ESCA STUDIES OF CORE AND VALENCE ELECTRONS IN GASES AND SOLIDS

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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�

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 spectrosco- pique 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&me n'a pas ete tres utilise pour explorer la structure Blectronique et les processus electroniques. Cependant en raison du developpement ces dernikres annees de moyens experimen- taux 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 electro- static 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 C4-255

Binding energies for the noble gases (eV)

c / 1 2 0 ~ , atoms, from the outer shells and as far into the atomic

(*) Reference value (from optical spectra).

400-

loo-

1000 2m0'

o-

He Mg K a

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 radia- tion, for example Cu Kcc. The electron binding energies

30 20

' ev obtained for the noble gases are given in Table I (see Ne ^1s also ref. [2]). One can conveniently study the entire electronic structure by use of one and the same ins- trument and at a resolution close to the limit set by

k 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

1 . I

880 870 860 " &I

Lo

30

20

' e v 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.

C Z

I>

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 radia-

690 680 670

210" 140" 70 tion at a pressure of 0.5 torr. The main peak (0) is the Ne l s line

BINDING ENERGY 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 FIG. 2. - ESCA spectra from the noble gases excited energies (numbers 1-12) are due to shakeup, shake-off and

by Mg Ka X-radiation. inelastic scattering.

C4-256 C. NOR 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 remai- ning 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 shake- up 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

lines 9 and 11 as a lower and upper state in

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 consis- tently 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 eIec- tron in Ne' 1 s 2 s2 2 p6 since the relativistic correction

is an order of magnitude smaller, i. e. E,,,, E 1.8 eV, which is a reasonable numerical value.

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 bin- ding energy of electron i (i = 1 is initial state vacancy in the Auger process, i = 2, 3 are the final state vacan- cies). 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 spec- trum of a noble gas figure 4 shows a high resolution

FIG. 4. - Autoionization electron spectrum from argon showing four identified series of lines.

410.

lwo-

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.

ARGON

I

,

-

1 I

ESCA STUDIES O F CORE AND VALENCE ELECTRONS IN GASES AND SOLIDS C4-257

KINETIC ENERGY

-

eV 16.00 15.75

BINDING ENERGY

XENON

KINETIC ENERGY

-

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.

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 perfor- med, 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

CARBON AUGER co

25000

O L L L

I

M t60

no

KlNErlC ENERGY

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.

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.

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 consi-

derably larger than that of the C I s line, and much

C4-258 C . NORDLING

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 spec-

trum 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 non- bonding 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 transi- tions 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 depen- dence 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 irra- diating the gas with Mg Ka. It shows the 1 s lines of

I I

710 715' 840 845 " 1000 1005 eV KINETIC ENERGY

~ 1 4 0 s

FIG. 8. - ESCA spectrum from air excited by Mg Ka radia- tion. 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.

AI I?

01s Nls A2P

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 spectro- scopy 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

L ^{I I} J

eV 545 540

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 pos- sible 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 res- pective elements and valence states. Because of all these chemical implications the acronym ESCA (Elec- tron 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 spec- trum. 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 infor- mation 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

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 0 H H

I I1 I I

F - C - C - 0 - C - C - H

I I I

F H H

J w ^' CARBON 1s

I ^{I I} J

1190 1195 eV

KINETIC ENERGY

- ^{I I I}

CV 295 290 285

BIND lNG ENERGY

FIG. 10. - Electron spectrum from carbon in ethyl trifluo- roacetate. All four carbon atoms in this molecule are distinguis- hed in the spectrum. The lines appear in the same order'from left to right as do the corresponding carbon aton~s in 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 charac- ters 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 hybri-

dizations. Compounds with as simple substituents as

C4-260 C. NORDLING

FIG. 11. - AE plotted against q p for carbon of tetragonal and trigonal types of hybridization in molecules with small inductive effects. Open circles represent chlorine compounds for which q p has been cak ulated using an uncorrected value for the electro-

negativity 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 calcu- lations 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 assump- tion 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 experimen- tal chemical shifts and orbital energy shifts obtained from ab initio MO LCAO SCF calculations on some

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).

I ^I

0 5 10 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 calcu-

lations.

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 elec- tron 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 determi-

ned 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 dis-

tribution 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

ESCA STUDIES OF CORE AND VALENCE ELECTRONS I N GASES AND SOLIDS C4-261

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-

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.

I

10

ev

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.

mental Cls shifts with shifts calculated in this way from the ab initio wavefunctions and CND0/2 wavefunc- tions, 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 reaso- nably well with calculated 1 s-2 p electrostatic repul- sion integrals. With an atomic Hartree-Fock wave- function 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)

CYSTINE S2p(AIKu)

1310 1315 1320 1325 eV

KINETIC ENERGY

1 I ^{I I}

eV 170 165 160

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.

C4-262 C . NORDLING 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

HOOC-CH-CH2-S-S-CH2-CH-COOH

I 1

0 I

NH2 NH2

A number of other structure problems more compli- cated 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. Experimen- tal data have been lacking and there is serious disa- greement between the different theories that have been proposed. ESCA measurements on the core level shifts in various transition metal carbides and related com- pounds have zhown that electrons are transferred from the metal to the carbon 161, see figure 16.

M ^{T i metal}

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 oxida- tion states of platinum in the complexes, as determined from the binding energy data, were ordered and the

investigation confirmed that coordination of Iigands to the metal results in a significant charge transfer from metal to ligand, see figure 17.

BINDING ENERGY (eV)

FIG. 17. - Platinum 4 f7/2 energies in a series of metal-organic complexes.

0

K

EszSZa

'ma Eels

02s

BINDING ENERGY

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.

FIG. 19. - Valence band spectra from transition metals.

ESCA STUDIES OF CORE AND VALENCE ELECTRONS IN GASES AND SOLIDS C4-263 We have also recently applied the ESCA technique

to some crystal and solid state phenomena. For exam- ple, 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 spectros- copic data [lo], see figure 18, and the valence bands

in transition metals and other solids have been inves- tigated [I], [ll]. Figure 19 shows the valence band spectra obtained from the transition metals. This work will be discussed in more detail in another contri- bution 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.), LING (C.) and LINDBERG (B. J.), Spectrochim.

NORDBERG (R.), HAMRIN (K.), HEDMAN (J.), Acta, 1967, 23, 2015.

JOHANSSON (G.), BERGMARK (T.), KARLSSON (S.-E.), [6] RAMQVIST (L.), HAMRIN (K.), JOHANSSON (G.), FAHL- LINDGREN (I.), LINDBERG (B.), ESCA, Atomic, ^MAN (A.) and NORDLING (C.), J. Phys. Chem.

Molecular and State Structure studied by ESCA Solids, 1969, 30, 1835.

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DISCUSSION

Mr. DAS GUPTA. - Why Calbon behaves as an Reply to question from Nagel : We are presently acceptor rather than a donor ? On alloying carbon measuring valence bands and core electron shifts in with iron group alloys carbon usually behaves as a some palladium alloys. To my knowledge there have donor rather than an acceptor. not yet been any measurements reported on core

Reply to questionfrom Das Gupta : I have not seen the electron shifts in other metal alloys.

evidence you refer to that carbon usually behaves as a C. Nordling.

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 t o 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.

DAVID J. 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 example, aluminum a ~ o m s in alloys should be reflected 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

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~ed electrons or does the valence band extend t o low energies ? Is it possible to determine the botton of the balence band by your method ?

Reply to questionfrom Wiech : Inelastically scattered electrons contribute t o 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.

in order to determine conduction electron distribution ? C. Nordling.