Competing dissociation channels in the photolysis of S2Cl2 at 235 nm

Article abstract of DOI:10.1063/1.1496465

The photodissociation of disulfur dichloride at 235 nm was investigated by three-dimensional (3D) imaging of the chlorine product recoil in its ground state and excited spin-orbit state. The resonance increasing muliphoton ionization and time-of-flight techniques were used. The dependence of anisotropy parameter on the fragment recoil velocity was determined due to the technique where the 3D momentum vector of a single reaction product was found.

Full text of DOI:10.1063/1.1496465

Competing dissociation channels in the photolysis of S 2 Cl 2 at 235 nm
Tina S. Einfeld, Christof Maul, Karl-Heinz Gericke, and Alexei Chichinin
Citation: The Journal of Chemical Physics 117, 4214 (2002); doi: 10.1063/1.1496465
View online: http://dx.doi.org/10.1063/1.1496465
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 117, NUMBER 9
1 SEPTEMBER 2002
Competing dissociation channels in the photolysis of S2Cl2 at 235 nm
Tina S. Einfeld, Christof Maul, and Karl-Heinz Gerickea)
¨
¨
Institut fur Physikalische und Theoretische Chemie, Technische Universitat Braunschweig,
Hans-Sommer-Strasse 10, D-38106 Braunschweig, Germany
Alexei Chichinin
¨
¨
Institut fur Physikalische und Theoretische Chemie, Technische Universitat Braunschweig,
Hans-Sommer-Strasse 10, D-38106 Braunschweig, Germany and Institute of Chemical Kinetics
and Combustion, 630090 Novosibirsk, Russia
͑Received 15 January 2002; accepted 5 June 2002͒
The photodissociation of disulfur dichloride (S2Cl2) at 235 nm has been studied by
2
three-dimensional ͑3D͒ imaging of the chlorine product recoil in its ground state P3/2 Cl and
͓
͔
2
*
excited spin–orbit state P1/2 Cl employing the resonance enhanced multiphoton ionization and
͓
͔
time-of-flight techniques. The photodissociation proceeds mainly along the three channels forming
S2ϩ2Cl ͑1͒, S2ClϩCl ͑2͒, and 2 SCl ͑3͒ photoproducts where slow and fast Cl fragments are
released in ͑1͒ and ͑2͒, respectively. The relative yield of channel ͑1͒ with respect to channel ͑2͒ was
*
*
*
*
͔
determined to be 1.2:1.0. The yield of Cl , ␾(Cl )ϭP͑Cl )/ P(Cl)ϩP(Cl ) , was found to be
͓
*
0.35. The obtained state-specific velocity distributions of Cl and Cl are mainly different in the high
*
energy range: For Cl the two dissociation channels are almost equally present, whereas in the case
of ground state Cl the contribution of dissociation channel ͑2͒ is of minor importance. The
dependence of the anisotropy parameter ␤ on the fragment recoil velocity was directly determined
due to the novel technique where the 3D momentum vector of a single reaction product is observed.
For both spin–orbit states the anisotropy parameters differ for slow, intermediate, and fast chlorine
atoms. The observed ␤ values change from zero to slightly negative values up to positive values
with increasing kinetic energy. These observations can be explained by two overlapping dissociation
channels, where the two-body channel ͑2͒ releases the chlorine atom with high kinetic energy and
a positive ␤ parameter via an excited 1A state, whereas the three-body channel ͑1͒ proceeds mainly
sequentially, where the first Cl atom is released with intermediate speed and a slightly negative ␤
1
value via an excited B state, while the second Cl product atom in the decay of S2Cl is released
isotropically with slow recoil velocities. © 2002 American Institute of Physics.
͓DOI: 10.1063/1.1496465͔
Tokue et al.5 studied the photoabsorption cross section
I. INTRODUCTION
and the fluorescence excitation spectrum of S2Cl2 vapor in
the range 110–200 nm using synchrotron radiation. They as-
signed a number of broad bands in the 120–170 nm region as
Rydberg transitions. The predominant photodissociation pro-
cess in the 120–155 nm region was the three-body decay
͑1͒,13 where S2Cl2 splits into three photofragments. Chas-
ovnikov et al.6 observed atomic Cl fragments at a dissocia-
tion wavelength of 266 nm by laser magnetic resonance
while Tiemann et al.7 studied the SCl radical generated by
the photolysis at 248 nm by infrared diode laser spectros-
copy. It was found that the photolysis yields the SCl radical
in its electronic ground state via channel ͑3͒. Later Park
et al.8 studied the photodissociation of S2Cl2 at 308, 248, and
193 nm where the radical channel ͑2͒ was observed. In ad-
dition, Park et al.14 used the photolysis of S2Cl2 at 248 nm as
a source of Cl radicals to study the reaction dynamics of
chlorine atoms with deuterated cyclohexane. The radical de-
cay channel ͑2͒ was confirmed by Lee et al.9 by translational
spectroscopy at 308 nm. It was observed that S2Cl2 dissoci-
The UV absorption spectrum of S2Cl2,1–3 shown in Fig.
1, starts at a wavelength of 350 nm with a first maximum
around 300 nm followed by a broad absorption peak at 260
*
nm. The broad continuum is attributed to the n→␲
and
SϪS
*
the n→␴
transition, where the nonbonding orbitals can
S–Cl
be located at the S or the Cl atoms. Below 230 nm, another
absorption band starts to rise which has not yet been
assigned.2 The ground state of S2Cl2 is of C2 symmetry as
determined by electron diffraction and is shown in Fig. 2.4
The photolysis of the S2Cl2 molecule has been the sub-
ject of several experimental investigations for decades cov-
ering the whole spectral range from 110 to 514 nm.5–12 The
following dissociation channels were observed:
S2Cl2→S2ϩClϩCl,
S2Cl2→S2ClϩCl,
S2Cl2→2SCl,
͑1͒
͑2͒
͑3͒
͑4͒
S2Cl2→SCl2ϩS.
ates via channel ͑2͒ with a mean translational energy E of
͗
͘
T
65 kJ/mol and the measured anisotropy parameter ␤ was 0.4
Ϯ0.2. Lee et al. concluded that S2Cl2 undergoes a fast dis-
a͒
Author to whom all correspondence should be addressed.
0021-9606/2002/117(9)/4214/6/$19.00
4214
© 2002 American Institute of Physics
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J. Chem. Phys., Vol. 117, No. 9, 1 September 2002
The photolysis of S2Cl2
4215
FIG. 1. Absorption cross section of S2Cl2 based on the publication of Speth
et al. ͑Ref. 1͒. The wavelength used in the present experiment is marked by
an arrow.
sociation process after excitation via a parallel transition.
However, the interest in the photodissociation of S2Cl2 ex-
ceeded the UV region, as Chiu and Chang10 studied the reso-
nance Raman and fluorescence spectra of the S2Cl radical
which was generated by the photodissociation of S2Cl2 using
514.5 nm laser radiation. They confirmed that the radical
channel ͑2͒ dominates at 514.5 nm, which is also the case in
the photodissociation at 308 nm.
FIG. 2. Structure of S2Cl2 ͑Ref. 2͒ and the geometry of both transition
dipole moments ␮͑a͒ parallel to the symmetry axis C2 and ␮͑b͒ lying in the
plane perpendicular to the C2 axis.
Recent work includes two studies by Tiemann and
co-workers.11,12 Lindner et al.11 studied the major dissocia-
tion channels of S2Cl2 in the range between 280 and 240 nm
by using laser induced fluorescence and resonance enhanced
multiphoton ionization and time-of-flight techniques
͑REMPI-TOF͒ detection techniques on the state-specific
photofragments S and S2. They concluded that the high-
energy edge of the main absorption maximum is mainly the
result of a fragmentation either into S2ϩCl2, where Cl2 is in
more efficient forming S2 and 2Cl with a Cl atom transla-
tional energy distribution of E ϭ42 kJ/mol, which is in
͗
͘
T
agreement with the value derived by Park et al.8 However,
the former interpretation given by Park that the observed Cl
are released via a radical channel at 193 nm was misleading.
The observed ␤Cl value of Ϫ0.3 hints at an involvement of a
1
higher B state in the excitation.
The former studies show that the dissociation of S2Cl2
following absorption in the main band between 230 and 280
nm is a highly complex process involving several excited
potential energy surfaces and all decay channels ͑1͒–͑4͒ are
in competition with each other. In order to elucidate the dy-
namics behind this complicated behavior more detailed ex-
perimental data are required. Since we are able to observe
the three-dimensional ͑3D͒ momentum vector of a single
state-selective photofragment by applying a novel 3D imag-
ing technique15,16 the unsolved questions in the photodisso-
ciation of S2Cl2 are a promising task to study. There are the
relative yield of the reactions, the spin–orbit branching ratio
the 3
͟ state, or into S2ϩ2Cl (1). The long wavelength part
of the absorption spectrum is connected to the channel
S2ClϩCl ͑2͒ and toward shorter wavelength a complex com-
petition between different dissociation channels is expected.
The involvement of SCl as an intermediate product is less
favorable. In addition to the above-described channel, the
dissociation into SCl2ϩS along channel ͑4͒ was observed.
Lee et al.12 studied the photodissociation of S2Cl2 at 248
and 193 nm by translational spectroscopy. The SCl product
was detected with a relative yield of 20%. Due to the mea-
sured anisotropy parameter ␤SCl of 1.6 at 248 nm and 0.6 at
193 nm, they concluded that a rapid S–S bond fission takes
*
of Cl /Cl, the translational energy disposal, the different in-
1
volved upper states, and the mechanism of the decay. To this
end, the energy dependence of the anisotropy parameter ␤ is
the most valuable tool to analyze the dissociation process
including the dissociation energy D0(ClS2–Cl).
place on the excited B state. Lee et al. observed that the
S–Cl bond fission is predominant with a factor 3.0 higher
than the S–S bond fission. Via reaction ͑2͒ two separate
product Cl atom translational energy distributions of E
͗
͘
T
ϭ42 and 126 kJ/mol were found. The slow S2Cl fragments
undergo a secondary dissociation to form isotropically dis-
tributed S2 and Cl which probably arise from the S2Cl
ground electronic state, whereas the fast components more
likely originate from a mixed excitation of A and B states
of S2Cl2. In addition, they observed the dissociation channel
͑4͒. At 193 nm the three-body decay channel ͑1͒ becomes
II. EXPERIMENT
A more detailed description of the experimental setup
and the novel position sensitive detector ͑PSD͒ has been
published elsewhere.15,16 Briefly, it consists of a combination
of a home-built single-field time-of-flight ͑TOF͒ mass spec-
1
1
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4216
J. Chem. Phys., Vol. 117, No. 9, 1 September 2002
Einfeld et al.
trometer and a position sensitive detector.17–19 The spectrom-
eter was evacuated to a base pressure of ϳ10Ϫ8 mbar by a
turbomolecular pump system. Disulfur dichloride was con-
stantly cooled to Ϫ10 °C ͑gas pressure: ϳ1 mbar at Ϫ10 °C͒
to prepare a mixture of 0.03% S2Cl2 in helium which was
flowing through the cooling vessel. Measurements of the
room temperature sample were performed at a pressure of
10Ϫ7 mbar, corresponding to a S2Cl2 density of 106/cm3.
Measurements of jet-cooled samples at a temperature of 8 K
agree with the results from the bulk measurements, but were
not further analyzed due to background interference.
Simultaneous dissociation and state-selective detection
of chlorine atoms were performed using one dye laser
pumped by a Nd:YAG laser ͑Coherent, Infinity 40 100͒. The
dye laser ͑Lambda Physik, Scanmate͒ was operated with
Coumarin 47 at a repetition rate of 100 Hz, its light was
frequency-doubled by a BBO crystal and focused by a 20 cm
lens in order to decrease the reaction volume to 5ϫ10Ϫ4
mm3. The output window was arranged in a Brewster angle
configuration of 54° to reduce the internally reflected light.
The energy of the frequency-doubled light amounted to 5–10
␮J per pulse. The energy was kept low to obtain approxi-
mately one fragment signal per ten laser pulses to avoid ki-
netic energy transfer to the fragments due to space charge
effects and saturation of the dissociation step. The laser
beam, the molecular beam, and the detector axes were mu-
tually orthogonal in the interaction region. Ultimate care was
taken to overlap the light and the molecular beam which was
checked frequently by monitoring of NO via ͑1ϩ1͒ REMPI
at 226 nm and optimization of the signal intensity.20 The
polarization of the laser was changed by a half wave plate in
order to investigate the spatial fragment distribution. Typi-
cally the acceleration voltage was 800 V in the acceleration
tube of the TOF spectrometer corresponding to an accelera-
tion field of 16 kV/m.
photoproducts, two decay channels will be discussed in the
following, i.e., reaction ͑1͒, which is called the three-body
decay channel, and reaction ͑2͒, which is named radical or
two-body decay channel:
S2Cl2→S2ϩ2Cl, ⌬H0ϭ 383Ϯ4͒ kJ/mol,
͑1͒
͑2͒
͑
S2Cl2→S2ClϩCl, ⌬H0ϭ 214Ϯ10͒ kJ/mol.
͑
The dissociation enthalpies were calculated from the stan-
dard enthalpies of formation (⌬fH0)23 of the molecules and
radicals involved in the process and the errors were calcu-
lated according to the given errors of the individual standard
enthalpies. The enthalpies were calculated for the spin–orbit
ground state Cl. The ⌬H0 required for formation of one or
*
two Cl atoms is higher by 10.6 and 21.2 kJ/mol, respec-
tively.
In the following, five aspects characterizing the photo-
dissociation dynamics will be discussed: the spin–orbit
branching ratio, the velocity distribution, the anisotropy pa-
rameter ␤, the maximal lifetimes of the excited states and the
translational energy disposal ET /Eavl via the different decay
channels.
A. Spin–orbit branching ratio
The spin–orbit branching ratio was obtained by scanning
*
the laser over the two resonance transitions of Cl and Cl .
The measurements were repeated at different laser light in-
tensities. Integrating the area under the Doppler profiles re-
*
sults in a signal ratio S͑Cl ͒/S͑Cl͒ of 0.50Ϯ0.04. Taking the
ratio of transition probabilities B of 1.0622 into account we
*
*
determined a Cl yield of ␾(Cl )ϭ0.35Ϯ0.03, where ␾ is
defined as the ratio of the number of excited state atoms
*
P(Cl ) to the total number of released chlorine atoms:
*
*
*
͔
␾(Cl )ϭP͑Cl )/ P(Cl)ϩP(Cl ) . The branching ratios
͓
given by Tiemann et al.24 ͑0.17͒ and Park et al.8 ͑0.21͒ at
248 nm are somewhat lower, which is probably due to the
different excitation wavelengths as Park et al.8 concluded
that the relative yield as well as the total Cl atom production
of the S2Cl2 molecule varied with the wavelength studied.
Ϫ1
2
The PJ state of the chlorine atom is split by 882 cm
due to spin–orbit coupling into Cl(2P3/2) and Cl ( P1/2).
Both states were detected by a ͑2ϩ1͒ REMPI process. The
ground state was probed via the (2D3/2 2P3/2) transition at
235.336 nm, the excited state by the (2P1/2 2P1/2) transi-
tion at 235.205 nm.21,22 Typically the dye laser was scanned
over a range of Ϯ0.003 nm around the Cl atom transition
accounting for the Doppler broadening. Signals were digi-
tized by time-to-digital converters, accumulated over 2
ϫ105 laser shots, and saved online by a personal computer.
Details of the analyzing procedure are described elsewhere.16
The PSD includes a delay-line anode introduced into the
spectrometer chamber right behind the double stage micro-
channel plates. The PSD allows one to monitor all three
components of the momentum vector from the measured po-
sition of the particles on the detector and the corresponding
TOF. Therefore, a full 3D velocity distribution is observed
and complete information about the kinetic energy distribu-
tion and the velocity dependent ␤ parameter can be obtained.
2
*
B. Speed distribution
In Fig. 3 the speed distributions and the speed dependent
*
anisotropy parameter ␤ for Cl and Cl are presented. This
one-dimensional presentation is obtained via integration of
the 3D data. Cl in its ground state is mainly produced at low
speeds and peaks at 1200 m/s. Only a tail is observed in the
high speed region above 2000 m/s. The bimodal character is
*
more pronounced for spin excited Cl where the two speed
regions are clearly separated. Here, the bimodal character
unambiguously divides the speed distribution in two parts,
*
where Cl is mainly released with high kinetic energy. The
*
speed distribution of slow Cl atoms which are a minor con-
tribution peaks at 1500 m/s, whereas the peak for the fast
atoms is at 2700 m/s. The small shoulder above 3600 m/s
belongs to a small contribution of 37Cl. The two speed re-
gions correlate with the two different decay channels ͑1͒ and
͑2͒, where the energetic border between the two channels
with respect to the dissociation enthalpies is at 2250 m/s.
III. RESULTS AND DISCUSSION
Upon excitation at 235 nm the photodissociation of
S2Cl2 may proceed via different decay channels which are
1
*
*
Accordingly, the fast Cl and Cl atoms are released via the
well characterized by Speth et al. As we probe Cl and Cl
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J. Chem. Phys., Vol. 117, No. 9, 1 September 2002
The photolysis of S2Cl2
4217
The curve shape of the speed dependent ␤ parameter has
a trimodal character for both ground and excited chlorine
atoms. At low velocities ͑Ͻ500 m/s͒ the ␤ value is close to
zero, decreasing to a value of Ϫ0.15 with increasing speed
up to 1750 m/s. The ␤ value increases strongly above 2000
*
m/s to a value of 0.45Ϯ0.12 and 0.24Ϯ0.12 for Cl and Cl ,
*
respectively. In the case of Cl the change from Ϫ0.15 to
ϩ0.24 at about 2000 m/s correlates with the determined limit
in the speed distribution which divides the bimodal speed
distributions in two parts. Agreeing with the above-given
suggestion that the bimodal character in the speed distribu-
tion reflects the different decay channels, the dependence of
the ␤ values on the speeds supports this view and gives us an
insight into the dynamics of the photodissociation.
We will start our study for ground state Cl. At high ve-
locities the Cl atoms have to be released via a two-body
decay for energetic reasons where the electronic transition
leads to a positive ␤ parameter of 0.45Ϯ0.12. This positive ␤
1
value suggests that the initially excited state is of A sym-
1
metry, because the transition dipole moment ␮ for the A
1A transition is oriented parallel to the C2 axis, which
means perpendicular to the line connecting the two S atoms
and bisecting the Cl–Cl angle. The geometry and the corre-
sponding dipole moments are shown in Fig. 2. A theoretical
limit of 0.48 for the ␤ parameter for instantaneous decay is
estimated from the ground state symmetry with a S–S–Cl
bond angle of 45.5°. If only rotation of the parent molecules
about its major principal axis is responsible for the reduction
of the ␤ parameter, the lifetime ␶ can be estimated based on
the relationship:27
FIG. 3. Speed distribution for ͑a͒ ground state Cl(2P3/2) and ͑b͒ excited
2
*
state Cl ( P1/2) atoms produced in the photodissociation of S2Cl2 at 235
nm. The dependence of the ␤ parameter on the Cl fragment kinetic energy
͑right scale͒ is shown by curves with error bars. In addition, the fit of the
␩
speed dependent ␤ parameter ͑dashed line͒ for ground state Cl(2P3/2) ͑a͒
␶ϭ
,
͑5͒
2
*
and excited state Cl ( P1/2) ͑b͒ to the experimentally observed data ͑closed
␻
A
circles͒ on the assumption of two underlying decay mechanisms. Three
Gaussians were fitted to the trimodal speed distribution. One describes the
fast chlorine fragments produced in a two-body decay with a ␤ parameter of
where
expϭP2 ␩͒␤ , ␻ ϭ
*
0.45 and 0.24 for Cl and Cl , respectively. The other two Gaussians de-
kT
IA
scribe the three-body decay with a ␤ parameter of Ϫ0.25 for the chlorine
fragment released in the first step and a ␤ parameter of zero for the slow
chlorine fragment released in the second step.
*
␤
.
͑
ͱ
A
Here ␩ is the angle between the velocity vector that would
result if the molecules were not rotating and the velocity
vector that actually results, ␻ is the angular rotational fre-
quency, ␤exp the experimentally observed anisotropy param-
*
radical decay channel ͑2͒ and the slow Cl and Cl atoms are
A
released via the three-body decay channel ͑1͒. In the case of
ground state Cl the contributions of the radical and the three-
body decay channel are estimated to be 17% and 83%, re-
spectively, whereas in the case of excited state Cl the con-
tributions are found to be 55% and 45% for the radical and
three-body decay, respectively. Detailed information of the
*
eter, and ␤ its theoretical limit. Assuming room tempera-
ture of 300 K, with IAϷ7ϫ10Ϫ45 kg m2 the mean rotational
frequency ␻AϷ8ϫ1011 sϪ1, which corresponds to an upper
limit of the lifetime ␶ of 270 fs from the experimental ␤exp
value of 0.45.
*
In order to extract the ␤ parameters belonging to the
speed region below 2500 m/s and therefore to the three-body
*
speed distributions for Cl and Cl is given in Table I.
*
decay, three Gaussians were fitted to both the Cl and Cl
C. Anisotropy parameter ␤ and lifetimes of the
excited states
speed distributions. As the ␤ parameter is bimodal in the
speed region below 2500 m/s, it is expected that the three-
body decay occurs in a sequential way. Consequently one
Gaussian was fitted to the high speed region ͑Ͼ2500 m/s͒,
describing the two-body decay and two Gaussians were fitted
to the low speed region ͑Ͻ2500 m/s͒, describing both steps
of the sequential decay: ͑1a͒ S2Cl→S2CϩCl and ͑1b͒
S2Cl→S2ϩCl. The Gaussian fits are also shown in Fig. 3.
The contributions of the Gaussians are used to simulate the
The spatial fragment distribution P( ,␪)ϰf( )(1
v
v
ϩ␤( )P (cos ␪))25,26 is characterized by the velocity de-
v
2
pendent anisotropy parameter ␤( ) ranging from Ϫ1 ͑per-
v
pendicular transition͒ to ϩ2 ͑parallel transition͒, where ␪ is
the angle of the polarization vector of the dissociating laser
with the product recoil velocity vector, and P2 is the second
Legendre polynomial: P2(x)ϭ 12(3x2Ϫ1).
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4218
J. Chem. Phys., Vol. 117, No. 9, 1 September 2002
Einfeld et al.
*
TABLE I. Characteristic data describing the speed distribution, the ␤ parameter of the Cl and Cl fragments, and the kinetic energy release in the
photodissociation of S2Cl2. The kinetic energy release is determined for the two- and three-body decay separately. The analysis consists of three Gaussians
assuming a two-body decay ͑2BD͒ and a sequential three-body decay ͑3BD͒. The calculation of Eavl is based on an initial internal energy content of the S2Cl2
molecule of 8 kJ/mol at room temperature.
Gaussian contribution
Full width at
E
͘
T
half maximum
Eavl
͗
Fragments
Cl
Type
Decay channel
͑kJ/mol͒
͑kJ/mol͒
f ϭ E /E
avl
␤
Center ͑m/s͒
͑m/s͒
͗
͘
r
T
2BD
3BD
S2Cl2→S2ClϩCl
͑a͒ S2Cl2→S2ClϩCl
͑b͒ S2Cl→S2ϩCl
302
136
166Ϯ10
68Ϯ10
0.55Ϯ0.03
0.50Ϯ0.03
0.45Ϯ0.12
Ϫ0.25Ϯ0.07
0Ϯ0.05
2650
1750
980
950
950
800
*
Cl
2BD
3BD
S2Cl2→S2ClϩCl
͑a͒ S2Cl2→S2ClϩCl
͑b͒ S2Cl→S2ϩCl
292
126
169Ϯ10
64Ϯ10
0.58Ϯ0.03
0.51Ϯ0.03
0.24Ϯ0.12
Ϫ0.25Ϯ0.07
0Ϯ0.05
2670
1710
1030
980
750
780
1
observed curve shape of the ␤ dependence on the speed.
Since the fit functions overlap in the medium energy range,
the ␤ parameter is accordingly reduced. The obtained simu-
lation is shown in Fig. 3. The best values for the ␤ param-
eters are 0, Ϫ0.25, and 0.45 for ground state chlorine de-
scribing slow and intermediate Cl via three-body decay and
fast Cl via two-body decay, respectively. In the case of ex-
another repulsive B state may exist. Both observations are
in agreement with our present measurements. At 308 nm the
Cl photoproducts were exclusively released via an excited 1A
surface yielding a positive ␤ parameter of 0.4Ϯ0.2. The
same ␤ value is observed in the present measurement for the
1
ground state Cl which is released solely via a A state. At
248 nm Cl atoms were released via both excited states A
and B with a positive and negative ␤ parameter, respec-
1
1
*
cited state Cl the values are 0, Ϫ0.25, and 0.24, respec-
tively. From the negative ␤ parameter in the low speed range
tively. Since Lee et al. measured the averaged ␤ parameter, a
value of zero was observed. The same value would be ob-
served as an average at 235 nm, if there would not be the
possibility to examine the speed dependence of the anisot-
ropy parameter ␤.
it can be concluded that the three-body decay proceeds via an
1
excited state of B symmetry. The value of Ϫ0.25 is in very
good agreement with former translational spectroscopy mea-
surements at 193 nm by Lee et al.,12 who observed that S2Cl2
decays rapidly in S2 and 2Cl with a ␤ parameter of Ϫ0.3 on
In summary, the branching ratio of reactions ͑1͒ and ͑2͒
1
*
an excited B state.
was measured to be 2.45:1.0 and 0.4:1.0 for Cl and Cl ,
These observations hint at a fast dissociation, where the
molecule S2Cl2 decays into two fragments on an excited A
respectively. The simultaneous absorption into the different
decay channels is illustrated in Fig. 4 in order to emphasize
the competing dissociation channels at 235 nm. The three-
body decay is found to be sequential as the second Cl atom is
released isotropically with a ␤ value close to zero, which
1
state and mainly into three fragments on an excited 1B state.
This is supported by the overall shape of the absorption spec-
trum which is smooth, displaying no structure on a typical
scale for vibrational spacing.1 The continuous absorption is a
good indication that direct dissociative processes take place
in this region.
*
means the intermediate S2Cl lives long enough on a rota-
tional time scale to lose the initial alignment of the parent
S2Cl2 molecule and the second Cl atom is ejected without
preferred spatial orientation.
*
In the case of excited state Cl the observed ␤ parameter
in the high speed region is 0.24Ϯ0.12. The decrease of the ␤
parameter in comparison with the limiting value may hint at
1
a small contribution of the excited B state associated with
*
the three-body decay releasing fast Cl atoms. This contri-
bution is not unlikely since a mixed excitation at 235 nm can
be expected due to the location at the high-energy end of the
strong absorption at 256 nm. By the reduction of the ␤ pa-
1
*
rameter the contribution of fast Cl being released via a B
state could be estimated to be ϳ6% of the total assuming the
1
␤ value of Ϫ0.25 for Cl atoms released via the B state as
*
observed for the intermediate speed Cl and Cl atoms. Lee
et al.12 also favored a mixed excitation as the reason for their
observed ␤ parameter of zero for fast S2Cl products at 248
nm. In a former study of Lee et al.,9 S2Cl2 was photodisso-
ciated at 308 nm and the primary dissociation process
S2Cl2→S2ClϩCl proceeds via a slightly positive ␤ parameter
FIG. 4. Diagram illustrating the simultaneous absorption into the excited
states 1A and 1B and the different decay channels including the spin–orbit
states of Cl. The contributions refer to the total number of chlorine frag-
ments.
1
of 0.4Ϯ0.2 via a A state through the excitation of an (nCl
*
*
→␴S-Cl) or (nS→␴S-Cl) transition. Lee et al. speculated due
to the existence of many nonbonding electrons in S2Cl2 that
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J. Chem. Phys., Vol. 117, No. 9, 1 September 2002
The photolysis of S2Cl2
4219
IV. CONCLUSION
ported by the Fonds der Chemischen Industrie, the Alexander
von Humboldt Stiftung, and the German–Israeli Foundation
͑GIF͒. Financial support by the Deutsche Forschungsgemein-
schaft is gratefully acknowledged.
Using 3D imaging spectroscopy, we have measured the
competing dissociation channels to release S2ClϩCl or
S2ϩ2Cl in the photolysis of S2Cl2 at 235 nm. The main
experimental findings are summarized as follows.
͑1͒ Overall the branching ratio of reaction ͑2͒ releasing
S2ClϩCl and reaction ͑1͒ releasing S2ϩ2Cl was determined
to be 1.0:1.2. If the two spin–orbit states are viewed sepa-
rately, the branching ratio decreases for ground state Cl to
1 R. S. Speth, R. Niemann, and E. Tiemann, Chem. Phys. 229, 309 ͑1998͒.
2
¨
F. Feher and H. Munzer, Chem. Ber. 96, 1131 ͑1963͒.
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4 B. Beagley, G. H. Eckersley, D. P. Brown, and D. Tomlinson, Trans.
Faraday Soc. 65, 2300 ͑1969͒.
*
ϳ1.0:2.45 and increases for excited Cl to ϳ1.0:0.4.
2
*
͑2͒ The yield of Cl in the excited spin–orbit state P1/2
5 I. Tokue, A. Hiraya, and K. Shobatake, Chem. Phys. Lett. 153, 346 ͑1988͒.
6 S. A. Chasovnikov, A. I. Chichinin, and L. N. Krasnoperov, Chem. Phys.
116, 91 ͑1987͒.
*
was found to be ␾(Cl )ϭ0.35Ϯ0.03.
͑3͒ The translational energy disposal for reaction ͑1͒ was
determined to be 0.53Ϯ0.03 and 0.54Ϯ0.03 and for reaction
7 E. Tiemann, H. Kanamori, and E. Hirota, J. Mol. Spectrosc. 137, 278
͑1989͒.
*
͑2͒ 0.55Ϯ0.03 and 0.58Ϯ0.03 for Cl and Cl , respectively.
8 J. Park, Y. Lee, and G. W. Flynn, Chem. Phys. Lett. 186, 441 ͑1991͒.
9 Y. R. Lee, C. L. Chiu, and S. M. Lin, Chem. Phys. Lett. 216, 209 ͑1994͒.
10 C. Chiu and H. Chang, Spectrochim. Acta, Part A 50, 2239 ͑1993͒.
11 J. Lindner, R. Niemann, and E. Tiemann, J. Mol. Spectrosc. 165, 358
͑1994͒.
*
͑4͒ The two-body decay, releasing fast Cl ͑11%͒ and Cl
͑19%͒, occurs mainly on a repulsive 1A state through an
electronic transition with a ␤ parameter of 0.45 and 0.24 for
*
12 Y. R. Lee, C. L. Chiu, E. Tiemann, and S. M. Lin, J. Chem. Phys. 110,
6812 ͑1999͒.
Cl and Cl , respectively. The reduced ␤ value in the case of
1
1
*
Cl is probably due to mixed excitation of A and B, where
6% of the observed Cl are released via the excited B state.
13 C. Maul and K.-H. Gericke, Int. Rev. Phys. Chem. 16, 1 ͑1997͒.
14 J. Park, Y. Lee, J. F. Hershberger, J. M. Hossenlopp, and G. W. Flynn, J.
Am. Chem. Soc. 114, 58 ͑1992͒.
1
*
͑5͒ The three-body decay, releasing relatively slow Cl
1
15 A. Chichinin, T. Einfeld, C. Maul, and K.-H. Gericke, Rev. Sci. Instrum.
73, 1856 ͑2002͒.
*
͑54%͒ and Cl ͑16%͒ atoms, occurs on a repulsive B state
through an electronic transition with a ␤ parameter of Ϫ0.25
in the first step. The decay has a sequential character and the
16 T. Einfeld, A. Chichinin, C. Maul, and K.-H. Gericke, J. Chem. Phys. 116,
2803 ͑2002͒.
*
second step Cl or Cl atoms are released isotropically ͑␤
17 S. E. Sobottka and M. B. Williams, IEEE Trans. Nucl. Sci. 35, 348 ͑1988͒.
18 O. Jagutzki, V. Mergel, K. Ullmann-Pfleger, L. Spielberger, U. Meyer, R.
ϭ0͒.
¨
¨
Dorner, and H. Schmidt-Bocking, Fast Position and Time-Resolved Read-
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Imaging Spectroscopy Vol. IV ͑San Diego, CA, 1998͒, p. 322.
19 M. Lampton, O. Siegmund, and R. Raffanti, Rev. Sci. Instrum. 58, 2298
͑1987͒.
All observations can be rationalized in terms of a simul-
1
1
taneous excitation on a B and A surface. Accordingly, the
1
main B state excitation would lead to three fragments, S2
and two ground-state chlorine atoms. Originally from the 1B
state, nonadiabatic coupling might be responsible for a small
production of excited spin–orbit Cl atoms via two- and
20 J. Danielak, U. Domin, R. Kepa, M. Rytel, and M. Zachwieja, J. Mol.
Spectrosc. 181, 394 ͑1997͒.
1
three-body decay. The main B state excitation could be ac-
21 S. Arepalli, N. Presser, R. Robie, and R. Gordon, Chem. Phys. Lett. 108,
88 ͑1985͒.
1
companied by a minor excitation of a A state, quickly de-
22 P. M. Regan, S. R. Langfold, D. Ascenzi, P. A. Cook, A. J. Orr-Ewing, and
M. N. R. Ashfold, Phys. Chem. Chem. Phys. 1, 3247 ͑1999͒.
23 M. W. Chase, J. Phys. Chem. Ref. Data Monogr. 9, IϩII ͑1998͒.
24 E. Tiemann, H. Kanamori, and E. Hirota, Annual Review, Institute for
Molecular Science, Okazaki, Japan, 1988.
caying into S2ClϩCl, with Cl atoms generated in both spin–
orbit states in almost equal amounts.
ACKNOWLEDGMENTS
25 R. N. Zare, Mol. Photochem. 4, 1 ͑1972͒.
26 M. Mons and I. Dimicoli, J. Chem. Phys. 90, 4037 ͑1989͒.
27 G. E. Busch and K. R. Wilson, J. Chem. Phys. 56, 3638 ͑1972͒.
The authors are grateful to Dr. R. Aures for numerous
stimulating discussions. These studies were generously sup-
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