On the high-resolution HeI photoelectron spectrum of Cl2O

Article abstract of DOI:10.1016/S0009-2614(97)01407-3

The high-resolution HeI (58.4 nm) photoelectron spectrum of dichlorine monoxide, Cl₂O, has been recorded in the region of the four lowest-energy ionic electronic states. Formation of the ion in its ground and excited electronic states is accomp

Full text of DOI:10.1016/S0009-2614(97)01407-3

6 March 1998
Ž
.
Chemical Physics Letters 284 1998 452–458
On the high-resolution HeI photoelectron spectrum of Cl2O
a,1
b
a
a,2
F. Motte-Tollet , J. Delwiche , J. Heinesch , M.-J. Hubin-Franskin ,
c
c
c
d
J.M. Gingell , N.C. Jones , N.J. Mason , G. Marston
a
Laboratoire de Spectroscopie d’Electrons diffuses, UniÕersite de Liege, Institut de Chimie-Bat. B6c, B-4000 Liege, Belgium
´
´
`
ˆ
`
b
Thermodynamique and Spectroscopie, UniÕersite de Liege, Institut de Chimie-Bat. B6c, B-4000 Liege, Belgium
´
`
ˆ
`
c
Department of Physics and Astronomy, UniÕersity College London, Gower Street, London WC1E 6BT, UK
d
Department of Chemistry, UniÕersity of Reading, Whiteknights, P.O. Box 224, Reading RG6 6AD, UK
Received 27 November 1997
Abstract
Ž
.
The high-resolution HeI 58.4 nm photoelectron spectrum of dichlorine monoxide, Cl2O, has been recorded in the region
of the four lowest-energy ionic electronic states. Formation of the ion in its ground and excited electronic states is
accompanied in each case by vibrational excitation. In particular, the vibrational structure of the first and second excited
states of Cl2Oq is resolved. Analysis of the vibrational progressions associated with formation of the various ionic states has
been completed, allowing confirmation of the symmetry and bonding characteristics of the four highest-energy occupied
molecular orbitals of Cl2O. q 1998 Elsevier Science B.V.
1. Introduction
the catalytic loss of stratospheric ozone. In this
Ž
Letter, we report recent high-resolution HeI 58.4
The spectroscopy and photochemistry of chlorine
oxides have received considerable attention in recent
years due to the involvement of these molecules in
the chemical transformations that lead to depletion of
ozone in the Earth’s stratosphere. Dichlorine monox-
ide, Cl2O, is currently thought to play, at most, a
minor role in the chemistry of the polluted strato-
sphere but is related structurally to the oxides of
chlorine that play a significant role. In addition, it is
the anhydride of HOCl, a reservoir for active chlo-
rine in the stratosphere, and is also widely used as a
laboratory source of ClO, a radical that is central to
.
nm photoelectron spectra of Cl2O and new ioniza-
tion energy values for the four lowest-energy elec-
tronic excited states of the molecular ion.
There is only one reported measurement of the
Cl2O photoelectron spectrum which was performed
w x
by Cornford et al. 1 at an energy resolution of
about 40 meV. Eight valence ionization energies
were determined, the first being localised at an adia-
batic energy value of 10.94 eV. Within the experi-
mental resolution, only the lowest-ionization energy
band of the spectrum exhibited vibrational structure,
assigned by the authors to the excitation of two
progressions involving the symmetric stretch vibra-
tional mode n1 and the bend vibrational mode n2.
The associated frequencies were slightly greater in
1 Collaborateur scientifique FNRS.
2 Directeur de recherche FNRS.
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
Ž
.
PII S0009-2614 97 01407-3

(
)
F. Motte-Tollet et al.rChemical Physics Letters 284 1998 452–458
453
2
q
w x
the ionic ground electronic state than in that of the
neutral, suggesting a slightly antibonding character
to the outermost occupied molecular orbital. The
photoelectron spectrum also showed a sharp peak at
12.79 eV assigned to ionization of a chlorine 3p
nonbonding electron. Recently, a discharge flow-
photoionization mass spectrometer apparatus coupled
to a synchrotron radiation source allowed the deter-
mination of the adiabatic ionization energy of Cl2O
and Grimm’s algorithm 8 . The Ar P3r2 peak was
used as the instrumental profile.
Gaseous Cl2O was prepared by slowly passing a
stream of chlorine through a reaction column packed
with dried mercuric oxide 9 . The spectrum of the
effluent gas showed the presence of only Cl2 impu-
rity. The best Cl2OrCl2 ratio was obtained by carry-
ing out the reaction at room temperature and purify-
ing Cl2O through multiple distillation in vacuo at
y808C.
w x
w x
equal to 10.909"0.016 eV 2 .
The HeI photoelectron spectrum of Cl2O was
recorded between the 10.5 and 13.5 eV ionization
energies, spanning the four lowest-energy electronic
excited states of the molecular ion. The 20 meV
energy resolution of our spectrometer allowed us to
resolve the vibrational structure of each electronic
band. The present work follows our recent study on
3. Cl2O molecule
w
x
Microwave spectroscopy 10,11 and electron
w
x
diffraction experiments 12,13 have shown that Cl2O
is bent in its electronic ground state and belongs to
the C2v symmetry group. The equilibrium parame-
w x
the VUV photoabsorption spectroscopy of Cl2O 3 .
Ž
.
Ž
.
ters are re Cl–O s0.169587 nm and /e Cl–O–Cl
s110.8868. The three normal vibrational modes have
Ž . Ž .
A1 n1 and n2 and B1 n3 symmetries, respec-
2. Experiment
Ž
tively. Their corresponding wavenumbers and ener-
.
The spectrum was recorded using a photoelectron
spectrometer equipped with a 1808 hemispherical
electrostatic analyser working in the constant pass
gies derived from infrared and Raman spectro-
scopies are 642 cmy1 0.0796 eV for n1, 296 cm
y1
Ž
.
Ž
0.0367 eV for n2 and 686 cmy1 0.0851 eV for n3
Ž
.
.
w x
w
x
energy mode and detailed elsewhere 4 . The spec-
14,15 .
trum was scanned by sweeping a retarding voltage
between the chamber and the entrance slit of the
analyser. Accuracy of the voltage ramp was recently
improved by the use of a 18 bit DrA converter
covering a 0–9.9999 V range. The photons were
produced through a dc discharge in He in a two-stage
differentially pumped lamp. The whole spectrometer
is under the control of a Macintosh computer which
also allows treatment and analysis of the data.
The energy resolution of the spectrometer was set
to about 20 meV and the calibration of the ionization
energy scale was carried out by observing the ÕX s0
levels of the X 2 Sqg Nq2 and X 2 Pg Clq2 electronic
w x
states which are located at 15.579 5 and 11.480 eV
w x
6 , respectively. The accuracy of the scale was
estimated to be "0.006 eV. The spectrum was cor-
rected for the transmission of the analysing system.
Deconvolution of the spectrum allowed a number
of features appearing as shoulders in the raw spec-
trum to be clearly resolved and hence to provide
more precise energy values. The procedure employed
Fig. 1. The four highest-energy occupied orbitals of the Cl2O
molecule illustrated in terms of their constituent atomic orbitals.
w x
was the ratio method of Van Cittert 7 with Allen

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)
454
F. Motte-Tollet et al.rChemical Physics Letters 284 1998 452–458
Ž
.
Ž .
Fig. 2. HeI 58.4 nm photoelectron spectrum of Cl2O in the region of the four lowest-energy electronic states of the molecular ion. a Raw
Ž .
spectrum; b deconvoluted spectrum.
1
˜
Cl2O is a 20-valence electron molecule. The ener-
getic ordering of its outermost valence molecular
orbitals has been probed by earlier photoelectron
is X A1 and its electronic configuration is expected
to be:
2
2
2
2
core ... 2a
9a
7b
3b
1
Ž
.
Ž
2 . Ž 1 . Ž 2 . Ž
.
w x
spectroscopy 1 and a recent Hartree–Fock theoreti-
w
x
cal calculation 16 . The ground state of the molecule
.
Table 1
a
Ž
.
Energy peak values eV and analysis of the vibrational structure associated with the ionic ground electronic state
Ž
.
Ž
.
Ž
.
Ž
.
n
nn1
D E n1
nn1 qn2
D E n1
D E n2
nn2
D E n2
0
1
2
3
4
5
6
7
8
9
10
11
12
10.887
10.971
11.054
11.137
11.218
11.296
11.375
y
10.930
11.012
11.096
11.178
11.258
11.335
y
0.043
10.887
10.930
10.971
11.012
11.054
11.096
11.137
11.178
11.218
11.258
11.296
11.335
11.375
y
0.084
0.083
0.083
0.081
0.078
0.079
0.082
0.084
0.082
0.080
0.077
0.043
0.041
0.041
0.042
0.042
0.041
0.041
0.040
0.040
0.038
0.039
0.040
b
Ž
.
Ž
11.419
0.084
b
Ž
.
Ž
.
13
11.419
0.044
hot bands
10.807
10.851
0.080
0.036
a Estimated uncertainty: "0.006 eV.
b
Y
X
1
The energy of this vibrational peak coincides with that of the Õ s1 X Sg ™Õ s0 X 2 Pg electronic transition in chlorine.
Ž
.
Ž
.

(
)
F. Motte-Tollet et al.rChemical Physics Letters 284 1998 452–458
455
Each of these four molecular orbitals has been
4.1. First band
w
x
predicted by Nelson et al. 17 to contain significant
Cl p character. Alternatively, information on the
nature of the molecular orbitals can be obtained
using Walsh’s treatment of a 20-valence electron
The 10.7–11.4 eV ionization energy range of the
photoelectron spectrum is enlarged in Fig. 3. It is
composed of a broad electronic band involving
well-resolved vibrational structure, clearly visible in
the deconvoluted spectrum. The band relates to the
loss of an electron from the highest-occupied molec-
w
x
non-hydride AB2 molecule 18 . Pictorial representa-
tions of the four outermost occupied molecular or-
bitals are illustrated in Fig. 1 with descriptions of the
expected approximate bonding or antibonding char-
Ž
.
ular orbital HOMO and hence to the formation of
the molecular ion in its ground electronic level.
Energies of the vibrational peaks are tabulated in
Table 1 and are compared to the adiabatic and
vertical ionization energy values previously reported
Ž
.
Ž
.
acter along the Cl–O and Cl–Cl coordinates. The
molecular orbitals 3b1 and 9a1 are built respectively
from px and pz orbitals centred on each atom and
overlapping out-of-phase. They correlate with the
molecular orbital pu) in the symmetry group D`h of
the linear molecule. Molecular orbitals 7b2 and 2a2
are linked to the molecular orbital pg in the linear
configuration. They are formed from px and pz
orbitals centred on each chlorine atom, respectively
and can be viewed in a first approximation as non-
w x
in the photoelectron 1 and photoionization effi-
w x
ciency spectra 2 in Table 2. Our adiabatic and
vertical ionization energies are both lower than the
earlier values. In particular, the energy differences
Ž
.
bonding Cl–O 3p chlorine orbitals.
4. Results and discussion
The raw photoelectron spectrum of Cl2O between
10.5 and 13.5 eV ionization energies is displayed in
Fig. 2a, while Fig. 2b shows the spectrum obtained
after deconvolution, in which the vibrational features
are better resolved. In the following results, the
energy positions of the peaks as well as the figures
w
x
3–5 will relate to the deconvoluted spectrum.
The electronic band extending from an ionization
energy of 11.4–11.8 eV is attributed to the chlorine
impurity and linked to the formation of the Clq2 ion
in its ground electronic state X 2 Pg Fig. 2 . This
impurity does not interfere with the Cl2O photo-
electron spectrum since there are no Cl2Oq ionic
Ž
.
electronic states in the energy region of the X 2 Pg
q
w x
state 1 . Higher-energy Cl2 states are expected to
w
x
appear above 13.5 eV 6,19,20 and so are not
observed in the present spectrum.
Analysis of the vibrational envelope of the bands
provides information about the bonding character-
istics and hence the symmetry of the four outermost
occupied molecular orbitals of the neutral molecule
in its electronic ground state. Analyses of the four
observed transitions are presented in the following
sections.
Ž
.
Fig. 3. HeI 58.4 nm photoelectron spectrum of Cl2O in the
region of the ionic ground electronic state X 2 B1.
˜

(
)
456
F. Motte-Tollet et al.rChemical Physics Letters 284 1998 452–458
Table 2
Ž
.
Cl–Cl slightly bonding predicted character of the
. w
Assignment of the four lowest-energy electronic states of Cl2Oq
Ž
x
outermost molecular orbital Fig. 1 18 . The n1
frequency is higher in the molecular ion ;0.083
eV than in the neutral molecule 0.0796 eV indicat-
Ionic State AIE
VIE
Cornford et al.Thorn et al.
. Ž . Ž
Ž
Ž
.
AIE VIE AIE
.
Ž
.
2 B1
2
10.887 10.971 10.94 11.02 10.909"0.016 eV
˜
Ž .
ing Cl–O antibonding character, consistent with
X
˜
Ž
the predicted shape of the 3b1 molecular orbital Fig.
A B2
12.016 12.297 y
F12.453 12.593 y
12.742 12.742 y
12.37
12.65
12.79
y
y
y
2
˜
.
1 and confirming the ionic electronic ground state
B A1
2
2
˜
˜
Ž
.
as X B1 Table 2 and Fig. 2 .
C A2
The adiabatic and vertical ionization energies, AIE and VIE,
respectively, are also tabulated and compared to the literature data
4.2. Second and third bands
w x
w x
of Cornford et al.
Ž
1 and Thorn et al. 2 . Energies are in eV
.
estimated uncertainty: "0.006 eV .
The second and third electronic bands of the Cl2O
photoelectron spectrum extend from 12.0 to 12.7 eV
ionization energies, as displayed in Fig. 4. Both
bands exhibit vibrational structure not previously
resolved. The features correspond to the formation of
the molecular ion in its first and second excited
electronic states. The vibrational pattern of the sec-
ond band, less well resolved than in the first band,
suggests that the corresponding ionic electronic state
has a shorter lifetime than the ground ionic elec-
tronic state, with predissociation being a likely cause.
Both vibrational envelopes have been analysed in
terms of a long progression involving excitation of
w x
with the previous work of Cornford et al. 1 are
Ž
significant Ds0.053 and 0.049 eV for the adia-
.
batic and vertical energies, respectively . However,
our 10.887"0.006 eV adiabatic energy value agrees
well, within the errors bars, with the recent 10.909"
w x
0.016 eV value of Thorn et al. 2 . The intensity
alternation of the vibrational peaks leads us to inter-
pret the band envelope in terms of two progressions
involving the n1 and n2 vibrational modes, i.e.
either nn1 and nn1 qn2 , as previously proposed by
w x
Cornford et al. 1 , or nn1 and nn2 , as detailed in
Ž
.
Table 1. However, if the nn2 progression is excited,
the n2 bending mode Fig. 4 and Table 3 . Excita-
it would be quite long, in contradiction with the
tion of this mode demonstrates a change in the
2
2
˜
˜
Ž
.
Fig. 4. HeI 58.4 nm photoelectron spectrum of Cl2O in the region of the ionic first and second excited electronic states A B2 and B A1.

(
)
F. Motte-Tollet et al.rChemical Physics Letters 284 1998 452–458
457
cited state of the Cl2Oq molecular ion would then
Table 3
a
Ž
.
2
Energy peak values eV and analysis of the vibrational peaks
associated with the first and second excited electronic states of the
molecular ion A B2 and B A1, respectively
˜
Ž
.
be the B state Fig. 2 and Table 2 . Its first observ-
Ž
able vibrational peak is at 12.453 eV Fig. 4 and
Table 3 . The corresponding adiabatic Õ s0 level
may be hidden in the high-energy side of the second
photoelectron band Fig. 4 . This B A1 assignment
then leaves the second 12–12.4 eV band as arising
from ionization of an electron from the 7b2 Cl–Cl
2
2
˜
˜
X
.
n
nn2
D E
2
2
˜
A B2
˜
Ž
.
0
1
2
3
4
5
6
7
8
12.016
12.052
12.085
12.123
12.159
12.193
12.227
12.263
12.297
12.330
12.367
12.404
y
0.036
Ž
.
Ž
.
0.033
0.038
0.036
0.034
0.034
0.036
0.034
0.033
0.037
0.037
0.028
antibonding molecular orbital and to the formation of
2
˜
Ž
.
the A B2 ionic state Fig. 2 and Table 2 . Therefore,
we assign the low-intensity peak at 12.016 eV to the
Ž
.
Õs0 term Tables 2 and 3 .
4.3. Fourth band
9
10
11
The energy region relating to the loss of an
b
Ž
.
Ž
.
.
12
12.432
Ž
.
electron from the HOMO-3 molecular orbital is
enlarged in Fig. 5. It comprises a sharp intense peak
2
˜
B A1
n
nq1
nq2
nq3
nq4
nq5
nq6
nq7
12.453
12.488
12.524
12.560
12.593
12.629
12.667
12.703
y
0.035
0.036
0.036
0.033
0.036
0.038
0.036
b
Ž
.
Ž
a Estimated uncertainty: "0.006 eV.
b Peak structure appearing as a shoulder.
Ž
.
Ž
.
Cl–O–Cl angle, and therefore along the Cl–Cl
coordinate, between the neutral ground and the ionic
excited electronic states. Of the outermost molecular
orbitals, 7b2 and 9a1 are both candidates for the
Ž
.
originating orbital, 7b2 being Cl–Cl antibonding
Ž
.
and 9a1 bonding along the Cl–Cl coordinate. How-
ever, as the n2 vibration frequency as about the
same in the neutral and ionic states, we cannot use
this to distinguish between the two possible orbitals.
The same arguments apply to the third electronic
band around 12.6 eV. However, this band closely
resembles the first ionization band in the photo-
w
x
electron spectrum of sulphur dioxide 21 which has
been assigned to the ionization of an electron from
Ž
.
the 6a1 O–O bonding HOMO orbital. Since the
sulphur dioxide and dichlorine monoxide molecules
have, to a first approximation, the same valence
w
x
molecular orbitals 18 , we relate the 12.6 eV elec-
tronic band to the loss of an electron from the 9a1
Ž
.
Fig. 5. HeI 58.4 nm photoelectron spectrum of Cl2O in the
2
Ž
.
˜
Cl–Cl bonding molecular orbital. The second ex-
region of the ionic third excited electronic state C A2.

(
)
458
F. Motte-Tollet et al.rChemical Physics Letters 284 1998 452–458
Table 4
dom. MJHF and FMT wish to acknowledge the
Fonds National de la Recherche Scientifique for
position and support. JMG and NCJ recognise the
provision of EPSRC postgraduate studentships. NJM
recognises the support of the Royal Society. FMT,
MJHF, NJM, JMG and NCJ are grateful to NATO
for a collaborative research grant. The authors also
wish to acknowledge Prof. I.C. Walker for her con-
tinuous interest to the present work as also for her
comments.
a
Ž
.
Energy peak values eV and analysis of the vibrational structure
associated with the ionic third excited electronic state C A2
2
˜
n
nn1
D E
n2
D E
0
1
2
12.742
12.818
12.894
y
0.076
0.076
y
y
b
Ž
.
Ž
.
12.78
0.038
a Estimated uncertainty: "0.006 eV.
b Peak structure appearing as a shoulder.
located at 12.742 eV with, at its high-energy side,
only a few peaks of much lower intensity involving
excitation of a small vibrational progression in the n1
References
Ž
.
symmetric stretching vibrational mode Table 4 .
Excitation of one quantum of n2 might be visible as
a shoulder at 12.78 eV. This band shape is character-
istic of the ionization of an electron nonbonding
w x
1
A.B. Cornford, D.C. Frost, F.G. Herring, C.A. McDowell, J.
Ž
.
Chem. Phys. 55 1971 2820.
w x
2
R.P. Thorn Jr., L.J. Stief, S.-C. Kuo, R.B. Klemm, J. Phys.
Ž
.
Chem. 100 1996 14178.
w x
3
F. Motte-Tollet, M.-P. Ska, G.M. Marston, I.C. Walker,
M.R.F. Siggel, J.M. Gingell, L. Kaminski, K. Brown, N.J.
Ž
.
Ž
.
along both Cl–O and Cl–Cl coordinates. It is
therefore attributed to the loss of an electron from
the 2a2 molecular orbital, built mainly from the
Ž
.
Mason, Chem. Phys. Lett. 275 1997 298.
w x
4
J. Delwiche, P. Natalis, J. Momigny, J.E. Collin, J. Electron.
Ž
.
chlorine 3p nonbonding orbitals. The corresponding
Spectrosc. 1 1972 219.
2
w x
Ž .
5
R. Stockbauer, J. Chem. Phys. 70 1979 2108.
˜
ionic electronic state would then be the C A2 state,
w x
Ž
.
6
H. Van Lonkhuyzen, C.A. DeLange, Chem. Phys. 89 1984
Ž
with an adiabatic energy of 12.742 eV Fig. 2 and
313.
.
Table 2 .
w x
Ž .
P.H. Van Cittert, Z. Phys. 69 1931 298.
J.D. Allen Jr., F.A. Grimm, Chem. Phys. Lett. 66 1979 72.
7
w x
Ž .
8
w x
Ž .
J.J. Renard, H.I. Bolker, Chem. Rev. 76 1976 487.
9
w
w
x
10 G.E. Herberich, R.H. Jackson, D.J. Millen, J. Chem. Soc. A,
5. Conclusion
1966, p. 336.
x
11 M. Nakata, M. Sugic, H. Takeo, C. Matsumura, T. Fukuyama,
The photoelectron spectrum of Cl2O has been
recorded in the region of the ionic ground electronic
state and of its three lowest-energy excited electronic
states. The high-energy resolution of the spectrome-
ter allowed us to observe the vibrational envelope of
each ionic state. Analysis of the vibrational progres-
sions has given information on the nature of the
ionized orbitals and confirms the ground state elec-
Ž
.
K. Kuchitsu, J. Mol. Spectrosc. 86 1981 241.
w
w
x
Ž
.
12 J.D. Dunitz, K. Hedberg, J. Chem. Soc. 72 1950 3108.
x
Ž
.
13 L. Bru, P. Rodriguez, M. Cubero, J. Chem. Phys. 20 1952
1069.
w
w
w
w
x
14 T. Shimanouchi, Tables of Molecular Vibrational Frequen-
Ž
.
cies US GPO, Washington, DC, 1972 .
x
15 Y. Xu, A.R.W. McKellar, J.B. Burkholder, J.J. Orlando, J.
Ž
.
Mol. Spectrosc. 175 1996 68.
x
16 S.L. Nickolaisen, C.E. Miller, S.P. Sander, M.R. Hand, I.H.
Ž
.
Williams, J.S. Francisco, J. Chem. Phys. 104 1996 2857.
tron configuration of the molecule to be
x
2
.2 Ž .2 Ž .2 Ž
.
17 C.M. Nelson, T.A. Moore, M. Okumura, T.K. Minton, J.
Ž
. . . 2a2 9a1
7b2 3b1
.
Ž
.
Chem. Phys. 100 1994 8055.
w
w
x
18 A.D. Walsh, J. Chem. Soc., 1953, p. 2266.
x
19 A.B. Cornford, D.C. Frost, C.A. McDowell, J.L. Ragle, I.A.
Ž
.
Acknowledgements
This research has been supported by the Patri-
Stenhouse, J. Chem. Phys. 54 1971 2651.
w
x
20 A.J. Yencha, A. Hopkirk, A. Hiraya, R.J. Donovan, J.G.
Goode, R.R.J. Maier, G.C. King, A. Kvaran, J. Phys. Chem.
Ž
.
99 1995 7231.
moine of the University of Liege and by the Fonds
`
w
x
21 D.W. Turner, C. Baker, A.D. Baker, C.R. Brundle, Molecu-
National de la Recherche Scientifique. It has been
funded in part by the EPSRC of the United King-
Ž
lar Photoelectron Spectroscopy Wiley–Interscience, Lon-
.
don, 1970 .