Detection of CISO with time-resolved Fourier-transform infrared absorption spectroscopy

Article abstract of DOI:10.1063/1.1641007

A step-scan Fourier-transform infrared spectrometer was used to detect time-resolved spectrum of ClSO. The rotational structure with the Q-branch at 1162.9 cm^-1 was revealed by a spectrum with a resolution of 0.3 cm^-1. The geometry, vibrational and rotational parameters of ClSO were predicted by calculations with density-functional theory. The results show that the CISO produced from photolysis of Cl₂SO at 248 nm is internally hot.

Full text of DOI:10.1063/1.1641007

Detection of ClSO with time-resolved Fourier-transform infrared absorption
spectroscopy
Li-Kang Chu, Yuan-Pern Lee, and Eric Y. Jiang
Citation: The Journal of Chemical Physics 120, 3179 (2004); doi: 10.1063/1.1641007
View online: http://dx.doi.org/10.1063/1.1641007
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 120, NUMBER 7
15 FEBRUARY 2004
Detection of ClSO with time-resolved Fourier-transform
infrared absorption spectroscopy
Li-Kang Chu
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
Yuan-Pern Leea)
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan,
and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
Eric Y. Jiang
Thermo Electron Corporation, Madison, Wisconsin 53711
͑Received 31 October 2003; accepted 20 November 2003͒
ClSO was produced as an intermediate upon irradiating a flowing mixture of Cl2SO and Ar with a
KrF excimer laser at 248 nm. A step-scan Fourier-transform infrared spectrometer coupled with a
small multipass absorption cell was employed to detect time-resolved absorption spectrum of ClSO.
A transient spectrum in the region 1120–1200 cmϪ1, which diminished on prolonged reaction, is
assigned to the S–O stretching (␯1) mode of ClSO. A spectrum with a resolution of 0.3 cmϪ1
partially reveals rotational structure with the Q-branch at 1162.9 cmϪ1. Calculations with
density-functional theory ͑B3LYP/aug-cc-pVTZ͒ predict the geometry, vibrational, and rotational
parameters of ClSO. An IR absorption spectrum of ClSO simulated based on predicted rotational
parameters agrees satisfactorily with experimental results. ClSO produced from photolysis of Cl2SO
at 248 nm is internally hot. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1641007͔
X 3⌺Ϫ state of SO ͑vр6͒ is vibrationally inverted at ϭ2 or
INTRODUCTION
v
3.6,7 At 248 nm, the Cl-elimination channel dominates
͑Ͼ96.5%͒, whereas the molecular-elimination channel yield-
ing SO predominantly in the b 1⌺ϩ state accounts for only
ϳ3.5%; the three-body dissociation channel has an insignifi-
cant contribution.7,8 ClSO produced in reaction ͑2͒ under-
goes further dissociation
Thionyl chloride (Cl2SO) is commonly used in organic
syntheses and serves also as an excellent ligand coupling
reagent.1 In high-capacity lithium batteries it promotes cy-
cling efficiencies of alkali metals in electrolytes.2 The pho-
todissociation dynamics of Cl2SO have been investigated ex-
tensively because of the importance in understanding the
competition among processes involving concerted and step-
wise three-body dissociation, and two-body dissociation; dis-
sociation products SO and Cl can be probed readily.3–10
UV absorption11 of Cl2SO shows an onset ϳ300 nm and
two maxima near 194 nm (␴ϭ1.3ϫ10Ϫ17 cm2) and
244 nm (␴ϭ7.1ϫ10Ϫ18 cm2), corresponding to transitions
ClSO→ClϩSO
͑4͒
to form Cl and SO with an isotropic angular distribution.8
Employing the REMPI ͑resonance-enhanced multiphoton
ionization͒–TOF ͑time-of-flight͒ technique to probe SO and
Cl, Roth et al.10 found that photodissociation of Cl2SO at
235 nm proceeds via two potential-energy surfaces A and
Љ
*
*
*
␴
nCl and a combination of ␴
nS and ␴
S–Cl
S–Cl
S–O
A . Dissociation on the A surface produces Cl (2P3/2) and
Ј
Љ
*
nS , respectively; the symbol
indicates antibonding
Cl (2P1/2) in nearly equal proportions in conjunction with
orbitals.4,10 At 193 nm, the major channel for photodissocia-
tion involves a concerted three-body dissociation,
SOCl radical which carries about half the available energy as
internal energy, whereas dissociation on the A surface pro-
Ј
Cl2SO→2ClϩSO,
͑1͒
duces mainly 2ClϩSO via sequential three-body decay. Al-
though these investigations of photodissociation dynamics
involved laser-induced fluorescence ͑LIF͒,6,7 REMPI,10 and
diode-laser absorption5 techniques to probe Cl and SO, no
direct spectral detection of ClSO has been reported. ClSO
was probed only with a mass spectrometer in an investiga-
tion using photofragment translational spectroscopy, but only
about one-fifth of ClSO survived electron bombardment.8
ClSO has been characterized with electron paramagnetic
resonance ͑EPR͒,12 far infrared laser magnetic resonance,13
and microwave spectroscopy.14 Li predicted geometries,
vibrational frequencies, and energies of ClSO in both
accounting for more than 80% of the overall decay.7,8 The
two-body Cl-elimination channel,
Cl2SO→ClSOϩCl,
has a branching ratio ϳ17%, whereas the molecular-
͑2͒
elimination channel
Cl2SO→Cl2ϩSO
͑3͒
is less than 3%. About one-fifth of SO produced in channel
͑1͒ is electronically excited in the a 1⌬ state;5 the ground
its ground (X 2A ) and first electronically excited states
a͒
Љ
Author to whom correspondence should be addressed. Electronic mail:
yplee@mx.nthu.edu.tw
(A 2A ) using various quantum-chemical calculation
Ј
0021-9606/2004/120(7)/3179/6/$22.00
3179
© 2004 American Institute of Physics
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3180
J. Chem. Phys., Vol. 120, No. 7, 15 February 2004
Chu, Lee, and Jiang
methods.15 Vibrational wave numbers of the X 2A state of
spectral region so that undersampling decreases the number
of data points, hence the duration of data acquisition. For
survey spectra in the range 960–1430 cmϪ1 at a resolution of
2.5 cmϪ1, 516 scan steps were completed within an acquisi-
tion period ϳ25 min, whereas for high-resolution spectra in
the range 1050–1370 cmϪ1 at a resolution 0.3 cmϪ1 data
acquisition with 2214 scan steps requires ϳ110 min.
At the later stage of experiments, a new step-scan spec-
trometer ͑Thermo Nicolet, Nexus 870͒ with a fast MCT de-
tector ͑20 MHz͒ was employed. The spectrometer controls
mirror positions to within Ϯ0.2 nm ͑Ref. 27͒ and is equipped
Љ
ClSO are predicted to be ␯1 (S–O stretch)ϭ1099 cmϪ1
␯2 (Cl–S stretch)ϭ477 cmϪ1
and ␯3 (ClSO bend)ϭ294
,
,
cmϪ1. To our knowledge, no vibrational spectrum of ClSO
has been reported. Hence it would be interesting to develop
an infrared detection technique to investigate further the pho-
todissociation dynamics of Cl2SO and reaction kinetics in-
volving ClSO.
We recently coupled a step-scan Fourier-transform spec-
trometer with a multipass absorption cell to record time-
resolved infrared ͑IR͒ absorption spectra of reaction interme-
diates or vibrationally excited species in the gas phase.16–18
Compared with time-resolved Fourier-transform spectros-
copy ͑TR–FTS͒ in emission mode,19–25 TR–FTS in absorp-
tion mode provides additional information on nonemitting
states or species, particularly on molecules in their ground
vibrational state. Here we report an application of step-scan
TR–FTS to record IR absorption spectra of the intermediate
ClSO upon photodissociation of Cl2SO.
with
a
14-bit digitizer ͑Gage Applied Technology,
CompuScope 14100, 108 sample sϪ1). Typically, 150 data
points were acquired at 1 ␮s integrated intervals ͑100 dwells
at 10 ns gate width͒ after photolysis; the signal is typically
averaged over 60 laser shots at each scan step. With its
present software, the data acquisition time is typically about
30% greater than that with Bruker spectrometer, but the ratio
of signal to noise appears to be superior to previous measure-
ments. For survey spectra in the range 850–1500 cmϪ1 at a
resolution of 1.5 cmϪ1, 1200 scan steps were completed
within ϳ70 min, whereas for high-resolution spectra in the
range 1050–1343 cmϪ1 at a resolution 0.3 cmϪ1 data acqui-
sition with 2592 scan steps requires ϳ170 min.
EXPERIMENTAL SECTION
A commercial step-scan Fourier-transform spectrometer
͑Bruker, IFS66v/S or Thermo Nicolet Nexus 870͒ operating
in absorption mode is employed.16,17 A White cell with a
base path length of 20 cm and a maximal effective path
length of 8 m was placed in the sample compartment of the
spectrometer; the volume of the cell is ϳ1600 cm3. The
housing of the White cell was modified to accommodate two
rectangular ͑3ϫ12 cm2͒ quartz windows to pass photolysis
beams that propagate perpendicular to multipassing IR
beams. The photolysis laser beam passes these quartz win-
dows and is multiply reflected between a pair of rectangular
laser mirrors installed externally but parallel to the quartz
windows. A KrF excimer laser ͑Lambda Physik, LPX110i,
21 Hz͒ emitting at 248 nm is employed for photodissocia-
tion; typical energy employed is ϳ85 mJ pulseϪ1 with a
beam dimension ϳ6ϫ11 mm2. The efficiency of photolysis
of Cl2SO is estimated to be ϳ50% based on the absorption
Cl2SO ͑Ͼ99%, Fluka Chemika͒ and Ar ͑99.9995%,
AGA Specialty Gases͒ were used without further purifica-
tion.
THEORETICAL CALCULATIONS
The equilibrium geometry, vibrational frequencies, and
IR intensities were calculated with B3LYP density-functional
theory using the GAUSSIAN 98 program.28 The B3LYP method
uses Becke’s three-parameter hybrid exchange functional
with a correlation functional of Lee, Yang, and Parr.29,30
Dunning’s correlation-consistent polarized-valence triple-
zeta basis set, augmented with s, p, d, and f functions
31,32
͑aug-cc-pVTZ͒
was applied in these calculations. Ana-
lytic first derivatives were utilized in geometry optimization,
and vibrational frequencies were calculated analytically at
each stationary point.
Calculated geometry, rotational parameters, vibrational
wave numbers, and IR intensities are compared with those of
previous work in Table I. The structure and rotational axes
are shown in Fig. 1͑A͒. The Cl–S bond length of 2.099 Å
predicted in this work is slightly greater than those predicted
cross
section
of
Cl2SO
at
248 nm (ϳ7
ϫ10Ϫ18 cm2 moleculeϪ1),11 the effective path length ϳ17
cm, and the laser fluence ϳ1.6ϫ1017 photons cmϪ2. Flow
rates
are
F
Cl SOϭ0.29–0.57
and
FArϭ2.46
2
–19.2 STP cm3 sϪ1
;
STP denotes standard temperature
͑273.15 K͒ and pressure ͑1 atm͒. The total pressure was
1.85–29.7 Torr, with partial pressure of Cl2SO in the range
0.35–0.65 Torr.
*
*
previously with QCISD/6-31G ͑2.088 Å͒ and MP2/6-31G
͑2.092 Å͒, but smaller than that ͑2.119 Å͒ predicted with
MP2/6-311G͑2d͒.15 The predicted S–O bond length of 1.479
Å is smaller than the value 1.499 Å predicted with
Derivation of time-resolved difference absorption spec-
tra from interferograms recorded with ac- and dc-coupled
signals is described previously.16,26 After preamplification,
the ac-coupled signal from the MCT detector ͑Kolmar,
Model KMPV11-1-J2, 20 MHz͒ was further amplified ͑Stan-
ford Research Systems, Model SR560, using bandwidth
300–1 MHz͒ 10 times, whereas the dc-coupled signal was
not further amplified before being sent to the internal 16-bit
digitizer (2ϫ105 sample sϪ1) of the spectrometer. Typically,
200 data points were acquired at 5 ␮s intervals after photoly-
sis; the signal is typically averaged over 60 laser shots at
each scan step. Proper optical filters serve to define a small
*
QCISD/6-31G , but greater than values 1.457 and 1.449 Å
*
predicted with MP2/6-31G and MP2/6-311G͑2d͒, respec-
tively. The predicted bond angle of 109.9° is similar to the
*
value 109.3° predicted with QCISD/6-31G , but much
smaller than those predicted with MP2. The difference in
geometry results in variations of rotational parameters less
than 9% for the A parameter, and 3% for the B and C param-
eters, as listed in Table I. Rotational spectral parameters pre-
dicted in this work are smaller by ϳ4% than those derived
with microwave spectroscopy.14
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J. Chem. Phys., Vol. 120, No. 7, 15 February 2004
Detection of ClSO
3181
TABLE I. Comparison of geometry and vibrational wave numbers of 35ClSO derived from various theoretical
calculations.
B3LYP
QCISD
MP2
MP2
*
*
Experiment
/aug-cc-pVTZ
/6-31G
/6-31G
/6-311G͑2d͒
r
r
Cl–S /Å
S–O /Å
2.090a
1.418a
2.099
1.479
109.9
2.088
1.499
109.3
2.092
1.457
112.6
1.1566
0.1439
0.1280
2.119
1.449
112.1
1.1478
0.1419
0.1263
ЄClSO/deg
A/cmϪ1
107.8a
1.094 73
0.151 88
0.133 17
1162.9b
1.061 51
0.145 67
0.128 09
1156.1
1.0280
0.1469
0.1285
1099
B/cmϪ1
C/cmϪ1
␯1 /cmϪ1
c
c,d
͑90.6͒
͑48͒
␯2 /cmϪ1
␯3 /cmϪ1
Reference
467.0
͑97.1͒
291.2
͑2.9͒
This work
477
͑100͒
294
14
15
15
15
aThe geometry is derived by assuming a bond angle of 107.8°. If a bond angle of 109.9° is used, rCl–S
ϭ2.055 Å and rS–Oϭ1.479 Å.
bThis work.
cIR intensities ͑in km molϪ1͒ are listed in parentheses.
dIR intensities relative to the most intense band ͑set to 100͒.
The smaller S–O bond length calculated with B3LYP/
aug-cc-pVTZ is consistent with a larger wave number ͑1156
cmϪ1͒ predicted for the S–O stretching mode, as compared
associated dipole derivative is shown in Fig. 1͑B͒. Wave
numbers predicted in this work for the other two vibrational
modes, 467 cmϪ1 for Cl–S stretching and 291 cmϪ1 for
ClSO-bending modes, are similar to those reported previ-
ously, but are beyond our range of detection.
with a value of 1099 cmϪ1 predicted with QCISD/6-31G .
*
Predictions of vibrational wave numbers for the S–O stretch
using B3LYP/aug-cc-pVTZ are estimated to be accurate to
within 1%, based on results of SSO ͑calculated 1169.0 cmϪ1
,
EXPERIMENTAL RESULTS AND DISCUSSION
gas phase 1166.5 cmϪ1͒,33,34 c-OSNO ͑calculated 1153.3
cmϪ1, N2 matrix 1156.1 cmϪ1͒, and t-OSNO ͑calculated
1173.3 cmϪ1, N2 matrix 1178.0 cmϪ1͒.35 If we use the same
ratio of experimental to calculated wave numbers for FSO,
1215/1165ϭ1.043, reported by Li using identical method, we
derive a revised value of 1146 cmϪ1 for the S–O stretching
mode based on a value 1099 cmϪ1 calculated with
As a test, conventional cw measurements were per-
formed with a static cell containing 0.110 Torr of Cl2SO.
Traces ͑A͒ and ͑B͒ of Fig. 2 represent partial IR absorption
spectra at a resolution of 0.25 cmϪ1 recorded before and after
laser irradiation at 248 nm ͑20 Hz͒ for 60 s, respectively.
Cl2SO absorption is characterized by an intense band near
1251.6 cmϪ1 (␯1 , S–O stretch͒.36 Absorption of SO2 as the
end product is clearly visible in the region 1300–1400 ͑in-
tense͒ and 1100–1200 cmϪ1 ͑weak͒; these absorption bands
are consistent with previous literature values for the asym-
*
QCISD/6-31G ; this revised value is only ϳ1% smaller than
our predicted value of 1156 cmϪ1. Hence, we expect that the
vibrational wave number for the S–O stretching mode of
ClSO is likely in the range 1146–1166 cmϪ1. Predicted dis-
placement vectors for the S–O stretching (␯1) mode and the
FIG. 1. Molecular structure and rotational axes of ClSO ͑A͒, and displace-
ment vectors for the S–O stretching (␯1) mode and associated dipole de-
rivative of ClSO ͑B͒ predicted with the B3LYP/aug-cc-pVTZ method. Bond
lengths are in Å and bond angle is in degree.
FIG. 2. Survey cw IR absorption spectra of Cl2SO ͑0.110 Torr͒ in a static
cell. ͑A͒ before irradiation; ͑B͒ after laser irradiation at 248 nm for 60 s ͑20
Hz, 100 mJ cmϪ2͒. Path length is 6.4 m and resolution is 0.25 cmϪ1
.
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3182
J. Chem. Phys., Vol. 120, No. 7, 15 February 2004
Chu, Lee, and Jiang
metric ͑1361.8 cmϪ1͒ and symmetric ͑1151.4 cmϪ1͒ S–O
stretching modes of SO2 .37 No absorption band in the static-
cell experiment may be ascribed to ClSO.
Spectra of internally excited Cl2SO
A representative three-dimensional ͑3D͒ plot of tempo-
rally resolved survey difference spectra at 2 ␮s intervals after
laser irradiation of a flowing mixture of Cl2SO/Ar ͑Х1/4.2,
1.85 Torr͒ at 248 nm is shown in Fig. 3͑A͒ ͑resolution 1.5
cmϪ1͒. In these difference spectra, features pointing up indi-
cate production, whereas those pointing down indicate de-
struction. The downward features near 1251 cmϪ1 is due to
loss of parent molecules. A new feature in the spectral range
1120–1200 cmϪ1 appears immediately after irradiation, then
decays with time; it will be discussed in the next section.
On each side of the downward parent band, more pro-
nounced on the smaller wave number side, there are new
features pointing upward, then decays with time. These new
features are readily assigned as absorption of Cl2SO parent
in highly excited rotational and vibrational levels, which
were produced via internal conversion from the initially pre-
pared electronically excited state; a population greater than
thermal distribution before photolysis yields upward absorp-
tion features. We found that such upward features on both
sides of absorption bands of the parent are a common obser-
vation in the absorption spectrum of a photolyzed species
after irradiation. In many cases these two side-lobes interfere
with nearby absorption bands of dissociation products and
hamper their detection.
Figure 3͑B͒ shows a representative 3D plot for spectra
recorded at 2 ␮s intervals after photolysis of Cl2SO ͑0.355
Torr͒ and excessive Ar ͑24.35 Torr͒ in a flowing mixture. In
the 1200–1300 cmϪ1 region where Cl2SO absorbs, the up-
ward side-lobes due to increase in populations of internally
excited states diminish and the width of the downward fea-
ture decreases, because of efficient relaxation under high
pressure.
FIG. 3. Time-resolved survey IR absorption spectra of a flowing mixture of
Cl2SO/Ar upon photolysis at 248 nm ͑21 Hz, 150 mJ cmϪ2͒ displayed in
three-dimensional mode. ͑A͒ 1.85 Torr, Cl2SO/Arϭ1/4.2; ͑B͒ 24.71 Torr,
Cl2SO/Arϭ1/69. Path length is 6.4 m and resolution is 1.5 cmϪ1. Traces
start at 2 ␮s after irradiation and are separated at 2-␮s intervals.
Spectra of ClSO
Even though the two-body dissociation channel ͓reaction
͑2͔͒ dominates photodissociation of Cl2SO at 248 nm, ClSO
produced upon photolysis is internally excited so that a sub-
stantial portion undergoes further dissociation to form Cl and
SO.7,8,10 In order to quench the internal excitation and stabi-
lize ClSO, we added excessive Ar in the system. The 3D
plots shown in Figs. 3͑A͒ and 3͑B͒ for spectra recorded after
photolysis of Cl2SO and Ar indicates an enhanced peak in-
tensity with narrower widths for the new feature in the
1135–1190 cmϪ1 region when excessive Ar was added.
Transient absorption spectra with an improved resolution
of 0.3 cmϪ1 were recorded upon irradiation of a flowing
mixture containing Cl2SO/Ar(1/45) at 29.7 Torr; the spec-
trum averaged over 10–40 ␮s after photolysis is shown in
trace͑A͒ of Fig. 4. The Q branch is clearly resolved from P
and R branches, but rotational structures of P and R branches
are unresolved. No satisfactory spectrum at resolutions better
than 0.3 cmϪ1 was obtained because the ratio of signal to
noise decreases and the duration of data acquisition increases
substantially.
FIG. 4. Comparison of observed absorption spectrum of ClSO and simu-
lated spectra; ͑A͒ spectrum at resolution 0.3 cmϪ1 and integrated for 10–40
␮s after 248-nm laser irradiation ͑21 Hz, 130 mJ cmϪ2͒ of a flowing mixture
of Cl2SO/Arϭ1/45 at 29.7 Torr; ͑B͒ simulated spectra based on rotational
parameters obtained from microwave spectra ͑Ref. 14͒; see text; ͑C͒ a-type
component; ͑D͒ b-type component; ͑E͒ spectrum of SO2 at 298 K ͑from
Hitran database͒ ͑Ref. 39͒.
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J. Chem. Phys., Vol. 120, No. 7, 15 February 2004
Detection of ClSO
3183
The wave number of the maximum of the Q branch,
1162.9 cmϪ1, is within the expected range, 1146–1166 cmϪ1
,
for the S–O stretching mode of ClSO, and close to the value
of 1156 cmϪ1 predicted with the B3LYP/aug-cc-pVTZ
method. Although the SO product also absorbs in this region,
observed new features are not due to SO because SO absorbs
at 1137.9 cmϪ1 ͑Ref. 38͒. The observed transient absorption
spectrum fits poorly with the spectrum of the end product
SO2 which absorbs at 1361.8 and 1151.4 cmϪ1; spectrum of
the band at 1151.4 cmϪ1 is shown in trace ͑E͒ of Fig. 4 for
comparison.39 Considering that observed peak wave numbers
are near that predicted for ClSO with theory but not for other
possible products SO and SO2 , and that these new features
were enhanced at high pressures, we conclude that the ob-
served transient absorption is likely due to ClSO.
FIG. 5. Comparison of transient absorption spectra averaged over 10–20 ␮s
after 248 nm irradiation of Cl2SO; ͑A͒ 4.76 Torr, Cl2SO/Arϭ1/8.5; ͑B͒
22.50 Torr, Cl2SO/Arϭ1/44. Pathlength is 6.4 m and resolution is 1.0 cmϪ1
.
CONCLUSION
Simulation of the ␯ absorption band of ClSO
1
We demonstrate an application of using time-resolved
Fourier-transform absorption technique to detect the S–O
stretching band of the transient species ClSO upon photoly-
sis of gaseous Cl2SO. Although current detectivity limits the
resolution to 0.3 cmϪ1 in this experiment so that fully re-
solved rotational spectrum is unavailable, our spectrum con-
forms satisfactorily to a simulation based on rotational pa-
rameters derived from microwave spectroscopy; the
vibrational wave number is also consistent with that for the
S–O stretching mode of ClSO predicted with theoretical cal-
culations. Spectrum recorded at low pressure indicates that
ClSO is internally excited. This band might be probed di-
rectly in future chemical-kinetic experiments on reactions in-
volving ClSO if there is no interference from SO2 .
As it is unlikely to derive rotational parameters from
observed spectra with the present spectral resolution, we
simulate the band contour to compare with observed spectra.
Rotational axes a and b of ClSO are shown in Fig. 1͑A͒; the
c axis is perpendicular to the molecular plane. Because the
S–O stretching mode alters the dipole moment mainly along
the a-axis, a-type absorption lines are expected to be domi-
nant. The projection of the displacement vector onto a and b
axes are 0.78:1.00, whereas the projection of the dipole de-
rivative onto these axes are 1.00:0.35.
The spectrum at 350 K was simulated with the SPECVIEW
program40 using rotational parameters A, B, and C derived
from microwave experiments,14
Jmaxϭ120, and a Doppler
line shapes with FWHMϭ0.3 cmϪ1. Contributions from
37ClSO are also included even though the 37Cl-isotopic shift
for the S–O stretching mode is small. Ratios of rotational
ACKNOWLEDGMENTS
parameters of the upper ( ϭ1) and the lower ( ϭ0) state
v
v
We thank the National Science Council of Taiwan ͑Grant
No. NSC92-2113-M-007-034͒ and MOE Program for Pro-
moting Academic Excellence of Universities ͑Grant No. 89-
FA04-AA͒ for support, and V. Stakhursky and T. A. Miller
for providing the SpecView software for spectral simulation.
(A /A , B /B , and C /C ) are estimated to be 0.9906,
Ј
Љ
Ј
Љ
Ј
Љ
0.9993, and 0.9982; relative ratios of these values are deter-
mined according to the displacement vector of the S–O
stretch, and the A /A value is varied to derive the best fit.
Ј
Љ
Simulated a-type and b-type spectra are shown in traces ͑C͒
and ͑D͒ of Fig. 4, respectively. A simulated spectrum of
ClSO using a ratio of 1.00:0.35 for a-type and b-type com-
ponents is shown in trace ͑B͒; it agrees satisfactorily with
experimental observation in trace ͑A͒, with most character-
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