, ed. D.A. Shirley (North-Holland, Amsterdam, 1972) p. 105. I;. Siegbahn, C. Nordling, G. Johansson,J. Hedman, P-F. Hed&. K. Hamrin, U. Gelius,T. Bergmark, L-0. Werme, R. Manne and Y. Baer, ESCA applied to free molecules (North-Holland. Amsterdam, 1969). W-E. Moddeman, T-A. Carlson, MO. Krause and B.P. Pullen, J. Chem. Phys. 5.5 (1971) 2317. L. Ungier, J .K. Gimzewski and T.D. Thomas, to be published. \%‘.A.Goddard and J. Allison, unpublished. L.C. Snyder and H. Basch. Molecular wave functions and properties (Wiley, New York, 1972). A-P. Hitchcock and C-E. Brion. J. Electron Spectry_ 18 (1980) 1. C-R. Brundle, P.S. Bagus, D. Mensel and K. Herman, Phys. Rev. B24 (1981) 7041_