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ESCA STUDIES OF CORE AND VALENCE
ELECTRONS IN GASES AND SOLIDS
C. Nordling
To cite this version:
C. Nordling. ESCA STUDIES OF CORE AND VALENCE ELECTRONS IN GASES
AND SOLIDS. Journal de Physique Colloques, 1971, 32 (C4), pp.C4-254-C4-263.
<10.1051/jphyscol:1971447>. <jpa-00214648>
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Submitted on 1 Jan 1971
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JOURNAL DE PHYSIQUE
Colloque C4, supplkment au no 10, Tome 32, Octobre 1971, page C4-254
ESCA STUDIES OF CORE AND VALENCE ELECTRONS
IN GASES AND SOLIDS
C. NORDLING (*)
Institute of Physics, Box 530, S-751 21 Uppsala 1, Sweden
Rbsumb. - Les processus electroniques dans les systkmes atomiques sont habituellement
associes a l'kmission ou l'absorption de photons ou a remission d'electrons. L'ktude spectroscopique peut par consequent s'effectuer par l'analyse du rayonnement electromagnetique ou par la
mesure de 1'6nergie cinetique des klectrons. Alors que la spectroscopie electromagn8tique dans le
domaine optique se pratique depuis des siecles et depuis plusieurs decades pour les rayons X,
l'electron lui-m&men'a pas ete tres utilise pour explorer la structure Blectronique et les processus
electroniques. Cependant en raison du developpement ces dernikres annees de moyens experimentaux pour I'analyse precise des spectres d'klectrons, ce type de spectroscopie a produit des rksultats
tres encourageants qui montrent que la spectroscopie basee sur l'observation directe des electrons
est une methode efficace d'etude des systkmes atomiques et mol6culaires. La spectroscopie des
electrons fournit egalement des renseignements qu'on ne peut obtenir par d'autres types de mesure
et il y a u n grand nombre d'applications de ce nouveau type de spectroscopie.
Noys exposerons brikvement le travail effectue par notre groupe k Uppsala dans le domaine de la
spectroscopie des electrons pour les atomes et les mol6cules. Un expose plus etendu peut 6tre trouv6
dans les rkfkrences [I] et [2].
Abstract. - Electronic processes in atomic systems are usually associated with theemission or
absorption of photons or the emission of electrons. The spectroscopic study of these processes can
therefole be made by the analysis of the electromagnetic radiation or by a measurement of the
kinetic energies of electrons. While electromagnetic spectroscopy in the optical region has been
made for centuries and in the X-ray region for many decades the electron itself has not been used
very much to probe the electronic structure and the electronic processes. However, following the
development in recent years of experimental devices for the exact analysis of electron spectra this
type of spectroscopy has now produced some very encouraging results which indicate that the
spectroscopy based on the direct observation of the electrons is a powerful method for the study
of atomic and molecular systems. Electron spectroscopy also produces information which cannot
be obtained by other types of measurement and there is a multiplicity of applications for this new
type of spectroscopy.
A brief account will be given of the work which has been done by our group at Uppsala in the
field of electron spectroscopy for atoms and molecules. A more comprehensive account until the
present year is given in references [I] and [2].
1. EIectron binding energies and photoionization
dynamics in the noble gases. - Different modes have
been used>to excite the electron spectra, viz X-rays,
UV-radiation, and electron impact, and the energy
(or momentum) analysis of the spectra is made in
double focussing electron spectrometers of electrostatic or magnetic type, see figure 1. When photons
are used for the excitation the kinetic energies of the
expelled electron are
where Eb is the electron binding energy (ionization
energy). With X-ray quanta one can liberate electrons
from all parts of the electronic structure, i. e. one can
study both the atomic core and the valence electrons
in molecules and solids. This is the mode of excitation
that we have used in most cases. UV excitation has the
advantage that electron lines with smaller inherent
SPECTROMETER
FIG. 1.
- Different modes
of excitation of electron spectra.
widths can be obtained but the technique is limited
to the outermost parts of the electronic structure.
X-ray induced electron spectra from the noble
gases are shown in figure 2. The spectra map out in
some detail the electronic structure of the noble gas
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971447
ESCA STUDIES OF CORE AND VALENCE ELECTRONS IN GASES AND SOLIDS
c/120~,
400-
-
Mg K a
He
loo30
2m0'
1000
o-
Ne
k
880
20
'
' ev
20
'
'ev
1s
1
870
.
I
860
" &I Lo
'
'
30
'
C
I>
Z
C4-255
atoms, from the outer shells and as far into the atomic
core as attainable with the quantum energy of the
magnesium Ka radiation, 1 253.6 eV. Electron emission
from deeper lying shells was induced by harder radiation, for example Cu Kcc. The electron binding energies
obtained for the noble gases are given in Table I (see
also ref. [2]). One can conveniently study the entire
electronic structure by use of one and the same instrument and at a resolution close to the limit set by
inherent width of the atomic levels. For example, the
observed width at half maximum intensity of the neon
1 s line is 0.80 eV which is near the natural width
of the exciting X-radiation. The spectrometer window
was in this case approximately 0.2 eV. This high
resolution is valuable for a more detailed study of e. g.
the dynamics of the K photo emission. Figure 3
8
100-
100
8 1
KINETIC ENERGY
100
FIG.3. - Neon 1 s electron spectrum excited by Mg K radia690
680 670 "
210" 140" 70
BINDING ENERGY
FIG. 2. - ESCA spectra from the noble gases excited
by Mg Ka X-radiation.
tion at a pressure of 0.5 torr. The main peak (0)is the Ne l s line
excited by Mg Kal,z. The M g KX-ray satellites and KB radiation
give the peaks with higher kinetic energy. The peaks with lower
energies (numbers 1-12) are due to shakeup, shake-off and
inelastic scattering.
Binding energies for the noble gases (eV)
(*) Reference value (from optical spectra).
C. NOR
C4-256
shows the electron spectrum from neon over an energy
range of 130 eV around the Ne 1 s line. On both sides
of the main line a number of satellite lines are observed.
The intensities of these lines are less than 10 per cent
of the main line which in the figure has been reduced
in intensity by a factor of 20.
The satellite lines have essentially three different
origins. The high energy lines are due to the high
energy satellite lines in the incoming X-radiation.
Photoelectrons from the neon 1 s shell induced by
more energetic X-radiation will consequently have
higher kinetic energy. The low energy part of the
spectrum contains a number of fairly sharp peaks
superposed on a broad continuum starting sharply
around 362 eV and extending some 50 eV toward
lower electron kinetic energies. The intensity of the
continuum and the distinct features 1 to 4 in figure 3,
measured relative to the main line, is found to be
pressure dependent. These features are therefore
interpreted as due to secondary collisions between
ejected photoelectrons and neutral atoms. The remaining features 5 to 1 2 in the spectrum are independent of
pressure and are due to ionization processes at which
a valence electron is simultaneously ejected or excited.
The former process is usually called shake-off and for
the latter we have used the term shake-up. It turns out
that lines 7-1 1 are due to shake-up processes to states
of the type 1 s 2 s2 2 p5 np2S and line 12 to a shakeup process of type 1 s 2 s 2 p6 ns2S.
Lines 5 and 6 are interpreted as the Ka,,, satellite
lines of 7 and 8 9. The complete shake-off of a 2 p
electron, i. e. the ionization limit of the first term series,
occurs close to line 11 at an excitation energy of 47 eV.
The shake-up states have three unpaired electrons.
Using a multiconfigurational SCF procedure the term
splitting could be calculated for n = 3,4 and 5. Thus,
lines 7 and 8 could be identified as a lower and upper
doublet state, respectively, in the term
+
is an order of magnitude smaller, i. e. E,,,,
which is a reasonable numerical value.
E
1.8 eV,
2. Autoionization and Auger electron spectra. Electrons are used to excite autoionization and Auger
electron spectra from gases. (Auger electron spectra
are also excited by photons.) The electron energies are
then independent of the energy of the bombarding
particles :
E" denotes the energy (above the first ionization energy)
of the excited atom in the autoionization process. E+ is
the energy of an atomic or molecular ion with an
inner (Auger) or outer (autoionization) shell vacancy,
E C + is the energy of the doubly ionized atom or
molecule in the Auger process. E,(i) denotes the binding energy of electron i (i = 1 is initial state vacancy
in the Auger process, i = 2, 3 are the final state vacancies). Thus from autoionization and Auger electron
spectra one obtains complementary information to
that obtained from photoelectron spectra : from the
autoionization spectra one obtains information on
highly excited states of the neutral atom or molecule
and from the Auger spectra one obtains information
on doubly ionized atoms and molecules.
As an example of an autoionization electron spectrum of a noble gas figure 4 shows a high resolution
410.
ARGON
lwo-
,
-
4
,
.
1
-
I
I
lines 9 and 11 as a lower and upper state in
FIG.4. - Autoionization electron spectrum from
showing four identified series of lines.
and line 10 as the lower state in Ne' 1 s 2 s2 2 p5 5 p.
Compared to the experimental spectrum the calculated
energies reIative to the main Ne 1 s lines are consistently displaced toward lower energies by about 1.8 eV.
The spacing between the states is very close to that
experimentally observed, however. These findings
may be explained in the following way : The main line
represents a state with one 2 p electron pair more than
he shake up states. There will thus be a larger electron
correlation energy for the main 1 s hole state than for
the shake up states. If the SCF calculations, which
omit the correlation energy, are accurate enough, the
difference found between the observed state energies
and the calculated ones should yield an approximate
value of the electron correlation energy for a 2 p eIectron in Ne' 1 s 2 s2 2 p6 since the relativistic correction
argon
study of argon. Several Rydberg series can be identified
in this spectrum with series limits corresponding to
ionization of a 3 s electron in the initiaI state. The
appearance of two Rydberg series for each of the
initial state configurations 3 s 3 p6 nd and 3 s 3 p6 np
can be assigned to the spin-orbit splitting of the 3 p
shell. The splitting is 0.18 eV and the intensity ratio
between the two Rydberg series of each configuration
is 2 : 1. It is interesting to note that these findings are
in close agreement with what we previously have
observed by means of photoionization of argon
using the He resonance radiation at 21 eV for excitation
This doublet is shown in figure 5. The resolution in
this experiment, defined as the full width at half
maximum height of the electron line, is I3 meV.
ESCA STUDIES OF CORE AND VALENCE ELECTRONS IN GASES AND SOLIDS
C4-257
Molecular Auger and autoionization electron spectra
have been little studied so far. Evidently, the electron
source for excitation need not be monokinetic (as
is the case for studies of discrete energy loss spectra of
electrons). In the experiments which we have performed, the electron source has been a beam from an
electron gun with an energy of a few keV traversing
the gas target chamber perpendicular to the emission
angle of theelectrons to be studied. In this way we have
recorded at high resolution Auger electron spectra
from all the noble gases. These spectra are very line
rich and can in part be compared to transitions in UV
spectra. An interesting field from a chemical point of
view is the study of such spectra emitted from molecules.
We have been able to observe chemical shifts in such
spectra both for solids and gases, although in general
these spectra are much more complex and more
difficult to interpret than the photo electron spectra.
We have also found evidence for a vibrational structure
in Auger electron lines as well as in autoionization
electron lines. An example of this is the carbon Auger
electron spectrum of CO, figure 6. The left part of
co
CARBON AUGER
25000
O
KINETIC ENERGY
-
L
L
L
M
t60
no
I
eV
KlNErlC ENERGY
I
I
FIG. 6. - Part of the carbon Auger and autoionization
electron spectrum from CO excited by electron impact. The
insert figure shows the vibrational structure in some of the
autoionization lines.
16.00
15.75
BINDING ENERGY
eV
the spectrum is the Auger part showing a closely
spaced vibrational structure at the right side of a
strongly excited single Auger line. Further out to the
higher energy side of the spectrum there are a few
autoionization electron groups with clearly resolved
vibrational components.
XENON
1325
KINETIC ENERGY
1100
1271
ENDING EtLERCI
llS0
1225
FIG. 5. - Electron spectra of argon and xenon showing the
spin-orbit splitting of the 3 ps(2P) and 5 p5(2P) term, respectively.
The spectra were excited by helium resonance radiation.
3. Core and valence electron spectra in molecules. As mentioned previously one can study both the core
and the valence electron structure when X-rays are
used to excite the electron spectra. Figure 7 shows an
electron spectrum from carbon tetrafluoride, excited
by Mg Ka. Between 1 200 eV and 1 250 eV kinetic
energy we find the valence molecular orbitals which in
this molecule derive mainly from the atomic 2 s and
2 p orbitals of carbon and fluorine. The two lines at
kinetic energy 952 eV and 558 eV are the core electron
lines C 1 s and F 1 s. One interesting feature of the
core electron lines is that the width of F 1 s is considerably larger than that of the C I s line, and much
C . NORDLING
C4-258
~140s
AI I?
01s
FIG. 7. - ESCA spectrum from carbon tetrafluoride excited
by Mg Ka radiation. The atomic-like core orbitals F 1 s and C 1 s,
and the valence molecular orbitals are seen in this ESCA spectrum as well as the fluorine K Auger electrons.
larger than the K level width in this region of the
periodic system. The spectrum in figure 7 also shows
the fluorine K Auger spectrum in the energy interval
between 630 eV and 660 eV.
The 32 valence electrons in CF, are distributed
among 16 molecular orbitals but due to the high
symmetry of the molecule several of these orbitals
are degenerate and only 7 different ionization energies
are observable in the valence electron spectrum. Three
of these states have previously been studied in UV
excited spectra and are attributed to mainly nonbonding molecular orbitals. For an understanding
of the chemical bonding it is of interest to study also
the bonding orbitals. In figure 7 these deeper orbital
states can now be seen. The deepest lying valence
orbitals are 1 a, and 1 t, which are mainly of F 2 s
character. There is a small bonding C 2 s contribution
to the 1 a, orbital and a similar C 2 p contribution to
the triply degenerate 1 t, orbitals. The large width
of the deepest valence orbitals can be explained by an
increased inherent width due to radiationless transitions of Coster-Kronig type and, to a lesser extent, to
vibrational structure of these strongly bonding orbitals.
The 2 t, has mainly C 2 p-F 2 pa bonding character
while 2 a, has C 2 s-F(2 s 2 p,) a bonding character.
The non-bonding orbitals 1 e, I t,, and 3 t, are seen to
the right in the spectrum.
For the interpretation of the molecular orbital
spectra we have employed semiempirical (CNDO)
and ab initio caIculations of the valence electron
configuration and the parentage of the molecular
orbitals. A particularly useful feature of the X-ray
mode of excitation in this context is the strong dependence of the photoemission cross-section on the
symmetry of the orbitals.
An interesting line splitting is observed in the core
electron spectra from paramagnetic molecules. This
splitting is shown for molecular oxygen in figure 8.
The spectrum was obtained by letting air into the
collision chamber at a pressure of 0.1 torr and irradiating the gas with Mg Ka. It shows the 1 s lines of
+
I
710
A2P
Nls
715' 840
845
" 1000
1005
eV
I
KINETIC ENERGY
FIG.8. - ESCA spectrum from air excited by Mg Ka radiation. The oxygen l s line shows spin splitting (paramagnetic
molecule). Argon is detected through its 2 p electrons and the
spin-orbit splitting is well resolved.
oxygen and nitrogen and also the 2 p spin-doublet in
argon which in this case had a partial pressure of
torr. The oxygen 1 s line is split into two components,
1.1 eV apart and with the intensity ratio 2 : 1. The two
lines correspond to the quartet and doublet states
respectively in which the oxygen molecule can be left
upon emission of a 1 s electron. Due to the exchange
interaction between the 1 s core electron (spin s = +)
and the two unpaired outer electrons (resulting spin
S = 1) the two states have different energies and give
rise to two lines with intensities proportional to the
weights of the states (4 :2). In the case of 0, one can
calculate this t( spin splitting )) by use of the vector
coupling method. The energy difference between
the ( S 3) and ( S - 3) states is obtained as
+
where K,, is the exchange integral defined by
In this expression for the exchange integral s(i) and
p(i) denote the 1 s and n, 2 p wave functions. The spin
splitting of core electron lines has also been studied in
transition metals with unfilled d-bands [3].
The spin splitting disappears when oxygen is bound
chemically to other atoms in a diamagnetic molecule.
This is illustrated by the 0, and H 2 0 spectra in
figure 9. However, chemical binding introduces a new
feature in the core electron spectra which is of great
importance, viz. a change in energy of the lines,
characteristic of the chemical bonds with other atoms
in the molecule. For the oxygen 1 s line this chemical
shift >) is 3.5 eV between (the quartet state of) molecular
oxygen and oxygen in H,O, see figure 9. The chemical
shift effect and its application to molecular spectroscopy will be discussed in the following section.
In conclusion the valence electron spectra contain
information on the electrons that take part in the
chemical binding and the chemical shift in the core
ESCA STUDIES OF CORE AND VALENCE ELECTRONS IN GASES AND SOLIDS
C4-259
figure 10, showing carbon in ethyl trifluoroacetate. All
four carbon atoms in this molecule are distinguished
in the spectrum. The lines appear in the same order
from left to right as do the corresponding carbon
atoms in the structure that has been drawn in the
figure.
F-C
F
0
I
I1
H
eV
I
545
I
540
I
-C-0-C-C-H
I
I
F
L
H
I
I
H
H
J
BINDING ENERGY
-
FIG.9.
Oxygen 1 s electron lines from a mixture of oxygen
gas and water vapour. The water line is shifted towards lower
dinding energy in the water molecule and the spin splitting is
remov-d.
electron spectra, vide infra, contain information on
chemical binding and molecular structure. Moreover,
since in the electron spectrum each element of the
sample makes its characteristic contribution, it is possible to identify the different atomic species contained
in the sample and the intensities of the core electron
lines are measures of the number of atoms of the respective elements and valence states. Because of all
these chemical implications the acronym ESCA (Electron Spectroscopy for Chemical Analysis) is used for
the spectroscopy which is based on X-ray induced
emission of electrons,
4. Chemical shifts. - When an atom is bound
chemically to other atoms there is a,change of wave
function for the valence electrons. ConsequentIy there
is a change in the interaction between the valence
electrons and the core electrons and this induces a
slight change in the binding energies of the core (and
valence) electrons. Therefore a change in chemical
environment of an atom is relayed to its core electrons
and can be observed as a line shift in the ESCA spectrum. These chemical shifts of the inner electron
energies can now be measured and are likely to assist
in the solution of many problems in chemistry. A
particular feature of ESCA is that one obtains information on chemical and molecular dynamics by
measuring a quantity that remains essentially atomic
in character. Thus one can move the area of inspection
from one atomic species to the other in the molecular
structure.
A conspicuous chemical shift spectrum is given in
w
J
'
CARBON 1s
I
CV
I
J
I
eV
1190
1195
KINETIC ENERGY
I
295
I
I
290
285
BINDlNG ENERGY
FIG. 10. - Electron spectrum from carbon in ethyl trifluoroacetate. All four carbon atoms in this molecule are distinguished in the spectrum. The lines appear in the same order'from
left to right as do the corresponding carbon aton~sin the structure
that has been drawn in the figure.
In the application of ESCA to chemical problems,
it is desirable to be able to explain the chemical
shifts by means of a simplified chemical language.
By making use of the electronegativity concept in a
quantitative manner this has turned out to be possible.
Correlations between chemical shifts and an atomic
charge parameter q, have been empirically established.
The parameter q, is the sum of the partial ionic characters of the bonds formed by the atom. The partial
ionic characters are obtained from the electronegativity
differences between the atoms forming the bonds, by
use of the relationship by Pauling as described and
discussed in ref. [I].
In figure 11 the chemical shift AE of the carbon 1 s
line has been plotted against q, for a selection of solid
carbon compounds with sp3 and sp2 types of hybridizations. Compounds with as simple substituents as
C4-260
C. NORDLING
small carbon molecules [4]. Extensive basis sets of
Gaussian type functions were used. For hydrogen
six s-type functions contracted to two, and for the
first row atoms eleven s-type functions contracted to
five and seven p-type functions contracted to three
were used. All points fall very close to the straight
line which has a slope of 1.09. The closer calculations
approach the Hartree-Fock limit the more does the
orbital shift (- A&)approach the experimental shift
(BE) (slope 1.00).
FIG. 11. - AE plotted against qp for carbon of tetragonal and
trigonal types of hybridization in molecules with small inductive
effects. Open circles represent chlorine compounds for which
qp has been cak ulated using an uncorrected value for the electronegativity of chlorine.
possible were chosen in order to avoid secondary
effects on the shifts of the binding atoms [4]. The
electronegativities used are those given by Pauling
except for chlorine and bromine. The points for
compounds containing chlorine indicate that the
electronegativity for this element, when bound to
carbon, is not well described by the Pauling value.
Inner electron binding energies of free molecules
can be determined to within a few electron volts from
ab initio quantum mechanical calculations within the
Hartree-Fock (SCF) approximation. Separate calculations are required for the neutral state and the ionized
state, and the electron binding energy is taken is the
difference in total energy between the two states.
Inner electron shifts obtained this way so far are
accurate to within a few tenths of an electron volt.
The high accuracy depends on a cancellation of errors
due to relativistic and electron correlation effects
which for inner electrons are unaffected by changes
in the chemical environment.
Electron binding energies can be obtained also from
Hartree-Fock calculations on the neutral systems
through Koopmans theorem. The additional assumption over separate calculation on neutral and ionized
systems is that the remaining electron of the ion can
be described by the same orbital wavefunctions as in
the initial state. The binding energies obtained in this
way for inner levels of light element are systematically
10-20 eV larger than those found experimentally.
Moreover, they are very sensitive to the size and
optimization of the basis set used. For this reasofi
comparisons of calculations with different basis sets
become meaningful only if the calculations are close
to the Hartree-Fock limit. Systematic series of inner
electron binding energy shifts from Koopmans theorem
are now available and have been compared with
experiments in our recent monograph 121.
Figure 12 shows the correlation between experimental chemical shifts and orbital energy shifts obtained
from ab initio MO LCAO SCF calculations on some
I
0
5
10
I
eV
EXPERIMENTAL CHEMICAL SHIFT
FIG. 12. - Comparison between carbon 1 s energy shifts
measured in the gaseous state of some small carbon molecules
and the shifts obtained from ab initio MO-LCAO-SCF calculations.
A simplification of the theoretical calculation of
binding energy shifts can be made with the help of an
electrostatic potential model [I], [2]. Through this
model the binding energy shifts are related to the electron distribution of the neutral molecule. The model
is purely classical although it can be described and
used in terms of quantum mechanics.
In the potential model the chemical shift is determined by a change in potential for the core electron. This
potential can be considered as a superposition of two
potentials. The first, which generally is the dominating,
originates from the change in electronic distribution
around the particular nucleus being studied within the
molecule. The second potential, which we may call
the molecular potential, is set up by the charge distribution from the rest of the molecule. The molecular
potential is easily estimated having condensed the
charges to point charges. The first potential obviously
is not well described by a point charge at the position
of the nucleus. However, from comparisons with free
atoms and ions it can be expected to be approximately
proportional to the charge of the atom. The expression
for the core level shift then becomes
C4-261
ESCA STUDIES OF CORE AND VALENCE ELECTRONS I N GASES AND SOLIDS
where
The first term in (4) represents the potential from the
charge at the atom considered while the second term,
the molecular potential, accounts for the potential
from the rest of the molecule. The third term is a
constant related to the choice of reference level. The
constant k is approximately equal to the electrostatic
interaction integral between the considered core orbital
and a valence atomic orbital in the same atom. This
integral is close to the expectation value < llr > for
a valence electron [I], [2].
Figures 13 and 14 show the correlation of experi-
mental Cls shifts with shifts calculated in this way from
the ab initio wavefunctions and CND0/2 wavefunctions, respectively. The charges from the ab initio
wavefunction are Mulliken gross atomic charges. The
constants k and 1 were in both cases determined from
a least squares fit of AE - V to kgi + 1. Thus values
for k, 18.3 eV from the ab initio wavefunctions and
23.5 eV from the CND012 calculations agree reasonably well with calculated 1 s-2 p electrostatic repulsion integrals. With an atomic Hartree-Fock wavefunction the value 21.2 eV is obtained while simple
Slater orbitals give 22.0 eV. The change of the Cls
binding energy upon the removal of a 2 p electron,
obtained from independentcal culations on atom free
the four and ion states involved is 18.8 eV.
5. Some applications. - The oxidation of cystine
provides a simple example of how the chemical shifts
in ESCA spectra can be utilized to solve chemical
structure problems [ 5 ] . The cystine molecule contains
two equivalent sulfur atoms :
HOOC-CH-CH2-S-S-CH2-CH-COOH
I
I
"0°
t
CYSTINE S-DIOXIDE
SZp(AIKa)
CALCULATED SHIFT (18.3q+V+3.0)
FIG. 13. - Comparison between measured shifts and shifts
calculated with the potential model using charges obtained from
ab initio calculations.
CYSTINE
S2p(AIKu)
1310
10
ev
I
CALCULATED SHIFT (23.5q+V+0.22)
FIG.14. -. Comparison between measured shifts and shifts
calculated with the potential model using charges obtained from
CNDOJ2 calculations.
1315
1320
1325
KINETIC ENERGY
1
I
eV
170
I
eV
I
165
160
BINDING ENERGY
FIG. 15. - Electron spectrum from the 2p shell of sulfur in
cystine S-dioride and cystine. The valence states of the sulfur
atoms can be determined from the spectra.
C . NORDLING
C4-262
If the cystine dioxide is synthetized two different
structure may be formulated. If one oxygen is attached
to each of the two sulfur atoms, the resulting compound
with equivalent sulfur atoms would give rise to a
single line in the electron spectrum. If both oxygens
are attached to one of the sulfur atoms, i. e. if the
disulfide dioxide has a thiolsulfonate structure, the
two sulfur atoms, having non-equivalent structural
positions, would give rise to two lines at different
energies in the electron spectrum. According to
figure 15 this is actually the case. Instead of one
single line as in the symmetrical cystine, two lines
are obtained from the 2p subshell in sulfur, one
unshifted and the other shifted by 4.0 eV. The
electron spectrum of cystine S-dioxide therefore gives
conclusive evidence for the thiolsulfonate structure :
0
t
1
investigation confirmed that coordination of Iigands
to the metal results in a significant charge transfer
from metal to ligand, see figure 17.
HOOC-CH-CH2-S-S-CH2-CH-COOH
I
I
0
NH2
NH2
A number of other structure problems more complicated than the above quoted example have already
been solved by means'of the ESCA technique. It is
likely that with the improved resolution now under
development still more detailed informations can be
obtained on structure problems for practical use.
The question of electron transfer between the metal
and the carbon atoms in transition metal carbides has
been a matter of discussion over the years. Experimental data have been lacking and there is serious disagreement between the different theories that have been
proposed. ESCA measurements on the core level shifts
in various transition metal carbides and related compounds have zhown that electrons are transferred
from the metal to the carbon 161, see figure 16.
BINDING ENERGY (eV)
FIG. 17. - Platinum 4 f7/2 energies in a series of metal-organic
complexes.
0
m
' a
EszSZa
K mission
Eels
02s
BINDING ENERGY
M
T i metal
FIG. 18. - Electron spectrum from Be0 excited with Mg EKE
radiation. The energy distributions obtained from K-emission
spectra are shown at the top of the figure.
BINDING ENERGY
FIG. 16. - Chemical shifts for titanium and carbon in Tic.
The shift of the Ti 2 P3/2 level indicates that titanium is more
positive in the carbide than in the metal ; the carbon 1 s shift
indicates that carbon is more negative than in the hydrocarbon
reference.
In the study of catalytic reactions much interest is
focussed on the binding of the metal in metal-organic
complexes. We have recently investigated by ESCA a
number of complexes of platinum, in which the metal
is in a formally low oxidation state [7]. Relative oxidation states of platinum in the complexes, as determined
from the binding energy data, were ordered and the
FIG. 19. - Valence band spectra from transition metals.
ESCA STUDIES OF CORE AND VALENCE ELECTRONS IN GASES AND SOLIDS
We have also recently applied the ESCA technique
to some crystal and solid state phenomena. For example, the angular variation in intensity of elastically
scattered electrons expelled by Mg Kcr from various
shells in a sodium chloride crystal have been studied [8].
The escape depth of electrons photoemitted from
a metal by X-rays has been measured [9]. Core lines
and valence bands of LiF, BeO, BN and graphite
have been studied and compared with X-ray spectroscopic data [lo], see figure 18, and the valence bands
C4-263
in transition metals and other solids have been investigated [I], [ll]. Figure 19 shows the valence band
spectra obtained from the transition metals. This
work will be discussed in more detail in another contribution to this conference [12].
Acknowledgement. - It is a pleasure for me to
acknowledge the cooperation of my colleagues at
Uppsala in the research work described in this
review.
References
[I] SIEGBAHN
(K.), NORDLING(C.), FAHLMAN
(A.),
NORDBERG
(R.), HAMRIN(K.), HEDMAN
(J.),
JOHANSSON
(G.), BERGMARK
(T.), KARLSSON
(S.-E.),
LINDGREN
(I.), LINDBERG
(B.), ESCA, Atomic,
Molecular and State Structure studied by ESCA
Nova Acta Regiae Soc. Sci. Upsaliensis, Ser. IV,
Vol. 20, 1967.
[2] SIEGBAHN
(K.), NORDLING(C.), JOHANSSON
(G.),
HEDMAN(J.), HEDBN (P. F.), HAMRIN(K.),
GELIUS(U.), BERGMARK
(T.), WERME
(L. O),
MANNE(R.), BAER(Y.), ESCA applied to free
molecules. North-Holland Publ. Co., AmsterdamLondon, 1969.
(C. S.) and SHIRLEY
(D. A.), FREEMAN
(A. J.),
[3] FADLEY
BAGUS(P. S.) and MALLOW
(J. V.), Phys. Rev.
LING (C.) and LINDBERG
(B. J.), Spectrochim.
Acta, 1967, 23, 2015.
[6] RAMQVIST
(L.), HAMRIN
(K.), JOHANSSON
(G.), FAHLMAN (A.) and NORDLING
(C.), J. Phys. Chem.
Solids, 1969, 30, 1835.
[7] COOK
(C. D.), WAN(K. Y.), GELIUS
(U.), HAMRIN
(K.),
JOHANSSON
(G.), OLSON(E.), SIEGBAHN
(H.),
NORDLING
(C.) and SIEGBAHN
(K.). J. Am. Cchem.
SOC.1971,93,1904.
[S] SIEGBAHN
(K.), GELIUS(U.), SIEGBAHN
(H.) and
OLSON(E.). Physica Scripta, 1970, 1, 272.
[9] BAER(Y.), HEDBN(P. F.), HEDMAN
(J.), KLASSON
(M.)
and NORDLING
(C.). in Solid State Comm., 1970,
8, 1479.
[lo] HAMRIN
(K.), JOHANSSON
(G.), GELIUS
(U.), NORDLING
Letters, 1969, 23, 1397.
(C.) and SIEGBAHN
(K.). Physica Scripta., 1970,
(U.), H E D ~(P.
N F.), HEDMAN
(J.), LINDBERG
1, 277.
[4] GELIUS
(B. J.), MANNE
(R.), NORDBERG
(R.), NORDLING [I11 BAER(Y.), H E D ~(P.
N F.), HEDMAN
(J.), KLASSON
(M.),
(C.) and SIEGBAHN
(K.). Physica Scripta., 1970,
NORDLING(C.) and SIEGBAHN
(K.), Physica
2, 70.
Scripta, 1970, 1, 55.
[5] AXELSON
(G.), HAMRIN
(K.), FAHLMAN
(A.), NORD- [12] BAER(Y.), This conference.
DISCUSSION
- Why Calbon behaves as an
Mr. DAS GUPTA.
acceptor rather than a donor ? On alloying carbon
with iron group alloys carbon usually behaves as a
donor rather than an acceptor.
Reply to questionfrom Das Gupta : I have not seen the
evidence you refer to that carbon usually behaves as a
donor in iron group carbides. The electronegativity
of Fe, Co and-Ni-(X = 1.8) is lower than that i f
carbon ( X = 2.5) and would rather suggest that carbon
is an acceptor. For the transition metal carbides that
we have studied (group IV b and V b ) there had been
great controversy as to the direction of the charge
transfer. Our ESCA results confirmed the prediction
that can be made from the electronegativities that
there is a transfer of charge from metal to carbon.
C. Nordling.
DAVIDJ. NAGEL.- Models exist for non-transition
metal alloys in which an electronegativity parametel is
used to calculate electron redistribution upon alloying.
The conduction electron concentration around, for
s alloys should be reflected
example, aluminum a ~ o m in
in core electron level shifts because the conduction
electron density partially determines the potential
experienced by core electrons.
Have core electron shifts been measured in alloys
in order to determine conduction electron distribution ?
Reply to question from Nagel : We are presently
measuring valence bands and core electron shifts in
some palladium alloys. To my knowledge there have
not yet been any measurements reported on core
electron shifts in other metal alloys.
C. Nordling.
Mr. WIECH.- In one of your slides you showed
the spectrum of the valence electrons of 12 metals.
In some of the curves the intensity at the high and the
low energy side is of the nearly same magnitude, in
others the intensity on the low energy side is high
than on the high energy side. Is this high intensity due
to inelastic scatte~edelectrons or does the valence band
extend to low energies ? Is it possible to determine the
botton of the balence band by your method ?
Reply to questionfrom Wiech : Inelastically scattered
electrons contribute to the intensity on the low energy
side of both core level and valence band spectra. The
low energy tails of core level peaks which are close in
kinetic energy to the valence band spectra can be
used as a measure of this contribution. We have not
yet applied this and other corrections that would be
necessary to determine the bottom of the band.
C. Nordling.