UV and visible emission spectra from the photodissociation of carbonyl sulfide using synchrotron radiation at 15-30 eV

Article abstract of DOI:10.1246/bcsj.74.1193

The photofragmentation of OCS at photon energies of 15-30 eV has been studied by dispersed fluorescence spectroscopy. The primary photon beam was monochromatized undulator radiation supplied from the UVSOR facility. The following emission-band systems hav

Full text of DOI:10.1246/bcsj.74.1193

c. Jpn.
e Chemical © 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 1193–1201 (2001) 1193
ciety of Ja-
n
e Chemical
ciety of Ja-
n
Headline Articles
UV and Visible Emission Spectra from the Photodissociation of Carbonyl
Sulfide Using Synchrotron Radiation at 15–30 eV
0
1
9
0
3
1
Emission Spectra from VUV Excitation of OCS
SJA8
K. Mitsuke et al.
09-2673
Koichiro Mitsuke* and Masakazu Mizutani
020
nuary
Department of Vacuum UV Photo-Science, Institute for Molecular Science and Department of Structural Molecular
Science, The Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585
0
1
1
(
Received January 22, 2001)
0
The photofragmentation of OCS at photon energies of 15–30 eV has been studied by dispersed fluorescence spec-
troscopy. The primary photon beam was monochromatized undulator radiation supplied from the UVSOR facility. The
.
.
5
8
+
2
2
+
2
2 +
following emission-band systems have been identified: OCS [A Π
Ω
(0,0,0) → X Π
Ω
(0,0,v
3
ꢀ)], CO (A Π
Ω
→ X Σ ),
+
2
+
2
3
3
+
2
2
CS (B Σ → A Π
Ω
), and CO(d ∆ → a Π). All of the transitions, except for OCS [A Π
Ω
→ X Π
Ω
], were first observed
+
2
in the vacuum UV photodissociation of OCS. The fluorescence excitation spectra for the OCS [A Π
Ω
(0,0,0) →
2
+
2
+
2
X Π
Ω
(0,0,v
3
ꢀ)] and CS (B Σ → A Π
Ω
) transitions were measured in the photon energy range of 15.1–15.75 and 21.8–
3
o
3 e
2
(
6 eV, respectively. The emission spectra obtained at 20.85 and 22.9 eV exhibit atomic transitions of S[nd D → 4p P
3 o
n = 6–9)], which result from the neutral dissociation of superexcited Rydberg states of OCS into S(nd D ) + CO. Pos-
sible excited states of the counterpart CO are discussed on the basis of the difference in the n distribution between the
two spectra.
3
+
3
3 −
When molecules are electronically excited by absorbing
vacuum ultraviolet (VUV) photons, they usually decay into
many sorts of ionic and neutral photofragmentation prod-
CO(aꢁ Σ , d ∆, e Σ ), respectively, produced by predissocia-
+ 2 +
tion of the Rydberg series converging to OCS (B Σ ). How-
ever, the emitting fragments were not definitely identified in
their work, since they employed only optical filters to confine
the wavelength region of the fluorescence. The energy-level
diagram in Fig. 1 summarizes the dissociation limits of OCS
for the formation of neutral and cation fragments. An increase
in Ehν is expected to drastically promote the formation of radi-
ative fragments. Nevertheless, at Ehν > 15 eV, the fluorescing
transitions which were dispersed and detected by a monochro-
1
–5
ucts.
In some cases, the fragments possess sufficient inter-
nal energy to de-excite radiatively by emitting UV or visible
fluorescence. In the early 1970’s, Lee and Judge accomplished
pioneering work on the fluorescence spectroscopy of various
6
,7
molecules photoexcited in the VUV region.
Later, Nenner
1
,8
9–12
and coworkers as well as Ukai, Hatano and coworkers
studied the direct dissociation and predissociation of ionic or
superexcited states produced by VUV excitation using syn-
chrotron radiation by means of fluorescence excitation spec-
troscopy and two-dimensional fluorescence spectroscopy. It is
widely accepted that fluorescence spectroscopy is an important
tool to determine the fragments and to clarify the mechanisms
governing the dissociation processes of diatomic and poly-
atomic molecules.
+
2
2
1
mator were limited only to OCS (A Π
Ω
→ X Π
Ω
) and CS(A Π
1
+ 13,15
→ X Σ ).
Moreover, there has been no experimental work
on the fluorescence spectroscopy of any neutral or cation frag-
ments from OCS at Ehν > 20 eV. As suggested from Fig. 1, the
VUV photon in this region has sufficient energy to cause a
prompt dissociation of OCS into a fragment ion emitting a UV
or visible photon.
Lee and Judge measured the dispersed fluorescence spectra
of carbonyl sulfide, OCS, by utilizing seven discrete emission
lines between Ehν = 15.48–19.46 eV and assigned the domi-
The present paper describes the fluorescence (emission)
spectra of OCS measured in the region Ehν = 15–30 eV. In or-
der to improve the detection efficiency, we developed an opti-
cal detection system consisting of spheroidal and spherical
mirrors in combination with an optical-fiber bundle. The de-
tection limit for the partial fluorescence cross section is esti-
+
2
2
nant features to the OCS [A Π
Ω
(0,0,0) → X Π
Ω 1 3
(v ,0,v )] tran-
13
sitions.
Here, Ehν denotes the excitation photon energy.
Tabché-Fouhaile et al. reported on the fluorescence excitation
1
4
−3
spectra of OCS in the region of Ehν = 11.1–18.5 eV. The ob-
mated to be less than 10 Mb. This improvement allows us to
served fluorescence emissions with wavelengths of 160–300
nm and 530–630 nm were ascribed chiefly to CS(A Π) and
perceive weak fluorescence signals of fragments from OCS
photoexcited in the VUV region.
1

1
194 Bull. Chem. Soc. Jpn., 74, No. 7 (2001)
HEADLINE ARTICLES
Fig. 1. Energy level diagram illustrating the adiabatic ionization potentials (Ref. 42) and dissociation limits of OCS. Data taken
from Refs. 19, 21, 28, 37, and 38 are used to calculate the dissociation limits (see also Table 1 of Ref. 43). All energies are mea-
1
+
sured with respect to the neutral ground state of OCS[X Σ (0,0,0)]. Thick, thin, and dashed arrows indicate the transitions identi-
fied in the synchrotron radiation photoexcitation (present study), in flowing rare gas afterglow reactions (Refs. 23–27 and 44), and
in the photoexcitation at the photon energy above 15 eV (Refs. 13 and 15), respectively.
harmonic. The concentration of the second-order light was esti-
mated to be no less than 10% at Ehν = 15 eV.
Experimental
All experiments were carried out at beam line 3A2 of the UV-
SOR facility in the Institute for Molecular Science. Details of the
The fluorescence was collected by an optical detection system,
as shown in Fig. 2. Spheroidal and spherical mirrors were mount-
ed so that their surfaces would face each other across the photoex-
citation region (PR), i.e. the source of the fluorescence. One focal
point of the spheroidal mirror fell at PR, while the other focal
point was at the surface of an optical-fiber bundle of 50 cm long.
The fluorescence light was reflected back to PR by the spherical
mirror with its focal point at PR, and was then focused onto the
surface of the fiber bundle by the spheroidal mirror. This detec-
tion system could collect light from about 62% of the full-sphere
solid angle. The fluorescence passed through the optical-fiber
bundle, whose transmission was estimated to be 55% at 400 nm.
Two types of monochromators were used to disperse the fluo-
rescence. In dispersed fluorescence spectroscopy we utilized a
300 mm focal-length imaging spectrograph (Roper Scientific,
SpectraPro-300i, Tzerny–Taner type) equipped with a liquid-ni-
trogen cooled CCD array detector of 26.8 mm wide (the disper-
sion direction) and 2 mm high. Unless otherwise mentioned, a
600 grooves mm− grating having a nominal blaze wavelength of
500 nm was used. The overall detection efficiency, including the
spheroidal and spherical mirrors, fiber bundle, and imaging spec-
5
,16
undulator radiation are described elsewhere.
Carbonyl sulfide
OCS was expanded under an effusive jet condition through a mul-
tichannel capillary plate, and subjected to the irradiation of mono-
chromatized undulator radiation (Ehν = 15–29.8 eV). Commer-
cial high-purity gas (Matheson, 97.5% pure) was used without any
further purification. The fundamental light of the undulator radia-
tion was dispersed by a monochromator of constant deviation
grazing-incidence type with a 2.2 m focal length. The monochro-
mator can cover a wide wavelength range of ~10–95 nm by inter-
−
1
changing three spherical gratings: G1 (600 grooves mm ) at
−
1
9
(
5.3–22.5 nm, G2 (1200 grooves mm ) at 53.8–12.4 nm, and G3
2400 grooves mm− ) at 27–9.9 nm. However, G1 was used in al-
1
most all experimental runs. The size of the monochromatized
2
light was ca. 1 × 1 mm at the intersection region. A typical pho-
ton intensity and spectral resolution of the synchrotron radiation
1
4
−1
−2
were 7.8 × 10 photons s cm and 2.4 Å (FWHM, 83 meV at
0.85 eV), respectively, with entrance- and exit-slit widths of 300
1
2
µm. There exists contamination of the light by passage of a sec-
ond order through the monochromator from the second undulator

K. Mitsuke et al.
Bull. Chem. Soc. Jpn., 74, No. 7 (2001) 1195
nent spin-orbit doublet structures, specified by Ω = 3/2 and
1
/2. A photoionization study at Ehν < 20 eV indicates that the
+
2
emission of OCS [A Π
in the UV and visible regions upon photoexcitation in OCS.
The CO (A Π
=
hand, no vibrational band of CO (A Π
when Ehν was lowered to 19.85 ± 0.04 eV. At this energy, only
Ω
(0,0,0)] gives the most intense bands
13
+
2
2 +
Ω
, vꢁ → X Σ , vꢀ) emission-band system up to vꢁ
4 clearly appears at Ehν ꢀ 20.85 ± 0.04 eV. On the other
+
2
Ω
, vꢁ) was identified
+
2
Ω
the vꢁ = 0 state of CO (A Π ) can be produced, since the ther-
1
9–21
3 e
mochemical thresholds
for the formation of S( P ) +
, vꢁ) with vꢁ = 0 and 1 lie at 19.70 ± 0.015 and
9.89 ± 0.015 eV, respectively. The vꢁ = 0 → vꢀ = 0 transi-
tion at 490 nm is relatively weak because of an unfavorable
+
2
CO (A Π
Ω
1
2
2
Franck–Condon overlap, and may be almost smeared out at
E
+
2
hν = 19.85 eV. Several authors have studied the CO (A Π ,
Ω
2 +
vꢁ → X Σ , vꢀ) emission-band system resulting from Penning
ionization or excitation transfer of OCS in collisions with rare
2
3–28
gas metastable atoms or ions, respectively.
investigation has been made on the production of CO (A Π ,
Ω
vꢁ) as an emitting species from photo-irradiated OCS mole-
In contrast, no
+
2
cules.
+
2
With increasing Ehv, the intensity ratio of the CO (A Π
Ω
, vꢁ
(0,0,0) →
3
ꢀ)] gradually increases. Another distinct trend of
2
+
+
2
→
X Σ , vꢀ) bands to that of OCS [A Π
(0,0,v
Ω
2
X Π
Ω
Fig. 2. Schematic diagram of the apparatus for dispersed flu-
orescence spectroscopy and fluorescence excitation spec-
troscopy. UR, monochromatized undulator radiation; PR
photoexcitation region (not to scale); ODS, optical detec-
tion system composed of spheroidal and spherical mirrors
and an optical-fiber bundle; CM, gold-mesh current moni-
tor; PM, photomultiplier.
the spectra is an enhancement of the broad feature encompass-
ing the 405–420 nm region. Somewhere between Ehν = 22.9
and 25.0 eV a reversal seems to occur in the intensities of this
+
2
2 +
feature at 405–420 nm and the CO (A Π
Ω
, vꢁ = 3 → X Σ , vꢀ
=
0) band around 400–405 nm. The fluorescence intensity in
the wavelength region of 400–410 nm was measured as a func-
tion of Ehν using the 100 mm focal-length monochromator.
The resultant fluorescence excitation spectrum of OCS is de-
picted in Fig. 4. The fluorescence intensity shows a slow onset
at 23 ± 0.5 eV. This observation leads us to infer that the new
feature at 405–420 nm in Fig. 3 can be attributed to the bands
−
3
trograph, was estimated to be (1.1 ± 0.3) × 10 with the en-
trance slit width of the spectrograph being 250 µm. When we ful-
filled fluorescence excitation spectroscopy by scanning the wave-
length of synchrotron radiation, we replaced the imaging spec-
trograph by a 100 mm focal-length monochromator (Ritu,
MC10N, 600 grooves mm− grating) and a photomultiplier tube
+
2
+
2
originating from the CS (B Σ → A Π
Ω
) transition, since the
for the formation of
CS (B Σ ) from OCS is 22.76 eV. A vibrational analysis of
1
9,29,30
thermochemical threshold
1
+
2 +
(
Hamamatsu, R464S). In this case, the overall detection efficien-
24
this band system was first carried out by Tsuji et al. in 1980.
cy, including the mirrors, fiber bundle, monochromator, and pho-
+
2 +
−
3
In their experiment, CS (B Σ ) fragments were formed by
tomultiplier tube, was estimated to be (1.4 ± 0.3) × 10 at the
entrance- and exit-slit widths of the monochromator of 2 mm. All
spectra given in the present paper were corrected by the wave-
length dependence of the relative detection efficiency.
+
thermal energy charge transfer from He to OCS in flowing
helium afterglow reactions. To compare the present data with
2
4
their high-resolution spectrum (∆λ = 0.08 nm) we acquired
an expanded dispersed fluorescence spectrum (Fig. 5) taken at
Results and Discussion
−1
E
hν = 25.05 eV. Here, a 1200 grooves mm grating with a
nominal blaze wavelength of 300 nm was mounted in the 300
mm focal-length imaging spectrograph. Figure 5 includes the
Observed Emission at Fluorescence Wavelengths of 360–
30 nm. Figure 3 shows dispersed fluorescence spectra at
the wavelength region 360–530 nm measured at five Ehν points
between 19.85 and 29.8 eV. The electronic transitions of two
5
2
4
reported positions of the ∆v = vꢁ − vꢀ = 0 sequences of the
two spin-orbit components (Ω = 3/2 and 1/2) of the
+
2
+
2
+
2
CS (B Σ , vꢁ → A Π
Ω
, vꢀ) transition. Weak maxima observed
kinds of ions are clearly observed: OCS [A Π
Ω
(0,0,0) →
2
+
2
2 +
at ~406 and ~411 nm in the present spectrum, which were re-
X Π
Ω
(0,0,v
3
ꢀ)] with v
3
ꢀ = 2–5 and CO (A Π
Ω
, vꢁ → X Σ , vꢀ).
producible on triplicate experimental runs, appear to be consis-
Quenching of the excited ions in collisions with ambient mole-
cules can be disregarded, because the sample pressure at the
+
2
+
2
tent with the accepted band features in the CS (B Σ → A Π
Ω
)
−
3
system.
intersection region was of the order of 10 Pa, and because
+
+
2
Cross Section for the OCS (A → X) Fluorescence. The
Ω
the fluorescence lifetime of OCS [A Π (0,0,0)] and
1
+
+
2
17,18
fluorescence cross section
σ
f
of OCS(X Σ ) for the
CO (A Π
Obviously, the observed transitions result from unimolecular
decay processes. The vibrational progression of the
Ω
, vꢁ) are ~0.17 and 2.1–3.0 µs, respectively.
+
2
2
OCS [A Π3/2(0,0,0) → X Π3/2(0,0,3)] transition at 394.5 nm
−
3
was evaluated to be 5 × 10 Mb at Ehν = 20.85 eV by cali-
+
2
+
+
2
2
brating its signal count rate against that of the N
2
(B Σ
u
, vꢁ =
OCS [A Π
Ω
(0,0,0) → X Π
Ω 3
(0,0,v ꢀ)] transition shows promi-

1
196 Bull. Chem. Soc. Jpn., 74, No. 7 (2001)
HEADLINE ARTICLES
Fig. 3. Dispersed fluorescence spectra of OCS encompassing the wavelength region 360–530 nm at five photon energies between
−
1
1
9.85 and 29.8 eV. The imaging spectrograph equipped with a 600 grooves mm grating (blaze wavelength = 500 nm) was used
−
1
at the entrance-slit width of 500 µm at Ehν = 19.85 eV and 250 µm at the other four photon energies. A 1200 grooves mm grat-
ing (blaze wavelength = 300 nm) was employed to obtain the dashed curve in the panel of Ehν = 22.9 eV. The thin vertical lines
+
2
2
indicate the vibrational progression in the antisymmetric stretch ν
3
mode of the OCS [A Π
Ω
(0,0,0) → X Π
Ω 3
(0,0,v ꢀ)] transition.
The number of the excited quanta are given in the panel of Ehν = 19.85 eV. The thick vertical lines indicate the band origins of the
+
2
2 +
CO (A Π
lower vꢀ vibrational states.
Ω
, vꢁ → X Σ , vꢀ) emission-band system. The (vꢁ,vꢀ) mark denotes the band due to the transition from the upper vꢁ to
2
+
0
→ X Σ
g
, vꢀ = 0) emission of N
2
at 389–392 nm. In the cal-
Here, n is the number density of the sample, ieff the effective
photon intensity at the interaction region, η the overall collec-
tion efficiency of the detection system, and V the interaction
ibration procedure, a N
citation region, instead of OCS. We adopt the following equa-
tion to connect σ with the signal count rate S:
2
sample was admitted into the photoex-
f
volume. The fluorescence cross section of N
2
for the
+
2
+
2
+
N
2
(B Σ
u
, vꢁ = 0 → X Σ
g
, vꢀ = 0) transition is assumed to
S = nσ
f
ieffηV.
(1)
be equal to the product of the partial photoionization cross sec-

K. Mitsuke et al.
Bull. Chem. Soc. Jpn., 74, No. 7 (2001) 1197
the first-order light to Ehν = 41.7 eV. We found that S/nieff at
E
hν = 41.7 eV is one order of magnitude lower than that at
0.85 eV. This manifests that S/nieff due to the second-order
light is negligibly low at Ehν = 20.85 eV.
2
+
2
2
The cross sections for OCS [A Π3/2(0,0,0) → X Π3/2
(
×
0,0,3)], obtained in a similar manner, were found to be (5–9)
10 Mb at the other photon energies given in Fig. 3. Lee
−
3
and Judge reported the corresponding cross sections of (1.0 ±
−
2
0
.1) × 10 Mb at six photon energies between 15.69–19.46
1
3
eV. It is therefore fair to say that the partial photoionization
cross section of OCS for the formation of OCS [A Π (0,0,0)]
Ω
+
2
depends rather weakly on the photon energy in the range of
from the associated ionization threshold of 15.075 eV to the
highest excitation energy in the present measurements, i.e.
2
9.8 eV.
The above statement is of course true only for the case of di-
Fig. 4. Fluorescence excitation spectrum of OCS obtained
by plotting the fluorescence intensity integrated over the
rect ionization. The cross section may drastically change
across the resonance positions that match the excitation ener-
gies for autoionizing superexcited states. This situation can be
recognized for the Rydberg series converging to
wavelength region of 400–410 nm as a function of Ehν
.
The thermochemical threshold for the formation of
+
2
+
3 e
CS (B Σ ) + O( P ) from OCS is indicated.
+
2 +
OCS [B Σ (0,0,0)]. Figure 6 shows a fluorescence excitation
+
2
2
spectrum for OCS [A Π
Ω
(0,0,0) → X Π
Ω
(0,0,3)] taken in the
+
2
+
8,31
tion of N
2
for the formation of N
2
(B Σ
u
, vꢁ = 0) and the
Ehν range of 15.1–15.75 eV. In this measurement, the 100 mm
focal-length monochromator was employed and the signal
counts due to the unresolved spin-orbit components, Ω = 3/2
and 1/2, were added together. The spectrum contains many
peaks whose assignments are indicated in Fig. 6, by reference
3
2
branching ratio of 0.7 for the vꢁ = 0 → vꢀ = 0 transition.
A
supplementary measurement at Ehν = 41.7 eV was made for
+
2
the normalized count rate, S/nieff, of the OCS [A Π3/2(0,0,0) →
2
X Π3/2(0,0,3)] emission in order to check the effect of the sec-
3
3,34
ond-order contamination of the light upon S/nieff at 20.85 eV.
to those given by Leclerc et al.
From the same procedure
−
1
Here, grating G2 (1200 grooves mm ) was utilized for tuning
as that described in the preceding paragraphs, we evaluated the
Fig. 5. Dispersed fluorescence spectra of OCS encompassing the wavelength region 375–431 nm at Ehν = 25.05 eV. The spectrum
was measured by using the imaging spectrograph equipped with a 1200 grooves mm− grating which has a nominal blaze wave-
length of 300 nm. The entrance-slit width was set to 250 µm. The thin short vertical lines indicate two spin-orbit components (Ω
1
+
2
2
=
3/2 and 1/2) of the vibrational bands of the OCS [A Π
Ω
(0,0,0) → X Π
Ω 3 3
(0,0,v ꢀ)] transition with v ꢀ = 3 and 4. The thin long
+
2
+
2
vertical lines indicate the two spin-orbit components of the vibrational bands of the CS (B Σ ,vꢁ → A Π
Ω
,vꢀ) transition (Ref. 24).
The (vꢁ,vꢀ) mark denotes the band due to the transition from the upper vꢁ to lower vꢀ vibrational states. The thick vertical lines in-
+
2
2 +
dicate the band origins of the CO (A Π
Ω
, vꢁ → X Σ , vꢀ = 0) emission-band system.

1
198 Bull. Chem. Soc. Jpn., 74, No. 7 (2001)
HEADLINE ARTICLES
+
duced not by the dissociation of OCS but by the neutral dis-
sociation of a superexcited state OCS* of Rydberg type. This
is because the primary excitation energy is lower than the dis-
3
o
sociation limits of OCS for the formation of S[nd D (n = 6–
+
2
+
19–21,37
9
)] + CO (X Σ ), 27.11–27.34 eV.
At Ehν = 20.85 ± 0.04 eV, the peaks located at 582, 596 and
6
18 nm are no longer observed on the dispersed fluorescence
spectrum, whereas there still remains the peak at 654 nm.
Hence, the upper limit of the excess internal energy of the
counterpart CO fragment is estimated to be 7.75 ± 0.06 eV by
3
o
37
using the excitation energy of 9.94 eV for the S(6d D ) state
and the dissociation energy of 3.16 ± 0.015 eV for the forma-
3
e
1
+
19,20
tion of S( P ) + CO(X Σ ) from OCS.
As listed in Table 1,
3
8
there are four states of the CO fragment whose transition en-
1
+
ergy (T00) for the 0–0 band with respect to CO(X Σ , vꢀ = 0) is
1
+
3
3
+
3
smaller than this upper limit, viz. X Σ , a Π, aꢁ Σ , and d ∆.
1
+
The CO(X Σ ) ground state is hardly suited for the counter-
3
o
part of S(6d D ) for the following reasons. Photoionization
Fig. 6. Fluorescence excitation spectrum of OCS obtained
by plotting the fluorescence intensity integrated over the
1
+
studies of OCS(X Σ ) have indicated that the potential energy
+
4 −
surface of the OCS ( Σ ) state correlating to the lowest disso-
wavelength region of 390–400 nm as a function of Ehν
.
+
4
o
1
+
2
ciation limit of S ( S ) + CO(X Σ) intersects the potential of
+ 2
The two spin-orbit components of the OCS [A Π
Ω
(0,0,0)
2
OCS (A Π
excited vibrational levels of OCS (A Π
5.36 eV are known to undergo conversion into the Σ state
Ω
) at an ionization energy of ca. 15.1 eV, since any
→
Ω 3
X Π (0,0,v ꢀ = 3)] band are considered to mainly con-
+
2
) between 15.18–
tribute to the observed fluorescence. The thick vertical line
indicates the thermochemical threshold for the formation
Ω
4
−
1
+
2
of the vibrational ground state of OCS (A Π3/2).
via avoided surface crossings through vibronic coupling to be
+
4
o
1
+
13,39,40
completely predissociated into S ( S ) + CO(X Σ ).
−
3 o
The Σ , Π, and ∆ components correlating to the S(6d D ) +
2
2
1
+
+ 4 −
fluorescence cross section for A Π3/2(0,0,0) → X Π3/2(0,0,3) to
be 0.04 Mb at the 5sσ Rydberg state (Ehν = 15.17 eV). This
value is larger by a factor of 4–8 than the cross section for the
same transition resulting from the direct ionization of OCS.
Observed Emission at Fluorescence Wavelengths of 530–
CO(X Σ ) limit may behave similarly to the OCS ( Σ ) state
along the OC–S dissociation coordinate, if the Rydberg elec-
tron of these neutral states has little effect on the bonding of
4
−
the Σ ion core. It is unlikely from these potential relations
3
o
1 +
that the S(6d D ) + CO(X Σ ) channel is accessible by the di-
rect dissociation of the primary OCS* that was substantiated at
Ehν ꢀ 20.85 eV in the present study. Furthermore, there is lit-
680 nm. Figure 7 shows the emission spectra obtained from
the photoexcitation at four Ehν energies from 20.85 to 29.8 eV.
In the region 530–680 nm, nine spectral features are discerned:
five of them are relatively broad and assignable to rovibration-
al bands, while the other four have narrow line shape. The
bands appearing in the wavelength regions 545–560 nm and
−
tle probability of the conversion of OCS* to the Σ , Π, and ∆
3
o
1 +
components correlating to S(6d D ) + CO(X Σ ), since the
+
4
o
1
+
+ 2
o
branching ratio of the S ( S ) + CO(X Σ ) to S ( D ) +
1
+
CO(X Σ ) channels rapidly decreases with increasing photon
6
→
18–635 nm can be assigned to the vꢁ = 0 → vꢀ = 1 and vꢁ = 0
energy above the dissociation limit of 15.36 eV for the latter
+
2
2
+
40
vꢀ = 2 transitions of CO (A Π → X Σ ), respectively, by
channel.
3
5
reference to the reported vibrational energy levels of the ions
It is, however, difficult to decide which to select as the most
3
o
and the Einstein A coefficients for the individual vibrational
bands. The rest of the broad peaks are attributable to the
proper counterpart of S(nd D ) among the other three states.
22
3
3
o
In the case of CO(d ∆), the dissociation limit for S(6d D ) +
CO(d ∆) is 20.62 ± 0.015 eV, as shown in Table 1. Hence,
3
3
35,36
3
CO(d ∆, vꢁ = 4–6 → a Π, vꢀ = 0) emission-band system.
+
2
2 +
The CO (A Π → X Σ ) transitions are present at every photon
most of the excess energy at Ehν = 20.85 ± 0.04 eV has been
converted preferentially to the electronic energy of CO. This
indicates that the involved potential energy curve(s) of OCS
should satisfy the following condition: the height of the as-
ymptote S(6d D ) + CO(d ∆) of the dissociation state
OCS(Dis) is included within the Franck–Condon region of the
superexcited state OCS* to which primary photoabsorption
takes place. The peaks of S(nd D ) with n ꢀ 7 are not ob-
served in the emission spectrum at Ehν = 20.85 ± 0.04 eV.
This suggests that the dissociation limit for S(7d D ) +
energy larger than 20 eV chosen in the present study, whereas
3
3
the CO(d ∆→ a Π) transitions are very weak at Ehν = 22.9 eV
and indiscernible at Ehν = 20.85 eV.
3
o
3
The peaks at 582, 596 and 618 nm are narrow and symmet-
ric, which suggests that they arise from atomic transitions. Us-
3
7
ing the Grotrian diagrams compiled by Bashkin and Stoner,
we assigned these peaks to the fluorescing transitions from the
3
o
3
o
3 e
three consecutive Rydberg states, S[nd D → 4p P (n = 7–
3
o
3
o
9
)]. Here, S(nd D ) is the member of the Rydberg series con-
+
4
o
3
verging to the ground S ( S ) and n denotes the principal
quantum number. A similar peak is also seen at 654 nm, and
assignable to the transition of the adjacent member of the Ryd-
CO(d ∆) of 20.73 ± 0.015 eV is too high to be reached from
OCS*, probably because the nuclear kinetic energy of few
hundreds of meV is released in dissociation.
We next examine the possibility of the S(6d D ) +
3
o
3
e
3
o
berg series, S(6d D → 4p P ). We can then derive a straight-
3
o
3
+
3
forward conclusion that the S(nd D ) Rydberg state is pro-
CO(aꢁ Σ and a Π) channels. The corresponding dissociation

K. Mitsuke et al.
Bull. Chem. Soc. Jpn., 74, No. 7 (2001) 1199
Fig. 7. Dispersed fluorescence spectra of OCS encompassing the wavelength region 530–680 nm at four photon energies between
1
2
0.85 and 29.8 eV. The imaging spectrograph equipped with a 600 grooves mm− grating (blaze wavelength = 500 nm) was used
+
2
2 +
at the entrance-slit width of 250 µm. The thick vertical lines indicate the band origins of the CO (A Π
sion-band system. The (0,vꢀ) mark denotes the band due to the vꢀ vibrational state of CO (X Σ ). The thin short vertical lines in-
Ω
, vꢁ = 0 → X Σ , vꢀ) emis-
+
2 +
3
3
dicate the band origins of the CO(d ∆, vꢁ = 3–6 → a Π, vꢀ = 0) band system. The (vꢁ,0) mark denotes the band due to the vꢁ vi-
3
3
o
3
e
3
o
brational state of CO(d ∆). The thin long vertical lines indicate atomic transitions S[nd D → 4p P (n = 6–9)] where S(nd D ) is
+
4 o
the member of the Rydberg series converging to the ground S ( S ). The features marked with the dotted lines are less unambig-
uous.
limits are located at 19.96 ± 0.015 and 19.11 ± 0.015 eV, re-
spectively, and therefore 0.89 ± 0.06 and 1.74 ± 0.06 eV, re-
spectively, are transferred to the vibrational energy of CO and/
or kinetic energy with which the fragments fly apart. The in-
tensity distribution of S(nd D ) is determined crucially by the
n-dependence of the nonadiabatic transition probabilities at the
avoided surface crossing points formed by the intersection of
the potential energy curves of OCS* and OCS(Dis). If the
Rydberg electron of OCS* interacts with the ion core and par-
ticipates directly into the electron-exchange mechanism that
controls the conversion at avoided surface crossing points
3
o
4
1
(participant predissociation), the energy width for the predis-

1
200 Bull. Chem. Soc. Jpn., 74, No. 7 (2001)
HEADLINE ARTICLES
1
+
Table 1. Transition Energies T00 from the Vibrational Ground State of CO(X Σ ) to
That of Low-Lying Electronically Excited States
a)
b)
3
o
c)
Electronic state of CO
T
00(CO)
T
00(CO)
D[S(6d D ) + CO]
1
+
X Σ
0
0
13.10
19.11
19.96
20.62
21.00
21.13
3
a Π
6.010
6.863
7.516
7.898
8.028
6.010
6.863
7.519
7.761
8.027
3
+
aꢁ Σ
3
d ∆
3
−
e Σ
1
A Π
1
+
The fourth column gives the dissociation limits D of OCS(X Σ ) for the formation
of S(6d D ) and the excited states of CO. All energies are in eV.
3
o
a) Taken from Ref. 38. b) Taken from the data reported in Ref. 35 on potential
energy curves for the electronic states of CO. c) Obtained from the sum of the dis-
1
+
sociation energy of 3.16 ± 0.015 eV (Refs. 19 and 20) of OCS(X Σ ) for the for-
3
e
1 +
mation of S( P ) + CO(X Σ ), the excitation energy of 9.94 eV (Ref. 37) for
3
o
S(6d D ), and T00(CO) in the second column (Ref. 38).
Table 2. Identified Emission-Band Systems in the Vacuum
UV Photoexcitation of OCS(X Σ ).
cation, Science, Sports, and Culture, and by a grant for scien-
tific research from Matsuo Foundation.
1
+
Vibrational level of
the upper state, vꢁ
Emitting species Electronic transition
References
+
2
2
OCS
A Π
Ω
→ X Π
Ω
0
+
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2
+
1
I. Nenner and J. A. Beswick, in “Handbook on Synchrotron
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+
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CS
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3
3
CO
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e
a)
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3
o
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4
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3
o
3 +
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3
5
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3
o
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6
7
8
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9
By dispersed fluorescence spectroscopy we have studied the
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the fluorescence spectra of Figs. 3 and 7. To our knowledge,
1
0
(
1
1
+
2
2
none of the transitions, except for OCS (A Π
Ω
→ X Π
Ω
), have
been reported before in the photoexcitation of OCS above 15
eV. We could offer evidence supporting the neutral dissocia-
1
1
2
3
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3
o
3
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3
+
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aꢁ Σ , and/or d ∆) with n = 6–9.
1
4
The authers are grateful to Professor Kazutoshi Fukui, Mr.
Takayoshi Kihara and the members of the UVSOR facility for
helpful advice and discussions during the course of the experi-
ments. This work has been supported financially in part by na-
tional funds appropriated for special research projects of the
Institute for Molecular Science, by a Grant-in-Aid for Scientif-
ic Research (Grant No. 10640504) from the Ministry of Edu-
7
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