Chemical Physics 132 ( 1989) 257-270
North-Holland,
Amsterdam
VARIABLE ENERGY PHOTOELECTRON STUDY
OF THE VALENCE LEVELS OF CFJX (X= F, Cl, Br, I) COMPOUNDS
BETWEEN 21 AND 200 eV PHOTON ENERGIES
J.D. BOZEK, G.M. BANCROFT ‘, J.N. CUTLER, K.H. TAN, B.W. YATES
Department of Chemistry and Centerfor Chemical Physics, University qf W&tern Ontario, London, Ontario, Canada N6A 5B7
and Canadian Synchrotron Radiation Facility, Synchrotron Radiation Center, University of Wisconsin,
Stoughton, WI 53589, USA
and
J.S. TSE
National Research Council of Canada 2zDivision of Chemistry, Ottawa, Ontario, Canada KlA OR9
Received
2 August
1988
The gas phase photoelectron
spectra of CFQ and CFrBr have been obtained between 20 and 170 eV, while the spectra of CFJ
and CF4 have been obtained between 100 and 170 eV and between 100 and 200 eV, respectively, using synchrotron
radiation.
Strong enhancements
in the photoionization
cross sections are observed near threshold in both CF,CI and CF,Br. These observations are in good accord with a previous measurement
on CF,Br but are contradictory
to a recent report on CF,Cl which shows
nearly all the valence level cross sections decrease monotonically
with increasing photon energy. Features observed in the experimental cross sections were analyzed using minima1 basis set ab initio and continuum MS Xcu calculations. The results show that
the cross section profiles are largely independent of the nature of the ligands. Low-energy resonant enhancements
are assigned to
excitations into antibonding-like
orbitals, while the broad and weak structures at high kinetic energy are ascribed to scattering of
the photoelectrons.
1. Introduction
phenomenon.
Initially these features were attributed
to shape re~dnance~ where the valence electron is momentarily trapped in a quasibound
continuum
orbital created by an effective or centrifugal potential
of the molecular ion [ 10-l 31. An alternative model
correlates the enhancement
in cross section with excitations into antibonding orbitals located in the continuum [ 141. Perhaps the best examples of these
concepts are provided by the partial photoionization
cross sections of Nz and COz. Resonances in orbitals
of g (gerade) symmetry in nitrogen have been attributed to the scattering of the electrons into an f (I= 3)
type continuum channel [ 15 1. An apparently different interpretation
of this phenomenon,
derived from
ab initio calculations,
shows that the resonant enhancement of the cross section is due to the excitation of a bound electron into the antibonding
o: or-
Knowledge of the variation in intensity of photoelectron bands as a function of photon energy provides important information
to further our understanding of the dynamics of the photoionization
process [ l-61. Recent studies of the valence level
photoionization
cross sections for a number of simple inorganic and organic molecules employing synchrotron radiation have shown that the energy dependence of the cross section is very complicated [ 291. Strong and sudden enhancements
in intensity are
often observed in the valence level spectra.
Several models have been proposed to explain this
’ To whom correspondence
’ Issued as NRCC 29893.
should be addressed.
O3Ol-OlO4/89/$O3.5O
0 Elsevier Science Publishers
( North-Holland
Physics Publishing Division )
B.V.
258
J.D. Bozek et (11./Photoelectron spectra qfCF,X
bital [ 16 1. The apparent discrepancy between the two
interpretations
is more one of terminology than one
of physical significance.
The continuum
MS Xcu
method is derived from a scattering theory [ 17 ] and
does not necessarily adhere to a molecular orbital description. It is important to realize that the f type
continuum
wave, which possesses three angular
nodes, has the same nodal structure as the antibonding N7 o: orbital. More significantly,
projection of
the Nz continuum f wavefunction onto the 0: molecular orbital shows that a strong resemblance exists
between the two. To further this argument, we consider the shape resonance observed in the photoionization cross sections of CO1 [ 18 1. The MS Xa calculations suggest that the shape resonance originates
from the scattering of the outgoing electron through
a predominating
I= 5 continuum
channel. Ab initio
calculations show that the resonance can be correlated with the C-O o: orbital [ 191. As the o: orbitals
in CO2 contain five angular nodes, they are formally
equivalent to an I= 5 partial wave.
A number of our recent experimental and theoretical observations
on polyatomic molecules such as
XeF, [3], CF4 and SiF4 [2], SF6 and SeF, [20],
Sn(CH3),andSi(CH,),
[5],andHg(CH,)2
[6] illustrate that neither of the two above concepts can
thoroughly explain the numerous above edge resonances which are experimentally
observed. First,
molecules such as XeF2, with no antibonding orbitals
in the continuum,
still give intense above edge resonances [ 3 1. Second, the valence band cross section
profiles for corresponding orbitals in analogous molecules (i.e. SF6 and SeF, [20]; Sn(CH3)J
and
Si( CH3)4 [ 51) are very similar and apparently independent of the central atom. Third, MS Xcr cross
section calculations
[ 2 1,221 on hypothetical species
with the central atom removed, such as “F2”, “Fe’)
exhibit similar resonance structure
and “(CO),”
(especially > 20 eV kinetic energy) to what is observed in the parent molecules XeF, [ 31, SF6 [ 201
and Cr (CO), [ 7 1, respectively. These calculations
strongly suggest that many resonances result from
multiple scattering of the photoelectron by the ligand
“cage”.
In an effort to broaden our understanding
of these
resonances, we are continuing our systematic investigation of the valence cross sections of a number of
closely related molecules. In the present study, our
(X= F, Cl. Br. I)
primary objective is to monitor the effect of reducing
the strength of the effective (centrifugal)
potential
created by the electronegative fluorine atoms in CFJ.
This is accomplished by replacing one fluorine atom
with the more electropositive chlorine or bromine atoms. As a result, we have experimental and theoretical partial cross sections for the whole CF3X (X = F,
Cl, Br, I) molecular series. Variable energy valence
level photoelectron
spectra of CF,Cl from 24 to 70
eV [23] andofCF,Brfrom
19to 117eV [24] have
been reported recently. Aside from an uncharacterized increase in all of the CF,Br partial cross sections
at z 60 eV, all the valence orbital cross sections were
found to be featureless and similar to those for CF,
[ 251. In contrast, our previously reported results for
CFII [4] showed considerable structure in the partial cross sections. In addition, no theoretical calculations were performed in the previous studies to help
characterize the partial cross sections of CF&l and
CF,Br. Since strong features in the cross section are
often observed near threshold, it is also desirable to
extend the measurements
for CF&l to lower kinetic
energies. Second, we wanted to extend the cross section measurements
for all the CF3X molecules to
z 200 eV photon energy in order to further explore
the high energy resonances which have been partially
characterized in the valence band spectra of SF, and
SeF, [ 201 and in CF$Zl [ 261. Such high energy spectra require very high resolution and have not previously been reported for any molecule. Finally, we
wanted to characterize the cross section profiles using theoretical MS Xcu calculations, and in particular
test the validity of the MS Xa method in the previously unexplored kinetic energy range between 100
and 200 eV.
2. Experimental methods
Tetrafluoromethane
(CF,), trifluorochloromethane (CF,Cl) and trifluorobromomethane
(CF,Br)
were purchased from Matheson and trifluoroiodomethane ( CF31 ) was purchased from PCR Research
Chemicals. All of the samples were used without further purification.
Photoelectron
spectra of the gaseous compounds up to 60 eV were obtained at the
Canadian Synchrotron
Radiation Facility (CSRF)
situated on the Tantalus I electron storage ring oper-
J.D. Bozek et al. /Photoelectron spectra of CF,X (X= F, Cl, Br, I)
ated by the University of Wisconsin at Stoughton,
Wisconsin. A 600 line/mm holographic grating from
JY Inc. was used in the Mark IV Grasshopper monochromator, limiting the minimum photon energy to
z 21 eV. The higher photon energy photoelectron
spectra were obtained after the CSRF beamline was
relocated to the Aladdin electron storage ring using a
1200 line/mm holographic grating. Spectra were accumulated with a photon resolution of z 1.7 8, on the
lower energy Tantalus ring and z 0.2 8, on the Aladdin ring. The optical elements of the beamline are
isolated from the interaction region of the photoelectron spectrometer by two stages of differential pumping, thus allowing the use of a free jet of the sample
gas [ 2 1. A Leybold-Heraeus
LHS- 11 electron spectrometer, mounted at the magic angle relative to the
crossed photon and molecular beams so that the photoelectron intensities are independent
of the asymmetry parameter p and the light polarization p [ 27 1,
was used to energy-analyze the photoelectrons.
Peak areas were obtained by fitting the spectra using an iterative procedure described previously [ 28 1.
Voigt functions, simulated by a linear combination
of Lorentzian-Gaussian
functions, along with a linear baseline were used in the fitting procedure. In this
study, all of the peak shapes were found to be very
close to Gaussian.
Branching ratios for the eight
highest occupied valence orbitals were obtained using the resulting peak areas (A,) and the branching
ratio definition (BR,=A,/CA,).
Using the total photoabsorption
cross section up to a photon energy of
70 eV [ 29 1, the experimental CF3C1 branching ratios
were converted to their corresponding
partial photoionization
cross sections, a,. Estimates of the contributions of the 1a,, 1e and 2a, partial cross sections
were subtracted from the total photoionization
cross
section in order to account for their absence from the
branching ratio data. Since the 1a,, le and 2a, orbitals consist primarily of F 2s and Cl 3s character,
atomic subshell cross sections [ 301 were used to estimate their contribution
to the total cross section.
The total photoionization
cross section of CF3Br is
not known over the photon energy range of interest
and hence the experimental
branching ratios could
not be converted to partial photoionization
cross sections. Similarly, at higher photon energies ( > 100
eV), the total photoionization
cross sections are not
known for any of the CF,X molecules and hence the
branching ratios could not be converted
photoionization
cross sections.
259
to partial
3. Computational details
For both CF$Zl and CFjBr, MS Xa [ 3 1,321 and
minimal basis set STO-3G ab initio calculations [ 331
were performed. The Xa! results were used to determine the theoretical partial photoionization
cross
sections for the valence orbitals, while the results of
the ab initio calculations were used primarily to identify the nature of the antibonding orbitals [ 341.
Geometrical data for CF&l and CF3Br were taken
from a gas phase electron diffraction and microwave
spectroscopic
study [ 351. For CF$l,
C-F bond
lengths of 1.3248 A, a C-Cl bond length of 1.7522 A
and a F-C-F angle of 108.57” were used; and for
CFjBr, C-F bond lengths of 1.3264 A, a C-Br bond
lengthof 1.9229AandaF-C-Fangleof
108.77” were
used in all the calculations. The Xa calculations for
CF4 and CF31 were performed with the same geometrical parameters used previously [ 2,3 1.
The sphere sizes used in the MS Xa calculations
are summarized in table 1. Atomic exchange parameters, aHF, were taker, from Schwarz’s tabulations
[ 361. The exchange p.$rameters for the inter- and
outer-sphere regions were obtained by averaging the
atomic values weighted by the number of valence
electrons for each atom. Atomic sphere radii were determined using the Norman procedure [ 37 ] and enlarged by 20%. The ground state converged potentials corrected for asymptotic behaviour with a Latter
tail [ 381 were used in the continuum calculations. In
calculating the cross sections, the I values for the final
states were enlarged for both the outer sphere and the
halide ligands as indicated in table 1 [ 39 1. All dipole-allowed
photoionization
processes were included in the calculations. It should be emphasized
that the continuum MS Xa method is a semi-quantitative model [ 11,401. Although the main features
observed in the cross section profiles are often correctly reproduced by the calculations, the predicted
magnitudes and positions may only be approximate
in nature. We therefore used the calculational results
only as a guide in the assignment and discussion of
the cross sections and branching ratios.
Gelius model calculations [ 4 1 ] for CF,Br (above
J. D. Boxk et al. /Photoelectron spectra 0-fCF.,X (X= b: Cl. Br. I)
260
Table 1
Parameters
used in the MS Xoc calculations
Region
of CF,Cl and CF,Br
.‘Y
1
R
%
(Y
l”,,”
initial
state
final
state
CF,CI
outersphere
C
Cl
P,
FZ
F,
-0.3834
0.0
_ 1.1520
2.5035
-0.7973
-0.7973
1.0333
0.0
3.1043
0.0
- 1.2248
- 1.2248
0.0
1.2103
0.0
0.0
2.0327
-2.0327
4.6200
1.4524
2.5211
1.5518
1.5518
1.5518
0.73699
0.75928
0.72323
0.73732
0.73732
0.73732
4
2
2
2
2
2
7
2
3
2
2
2
CF,Br
outersphere
C
Br
F,
F2
P,
-0.5020
0.0
- 1.2526
2.5065
-0.8065
-0.8065
1.3671
0.0
3.4111
0.0
- 1.2166
- 1.2166
0.0
0.0
0.0
0.0
2.0377
-2.0377
5.0000
1.4362
2.9243
1.5716
1.5716
1.5716
0.73323
0.75928
0.70606
0.73732
0.73732
0.73732
4
2
2
2
2
2
7
2
3
2
2
2
70 eV), CFJ and CF, (above 100 eV) were performed using orbital populations from the ab initio
calculations
and theoretical atomic cross sections
[ 301. The Gelius model calculations of molecular orbital cross sections are based on the product of atomic
orbital cross sections times the corresponding orbital
population.
4. Results and discussions
Representative photoelectron spectra of CF&l and
CF,Br obtained using 26,4 1,70 and 150 eV photons
are depicted in figs. 1 and 2, respectively, while high
energy spectra of CF, and CF31 obtained using 100
and 150 eV photons are given in fig. 3. The resolution of the high energy spectra is similar to that of the
lower energy spectra. Molecular orbital assignments
for CF3Cl and CF,Br, taken from previous HeI/II
experiments
[ 42,43 ] are indicated in figs. 1 and 2,
respectively, and are consistent with the results reported here.
The calculated charge distributions
from the MS
Xcu calculations for the valence levels of CF,Cl and
CF3Br are given in table 2. The 3a, and 2a, orbitals
are the only anomaly in the MO correlation belween
the two molecules, In CF,Cl, the 3a, orbital arises
primarily from an F parentage, while the 2a, orbital
is of primarily Cl parentage. In CF,Br, the 3a, orbital
Fig. 1. Photoelectron
spectra of CF,Cl at 26. 4 1. 70, and 150 eV
photon energies. The molecular orbital assignment is given in the
upper right-hand quadrant. The fitted peaks are from the iterative method described in the text.
corresponds primarily to contributions
from the Br
center and the 2a, orbital corresponds to a large contribution from the F atoms. These results lead us to
assign the 3a, and 2a, orbitals of CF3Cl to correspond to the 2a, and 3a, orbitals of CF,Br respectively. This result is of little consequence in this report, however, as only experimental data for the 3a,
J.D. Bozek et al. /Photoelectron spectra of CF,X (X= F, Cl* Br, I)
zoo--
3
lOO--
2
2
"T
2
b
P-
5
hv=?O
zoo-
e-V
15ot
I
z
lOO--
v
200
50 .-
,
0~~
l~;:~~~:;~;~:::;:l::::::::::::::::I
25
15
20
0
20
10 25
Energy
Binding
15
10
(eV)
spectra of CFSBr at 26, 4 1, 70, and 150 eV
Fig. 2. Photoelectron
photon energies. The molecular orbital assignment is given in the
upper right-hand quadrant. The fitted peaks are from the iterative method described in the text.
900
hv=lOO
cF3’
hv=150
eV
eV
i 400
Je, 4e
I
Ai
20,
300
‘5
*e
600..
200
10,
50,
,5e
'0,
/
100
0
4:::::;:i:::::t:::
hv=150
eV
20
4oo
I
Lw?::::::::::::::
25
15
Binding
10
13. The high photon energy cross sections and
branching ratios for CFJ are given as inserts in the
left-hand side of these figures. High photon energy
results for CF, are presented in fig. 14. In addition to
the MS Xa! theoretical results shown with solid lines,
Gelius model cross section results are indicated in the
figures with dashed lines. MS Xo theoretical values
of the cross sections for the la,, le and 2a, orbitals
of CF,Cl and CF3Br are presented in fig. 13. The tabulated experimental cross sections for CF$l and the
branching ratios for CF,Cl, CF3Br, CF31 and CF4 can
be obtained from the authors. Experimental
partial
cross sections and branching ratios for CF&l and
CF3Br from previous studies [ 23.241 are also illustrated in the figures where the data is available. In
order to facilitate a comparison of our results with
those reported previously, an additional figure, fig. 8,
consisting of the sum of contributions
from the overlapping 5a,, la2 and 4e bands of CF,Cl and the sum
of the 1a, and 4e bands of CF3Br has been included.
Examination
of the photoelectron
spectra in figs.
l-3 shows that, as expected, there are large changes
in the relative peak intensities at low photon energies. For example, the intensity of the 5e band changes
dramatically relative to the other photoelectron bands
in both CF$l and CF3Br between 26 and 4 1 eV (figs.
1 and 2). More surprisingly,
there are still large
changes in relative intensity at higher photon energies. The intensity of the spin-orbit split 5e band in
CF31, for example, changes relative to the 5a, band
between 100 and 150 eV (fig. 3 ). The intensity of the
1e band also changes relative to the 4tz and 1t , bands
in CF4 over the same energy range (fig. 3). The theoretical and experimental cross section and branching ratio results (figs. 4-13) illustrate more clearly
that there is a great deal of structure both near threshold and at higher photon energies. In contrast, Novak
et al., in their studies of CF3Cl [ 231 and CFSBr [ 241,
noted only an atomic-like monotonic decrease in cross
section. Since we are unable to directly measure partial cross sections, it is important to emphasize that
the branching ratios usually show the same basic
structure as the cross sections (see for example figs.
5 and 6), although the relative intensities of the features are often quite different in the two cases.
There are numerous examples of structure in the
cross section and branching ratio plots of the CF3X
(X=F, Cl, Br, I) molecules. A summary of the cal-
Jl.!tLi
400
25
Energy
20
15
10
(eV)
Fig. 3. Photoelectron
spectra of CF31 (top) and CF., (bottom) at
100 and 150 eV photon energies. The molecular orbital assignments are given in the right-hand plot. The fitted peaks are from
the iterative method described in the text.
orbitals of both molecules is presented here.
Experimental and theoretical partial photoionization cross sections and branching ratios over photon
energies from 20 to 170 eV for the valence orbitals of
CF,Cl and CFSBr are presented graphically in figs. 4-
261
J.D. Bozek et al. /Photoelectron spectra 0fCF.J
262
Table 2
Calculated
charge distributions
for the CF,Cl and CF,Br ground
(X= F, Cl, Br. I)
state valence molecular
orbitals
MO
Experimental
binding
energy (eV )
Outer
c
F
X
Inter
CF,CI
la,
le
2a,
3a,
2e
4a,
3e
4e
Ia2
5a,
5e
44.0
42.8
26.3
23.8
21.2
20.20
17.71
16.72
15.80
15.20
13.06
0.8
1.1
I.1
2.4
2.9
1.8
2.2
1.4
1.3
3.4
3.3
14.4
6.9
14.3
18.5
21.3
13.4
1.3
0.7
0.0
13.4
0.2
71.2
86.5
15.7
50.3
69.2
47.2
81.6
82.7
86.7
36.6
3.6
0.7
0.1
65.1
23.1
0.8
24.8
0.2
1.3
0.0
43.9
78.4
6.9
5.4
3.8
5.6
5.8
12.8
14.7
13.9
13.0
2.8
14.6
CF3Br
la,
le
2a,
3a,
2e
4a,
3e
4e
Ia2
5a,
5e
43.0
40.6
25.0
23.7
20.9
19.8
17.57
16.55
15.86
14.28
12.08
0.5
0.8
1.4
1.1
2.3
1.6
1.9
1.0
0.9
3.2
4.1
13,7
6.6
20.9
10.1
‘0.9
11.2
1.1
0.6
0.0
17.6
0.1
78.5
87.2
40.8
26.6
70.4
55.0
82.5
84.2
87.3
28.8
2.1
0.6
0.1
33.3
57.1
0.5
19.5
0.1
0.8
0.0
49.9
19.4
6.6
5.3
3.6
5.0
5.9
12.7
14.3
13.4
11.8
0.6
14.3
culated resonance positions for CF,Cl, CF3Br and
CFjI is included in table 3. In the low energy region
(kinetic energy =G10 eV), both the theoretical and
experimental
results for CF,Cl and CF3Br exhibit
sharp resonances in the 5e (fig. 4), 5a, (fig. 5) and
2e (fig. 11) orbitals. The CF&l experimental cross
section data for the la> orbital (fig. 6) also indicates
a low energy resonance. It is not reproduced in the
theoretical curve however, and may only be due to
the deconvolution
method used for this overlapping
peak. The MS Xa results also indicate sharp resonances near threshold for the 4e (fig. 7), 3e (fig. 9)
and 4a, (fig. 10) orbitals. The experimental
cross
section data for CF3C1 is definitely consistent with
these trends for the 3e and 4a, orbitals, but less delinite in the 4e orbital. In CF,Br, the MS Xa results
indicate sharp resonances near threshold for the 4e
(fig. 7) and 3e (fig. 9) orbitals, but the experimental
branching ratios begin at too high a photon energy to
validate these predictions.
In addition to these low energy resonances, a num-
ber of broad structures are apparent at higher energy
in the cross section and branching ratio plots for all
of the orbitals. For example, the 3e cross section and
branching ratio plots for CF&l and CF,Br (fig. 9 )
both have a very broad feature at r 60 eV kinetic energy. Other orbitals. such as the 4a, (fig. 10) and 2e
(fig. 11) orbitals also show weak peaks in the experimental branching ratio and theoretical cross section
data at ~50 eV kinetic energy for both CF,Cl and
CF3Br. Additionally,
the 5a, cross section and
branching ratio plots for all three CF3X (X = Cl, Br,
I) molecules (fig. 5) all show weak and broad features above 100 eV kinetic energy. For CF7C1. the errors in the experimental
data for the 5a, band are
fairly large due to overlap with the la: and 4e peaks.
Comparing the experimental
branching ratios with
the MS XU cross section for the sum of the overlapping bands (fig. 8) confirms the existence of a high
energy peak at a photon energy of = 125 eV. Similarly. there are strong indications
of a broad peak
above 100 eV kinetic energy in both the theoretical
J.D. Bozek et al. /Photoelectron spectra of CFjX (X= F, Cl, Br, I)
Photoelectron
Energy
(eV)
Photoelectron
50
50
100
150
Photon
50
Energy
100
50
(eV)
Energy
100
100
:
50
:
:
:
100
:
(eV)
,I,
150
Photon
100
(eV)
50
100
150
50
Energy
100
150
(eV)
Fig. 6. Experimental and theoretical partial photoionization
cross
sections (upper plots) and branching ratios (lower plots) for the
laz orbitals of CF&l (left), CF,Br (right), and CFJ (insert, left).
Partial photoionization
cross section data was not reported for
CFSCl or CF,Br in the previous studies [ 23,241 and hence could
not be included here.
Photoelectron
Energy
(eV)
o(Mb)
:
50
Energy
0
150
Photon
Photon
I:
Energy
150
OL.
150
Fig. 4. Experimental and theoretical partial photoionization
cross
sections (upper) and branching ratios (lower) for the 5e orbitals
of CF,CI (left), CF,Br (right) and CF,I (insert, left). The solid
data points correspond to our experimental
data; the open data
points to the previously reported data for CF,Cl [ 23 ] and CF,Br
[24]; the solid line to the theoretical
MS Xa results and the
dashed line to the Gelius mode1 results. The upper energy scale
is in terms of the photoelectron
kinetic energy, while the lower
scale gives the corresponding
photon energy.
Photoelectron
263
,J
150
(eV)
Fig. 5. Experimental and theoretical partial photoionization
cross
sections (upper plots) and branching ratios (lower plots) for the
5a, orbitals of CF,Cl (left), CF,Br (right), and CF,I (insert, left).
Partial photoionization
cross section data was not reported for
CF,Cl in the previous study [ 231 and hence could not be included here.
Energy
(eV)
Fig. 7. Experimental and theoretical partial photoionization
cross
sections (upper plots) an& branching ratios (lower plots) for the
4e orbitals of CF,Cl (left), CF,Br (right), and CFJ (insert, left).
Partial photoionization
cross section data was not reported for
CF,Cl or CF,Br in the previous studies [ 23,241 and hence could
not be included here.
and experimental branching ratios of the 4t2 orbital
in CF4 (fig. 14b). A theoretical interpretation of these
features is preseyted after a comparison of our data
J.D. Bozek et al. /Photoelectron spectra qf CF,X (X= 17 CL Br. I)
Photoelectron
2
o(Mb)
Energy
150
.OC
c
Energy
(eV)
Photoelectron
0
50
Energy
150
0
50
15s
100
Photon
150
100
50
Energy
Photoelectron
ICI0
‘50
350
(eV)
Fig. 10. Experimental and theoretical partial photoionization
cross
sections (upper plots) and branching ratios (lower plots) for the
4a, orbitals of CF,CI (left ). CF,Br (right ). and CFJ (insert. left).
(eV)
50
130
;
---rl------cI---+~----c1--~
Fig. 8. Experimental and theoretical partial photoionization
cross
sections (upper plots) and branching ratios (lower plots) for the
sums of the overlapping bands of CF,Cl ( 5a, + 1a1 + 4e, left) and
CF,Br ( laz+4e, right). These plots are included in order to facilitate a comparison
of our data with previously reported data
(see text).
(eV)
53
I
04
Photon
i
c
100
50
----
Energy
15”
(eV)
SO
?
:oo
Kx
o(Mb)
50
100
150
Photon
50
Energy
100
150
(eV)
5”
1 00
150
Photon
‘00
5c
Energy
IS0
(eV)
Fig. 9. Experimental and theoretical partial photoionization
cross
sections (upper plots) and branching ratios (lower plots) for the
3e orbitals of CFJJl (left). CF,Br (right), and CF,I (insert, left).
Fig. 11.Experimental and theoretical partial photoionization
cross
sections (upper plots) and branching ratios (lower plots) for the
2e orbitals ofCF,Cl (left), CF,Br (right), and CFJ (insert, leti).
both with previous worker’s data and the theoretical
results.
Although Novak et al. noted only a monotonic decrease in the cross sections of all orbitals for CF$l
[23] and CF3Br [ 241, there is overall good agreement between their results and those presented here
in the overlapping photon energy range. For example, there is very good agreement for the 5e orbital
between the two sets of data for both molecules, except below 30 eV photon energy. For CF&l, the cross
section and branching ratio data of Novak et al. is as
much as 50% lower than ours. In addition, since the
previously reported data for CF3Cl begins at 25 eV
photon energy [ 23 1, it does not support the existence
of a peak in the branching ratio data at z 26 eV, while
our data, which begins at a photon energy of 2 1 eV,
J.D. Bozek et al. /Photoelectron spectra of CF,X (X= F, Cl, Br, I)
Photoelectron
50
100
Energy
150
0
(eV)
50
1;
100
150
i : CF&
: : :t
Se,
10-
50
100
50
150
Photon
Energy
100
150
(eV)
Fig. 12. Experimental and theoretical partial photoionization
cross
sections (upper plots) and branching ratios (lower plots) for the
3a, orbitals of CFJJI (left), CF,Br (right), and CF,I (insert, left).
Fig. 13. Theoretical
MS Xa partial photoionization
cross sections for the la, (upper), 2e (middle), and 2a, (bottom) orbitals of CF,CI and CF,Br.
does. Novak et al.‘s experimental
cross section data
for CF&l is notably different from ours below x 30
eV for all of the orbitals except the summation plot
(fig. 8). The 5a, cross section illustrates other differences between the previously reported data and our
experimental
data. Most importantly,
the disconti-
265
nuity in the CF,Br cross section data at z 60 eV photon energy is observed in all of the CF3Br orbitals at
the same photon energy. This discontinuity
is likely
due to a normalization
error resulting from pressure
fluctuations
in -either the Ar calibrant and/or the
CF3Br gas. Sincelthis discontinuity is present in every
orbital, it does not affect the branching ratio data. Indeed, the CF3Br branching ratios calculated from
Novak et al’s data, while having rather large errors,
are in good agreement with our values (see figs. 4, 5,
8-12). The 5a,, la, and 4e photoelectron
bands
heavily overlap in CF,Cl, and the laZ and 4e bands
overlap in CF3Br. Novak et al. did not fit individual
peak areas to these bands and hence could not report
partial cross sections for the overlapping bands. Their
cross sections and branching ratios for the summation of these bands are in excellent agreement with
our data (fig. 8).
Agreement between the MS Xcu theoretical cross
sections and branching ratios with the experimental
values is remarkably good - at least as good as that
for any molecule in the literature. This agreement
confirms the orbital assignments of CF-,Cl and CF3Br.
It is perhaps somewhat surprising that the MS Xa!
method yields values in as good agreement with the
experimental results above 100 eV kinetic energy (see
fig. 4 for CF,Cl, CF,Br and CF,I) The theoretical
curves prove to be a considerable aid in identifying
features in the cross section and branching ratio data.
In the data for the 5a, orbital of CF$l there is a great
deal of scatter and error due to the inherently poor
resolution of this peak. This makes it difficult to discern trends in the experimental data. The experimental data is, however, consistent with the MS Xa! theoretical results.
The Gelius model results generally reproduce the
trends shown in the MS Xo theoretical results extremely well from 70 to 200 eV. For example, the cross
sections for the orbitals which consist primarily of
contributions
from the X (X=Cl, Br or I) ligands
show atomic-like behavior, dropping rapidly from
threshold to a fraction of a Mb at 100 eV (fig. 4 ). The
results for CF, between 100 and 200 eV (fig. 14a)
are particularly notable. Apart from some structure
above 100 eV for the 3t, and 4tz orbitals, the agreement between the Gelius model and MS X~J calculations is very good. .4t lower photon energies, agreement between the two calculation methods is not as
J. D. Bozek et al. /Photoelectron spectra of CF,X (.X= F, CL Br, I)
266
100
120
140
Photon
160
Energy
180
J
200
100
120
(eV)
140
Photon
160
Energy
180
200
(eV)
Fig. 14. (a) Theoretical MS Xcr (solid lines) and Gelius model (dashed lines) partial photoionization
cross sections for the five valence
orbitals of CF, over a photon energy range of 100-200 eV. (b) Experimental
and theoretical MS Xa (solid lines) branching ratios for
the five valence orbitals of CF,.
and theoretical cross sections for both CFICl and
CFJ3r are extremely similar to each other and to CF31
[ 41. All of the cross sections and branching ratios for
CFJl and CF,Br have been plotted on the same scale
to emphasize this similarity. From the Gelius model;
it is not surprising that the basic trends are similar
since corresponding
orbitals from the different molecules have similar atomic compositions
(table 2 ).
The best example of this is given by the 5e orbital
cross sections (fig. 4), which, apart from the near-
good - as has already been noted for CF, between 20
and 60 eV [ 441. The Gelius model calculations on
lone pair F orbitals for example (such as the CF, It , ,
4tz and le orbitals; or the 4e and 3e orbitals in CF,Cl,
CF,Br and CF31) show a broad cross sectional maximum at z 5 eV above threshold, while experimental
results and MS Xcr calculated cross sections often
have sharp low energy resonances and give broad
cross sectional maxima at > 15 eV kinetic energy.
We would like to emphasize that the experimental
Table 3
Kinetic energies of features
in the calculated
5e
5a,
Ia,
4e
3e
4a,
2e
3a,
‘) s=strong,
m=medium,
w=weak
MS Xa partial photoionization
cross sections of CF,CI, CF,Br and CFJ ”
CF,Cl
CF,Br
6m
9m, 29m, 125~
21m, 105~
2s, SW, 24m. 95vw
Zs, 9w, 27m, 55~
8m, 34w, 52vw
2s, 25m. 55~
9w, 24m
6m
9m, 26w, 45~~. 110~.
?lm, 10%~
2s, 23m, 105~~
2s, I lvw, 27m, 55~
Sm. 33~. 75~
2s. 24m, SOW
23m, 48vw
and vw=very
weak.
CFJ
145~
l6Ovw
160~~
I low
llOvw
135w
J.D. Bozek et al. /Photoelectron spectra of CF,X (X= F, Cl, Br, I)
edge resonances, show atomic-like behavior, dropping rapidly from = 40 Mb at threshold to x 2 Mb at
32 eV photon energy. Most cross sections, however,
do not show atomic-like behavior and yet they are
still almost identical in the three CF3X (X = Cl, Br or
I) molecules. The 5a, orbital cross sections and
branching ratios for CF$l and CF3Br both have sharp
resonances of = 5Mb at % 9eV kinetic energy followed by a broad asymmetric feature of ~2 Mb at
25-40 eV kinetic energy and a very broad feature at
zl25eV.
We now turn to a discussion of the origin of the
sharp low energy resonances ( < 10 eV kinetic energy). It is informative to use qualitative molecular
orbital arguments to help in deciding which antibonding levels might be important contributors to the
resonance behavior near threshold. The only localized unoccupied levels in CF$l and CF3Br will be
the antibonding
C-F, C-Cl and C-Br orbitals. Since
both the Cl and Br ligands are more electropositive
than the F atom, the energy of the antibonding
C-X
orbital will be lower than the corresponding
antibonding C-F orbital. In previous studies, it was
shown that the antibonding
C-F orbitals in CF, are
located very close to the ionization threshold [ 2 1,
hence, the antibonding
C-X orbital is expected to lie
below the continuum.
Vacuum ultraviolet
(VUV)
absorption studies of these molecules substantiate this
anticipation
[ 45 1. Resonant features observed in
CF3X compounds just beyond the ionization threshold may be correlated with the antibonding
C-F orbitals [ 341. For the Cxv CF3X molecules, the antibonding C-F orbitals transform as e and a, symmetry.
From dipole selection rules, the final state e is accessible from all valence orbitals. However, transitions
from valence orbitals of a, symmetry to the final state
a, are symmetry forbidden, Our minimal basis set ab
initio calculations
suggest that the a, orbital is at
slightly higher energy than the e orbital. We have calculated the e and a, antibonding
orbitals to be at 2.9
and 4.4 eV respectively in the continuum for CF3Cl
and 4.0 and 5.4 eV respectively into the continuum
for CFJBr.
Experimental observation of excitations into antibonding orbitals is governed by several factors. The
excitation will be intense only if the antibonding
orbital is spatially close to the originating orbital [ 461.
Additionally,
mixing between the antibonding
and
267
Rydberg orbitals of the same symmetry should also
be small [ 47 1. Extensive Rydbergization
[ 48 ] .of antibonding
orbitals
will distribute
the oscillator
strength among many excitations resulting in a broad
band rather than a single distinct transition. On the
other hand, in the presence of a potential barrier,
penetration of the diffuse Rydberg orbitals into the
inner molecular region is avoided and the antibonding orbitals may behave as a quasibond state [ 47,491.
In order to characterize the resonance structure in
the calculated partial cross sections, the phase shifts
[ 501 of the continuum channels for both CF&l and
CFSBr have been examined in detail. Near the ionization threshold, the e and a, continuum channels for
both molecules were found to exhibit a sudden change
in the eigenphase sum. In the e channel, an eigenphase sum change of =:n/2 radian was observed at
2.5 eV. A similar change in the eigenphase sum was
also detected in the a, channel at z 5 eV. Both of these
features satisfy Kreile’s first resonance criterium
[34]#‘. that the eigenphase sum of a continuum
channel must change by > 0.3~ over an energy range
of < 6 eV, and hence may be classified as shape resonances. As mentioned above, resonances in the e and
a, channels may also be identified with excitations
into the antibonding C-F orbitals. The energy ordering of the resonances are also in good accord with the
minimal basis set ah initio predictions.
The low energy features observed in the experimental cross sections and branching ratios of CF&l
and CF,Br may be assigned in light of the theoretical
results. For the low binding energy 5e and 5a, orbitals where measurements
started 6-10 eV above the
ionization threshold, the initial drop in cross section
is attributed to the tail of the e resonance. For the
higher binding energy 3e, 4a, and 2e orbitals, the e
resonance can be assigned to the peak observed at 35 eV kinetic energy. The position of the a, resonance
is more difficult to identify from the experimental
cross sections due to the scatter in the data. The
shoulder observed at z 10 eV in the 3e cross sections
and the weak peaks at = 8 eV in the 4a, may be good
candidates for the a, resonances in CF-,Cl and CF3Br.
ft’In a previous study on CFJ [ 481, due to a misinterpretation
of the change in eigenphase sums, the weak structures at = 15
20 eV were incorrectly assigned to resonances in the continuum a, and e channels.
268
J.D. Bozek et al. /Photoelectron
The observed energy separation between the e and a,
resonances is z 5 eV and can be compared with the
theoretical value of 2.5 eV.
Both experiment
and theory show broad feature(s) in the cross section above 10 eV from the
ionization
threshold. These features, although correctly predicted by the MS Xa calculations, cannot
be ascribed to shape resonances as the eigenphase
sums of the continuum
channels are smooth functions of the photon energy [ 341 for both CF&l and
CF,Br. Recently, it has been suggested that scattering
(diffraction ) of the photoelectron by the neighbouring atoms in a molecule [ 20-22,5 1 ] gives rise to these
features. Unlike core level X-ray absorption
tine
structure (EXAFS), where both forward- and backscattering are important, the forward scattering dominates for ionization in the valence level [ 521. Neglecting the phase shift contribution,
which will be
different for different orbitals, the period (Ak A- ’ )
of the modulation is then twice as long as in EXAFS
and is related to the interatomic distance r by [ 5 l551
Akr=2x.
(1)
We will apply this theory to the resonance structure
of CFJl, with distinct interatomic distances of 2.54
A (Cl-F), 2.15 A (F-F), 1.75 A (C-Cl) and 1.33.A
(C-F). Using eq. ( 1 ), the first maxima of the modulations in cross section are calculated to appear at
23,32,29 and 86 eV, respectively. Experimentally and
theoretically, broad maxima are observed at z 20 eV
in the laz, 4e. 2e and 3a, orbitals and at z 30 eV in
the 5a,, 3e and 4a, orbitals in good correlation with
the above simple calculation. Photoabsorption
data
beyond 70 eV photon energy is not yet available for
CF&l and hence the branching ratios cannot be converted to partial cross sections. There are also indications from the experimental
CF_$Zl branching ratios that there is an enhancement in the valence orbital
cross sections at z 50 eV kinetic energy in the 3e, 4a,
and 2e orbitals and at = 100 eV kinetic energy in the
5a,, 1a, and 4e orbitals. Several orbitals in CF,Br,
CF,I and CFJ also exhibit these high energy resonances at z 100 eV kinetic energy. Due to the similarity of the other CF,X molecules to CF,Cl, it is not
particularly informative at this point to include them
in this portion of the discussion. Also owing to the
spectra
qf CF,X (X= F, Cl, Br. I)
limited resolution of the experimental data, we will
not elaborate on this point any further.
In many respects, the description of the energy dependence of the valence level cross sections is very
similar to that for core level absorption spectra. Core
level photoabsorption
spectra are customarily
divided into three regions according to the energy beyond threshold [ 531. Close to the ionization threshold ( ~20 eV) is the near-edge region where strong
transitions (XANES) are often observed. Features in
this region have been attributed to multiple scattering of the photoelectron by the atoms neighbouring
the excitation site [ 541. Interference of the multiply
scattering waves results in localization of the continuum wavefunction effectively behaving like a quasibond orbital [ 561. The low energy resonances ( < 10
eV) in CF&l and CF,Br, which were attributed to
transitions into C-F antibonding
(quasibond)
orbitals, can be thought of as belonging to this region. In
the intermediate energy region ( > 20 eV), the wavelength of the outgoing photoelectron
is comparable
to the size ofthe molecule. In this region. the maxima
in the absorption can be related to the interatomic
distance [ 57 1. The higher energy resonance features
observed in the partial cross sections of CFQ, which
were attributed to photoelectron
diffraction can be
viewed as belonging to the same energy region. Finally, when the energy of the photoelectron
is very
large, the weak scattering condition is no longer met
and the XANES spectrum merges smoothly into the
EXAFS region [ 581. There is no complementary
region in our photoelectron
partial cross section and
branching ratio data as it only extends =: 150 eV beyond threshold.
5. Conclusions
The valence level photoelectron
spectra of CF3Cl
and CFsBr have been measured from 2 1 to 170 eV
using synchrotron radiation. In addition, the valence
level photoelectron spectra of CF,I;and CF, have been
measured from 100 to 170 eV and from 100 to 200
eV respectively. The branching ratios obtained are in
good accord with those reported previously except in
the low energy region which was not covered in the
previous CF,Cl study. A possible error in the normalization
of the previously reported partial pho-
269
J.D. Buzek et al. /Photoelectron spectra of CF,X (X= F, CI, Br, I)
toionization
cross sections of CF3Br is noted. Theoretical partial cross sections and branching ratios
obtained with MS Xcu calculations for all four CF3X
(X= F, Cl, Fr, I) molecules are generally in good
agreement with the experimental values. Strong enhancements in the cross sections of CF3Cl and CF3Br
are predicted near threshold and attributed to excitations into antibonding
C-F orbitals. These enhancements are confirmed by the experimental data,
The positions of the resonances are found to be similar in CFJCl and CF3Br, indicating that the nature of
the ligand does not substantially
affect the continuum states. Weak structures observed at higher photon energies in CF3C1 are shown to correlate with a
simple diffraction model based on the forward scattering of the photoelectron by the immediately neighbouring atoms. Similar observations have also been
made in studies of the cross sections of SF6 and XeF,.
The complementary
nature of the descriptions of the
energy dependence of the valence level cross sections
and core level absorption spectra is also noted.
The partial cross sections and branching ratios of
CF&l and CF,Br are similar to the lower energy (20100 eV photon energy) results obtained previously
for CF, and CF31. In the higher energy region ( 100175 eV photon energy) the results obtained for all
four CF3X (X = F, Cl, Br, I) molecules are again very
similar. A more detailed comparison of the theoretical results presented here with previous results for CF,
and CFJ along with a possible relationship of the high
energy features to the molecular geometry will be explored in a forthcoming paper.
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