Thiazyl chloride: An experimental and theoretical study of the valence shell HeI photoelectron spectrum

Article abstract of DOI:10.1016/S0301-0104(02)00966-7

High level (CASSCF-MRCI) ab initio calculations are used to investigate the structural, electronic and vibrational properties of the electronic ground state of thiazyl chloride (NSCl) and of the low-lying electronic states of NSCl⁺. A new high resolution HeI photoelectron spectrum of NSCl has been recorded in the 10-16 eV energy region. From the results of the calculations, the first band is assigned to the (1)²A′ state of NSCl⁺. The second one corresponds to the (2)²A′ and (1)²A″ states which are quasi-degenerate. Despite the high resolution, the two first bands show no vibrational fine structure. For the first one, Franck-Condon analysis shows that it is due to the overlapping of two vibrational progressions involving the S-Cl stretching and the NSCl bending modes. In the case of the second band, it is explained by the highly repulsive character of the potential energy surfaces of two states of NSCl⁺ in the Franck-Condon region of the neutral molecule. For the third band, which shows vibrational peaks, the calculation of Franck-Condon factors permits the determination of the adiabatic ionisation energy of the (3)²A′ electronic state of NSCl⁺ at 13.798 eV. Finally, the fourth band, which is due to three different ionic states with vibrational progressions, is too complicated to be assigned quantitatively.

Full text of DOI:10.1016/S0301-0104(02)00966-7

Chemical Physics 288 (2003) 95–104
www.elsevier.com/locate/chemphys
Thiazyl chloride: an experimental and theoretical study of
the valence shell HeI photoelectron spectrum
a
D. Duflot , N. Chabert , J.-P. Flament , J.-M. Robbe , I.C. Walker ,
a
a
a
b
b
J.H. Cameron , A. Giuliani , M.-J. Hubin-Franskin , J. Delwiche
c
c
d,
*
a
Laboratoire de Physique des Lasers, Atomes et Mol eꢀ cules (PhLAM), UMR CNRS 8523,
ꢀ
Centre dÕEtudes et de Recherches Lasers et Applications (CERLA),
Universit eꢀ des Sciences et Technologies de Lille, F-59655 Villeneuve dÕAscq Cedex, France
Department of Chemistry, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK
b
c
ꢀ
Laboratoire de Spectroscopie dÕElectrons Diffus eꢀ s, D eꢀ partement de Chimie, Institut de Chimie B6c, Universit eꢀ de Li eꢁ ge,
Sart Tilman, B4000 Li eꢁ ge 1, Belgium
d
Thermodynamique et Spectroscopie, D eꢀ partement de Chimie, Institut de Chimie B6c, Universit eꢀ de Li eꢁ ge, Sart Tilman,
B4000 Li eꢁ ge 1, Belgium
Received 8 March 2002; in final form 21 October 2002
Abstract
High level (CASSCF-MRCI) ab initio calculations are used to investigate the structural, electronic and vibrational
þ
properties of the electronic ground state of thiazyl chloride (NSCl) and of the low-lying electronic states of NSCl . A
new high resolution HeI photoelectron spectrum of NSCl has been recorded in the 10–16 eV energy region. From the
2
0
þ
results of the calculations, the first band is assigned to the (1) A state of NSCl . The second one corresponds to the
2
0 00
2
(
2) A and (1) A states which are quasi-degenerate. Despite the high resolution, the two first bands show no vibrational
fine structure. For the first one, Franck–Condon analysis shows that it is due to the overlapping of two vibrational
progressions involving the S–Cl stretching and the NSCl bending modes. In the case of the second band, it is explained
þ
by the highly repulsive character of the potential energy surfaces of two states of NSCl in the Franck–Condon region
of the neutral molecule. For the third band, which shows vibrational peaks, the calculation of Franck–Condon factors
2
0
þ
permits the determination of the adiabatic ionisation energy of the (3) A electronic state of NSCl at 13.798 eV. Fi-
nally, the fourth band, which is due to three different ionic states with vibrational progressions, is too complicated to be
assigned quantitatively.
Ó 2003 Published by Elsevier Science B.V.
PACS: 31.14.Ar; 33.60.Cv; 33.20.Tp
1. Introduction
*
Thiazyl chloride is the S-analogue of nitrosyl
chloride, NOCl. At room temperature, it exists as
a solid trimer, N Cl , in equilibrium with the
Corresponding author. Tel.: +32-4-3663435; fax: +32-4-
662941.
E-mail address: jdelwiche@ulg.ac.be (J. Delwiche).
3
3
S
3
3
0
301-0104/03/$ - see front matter Ó 2003 Published by Elsevier Science B.V.
doi:10.1016/S0301-0104(02)00966-7

96
D. Duflot et al. / Chemical Physics 288 (2003) 95–104
gaseous monomer. The trimer is a six-membered
ðNSÞ ring having all the chlorine atoms on the
3
same side of the ring. The monomer, characterised
by both microwave spectroscopy [1] and electron
s
diffraction [2], is a bent molecule (point group C )
whose geometry is given in Table 1. Its infrared
spectrum is known [3] (Table 2). Consistent with
the tabulated data, the S–Cl bond is relatively
weak (shared electron number ¼ 0.60, compared
with 2.14 for the N–S bond [4]) and photolysis of
NSCl in the near UV generates a chlorine atom
and an NS radical. However, there is little avail-
able information on the electronic states of NSCl.
The HeI photoelectron spectrum (PES) was first
reported by Cowan et al. [5] some years ago. It
showed four unstructured bands between about
1
0.5 and 15 eV ionisation energy. These were as-
signed to five ionisations from comparison with
the PES of thiazyl fluoride (NSF) together with the
results of semi-empirical extended H u€ ckel type
calculations. Subsequently, De Kock et al. [6]
published HeI and HeII spectra along with a NeI
spectrum. In the last, the third and fourth ionisa-
tion bands showed fine structure suggesting exci-
tation of the bend vibration, m
2
(band 3) and the m
1
stretch vibration (band 4) but the data were com-
plicated by the doublet nature of the NeI source. A
fifth ionisation was also reported. Cationic states
were assigned using relative intensity differences
between the HeI and HeII spectra and CNDO/2
calculations. Like Cowan et al., De Kock et al. [6]
proposed two closely spaced ionisations within the
second band, as did Allaf et al. from ab initio
computations [7]. All of the above workers concur
2
0
that the first cationic state is A followed by
0
2
00
2
A – A but thereafter, the state ordering is con-
fused. It should be noticed that all previous theo-
retical studies applied KoopmansÕ theorem to
assign the experimental bands.
As part of an investigation into the electronic
structure of NSCl, we have carried out a high
level ab initio theoretical study of the neutral
ground state molecule and its low-lying cationic
þ
states, NSCl . We have also re-measured the HeI
photoelectron spectrum, with energy resolution
superior to that previously obtained; the spec-
trum is analysed using the results of the compu-
tations.

D. Duflot et al. / Chemical Physics 288 (2003) 95–104
97
Table 2
Vibrational wavenumbers (cm ) of the electronic ground state X A of NSCl
a
À1
~
1
0
b
c
d
RHF RHF CASSCF CASSCF CASSCF CASSCF CASSCF Exp.
MRCI MRCI(D) MRCI(D)
cc-pVTZ cc-pVQZ cc-pVTZ cc-pVQZ cc-pVTZ cc-pVTZ cc-pVQZ
Exp.
Exp.
x
x
x
1
2
3
(stretch NS) 1415.8
(bend NSCl) 314.0
(stretch SCl) 472.0
1429.0
318.5
471.1
1323.8
267.1
391.7
991.3
1332.4
268.1
396.5
998.5
1313.9
272.1
416.3
1351.9
276.8
422.1
1364.7
281.3
424.8
ZPE
1101.8
1109.3
1001.2
1025.4
1035.4
m
m
m
1
2
3
(stretch NS) 1394.7
(bend NSCl) 313.9
(stretch SCl) 470.6
1406.9
317.8
464.6
1308.4
267.2
379.1
987.8
1315.9
265.0
400.6
990.8
1297.6
271.9
405.1
997.1
1335.2
272.7
422.3
1351.9
281.5
431.6
1324:5 Æ 1 1327.3 1326
271:6 Æ 1
414:8 Æ 1
1005.4
267.4
403.75 414
999.2
ZPE
1099.0
1094.7
1022.0
1035.1
a
1 2 3 1 2 3
x ; x , and x correspond to the harmonic vibrations, while m , m , and m correspond to the anharmonic vibrations.
b
c
Ref. [3].
Ref. [27].
Ref. [7].
d
2
. Computational details
calculations, the standard method is to generate
the excitations from a sub-space of the active
space, consisting of the CSFs whose weight in the
wave function is greater than a given threshold
(typically 0.01). This criterion, however, is some-
what arbitrary and cannot be used for the calcu-
lation of potential energy curves. To avoid this
problem, we used the excitation degree of the CAS
CSFs as the selection parameter. For example, in
All the ab initio calculations in this paper were
carried out with the MOLPRO program package
8]. Two Gaussian atomic orbital basis sets, taken
[
from Dunning [9], were used: the cc-pVTZ (98
basis functions) and the cc-pVQZ (173 basis
functions) basis sets.
The molecular orbitals (MOs) were optimised at
the RHF and CASSCF [10–12] levels. In the latter
case, the active space consists of the n ¼ 2 shell for
the N atom and the n ¼ 3 shell for the S and Cl
atoms, the inner shells being kept inactive. Thus,
1
0
~
the case of the X A ground state of NSCl, the
active space contains the HF closed shell reference
configuration plus all single, double, triple. . . up to
hexa excitations from the occupied orbitals to the
empty ones. With the cc-pVQZ basis set, the re-
sulting MRCI space contains about 5 millions of
contracted CSFs. If we impose that only the closed
shell HF configuration plus all the single and
double excitations (which correspond to 219 CSFs
among the 8029 of the full CAS) are generators of
the MRCI space, the size of this space is reduced
to about 1.8 million of uncontracted CSFs. When
1
0
~
for the X A ground state of NSCl, the active space
0
00
consists in 9 a MOs, 3 a MOs with 18 active
þ
electrons, leading to 8029 CSFs. For NSCl , there
are 17 active electrons and the size of the active
2
0
spaces are much larger (28,503 CSFs for A states
2
00
and 28,125 CSFs for A states). In order to take
into account the effect of dynamical correlation,
multi-reference configuration interaction (MRCI)
þ
[
13,14] calculations were performed, using the
a similar procedure is applied to the case of NSCl
CASSCF MOs as starting orbitals. We recall that
this method generates all single (S) and double (D)
excitations from the configurations state functions
states (open shell HF configurations), the reduc-
tion of the MRCI space is even more drastic, and
the calculations become feasible for our present
computational possibilities. In the following, this
type of MRCI calculations will be designed as
MRCI(D).
(
CSFs) of the active space. This leads to CI di-
mensions too large for our present computational
þ
possibilities, especially for NSCl . For example
0
2
þ
the MRCI space for the A states of NSCl con-
9
In order to determine the equilibrium geometry
1
0
~
of the X A ground state of NSCl, a grid of 35
tains about 3 Â 10 CSFs. To reduce the size of the

98
D. Duflot et al. / Chemical Physics 288 (2003) 95–104
points centred on the experimental geometry was
generated. The resulting energies (calculated at
RHF, CASSCF and MRCI levels) were then fitted
by a fourth-degree polynomial, using the SURFIT
program [15]. The resulting surfaces were then
converted to quadratic force fields in internal and
dimensionless coordinates using l-tensor algebra
4S Cl þ 2NH Cl ! S N Cl þ 8HCl þ 5S
2
2
4
3
2
2
3S
3
N
2
Cl
2
þ 3Cl
2
! 2N
3
S
3
Cl
3
þ 3SCl
The preparation was carried out in small bat-
2
ches, using 1/10th of the quantities recommended
in the original recipe. Consequently, the first re-
action step was completed in about 3 h (rather
than 10–12 h) and the intermediate S N Cl was
[
16] as implemented in SURFIT. The program
3
2
2
uses perturbation theory to obtain the harmonic
and anharmonic wavenumbers.
chlorinated directly. At stages when moist air had
to be excluded, oxygen-free nitrogen was flowed
through the system. The final batches of product
were stored in air-tight sample bottles. Portions
were transferred, as required, to a sample tube that
was connected directly to the ionisation chamber
of the photoelectron spectrometer. Rapid heating
under vacuum to 70 °C released the monomer.
A well known problem arising in the calcu-
lation of transition energies is the choice of the
orbitals for the description of the initial and final
states. Here, for the calculations of the electronic
þ
states of NSCl , three types of CASSCF MOs
were employed: the NSCl ground state MOs,
state-averaged (SA) MOs for the four lowest-ly-
þ
ing states of NSCl (for each irreducible repre-
0
00
3.2. Photoelectron spectrometer
sentation A and A ) and state-optimised MOs
for each ionic state.
The HeI photoelectron spectra were recorded
using an hemispherical energy analyser described
elsewhere [21]. The resolution was set to 20 meV,
measured as the full width at half height of
the P3=2 peak of the argon HeI photoelectron
spectrum.
Finally, in order to interpret the vibrational
structure of the spectral bands, Franck–Condon
factors calculations were carried out, using our
local implementation [17,18] of the method pro-
posed by Cederbaum and Domcke [19]. This
model requires the determination of the gradient
2
The resolution was further increased by de-
convolution using a procedure described by van
Cittert [22] and modified by Allen and Grimm [23].
The energy axis was calibrated with reference to
(
the j matrix) and the Hessian (the c matrix) of the
ionisation energy E at the neutral geometry:
v
ꢀ oE
ꢁ
ꢀ
ꢁ
2
v
1
o E
oQ oQ
v
ji ¼ 2À1=2
;
cij ¼ 4
:
oQ
i
i
j
HCl and N , present as impurities in some of the
2
0
0
spectra, using the values given by Kimura [24]. The
spectra have been corrected for the transmission of
the electron optics.
In the present work, the quadratic coupling
matrix c was neglected and only the j matrix was
computed (linear coupling approximation) at the
CASSCF/cc-pVQZ level.
4. Results and discussion
3
. Experimental
~
1
0
4
.1. The electronic ground state X A of the neutral
molecule
3
.1. Sample preparation
At the Hartree–Fock level, the electronic con-
0
The trimer of thiazyl chloride N
3
S
3
Cl
3
was
~
figuration of the X A ground state is
1
made using the method described by Jolly and
Maguire [20]. This is a two-step synthesis. First,
0
0
0
0
0
00
0
00
0
½
coreꢀ1a 2a 3a 4a 5a 1a 6a 2a 7a
The experimental equilibrium geometry was
disulfurdichloride, S
chloride, NH Cl producing S
2
Cl
2
, reacts with ammonium
Cl which then
N
2
2
determined by microwave spectroscopy [1] and X-
ray diffraction [2] (see Table 1). The two sets of
values agree well with each other. At the theoret-
4
3
reacts with chlorine, Cl , to give the solid product,
2
S N Cl .
3
3
3

D. Duflot et al. / Chemical Physics 288 (2003) 95–104
99
ical level, the only ab initio determination is that of
4.2. The ionic states
Ehrhardt and Ahlrichs [25], using the coupled pair
functional (CPF) method [26] but with the NSCl
angle kept frozen to 117.7° (Table 1). This study
emphasised the importance of polarization func-
tions (especially g AOs) for a good description of
the NSCl bond lengths.
Our own calculated geometries using the cc-
pVTZ and cc-pVQZ basis sets are presented in
Table 1. As expected for these methods, the RHF
bond lengths are underestimated while the
CASSCF ones are overestimated. In particular,
the S–Cl bond length is too large at cc-pVQZ/
4.2.1. Photoelectron spectrum
The HeI photoelectron spectrum is displayed in
Fig. 1. We note that bands 1 and 2 are structure-
less with FWHM of about 0.18 and 0.46 eV, re-
spectively. Concerning the second band, the results
of the calculations described below show that it is
the superposition of two displaced peaks with the
same FWHM. The result of the fitting of this band
by two identical Gaussian functions is displayed
in Fig. 2. The energy spacing is 0.224 eV and
the FWHM is 0.27 eV. Band 3 shows a vibra-
tional progression, with a wavenumber of about
ꢂ
CASSCF level by 0.07 A. The full MRCI and
MRCI(D) calculations using the cc-pVTZ basis set
À1
290 cm (0.036 eV) consistent with excitation of
the bending vibration m
of 271.6 cm
slightly improve the result with a S–Cl bond length
ꢂ
2
, which has a wavenumber
(0.034 eV) in the ground state
ꢂ
À1
around 2.2 A, which is too large by ꢁ0.04 A. Fi-
nally, the cc-pVQZ/MRCI(D) calculations give the
best results, with a remaining discrepancy between
ꢂ
theory and experiment of 0.015 A for RSCl
.
The vibrational wavenumbers of NSCl have
also been determined, using infrared spectroscopy
in the gas phase by M u€ ller et al. [3] and in argon
matrices by Peake and Downs [27] (see Table 2).
The three modes correspond, respectively, to the
N–S stretching mode (m
1
), to the N–S–Cl bending
).
mode (m ) and to the S–Cl stretching mode (m
2
3
The calculated wavenumbers, obtained at RHF,
CASSCF and MRCI(D) levels with both basis sets
are given in Table 2. Several well known trends can
be observed, such that the 10% overestimation of
the RHF values. The harmonic wavenumbers are
too high in most cases and the inclusion of an-
harmonicity improves the situation. The basis set
effects are less important than for the geometries,
the difference between the cc-pVTZ and cc-pVQZ
Fig. 1. The HeI photoelectron spectrum of NSCl.
À1
being less than 10 cm in most cases. Among the
three modes, the bending and S–Cl stretching
values are very close to the experimental ones. For
these two modes, anharmonic effects seem to be
weak since at all levels of theory, the differences
between the x and the m values are negligible. The
MRCI(D) results seem to give slightly poorer re-
sults, since the anharmonic values remain above
the experimental ones. This tends to indicate that
the restrictions of MRCI(D) method described
above slightly affect the topology of the potential
well around the equilibrium geometry.
Fig. 2. Results of the fitting of the second peak of the
experimental spectrum by two identical displaced Gaussians
functions.

100
D. Duflot et al. / Chemical Physics 288 (2003) 95–104
Table 3
Calculated vertical ionisation energies of NSCl (eV)
This work
a
a
b
c
d
e
Neutral MOs
SA MOs
Optimised MOs
Exp.
Exp.
Exp.
HF
EH
2
0
0
00
0
0
0
00
(
(
(
(
(
(
(
(
1) A
2
10.670
11.679
12.075
14.012
14.587
14.703
14.777
16.291
10.346
11.239
11.237
13.474
14.349
14.254
14.016
14.874
10.362
11.227
11.153
13.394
13.958
14.202
14.404
15.042
10.953
11.725
11.949
13.834
14.499
10.96
11.80
10.95 (1)
11.82 (2)
11.27 (7a )
12.48 (2a )
11.69 (7a )
12.89 (6a )
00
0
1) A
2
2) A
2
12.71 (6a )
15.92 (1a )
13.04 (2a )
14.33 (5a )
0
00
0
0
0
3) A
2
13.77
14.46
13.83 (2)
14.49 (1)
00
0
2) A
2
16.19 (5a )
17.25 (4a )
15.25 (4a )
0
4) A
2
00
00
3) A
2
4) A
a
MRCI(D) values obtained at the calculated MRCI(D)/cc-pVQZ equilibrium geometry of NSCl (see Table 1) and corrected of the
À1
ZPE (1035.1 cm or 0.128 eV, Table 2).
b
Ref. [5].
Ref. [6].
c
d
Koopmans MOs energies from [7].
Extended H u€ ckel calculations from [5].
e
neutral molecule. De Kock et al. [6] identified a
similar progression in the NeI spectrum, citing a
seems to give values closer to the experimental ones,
but this is an artefact: since the NSCl MOs are not
À1
þ
wavenumber of 250 Æ 30 cm (0:031 Æ 0:04 eV).
the best choice for the description of NSCl states,
the calculated energies are too high. Nevertheless,
all the calculations agree to assign without ambi-
À1
In band 4 vibrational intervals of about 1049 cm
À1
(0.13 eV) and 403 cm (0.05 eV) may be identi-
fied; these could relate to excitation of the two
stretching vibrations m and m (1325 cm or 0.164
eV and 415 cm or 0.0514 eV, in the neutral
ground state, respectively). However, as will be
shown later, this band is complex and its analysis
in terms of a single ionisation is simplistic.
2
0
guity the first band to the (1) A , the second one to
2
À1
00 0
2
the (1) A and (2) A states and the third one to the
0
1
3
À1
2
(3) A state. These results are in agreement with the
Koopmans values [5,7], except that Allaf et al. [7]
00
assigned the third band to a A state. Concerning
the fourth band, one can only say that it is due to
several electronic states. According to the theoreti-
cal results, there are four states in the 14–15 eV en-
ergy region corresponding to this band.
4
.2.2. Vertical ionisation energies
In Table 3, the vertical ionisation energies cal-
culated at the cc-pVQZ/CASSCF-MRCI(D) level,
using the three different types of MOs are re-
ported, as well as the results of previous calcula-
tions, based on KoopmansÕ theorem [5,7]. The
present experimental values have been determined
in the following way: for the first band, the ioni-
sation potential is taken as the maximum of the
band. The two values given for the second band
are extracted for the fit by two Gaussian functions.
For the third and fourth bands, the values re-
ported in Table 3 are those of the most intense
vibrational peaks.
Another problem concerns the ordering and the
2
00
2
0
energy splitting of the (1) A and (2) A states re-
sponsible for the second band. The three types of
calculations give contradictory results: the neutral
MOs give 0.4 eV, which seems too high; SA cal-
culations give quasi-degenerate values while opti-
mised MOs lead to DE ¼ 0:074 eV. Relying on this
latter value, which is the closest to the splitting
deduced from the fit of the experimental band
(0.224 eV), the ordering of the states would be
2
0
2
00
(2) A and (1) A .
The comparison between the three sets of calcu-
lations and the experiment shows that the theoreti-
cal results are underestimated, in most cases by
several tenths of eV. The use of the neutral MOs
4.2.3. Detailed investigation of the three first bands
of the photoelectron spectrum
Despite the good energy resolution used to
record the experimental HeI photoelectron

D. Duflot et al. / Chemical Physics 288 (2003) 95–104
101
etries, the SA-MRCI(D) energies of the three
states were recalculated. The results of these cal-
culations are reported in Fig. 3. In addition, a full
geometry optimisation of the lowest lying states of
þ
NSCl was performed at the CASSCF/cc-pVQZ
level. A Mulliken population analysis was also
carried out. The bond orders, which are not
available in MOLPRO, were obtained with the
HONDO package [29]. The results of these cal-
culations are given in Table 4.
2
0
Concerning the (1) A state, which is responsi-
ble of the first band, the potential energy curve
(
Fig. 3) presents two minima, the lowest lying
ꢂ
ꢂ
around 3.3 A and the secondary one around 2.0 A.
Indeed, geometry optimisations found the equi-
librium geometries indicated in the two first col-
umns of Table 4. In the following, this secondary
minimum will be denoted as (1a) to distinguish it
from the lowest lying one (1b). Since (1a) takes
place in the Franck–Condon zone of the neutral
Fig. 3. Calculated potential energy curves of the lowest lying
þ
states of NSCl (see text for details).
ꢂ
molecule (around 2.2 A, vertical line in Fig. 3), one
spectrum, no vibrational feature is observed for
the two first bands of the spectrum while well re-
solved vibational structure is observed in the third
band. In order to ascertain the reasons for such
behaviour, we explored the topology of the po-
should expect the presence of vibrational features
in the first band. In order to explain its absence, we
have calculated the Franck–Condon factors for
this state, using the force constant matrix of NSCl
1
0
þ
2
0
~
(X A ) and the j matrix of NSCl ((1a) A ) ob-
tained at the cc-pVQZ/CASSCF level. The simu-
lated band is in good agreement with the
experimental one as shown in Fig. 4. In fact, this
band involves excitation of both the S–Cl
stretching and the bending modes, which are very
close as shown in Table 2. The absence of fine
structure is due to the overlapping of two vibra-
2
0
tential energy surfaces of the lowest lying (1) A ,
2
00 0 þ
2
(
1) A and (3) A states of NSCl , which corre-
þ
1
þ
2
lates to the NS ðX R Þ þ Clð PÞ fragments (using
the cc-pVTZ basis set). To do this, we optimised
at CASSCF level) the RNS and h
for different (frozen) values of R for the lowest
(
coordinates
ðNSClÞ
SCl
2
0
þ
lying state (1) A of NSCl ; then, at these geom-
Table 4
Calculated properties (CASSCF/cc-pVQZ) of the lowest lying electronic states of NSCl
þ
þ
a
b
NSCl
NSCl
NSCl
NSCl
2
0
2
0
2
00
2
0
~
X A
1
0
~
X A
1
0
~
X A
1
0
(
1a) A
(1b) A
(1) A
(3) A
ꢂ
R
R
h
Q
Q
Q
NS (A)
1.524
2.046
1.447
3.342
118.68
+0.03
+0.93
+0.04
2.53
1.447
3.342
118.07
+0.03
+0.93
+0.04
2.53
1.470
2.171
111.72
+0.06
+0.87
+0.07
1.88
1.461
2.229
117.51
)0.27
+0.63
)0.36
2.34
1.444
2.140
1:450 Æ 0:0001
2:161 Æ 0:0001
117:42 Æ 0:05
ꢂ
SCl (A)
NSCl (°)
108.03
+0.01
+0.82
+0.17
1.83
114.4
N
)0.37
+0.73
)0.36
2.14
S
Cl
r
NS
r
SCl
r
NCl
a
1.02
0.07
0.01
0.06
0.01
0.06
0.89
0.19
0.72
0.15
0.60
HF/TZ + 2P from [4].
Ref. [1].
b

102
D. Duflot et al. / Chemical Physics 288 (2003) 95–104
Fig. 5. Third electronic band of the HeI photoelectron spec-
trum of NSCl. (a) Experimental curve and (dots) deconvoluted
curve (Â0:4). (b) Calculated curve (the energy of the most in-
tense peak is set to the experimental value).
Fig. 4. First electronic band of the HeI photoelectron spectrum
of NSCl. (a) Experimental curve. (b) Calculated curve (the
energy maximum is set to the experimental value).
tional progressions. Moreover, it should be no-
ticed that our calculations do not take into ac-
count the tunnelling effect through the potential
repulsive character of potential energy curves ex-
plains the absence of vibrational structure in the
spectrum for this band.
ꢂ
energy barrier occurring around 2.5 A (Fig. 3).
This should increase the width of the vibrational
peaks, at least on the high energy side of the band,
and contribute to the absence of fine features.
The situation is somewhat different for the third
band of the photoelectron spectrum, correspond-
2
0
ing to the (3) A state whose geometry and charge
properties are very close to the one of the neutral
ground state, as shown in Table 4. By comparison
with the wavenumber values in NSCl (Table 2), we
can attribute the observed vibrational features in
2
0
For the second band, assigned to the (2) A and
2
00
þ
the (1) A states of NSCl , inspection of Fig. 3
shows that, in the Franck–Condon zone of the
neutral ground state, their potential energy curves
2
0
are highly repulsive (the (2) A is in fact dissocia-
tive) and well above the dissociation asymptote.
2
this band of the photoelectron spectrum to m , the
bending vibration mode. Another indication is the
fact that the main difference between the geome-
2
0
2
00
Furthermore, the (2) A and (1) A potential en-
ergy curves have the same slope. After the reflec-
tion principle [28], it is possible to estimate from
this slope the width of the spectral bands. The
resulting value (0.35 eV) is consistent with the
value obtained in the fitting of the second experi-
mental band (0.27 eV) as shown in Fig. 2. The
þ
2
0
tries of the NSCl (3) A state and the neutral
ground state is the hðNSClÞ angle (Table 4).
In order to assign precisely the experimental
peaks, we have calculated the Franck–Condon
factors for this state, using the same method as for
the first band. The calculated shape of this third

D. Duflot et al. / Chemical Physics 288 (2003) 95–104
103
band is shown in Fig. 5(b) and can be compared to
the experiment shown in Fig. 5(a). Despite the
simplicity of the model, the overall agreement with
the experimental spectrum is good, the main dis-
crepancy being the relative intensity of the third
and fifth peaks. According to these calculations,
the two first peaks observed correspond to hot
high resolution spectrum, large scale (CASSCF-
MRCI) ab initio methods, and Franck–Condon
factors calculations. For the three first bands of
the spectrum, the theoretical results provide as-
signments that improve on those from previous
semi-empirical calculations. Geometry optimisa-
þ
tions of the lowest lying states of NSCl and
00
band type transitions from the (0, m ¼ 1; 2, 0)
Franck–Condon analysis are used to explain the
vibrational structure (or its absence) in the three
first bands. In particular, the theoretical simula-
tion of the vibrational progression observed in the
third band reveals the presence of hot bands and
leads to the conclusion that the adiabatic ionisa-
2
0
~
levels of the X A ground state of NSCl to the
1
2
0
ð0; 0; 0Þ level of the (3) A electronic state of
þ
NSCl . All the following observed peaks corre-
spond to transitions from the ð0; 0; 0Þ vibrational
0
level of NSCl to the ð0; m ; 0Þ levels of the ionic
2
2
0
þ
state. This gives a value of 13:798 Æ 0:002 eV for
tion energy of the (3) A electronic state of NSCl
is 13.798 eV.
2
0
the adiabatic formation of the (3) A electronic
state of NSCl . Moreover, the model predicts that
þ
0
0
0
the intensity ratio of the ðm ¼ 1 ! m ¼ 0Þ and the
2
2
0
0
0
2
ðm ¼ 0 ! m ¼ 1Þ transition should match the
2
Acknowledgements
Boltzmann factor expðÀm =kT Þ, i.e., about 0.27 in
2
the present case. This value is in good agreement
with the one (around 0.26) deduced form the ex-
perimental band shown in Fig. 5(a).
The Laboratoire de Physique des Lasers, Ato-
mes et Mol ꢀe cules (PhLAM) is a ‘‘Unit ꢀe Mixte de
Recherche du CNRS’’ of France. The Centre
In addition to the geometries, Table 4 shows the
Mulliken populations and bond orders of the
ꢀ
dÕEtudes et de Recherches Lasers et Applications
(
CERLA) is supported by the Minist ꢁe re charg ꢀe de
þ
lowest lying states of NSCl . For all the structures,
the positive charge is located on the sulphur atom.
la Recherche, the R ꢀe gion Nord/Pas-de-Calais of
France, and the Fonds Europ ꢀe en de D ꢀe veloppe-
ment des R ꢀe gions (FEDER). M.J.H.F. acknowl-
edges the Fonds National de la Recherche
Scientifique for a research position. ICW is grate-
ful to the Fonds National de la Recherche Scien-
tifique for funding her stay if the Li ꢁe ge
laboratories. The authors thank the referee for
very useful suggestions.
2
0
The (1a) and (3) A states, whose geometries are
rather close to the neutral one, the charge prop-
erties and bond orders are also similar to those of
2
0
2
00
NSCl. On the other hand, the (1b) A and (1) A
þ
states of NSCl are nearly indistinguishable. These
states are very weakly bound complexes between
þ
0
00
the Cl atom and a NS ion. The A –A splitting is
due to the removal of the electron from the in-
plane and out-of-plane p orbital which constitutes
the HOMO of the neutral NS molecule.
Finally, for the fourth band, one can only say
that it is very difficult to perform a detailed theo-
retical study, as this would require the determi-
nation of the ionisation energies, the absolute
electronic ionisation intensities and the vibrational
wavenumbers of at least three highly excited states.
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