129
Chemical Physics 107 (1986) 129-138
North-Holland, Amsterdam
UV ANGLERESOLVED
MIXED METHYLENE
PHOTOELECI’RON
SPECI-RA OF
DIHALIDES USING SYNCHFtOTRON
RADIATION
I. NOVAK, J.M. BENSON
Department
of Physics, King’s College, Strand
London WC2R 2LS, UK
and
A.W. POTTS
Department of Physics, King’s College, Strand London WCZR 2L.9, UK
and Daresbwy Laboratory Daresbwy, Warrington WA4 4AD, UK
Received 17 February 1986
Photoelectron spectra for the dihalomethanes CH,BrCl, CH,ClI and CH,Br,
have been recorded with photons in the
energy range 19-115 eV using Daresbury Laboratory Synchrotron Radiation Source. Ionization energies have been measured
for all valence orbitals falling within this energy range including Br 3d and I 4d orbitals. Asymmetry parameters have been
measured for all intense ionization processe s and characteristic Cooper minima observed for halogen lone pair orbitals. For
CH,ClI lone pair orbital B spectra show strong evidence of mixed halogen character although a similar situation is not
observed for CH,BrCl. Partial photoionization
cross sections are tabulated for the observed ionization processes. Detailed
spectra are presented for the molecule CH,ClI since this appears to be the first photoelectron study of this molecule.
1. Introduction
In recent years considerable effort has been
given to studies of molecular photoionization using
monochromatized synchrotron radiation. Angular
distribution parameters (/3) and valence band partial photoionization cross sections (a) have been
measured and their values compared with the results obtained from calculations. Oscillations in
the values of j3 and u over a wide photon energy
range have provided evidence of shape resonances,
Cooper minima and autoionization processes for a
variety of molecular systems. The position and
profile of these resonances have been studied for
photoionization
from individual molecules and
appear to reflect the atomic character of particular
molecular orbitals. Theoretical [l] and experimental [2,3] results reported indicate how changes of
atomic number and molecular orbital character
should affect the photoionization parameters p
and (I.
While it is desirable to study individual systems
in detail it is perhaps more effective to study the
relationship between photoionization parameters
and molecular orbital character by studying a
related group of molecules where controlled
changes may be made to particular molecular
orbitals. Only a few studies have adopted this
approach [4] although asymmetry parameters have
been used for some time as an aid to assignment
in photoelectron spectroscopy. The relationship of
photoionization cross section to the atomic character of molecular orbitals was first discussed by
Gelius [5] for valence shell photoionization cross
sections using soft X-ray radiation.
In this study we have chosen the methylene
dihalide molecules CH,BrCl and CH,ClI for
several reasons. The presence of different halogen
atoms in the same molecule can be expected to
produce well-defined and characteristic Cooper
minima in /3 spectra for the halogen lone pair
ionizations. Low molecular symmetry (C, point
0301-0104/86/$03.50 0 Elsevier Science Publishers B.V.
(North-Holland Physics Publishing Division)
130
I. Novak et al. / Photoelectron spectra
group) can on the other hand modify the MO
characters of the lone pairs by allowing considerable mixing between
them possibly
producing
complex /3 and u spectra. The presence of halogen
inner valence s and d shells gives additional
information about the applicability
of the one-electron model to s shell ionization and the photoionization process in Br 3d and I 4d shells.
The CH,Br,
photoionization
study has been
included with the present work in order to provide
a reference system of higher symmetry with which
to compare the results obtained for CH,BrCl and
CH,ClI.
The electronic
structures
of CH,Br,
and
CH,BrCl
have been studied previously
by photoelectron
spectroscopy
[6,7], however, no photoelectron studies have been reported for CH,ClI.
The assignment of CH,Br,
spectrum is based on
the recent results of MS Xe calculations
[8].
Several theoretical
and photoelectron
studies of
the methylene dihalides have been published (see
ref. [9]), however, there are inconsistencies
between assignments
given for the outermost
and
closely spaced lone pair orbitals.
Since these
orbitals are essentially halogen valence p in character such uncertainties
will not affect our discussion. We have therefore adopted the most recent
assignments.
The assignments
adopted for CH,
BrCl and CH,ClI spectra are based on empirical
grounds such as comparison
with spectra of related halomethanes,
relative band intensity variations and observed vibrational fine structure. The
molecular coordinate
system is defined in such a
way that the two halogen atoms lie in the yz
plane. The molecular
orbital numbering
system
includes all molecular orbitals which can be considered to be formed from the outermost occupied
ns and np orbitals of the constituent
atoms.
2. Experimental
The photoelectron
spectra studied in this work
were recorded at the vacuum UV photoelectron
work station established at SERC Daresbury SRS.
The details of the electron
spectrometer
and
monochromator
assemblies have been given previously [lo]. The spectra of samples were recorded
of mixedmethylene
dihalides
for two photoelectron
emission angles 8 = 0” and
90” to the direction of the principal E vector for
the elliptically
polarized
synchrotron
radiation.
Calibrant
gases Ar and He were used to obtain
spectra from which the polarization
of the photon
beam in the energy range 19-115 eV was determined. The calibrant
spectra were always recorded under experimental
conditions identical to
those utilized for sample spectra. Details of the
procedures used to calculate asymmetry parameters and partial photoionization
cross sections from
measured band areas have been outlined previously [lO,ll] and only a brief explanation
of the
procedure used in cross section measurements
is
given here. The cross section determination
is
based on recording the spectra of a calibrant gas,
e.g., He, Ne, Ar and sample under well-defined
experimental
conditions.
The absolute photoionization
cross section
of various
photoelectron
processes in the sample molecule can then be
calculated from the measured band areas of the
sample spectrum and the tabulated cross sections
for rare gases [12]. In some spectra spurious features were observed at certain photon energies.
They are caused by second-order
radiation
from
the toroidal grating monochromator.
These features were strong due to the high cross sections for
Br 3d-’ and I 4d-’ ionizations.
They obscured
parts of the valence spectrum at certain photon
energies but were easily identified. As a result of
this interference
gaps occur in the asymmetry
parameter curves and tabulated partial photoionization cross sections, Sample compounds
were
obtained
from the BDH Chemical Co. and Aldrich Chemical Co. Ltd. and used without further
purification.
The sample spectra were, however,
recorded and calibrated by using a high resolution
HeIcY spectrometer operating at 15 meV resolution
(fwhm). The He1 spectra obtained were identical
with those reported previously [6,7] and also with
spectra recorded at 21 eV photon energy with
synchrotron
radiation. The inner valence shell reusing known
gion in the spectra was calibrated
binding energies for Xe 4d-’ ionization compiled
by Krause [13].
I. Novak et al. / Photoelectron spectra
of mixedmethylene
131
dihalides
3. Results and discussion
x0.5
I
Br 3d
Photoelectron spectra of CH zBr, , CH,BrCl and
CH,ClI recorded at hv = 100 or 115 eV and t9 = 0”
are presented in figs. l-3. The spectra shown
correspond to 200 counts full scale after a total
accumulation of 50 scans. High-resolution (15 meV
fwhm) HeIa spectra of the lone pair region of
CH,ClI are shown in fig. 4. The features in the
spectra of CH,Br,
and CH,BrCl are associated
with the assignments made by Li et al. [8] and
Novak et al. [7] respectively. The spectrum of
CH,ClI is assigned on the basis of comparison
with the assignments deduced from MS Xa calculations for CH,Cl, and CH,I, [8]. The two sharp
features in the CH,ClI spectrum with binding
energies 9.76 and 10.35 eV can be associated with
ionizations from iodine lone pair orbitals 7a’ and
3a”, respectively. The band with two partially
resolved features at 11.34 and 11.57 eV can be
assigned to 2a” and 6a’ chlorine lone pairs respectively, while the three broad features observed at
13.87, 15.14 and 16.22 eV correspond to ionizations from (I bonding orbitals 5a’, 4a’ and la”
which have significant C-I, C-Cl and C-H character, respectively. The band with ionization energy of 20.4 eV can be assigned to ionization of an
orbital with predominantly C 2s character. This
empirical assignment is based on the comparison
of the CH,ClI spectrum with those of CH,BrCl,
CH,Br,
and CH,I,.
The high-resolution HeIa
I
x0.17
20
10
30
50
10
BMMi
ENERGY
64
80
70
90
WI
Fig. 2. The photoelectron spectrum of CH,BrCl
eV, e = 0”.
for hv =115
spectrum of CH,ClI (fig. 4) has revealed vibrational fine structure present in the lone pair bands.
The vibrational progressions with frequencies of
390, 380 and 400 cm-’ observed in the three lone
pair bands are attributed to the C-Cl stretching
mode which has a molecular ground state
frequency of 527 cm-’ [14]. A second progression
observed in the chlorine lone pair band with a
frequency of 1180 cm- ’ can on the basis of comparison with Raman spectrum [14] be described as
a CH,-scissoring mode whose molecular ground
state frequency is 1392 cm-‘. The vibrational fine
structure found in the lone pair ionization bands
indicates that there is a certain amount of mixing
between chlorine and iodine lone pairs and that
I
x0.2
Br 3d
I4d
CH2Br2
I
jjJ& ,;1 7jjf&&JOL
10
20
30
50
40
BINDING
ENEAY
60
70
80
lo
WI
Fig. 1. The photoelectron spectrum of CH,Br,
eV, O=U’.
20
30
for hv = 115
54
M
EWNCNG ENERGY
60
70
WI
Fig. 3. The photoelectron spectrum of CH,CII for hv = 100 eV,
e=oo.
132
I. Novak et al. / Photoelectron spectra
I
-
a
B 5
-
h
h
I
A
CH2N
h
$8
x)1) lo.2 104
W6
" 11.2 114
BkarG
ENWGYW)
Fig. 4. High-resolution
CH ,CII.
HeIa
1M
lH
photoelectron
12.0
spectrum
of
neither has purely atomic character.
For all three molecules studied features additional to those mentioned above have been found
in the spectra. They can be associated on energy
grounds with ionizations from molecular orbitals
with predominantly Cl 3s, Br 4s, I 5s Br 3d or I
4d character. Ionization energies measured for
these bands are in good agreement with values
calculated by Yeh and Lindau [15] for atomic
subshell binding energies. The one-electron description of ionization is known to be inadequate
for inner valence molecular orbitals but for simplicity it will be retained here. The ionization
energy for the Cl 3s-type la’ orbitals was found to
be 26.6 eV in both CH,BrCl and CH,ClI spectra.
For Br 4s-type orbitals measured binding energies
are 23.0 and 24.4 eV for the lb, and la, orbitals
of CH,Br,
and 23.5 eV for the 2a’ orbital of
CH,BrCl. The I Ss-type 2a’ orbital in CH,ClI has
a binding energy of 22.3 eV. In addition to broad
weak features several sharp very intense bands
with large binding energies have been observed.
They can be associated with Br 3d and I 4d
orbitals. Binding energies measured for 3d,,, and
states of CH,Br,
have values of 76.1 and
3d
77.b/‘eV, respectively while for corresponding states
in CH,BrCl the binding energies have been measured as 76.8 and 77.8 eV. Iodine 4d,,, and 4d,,,
states observed in the CH,ClI spectrum appear at
56.9 and 58.8 eV. In the vicinity of these intense
d-shell bands several broad featureless bands were
recorded. In bromo-substituted
molecules their
binding energy was dependent on the energy of
ofmixedmethylene
dihalides
the incident photons. This strongly suggests that
the features (labelled A) in figs. 1 and 2 with
electron energies of 48.8, 45.6 and 29 eV in the
CH,BrCl spectrum and 44.9 eV in the CH,Br,
spectrum correspond to Auger M,,,NN
transitions. The CH,ClI spectrum does not reveal features with Auger-type electron energy dependence.
The two weak bands with ionization energies of
62.6 and 64.4 eV can be interpreted as shake-up
states, i.e. describable only in terms of many-body
effects. The /I parameter measurement has not
been possible for halogen s orbital ionizations or
Auger bands due to their low intensity and signal/noise problems.
3.1. Asymmetry
measurements
The main orbitals of the halomethanes studied
can be classified for convenience into three groups.
The first group consists of non-bonding orbitals
with predominantly halogen p character, i.e.
halogen lone pair orbitals. The second group consists of u bonding orbitals with C 2p, H 1s and
halogen p character while the last group contains
core-like orbitals with mainly Br 3d or I 4d
character. The orbitals mentioned do not of course
conform rigidly to this classification especially in
view of low molecular symmetry which allows
mixing between orbitals from the first two groups.
A number of features observed in the spectra
overlap making individual j? measurements for
each band difficult. In such cases the total area of
the overlapped bands has been measured and used
to calculate a mean value of p. /I variations for
the various photoelectron bands in the spectra of
CH,Br,,
CH,BrCl and CH,ClI as a function of
photoelectron energy are shown in figs. 5-11.
3.2. Ionizations from halogen lone pair orbitals
Asymmetry parameters for ionization from the
halogen lone pair orbitals are shown in figs. 5-7.
B curves displayed show the existence of Cooper
minima at electron energies of 32, 60 and 48 eV
for chlorine, bromine and iodine lone pair orbitals,
respectively. The measured positions of the Cooper
minima can be compared with the values reported
for lone pair orbitals in Ccl, [16], CF,Br [ll] and
I. Novak et al. / Photoelectron spectra of mixed methylene dihalides
ELECTRON
t
EHRGY
133
IN
Fig. 5. /I as a function of electron kinetic energy for ionization
from the 4aI + la, + 2b, + 3b2 orbitals (0) of CH2Br2.
Fig. 7. B as a function of electron kinetic energy for ionization
from the 7a’ + 3a” orbitals (0) and 6a’ +2a” orbitals (A) of
CH,ClI.
CH,I [22]. The comparison tends to suggest that
the position of the halogen lone pair minimum is
largely unaffected by chemical environment. The
electron energy value at which a Cooper minimum
in the j3 curve can be expected thus seems to
depend mainly on the atomic number of the
halogen atom. A similar atomic number dependence of the position of Cooper minima has been
observed in the asymmetry parameter variations
for ionization from the outermost p orbitals of the
rare gases Ar, Kr and Xe. Several theoretical
models have been applied to the analysis of trends
in Cooper minima. We shall mention only the
recent calculations reported by Manson [l] and
Parpia et al. [17]. In the first study non-relativistic
Hartree-Slater wavefunctions were used to obtain
the positions of zeros in dipole matrix elements
describing photoionization for all elements of the
periodic system. The estimated trends in the Cooper minima thus obtained show qualitative agreement with the behaviour observed from B variations for the rare gases and halogen lone pairs of
the methylene dihalides. The second study by
Parpia et al. [17] used a more sophisticated relativistic .time-dependent local density approximation
for the outer shell photoionizations of the rare
gases. They achieved a nearly quantitative agreement with experimental data showing the sensitivity of Cooper minimum positions to relativistic
and correlation effects especially for heavy atoms.
In view of the similarity between the electronic
properties of halogen lone pairs and the outermost
p orbitals of rare gases the same effects can be
expected to play an important part in determining
the positions of Cooper minima in the methylene
dihalides. Besides the Cooper minima the overall
shape of the j? curve for lone pair orbital ionizations is important. Assuming, in the first approximation the validity of the Gelius model one
can expect two types of B curves. In halomethanes
containing only one kind of halogen atom
n = O-3 and X = Cl, Br, I) /3 curves
(CH,X,-,,,
for lone pair orbitals should strongly resemble one
another regardless of the molecular symmetry or
the degree of halogenation. j3 curves for Br lone
pair orbitals measured for CH,Br, or CF,Br are
indeed found to be very similar to that of CH,Br.
In the “mixed” halomethanes the profile of the /3
curves can be modified by the mixed halogen
character of the lone pair orbitals. In this work we
have studied two mixed halomethanes, CH,BrCl
+2CVQ
fib;*.*
AA;A.tAk ,.*..‘i”
“+‘I B(t
A
0
10
“kA,
AbAAA
20
30
.
40
ELECTRON
50
l
60
EHFUiY
70
80
90
loo
lil
IA’)
-1-
Fig. 6. j3 as a function of electron kinetic energy for ionization
from the 7a’ + 3a” orbitals (@) and 6a’ + 2a” orbitals (A) of
CH,BrCl.
134
I. Novak et al. / Phoioelectron spectra of mixed methylene dihalides
and CH,ClI. The shapes of j3 curves are distinctly
different in the two cases. In CH,BrCl p curves
for the chlorine and bromine lone pair orbitals
closely resemble those found in Ccl, or CH,Br
[16], respectively. However, j3 variations for chlorine and iodine lone pair orbitals deduced from
CH,ClI spectra display prominent shoulders in
the vicinity of the Cooper minima. For the chlorine j3 spectrum with a Cooper minimum at 32 eV
the shoulder occurs at around 52 eV while for the
iodine /3 spectrum with a Cooper minimum at
around 48 eV the shoulder occurs in the 32 eV
electron energy region. Since each shoulder coincides with the Cooper minima in the other halogen
/I spectrum this would appear to indicate considerable mixing between the halogen lone pair
orbitals.
The obvious deduction from this different behaviour might be that in case of chloroiodomethane interhalogen lone pair interaction (mixing) is considerably more pronounced than is the
case for bromochloromethane.
The correct explanation is likely to be more complicated. The
vibrational fine structure present in lone pair bands
and observed under high resolution indicates that
in both molecules significant mixing takes place
between lone pair orbitals of neighbouring halogen atoms. A possible reason for the difference in
the @ curves can be found in the shape of the
bromine lone pair /? curve. The p spectra observed
in CH,Br, show that at electron energies > 55 eV
p values vary by less than 0.3 units throughout a
40 eV electron energy interval. The Cooper minima
observed for chlorine and iodine lone pairs tend to
be much sharper and better defined. As a result
any mixing between chlorine and iodine lone pair
orbitals would be more readily detected through
the mixed character of the /3curve. The argument
proposed does not, however, rule out the possibility of greater overlap (and hence mixing) between
the more diffuse iodine 5p and chlorine 3p orbitals
than is possible between bromine 4p and chlorine
3p orbitals.
A further important feature of the p curves for
ionization from the lone pairs in CH,ClI is the
pronounced increase in j3 values at electron energies > 70 eV. The large values appear to be
associated with the interchannel coupling between
+l
0
I
.- Tl
O__m
al
3b
LO
’
50
’
60
10
60
90
ice no
ELECTRCN ENERGY (et’)
-1
Fig. 8. fi as a function of electron kinetic energy for ionization
from the 3a, +2b, + lb, orbitals (0) and 2a, orbital (A) of
CH,Br,.
I 5p and 4d electrons. The importance of this
correlation effect has recently been observed in
the photoelectron spectrum of CFJ [18]. Calculations performed for the lone pairs in HI have
demonstrated that the inclusion of interchannel
coupling with the continuum states corresponding
to ion states associated with a 4d hole is required
to fit the large values of j3 observed in this region
P91.
3.3. Ionizations from o bonding orbitals
/3curves for ionization from u bonding orbitals
are shown in figs. 8-10. A smooth monotonic
increase in j? values occurs in all u bonding
orbital ionizations. Many (I bands have not been
+ZT
+l
0
I
A
AAAAAA~A
AAJA
AA AAA
0..
.
l
l
f’
a*..*
f l*
CIpCl
ELECTRONENEffiY WI
-1 1
Fig. 9. /3 as a function of electron kinetic energy for ionization
from the 5a’ +4a’ + la” orbitals (0) and 3a’ orbital (A) of
CH,BrCI.
135
I. Novak et al. / Photoelectron spectra of mixed methylene dihalides
ably reduce values of asymmetry parameters for u
orbitals.
3.4. Ionization from bromine and iodine d shells
.
0
10
20
30
w)
50 .
60
70
Go
90
m
m
ELECTRONENERGY WI
-1
f
Fig. 10. /3 as a function of electron
tion from the 5a’ +4a’ + la” orbitals
CH,CII.
kinetic energy for ioniza(@) and 3a’ orbital (A) of
completely resolved in the spectra so that a mean
j3 value has been calculated instead. This averaging procedure possibly smoothes out some of the
shallow resonances which might have been associated with individual bands. The fl dependence for
5a’ + 4a’ + la” orbitals in CH,ClI does show a
broad shoulder between 22 and 45 eV electron
energy. Unfortunately, due to second-order radiation problems several points on the j3 curve are
missing precluding an accurate description of the
resonance. Halogen lone pair p character in these
(I bonding orbitals could have produced the shallow feature in this position. In the electron energy
region where this resonance appears both halogen
lone pair orbitals have low /3 values due to Cooper
minima so that any mixed character would prob-
+2-
CH2i3r2
cHpcl
CH2CII
A4
AA
+1 .’
l-i
.,
..
4&
P
10
20
30
40
50
60
70
FQ
90
100
r
-11
ELECTRON ENERGY WI
Fig. 11. j3 as a function of electron kinetic energy for ionixation from the Br 3d orbital (+) of CH,Brs,
Br 3d orbital of
CH,BrCl (0) and I4d orbital of CH,ClI (A)
Asymmetry parameters for Br 3d-’ and I 4d-’
ionizations are shown in fig. 11. The fi curves for
Br 3d ionizations are similar to each other in the
two molecules studied and also to the /i depen-
Table 1
Partial photoionization
CH,Br,
as a function
cross sections ‘) (in Mb) for orbitah
of photon energy
hv
(ev)
6a, + 2a,
+4b, +4b,
3a, + 2b,
+ lb,
19
22
25
28
31
34
37
40
43
46
49
55
60
65
70
75
80
85
90
95
100
105
110
115
92.4 f 18.2
64.6 f 9.7
30.2 f 5.9
28.0 f 4.3
12.9 f 2.4
6.1 f 1.2
3.4 f 0.1
2.4 f 0.5
1.8 f 0.3
1.5 f 0.3
1.2 f 0.2
1.0 f 0.2
0.5 f 0.1
0.8 f 0.2
0.20* 0.07
0.18* 0.06
0.16f
0.04
0.17f
0.04
0.45 f 0.06
0.39* 0.06
0.33*
0.06
0.30* 0.03
0.29f
0.03
0.26* 0.03
33.7
29.6
20.3
22.0
13.8
6.3
3.4
2.5
2.0
1.8
1.5
1.5
*5.1
f3.8
k4.1
*4.2
f2.9
il.3
kO.6
+0.4
*to.4
f0.3
*0.3
*0.3
0.8 kO.1
0.25 f 0.06
0.23 f 0.06
0.19*0.05
0.19*0.04
0.35 f 0.03
0.30*0.04
0.22 f 0.03
0.19*0.03
0.18kO.03
0.15j,O.O2
2a,
5.7
2.2
1.1
0.8
0.5
0.4
0.4
0.4
_
of
Br 3d
*1.1
*0.5
f0.3
kO.1
*0.1
fO.l
*to.1
*0.1
0.20 f 0.05
0.24 f 0.06
0.14*0.05
0.10*0.04
0.10*0.03
0.10*0.03
0.16f0.03
0.14kO.03
0.11*0.02
0.10*0.01
O.lOf0.01
0.07 f 0.01
1.7&0.4
2.5kO.5
2.7kO.5
3.5*0.7
4.2kO.8
4.4 f 0.8
a) The photoionization
cross section data have been given in
tabulated form because although resonances observed in the
/3 spectra are also present in e spectra changes in e by
several orders of magnitude
over the energy range studied
tend to make these effects less marked when displayed
graphically.
The experimental
uncertainties
quoted for absolute cross sections originate from statistical errors associated with band area measurements
and do not include
possible systematic errors due to sample pressure measurements, analyser transmission
function corrections
and contamination
of analyser surfaces. Relative cross sections are,
however, more reliable allowing the set of values to be
renormahzed
should better absolute values become available.
I. Novak et al. / Photoelectron spectra of mixed methylene dihalides
136
Table 2
Partial photoionization cross sections (in Mb) for orbitab of CHsBrCl as a function of photon energy
19
22
25
28
31
34
37
40
43
46
49
55
60
65
70
75
80
85
90
95
100
105
110
115
7a’ + 3a”
6a’ + 2a”
5a’ + 4a’ + la”
33.1 *6.6
24.8 k5.0
12.1 *2.1
10.6 f2.0
5.0 *1.1
2.6 kO.4
1.7 *0.2
1.0 *0.1
0.8 +O.l
0.7 *0.1
0.5 *0.1
0.21 f 0.02
0.28 f 0.02
0.27 f 0.02
0.25 f 0.02
0.21*0.02
0.21*0.02
0.17&0.02
0.21 f 0.02
0.26 f 0.02
0.19~0.02
0.16kO.02
0.16kO.02
0.16kO.02
36.5 f6.8
30.3 *5.2
14.1 *3.0
12.5 k2.2
5.3 fl.2
2.3 kO.4
1.3 *0.2
0.8 kO.1
0.7 kO.1
0.8 *O.l
0.5 *0.1
0.29 f 0.03
0.41 f 0.09
0.35 f 0.04
0.36 f 0.03
0.40 f 0.03
0.37 + 0.03
0.33 f 0.03
0.37 f 0.03
0.54 f 0.06
0.39 + 0.04
0.34 f 0.04
0.30*0.04
0.31 i-o.04
25.3 k4.1
17.4 L-3.5
17.3 +3.4
11.6 +2.1
5.9 +0.9
3.3 +0.6
2.1 +0.4
1.8 +0.3
1.8 f0.3
1.3 +0.3
0.6 IfIO.1
0.9 *0.1
0.6 +O.l
0.49 f 0.07
0.50*0.07
0.34 f 0.03
0.37 f 0.03
0.38 f 0.03
0.51*0.06
0.36*0.05
0.33*0.04
0.30*0.04
0.28 f 0.04
3a’
Br 3d
2.3 i0.4
1.6 +0.3
0.9 +0.2
0.7 rto.1
0.5 f0.1
0.4 +0.1
0.4 +0.1
0.30*0.07
0.19*0.03
0.15 *to.02
0.12 *0.02
0.15 *0.02
0.12*0.02
0.13+0.02
0.11+0.01
0.19+0.03
0.13 f 0.02
0.13 f 0.02
0.10 f 0.01
0.08 f 0.01
0.23
0.57
1.8
1.6
1.9
2.2
2.3
f 0.03
f 0.07
kO.3
*0.3
*0.4
*to.5
kO.5
Table 3
Partial photoionization cross sections (in Mb) for orbitals of CH,ClI as a function of photon energy
hv (ev)
7a’ + 3a”
6a’ + 2a”
5a’ + 4a’ + la”
19
22
25
28
31
34
37
40
43
46
49
55
60
65
70
75
80
85
90
95
100
105
110
115
29.3 It5.9
17.3 f3.5
6.6 f1.3
4.5 *0.9
2.0 *0.4
1.0 *0.2
0.8 kO.1
0.6 fO.l
0.43 f 0.09
36.8 f7.4
26.6 f5.0
12.2 *2.2
9.4 *1.9
4.2 *0.8
1.3 *0.2
0.7 *0.1
0.7 zko.1
0.55 f 0.09
25.2
24.2
16.9
15.0
9.2
3.1
1.6
0.34*0.04
0.43 f 0.08
0.27 f 0.05
0.30 f 0.06
0.57 f 0.12
0.45 f 0.08
0.48 f 0.08
040~0.08
0.31 f 0.05
0.21 f 0.04
0.18*0.03
0.19*0.03
0.13*0.02
0.47 f 0.08
0.39 f 0.05
0.47 f 0.08
0.25 f 0.04
0.24 f 0.05
0.45 * 0.10
0.38 f 0.07
0.37 f 0.07
0.33 f 0.06
0.28 f 0.05
0.24f0.04
0.21 f 0.04
0.26 f 0.04
0.21 f 0.04
3a’
14d
0.19 f 0.04
0.17 f 0.04
0.13 + 0.03
0.10+0.02
0.09 f 0.02
0.06 * 0.01
0.06 * 0.01
0.05 f 0.01
3.7f0.7
8.0f1.3
9.3*1.9
10.6 f 2.1
10.0 f 1.9
8.6*1.5
6.8 f 1.2
6.7 f 1.2
7.3 f 1.5
6.3 f 1.2
k4.8
f4.4
k3.4
f3.0
k1.8
+0.6
kO.3
_
1.1 *0.2
1.0 *0.2
0.86kO.15
0.50 * 0.10
0.43 f 0.08
0.57 f 0.11
0.46 f 0.08
0.42 f 0.07
0.35 *0.06
0.29 f 0.05
0.22f0.04
0.20*0.04
0.25 f 0.05
0.23 f 0.04
I. Novak et al. / Photoelectron spectra ojmixed
dence reported for CH,Br [20] showing the same
smooth monotonic decrease with electron energy.
The behaviour of /I parameters for I 4d ionization
is significantly different from the one observed for
bromine in’ the limited electron energy range
studied. A distinct resonance occurs in the /3 curve
at approximately 13 eV electron energy. It strongly
resembles the minimum found in methyl iodide
[21] and can on the basis of this comparison be
attributed to a shape resonance in the 4d --, ef
outgoing photoelectron channel.
4. Partial photoionization cross sections
The absolute partial photoionization cross sections measured for CH, Br, , CH, BrCl and CH,ClI
are given in tables l-3 for the 19-115 eV photon
energy range. The values were calculated using the
calibration procedure discussed previously 1111.
Possible systematic errors notwithstanding
the
tabulated values show an initial rapid decrease in
cross sections with increasing photon energy. The
absolute values can be compared with the cross
sections measured
for CH,I
[22], bromofluoromethanes [ll] and with calculations for
atomic subshell cross sections [15]. Due to the
overlap of bands corresponding to a bonding
orbital ionizations it was not possible to measure
their individual cross sections. Halogen lone pair
bands are reasonably well resolved and their measured cross sections are in good agreement with
the experimental and theoretical results reported
previously [11,22]. d shell partial cross sections
can be compared with results reported for methyl
bromide and iodide. The cross section variation
for the I 4d orbital in CH,ClI is in good agreement with the results obtained for methyl iodide
[21] and with the theoretical cross section values
computed by Yeh and Lindau [15]. When partial
photoionization cross sections measured for Br 3d
ionization in CH,Br, and CH,BrCl are compared
with available experimental values for methyl
bromide [20] and with calculated bromine 3d shell
cross sections it appears that the values given by
Carlson et al. [20] (fig. 2) for CH,Br may be too
large by a factor of ten. The discrepancy is possibly due to a drawing error since cross section
methylene dihafides
137
values for 3d- ’ ionization in Kr shown in this
diagram are also too large by a factor of ten.
5. Conclusions
From the results discussed in this work a few
general comments can be made concerning the
photoionization processes involving valence orbitals. It has been known for some time that the
independent particle approximation is not always
applicable to the description of photoionization
from inner valence and outermost core orbitals.
Correlation and relaxation effects in molecules
cannot be neglected. The main experimental evidence supporting the quantum-mechanical
prediction has come from studies using vacuum UV
or X-ray line sources of radiation. In the case of
the halomethanes very few experimental studies of
inner valence shell ionizations have been reported
[23]. To our knowledge no systematic study of the
inner valence shells has been performed utilizing a
vacuum UV tunable radiation source apart from
those reported by our groups [lo]. We have observed that inner valence orbital ionizations can
still be recognized on the “one orbital-one band”
basis. A similar conclusion has recently been
reached by Lindle et al. [24] based on the photoionization study of 3d and 3p subshell ionizations
of krypton. This does not suggest that the oneelectron picture is completely valid. It simply
points out the experimental fact that the splitting
of spectroscopic oscillator strengths is not always
sufficient to modify the relative intensities of
“main” and “satellite” bands to the extent where
they are of the same order of magnitude. Further
experimental studies of inner valence and outermost core level photoionizations
with better
counting statistics are required to explore this
point. Photoionization
studies of the various
halomethanes performed so far indicate that the
Gelius model (further elaborated by Yarzhemsky
et al. [25]) may be applicable to vacuum UV
photoionizations. The asymmetry parameter and
cross section variations for halogen lone pairs are
similar to a particular halogen atom regardless of
the molecular symmetry or chemical environment.
While the main resonance features like Cooper
138
I. Novak et al. / Photoelectron spectra of mixed methylene dihalides
minima will not experience a significant change
the overall shape of fi or u variations is modified
depending on molecular symmetry and bonding
properties. The above comments should be regarded as an attempt to organize the growing
body of experimental data along the lines which
would guide the design of future studies and should
not be regarded as a substitute for sophisticated
calculations designed to unravel details of photoionization processes.
Acknowledgement
We thank the SERC for generous financial
assistance and the staff of Daresbury Laboratory
for help and cooperation. In particular we would
like to express our gratitude to Drs. F. Quinn, J.B.
West, B. Dobson and Professors G.V. Marr and
I.H. Hillier for establishing the photoelectron work
station. JMB would like to thank SERC for his
studentship. IN thanks the “Ruder BoSkoviY Institute, Zagreb for leave of absence.
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