Emission spectrum of carbonyl sulfide in a helium afterglow

Abstract

Photoemission from the following excited species was observed from OCS in a helium afterglow at wavelengths from 200 to 500 nm: OCS+ (Ã2Π-X̃2Π), CO(a3Π-X1Σ+), CO+(A2Πi-X2Σ+, B2Σ+-X2Σ+), CS(A1Π-X1Σ+, a3Π-X1Σ+) and CS+(B2Σ+-A2Πi). The dependence of emission intensities on the electrostatic potential applied to an ion-collector grid indicated that OCS+(Ã-X̃) emission results from He(23S) Penning ionization, while CO+(A-X, B-X) emission is the result of dissociative charge-transfer ionization of OCS with He+ ions. The emission rate constants were estimated to be 5.1 ± 0.5 × 10−13 cm3 molecule−1 s−1 for OCS+(Ã) and about 9 × 10−12 cm3 molecule−1 s−1 for CO+ (A). Energy resonance was found to be an important factor for the thermal He+/OCS reaction.

Full text

International Journal of Mass Spectz-ometry nnd ion Physics, 34 (1980) 273-286
@ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands 273
EMISSION SPECTRUM OF CARBONYL SULFIDE IN A HELIUM
~~R~LO~
~S~~U TSUJI, ~~ORU ~ATSUO and YUKIO NIS~URA
Research Institute of Industrial Scicwce. Kyushu University 86, Hakozaki. Fukuoka 812
(Japan)
(Received 18 June 1979)
ABSTRACT
Photoemission from the following excited species was observed from OCS in a helium
afterglow at wavelengths from 200 to 500 nm: 0CS+(Z2~-Z2~), CO(a3n-XIX+),
CO*(A*ni-X*x+, B2E+-X2Z+), CS(A’n-X1x+, a3n-X1x:‘) and CS+(.B*~+-A*ni). The
dependence of emission intensi$es_on the electrostatic potential applied to an ion-collec-
tor grid indicated that OCS“(A-X) emission results from He(23S) Penning ionization,
while CO+(A-X, B-X) emission is the result of dissociative charge-transfer ionization of
OCS with He+ ions. The emission rate constants were estimated to be 5.1 + 0.5 x lo-l3
cm3 molecule-l s-r for OCS’(2i) and about 9 x lo-‘* cm3 molecule-’ s-r for CO’ (A)-
Energy resonance was found to be an important factor for the thermal Ile+/OCS reaction.
Although reactions of diatomic molecules in a helium afterglow have been
~vestigated extensively by emission spectrometry ] 123, comparatively few
studies have been made on polyatomic molecules. In order to study Penning
ionization and charge-transfer ionization of triatomic molecules, the simplest
polyatomic molecules, \rve have initiated an optical spectroscopic study in
the helium afterglow. The results obtained for H2S 133, SO, [lr], N,CI 151 and
CO, 163 have been reported in previous papers. This paper deals with results
for carbonyl sulfide, which is a linear triatomic molecule.
The emission spectrum of OCS in_ a helium afterglow has been measured
by Setser [ 73. He observed OCS’(A-X), CO”(A-X), CS(A-X, a-X) and
S(%--3P*, 3P1) emission in the 190-600 nm region. An important finding is
that OCS exhibits relatively strong CO*(A-X) emission extending up to U’ =
11. Based on energy relations, He’ or He; has been attributed as an impor-
tant source of the CO*(A-X) emission [S]. This result indicates that ionic
active species, the relative concentration of which was estimated to be only
~5% that of He(23S) [ 71, take part in the formation of some excited species
at helium pressure of -2 torr. Since both metastable atoms and ionic species
can energetically produce all ionic emissive products observed from OCS in

274
the helium afterglow, it is necessary to determine the active species responsi-
ble for a detailed analysis of the reaction mechanism.
In the present study, an ion-collector grid was placed between the dis-
charge section and the reaction zone for the purpose of reaction identifica-
tion. By use of the Len-collector grid and synchronous photon-counting
equipment, OCS’(A-X) and CO’(A-X, B-X) emission was found to result
from different ionization processes. The former results from He(23S) Pen-
ning ionization, while the latter is produced by the dissociative charge-trans-
fer ionization of OCS with He* ions. The emission rate constants were mea-
sured with reference to Nf;(B--X) emission in an OCS/N* mixture_ Charge-
transfer reactions of thermal He” ions with triatomic molecules are discussed
in terms of energy resonance and Franck-Condon factors.
EXF’ERIMENTAL
The flowing afterglow apparatus used in this study is shown in Fig. 1.
Active species generated by a microwave discharge of helium gas flowed
down into the stainless steel reaction cell by evacuating with a fast pump
(7000 I mm-“). The input power of the microwave discharge was 40-60 W.
Reagent gases stored in a reservoir (total vohune, about 16 1) were intro-
duced through a stainless steel nozzle (0.6 mm in diameter) 10-15 cm
downstream of the discharge section.
In order to eliminate ionic active species, an ion-collector grid was added
to the flow line as shown in Fig. 1. Two nickel electrodes (20 mesh, transpar-
ency about 0.90) were spaced at intervals of 2 mm. The efficiency of the
grid was checked by observing N;(C-X) emission in the 190-210 nm region
tie
Sample gas Quartz window
P.M. RI06 UH
Fig. 1. The schematic design of the flowing afterglow apparatus.

275
resulting from the He+/N2 charge-transfer ionization. When a suitable positive
or negative voltage was applied to the grid, the Ns(C-X) emission disap-
peared. The disappearance of the N;(C-X) emission on application of a
negative voltage is probably due to the fact that an ion-sheath is formed and
the effective transmission of ion flux is reduced [9]_
Another possible ionic active species in a helium afterglow is Hef. Piper et
al. [8a] analyzed the N:(B-X) and CO’(A-X, B-X) emission resulting from
the reactions of He(23S), He+ and He; with N2 and CO using a flowing after-
glow apparatus. They found that He: is an important excitation source at a
helium pressure of -4 torr. The helium pressure in the present experiment,
1.0-2.0 torr, is low enough that He”, is probably unimportant.
A pulse modulation system, which has been reported briefly [S], was em-
ployed to monitor the fluorescence resulting from thermal ion/molecule
reactions. The flux of ionic species in the discharge flow was modulated on
and off by pulsing the potential applied to the ion-collector grid. Photons
were alternatively counted up and down synchronously with the grid pulse.
The difference signal corresponding to the photons due to ion/molecule reac-
tions was accumulated and registered. The details of the apparatus will be
described elsewhere [lo].
The helium gas (99.99% nominal purity) was purified by passage through
a molecular sieve trap at 77 K before entering the discharge section. The
OCS (Matheson, purity >97.5%) and SF6 (Seitetsu Kagaku, purity >99.65%)
gases were outgassed before use. The N2 gas (Seitetsu Kagaku, purity
>99.999%) was used without further purification. The pressure in the reac-
tion chamber was monitored by a Pirani gauge calibrated against a McLeod
gauge. The pressure of the sample gases lay in the range 10-3-10-’ torr.
Spectroscopic observation was made through a quartz window with a
Shimadzu GE-100 monochromator and a HTV R106UH photomultiplier.
The optical detection system was calibrated in the 300-600 nm region using
a halogen lamp (Ushio Electronic Corporation, JPD-100-500CS), the spectral
h-radiance of which was determined at the Nippon Denki Keiki Kenteisho.
RESULTS AND DISCUSSION
Emission spectrum
UV and visible photoemission of the excited species resulting from OCS
has been investigated by impact of photons [ 11,121, electrons [ 133, protons
[ 143, hydrogen atoms [ 141 and metastable argon atoms [ 151. Photoemission
from the parent molecular ion, OCS*(z-X), and fragment species such as CI,
CII, SI, CO and CO+ have been observed and the excitation processes have
been studied. In the present study, reactions of OCS with metastable
He(23S) atoms and thermal He* ions were examined by analyzing the emis-
sion spectrum in the helium afterglow.
On the addition of OCS, a white-blue conical flame appeared in the heli-

276
um afterglow. One example of the recorded emission spectra in the 210-500
nm region is shown in Fig. 2. Since this spectrum was m easured without
applying a voltage to the g-rid, both He(23S) and He” are possible active
species. Most of the observed bands are attributable to photoemission from
the fragment species. The emission systems identified are summarized in
Table 1. The spectral observation in the 250-500 nm region is essentially
identical to that obtained by Setser [ 71. In the 200-250 nm region, not ana-
lyzed by Setser, weak CO’(B-X) and CO(a-X) bands are found.
In addition, a weak feature, not observed from OCS in any other excita-
tion sources, appears at -410 nm. Gauyacq and Horani [16] have detected a
similar band under electron impact on CS2 and tentatively assigned the fea-
tures at 406 and 411 nm to the CS+(B-A) (0,O) transition. However, Coxon
et al. [Sb] have argued against this interpretation. As shown later (see
Fig. 4), the excitation source of this emission from OCS was found to be
He’. By comparison with reported photoelectron spectroscopic data for the
CS radical, it was assigned to the Au = 0 sequence of the CS+(B*Z+--A’&)
system. The details of the assignment will be reported with the results for
CS, elsewhere [lo] _
CS (A-X)
210 250 300 350
Z He1 Co* (A-X) *
I
(L.0) : *
I
I 1 8 I I
350 400 450 500 nm
Fig. 2. A typical emission spectrum of OCS, resulting from collisions with a mixture of
He(23S) and He+ in a helium afterglow. Lines marked * are stray He(I) lines.

277
TABLE 1
Identified emission systems from OCS in a helium afterglow
Emitting species
ocs+
CO; (impurity)
co
co+
CS
CS+
Ekctronic transition
X* n---j72n
~q-j$n,
a3l-kxl C’
A2 Hz-X” c+
f32 x+--X’ x+
A I n-X’ c+
a3 n-x1 x+ l
B’x+-_A2,&
Vibrational level (IJ’) Excitation source
0 He (Z3S)
0 He ( Z3S)
0
O-11 He+
OS1 He+
O-5 He ( P3S),Hd
O-1
091 He+
l The (0,O) and (1,l) bands of the CS(n-X) transition are identified at 362 and 364 run,
respectively_ However, they are not indicated in Fig. 2 because of the weakhess of the
intensities.
Reaction identification
The available energies of He(23S) and He’ are 19.82 and 24.58 eV, respec-
tively 1171. The minimum energies required for various possible species
formed from OCS are listed in Table 2. From the energetics, He’ is the only
possible candidate for the production of CO’(A, u’ 2 1) and CO’(B), whereas
both He(23S) and He’ can produce OCS’(z) and CO’(A, u’ = 0). In order to
identify the active species responsible, the dependence of emission intensities
on the electrostatic_po_tential applied to the grid was measured. The band
intensities of OCS’(A-X), CO’(A-X) and CS(A-X) are plotted against the
grid potential in Fig. 3. The intensity of the OCS’(z-2) band is independent
of the potential, while that of the CO’(A-X) band rapidly decreases to zero
on application of positive or negative potential to the grid. The dependence
of the band intensity of CO’@-X) on the grid potential was similar to that
of CO’(A-X). These results suggest that OCS’(A-)i) emission results from
collisions with neutral species, whereas ionic species take part in the produc-
tion of CO’(A-X, B-X) emission.
Almost all ion flux is prevented from entering the reaction cell at a grid
voltage of about 20 or -10 V, as seen from Fig. 3. Therefore, emission spec-
tra induced by collisions with He(23S) or He+ were measured applying a DC
or AC 20 V to the grid in the present study. As an example, the expanded
emission spectra in the 375-435 nm region are shown in Fig. 4. In spectrum
(A), both He(23S’) and He’ are possible active species, while only one of
them is responsible for the spectra shown in (B) and (C). Spectrum (A) con-
sists of CO’(A-X), OCS’(A-3) and CS’(B-A) bands. In spectrum (B), OCS’

278
TABLE 2
Calculated minimum energies (E) required for formation of various products from OCS
Product Electronic state E (eV) * Ref.
OCS ÷ X'-l] 11.190 18
AsR 15.075 18
~2Z+ 16.044 18
~2~+ 17.955 18
÷
CO + S X,~ +, 3p 3.29 -+ 0.03 19
3.22 20
3.09 21
3.16 _+ 0.05 22
mean value
3.19 ± 0.09
a3i], 3p 9.20 _+ 0.09 23
CO+ + S Xs~. +, 3p 17.20 ± 0.09 23
A2Ni
(u' = 0), 3p 19.73 ~ 0.09 23
AsHi
(u' =
1), 3p 19.92_+ 0.09 23
AS~ni (um = 11), 3p 21.64 ± 0.09 23
ASlIi (u' = 0), 1D 20.88 ± 0.09 23
AsHi (v t = 11), ~D 22.79 ± 0.09 23
As[[i (v" = 0), IS 22.48 ± 0.09 23
A2[li (v' = 11), IS 24.39 +_ 0.09 23
Bs~ ÷ (v" = 0), 3p 22.86 ± 0.09 23
BS~ + (u' = 1), 3p 23.07 --k- 0.09 23
B2~ + (u' = 0), ID 24.01 ± 0.09 23
B2~ +(u' = 1), ID 24.21--+0.09 23
CO + S + X 1E÷, 48 13.55 ± ¢'.09
a3H,
48
19.56±0.09 23
C8 + O X'-Z +, 3p 6.85 12
a31] • 3,0 10.27 24
AII], 3p 11.66 12
CS ÷ + O XsZ +, 3p 18.18 25
A2]]i, 3p 19.64 16
Bs~. +, 3p 22.68 16
CS + 0 + X I ~+, 48 20.46
C + S + 0 3p, 3p, 3p 14.28 12,23
* The energies of atomic species were obtained from Moore [ 17].
bands are found, though CO + bands are absent. Conversely, although OCS +
bands disappear, CO + bands are observed in spectrum (C). These spectral
observations lead us to the snme conclusion as the grid-voltage dependence
of the emission intensities; that is, the active species responsible for OCS ÷
emission is He(2SS), while that responsible for CO+(A--X) emission is He ÷. As
seen from Table 2, emission from the CO+(A, u'= O) level due to the

279
It
em
-A-A- A
\
6
/ Ok
I._
P %,
_ ,.OP
*\ CO+(A-X)
--.-O-- 0 -0
P I I I ---?--Y--O-
-10 0 10 20
Grid potential (VI
Fig. 3. Dependence of relative emission intensities of OCS’(z-2; 395 nm), CO+(A-X;
380 nm) and CS(A-X; 258 nm) on the voltage applied to the ion-collector grid.
He(S”S)/OCS reaction is energetically possible. However, the dependence of
the band intensity on the grid voltage indicated that almost all CO’(A-X)
emission from the V’ = 0 level is produced by collisions with He’,
The identification of He’ as the ionic active species responsible was fur-
ther confirmed by observing the dependence of the intensities of CO*(A-X,
B-X) bands on the relative concentration or’ Je’. The relative concentration
of He+ was estimated from the (3, 10) band of Nz(C-X) emission in the
OCS/N, mixture, assuming that it is linearly proportional to the number den-
sity of He’. This ~s~ption is valid because the N”,(C-X) emission arises
from a primary reaction between He” and N2 in the helium afterglow [8a].

280
He1 CO’lA-Xl
(8) He-l OCS
* Y
OCS’ GGt) 0 Lr
1
CO*(A-X)
375 390 390 400 410 420 430 L35 “n-l
Fig. 4. Expanded emission spectra of OCS in the 375-435 nm region due to interaction
with (A) He( 23S) + He+; (B) He(23S), and (C) He+. The spectra (A) and (B) were obtained
at grid potentisls of 0 and 20 V, respectively, while the spectrum (C) was measured using
a pulse modulation system. Lines marked * are stray He(I) lines.
The concentration was controlled by the grid voltage. As shown in Fig. 5, a
satisfactory linearity is found between the emission intensity of CO”(A-X,
B-X) and the He’ concentration. From this result, it is clear that the ionic
species responsible for the CO’(A-X, B-X) emission is He*, and multiple
collision processes of He” are unimportant under the present experimental
conditions.
The dependence of emission intensities on the sample gas pressure was
measured in order to determine the number of target molecules which partic-
ipate in the reaction. Figure 6 shows the results for OCS’(A-%, CO’@--X)

281
0 0.5 1.0
z CO’(A-X)
5 / 0
CS(A-X)
0 2 4 6
[He*] OCS pressure ( 10m3 Torr)
Fig. 5. Dependence of emission intensities of CO+(A-X; 380 nm) and CO*(B-X, 230 nm)
on the relative He+ concentration. Nt(C-X, 198 nm) emission was measured % a monitor
of [He+].
Fig. 6. Dependence of emission intensities of OCS’(z-2; 395 nm), CO+(A-X; 380 nm)
and CS(A-X; 258 nm) on the sample gas pressure.
and CS(A-X). Since the band intensities of OCS+@-2) and CO’(A-X) are
linearly proportional to the sample gas pressure, only one target molecule
participates in the reactions in the present pressure range.
Summarizing all the results described above, it was concluded that
OCS*(2i-2) and CO+(A-X, B-X) emission result from the following pro-
cesses:
He(23S) + OCS + OCS’(2) + He + e- Penning ionization (1)
OCS+(A) -+ OCS+(-%) + hu
He+ + OCS + CO’(A) + S + He
I
Dissociative charge-transfer
+ CO*(B) + S + He ionization
CO+(A) + CO+(X) + hv
CO+(B) 4 CO+(X) + hv
(2)
In reaction (2), sulfur atoms in the 3P, ‘D and ‘S states are energetically
possible as the partner of CO’(A), while those in the 3P and ‘D states are pos-
sible as the partner of CO’(B). Since the formation of singlet and triplet
states of sulfur atoms satisfies the Wigner spin-conservation rule, neither
state can be excluded as possible candidates. Although the relative popula-

282
tion of sulfur atoms in each state cannot be determined in the present study,
the ‘S and ‘D states are most favorable for the formation of CO’(A) and
CO’(B), respectively, assuming that an energy-resonance requirement domi-
nates reaction (2).
Another dominant emissive product is CS(A). As shown in Fig. 3, the
dependence of the emission i@ensity of CS(A-X) on the grid potential is
different from that of OCS’(A--8) and CO+(A-X, B-X). Although the
intensity of CS(A-X) decreases with grid voltage, it levels off at a higher
positive or negative voltage. It is evident, therefore, that both neutral and
ionic species participate in the production of CS(A). Under the experimental
conditions used for the observation of the data given in Fig. 3, the contribu-
tion from ionic species is nearly the same as that from neutral species. How-
ever, the ratio depends significantly upon the experimental conditions,
mainly due to a change in the relative concentration of He’ and He(23S).
Figure 6 shows that the intensity of CS(A-X) increases non-linearly with the
sample gas pressure, and is proportional to about the 1.2 power of the pres-
sure. Secondary collision processes are therefore involved in the produc-
tion of CS(A ).
Ion/electron recombination processes play an important role in the pro-
duction of diatomic species from triatomic molecules in the helium after-
glow. Such examples are CO(a) from CO* [ 261, CS(A) from CS, [ 8b] and
OH(A) from Hz0 [ 271. The relative contribution of ion/electron recombina-
tion process(es) to the production of CS(A) from OCS was estimated by
adding an electron scavenger, SFB, to the helium afterglow. The emission
intensities from pure OCS and various mixtures of OCS/SF, ([SF,] /[OCS] =
0.1 - 0.5) were compared at identical experimental conditions. Since no sig-
nificant changes in the emission intensities of OCS’(1--8) and CO’(A-X)
were observed on addition of SF6, it is unlikely that He(23S) and Eie’ are
effectively quenched by SF+ Addition of SF, reduced the intensity of
CS(A-X) by a factor of 2 - 4 at grid voltages of 0 and 20 V. This result sug-
gests that ion/electron recombination process(es) are involved in the produc-
tion of CS(A) initiated by collisions with He(23.S) and He+, as follows
CS’ + e- --f CS(A)
OCS’ + e-+ CS(A) + 0
Combining the grid-voltage and pressure dependence of the emission
intensity with the additional effect of SF6, it seems that the mechanism of
the formation of CS(A) from OCS is rather complicated. Both He(23S) and
He’ are excitation sources of CS(A), and secondary collision processes
involving ion/electron recombination pathway(s) participate in the reaction.
Emission rates for the OCF(A-2) and CO’(A-X) systems
The emission rate constants for reactions (1) and (2), k(OCS, A) and
k(CO+, A), were measured with reference to the emission intensity of

283
N:(B-X) in various mixtures of OCS/N*. The rate constants were obtained
under experimental conditions such that the active species in the helium
flow were He(23S) and He‘. The following processes were used as the refer-
ence reactions.
He(2jS) + N2 + s(B, U’ = 0,l) + He + e- Penning ionization
N&3, u’ = 0, 1) + N:(X, u”) + hv
He’ + Nz + Nl(B, u’ = O-5) + He Charge-transfer ionization
N;(B, u’ = O-5) + N;(X, 0”) + hv
(3)
(4)
The rate constants were estimated by comparing the total emission intensity
of OCS*(?i-%) or CO’(A-X) writh that of N:(B-X). The N:(B-X) and
CO’(A-X) bands seriously overlapping with the other band systems and
those bands outside the region of the present experiment were calculated by
the use of available Franck-Condon factors [ 28]_ Since the rate constant for
the formation of N(B, U’ = 0, 1) through reaction (3) is 2.87 X lo-” cm3
molecule+ s-l 1291, we find k(OCS+, ?i) = 5.1 2 0.5 X lo-l3 cm3 molecule-’
S -I. The rate constant of reaction (4) has been measured using a pulsed ICR
cell with an optical detection device 1301. The upper limit has been esti-
mated at 9 X 10-l* cm3 molecule-’ s-l, and its probable value at about 4 X
lo-‘* cm3 molecule-’ s-l [30,31]. The N”,(B, u’ = O-5) vibrational distribu-
tion obtained fi-om the Au = -1 sequence in our flowing afterglow apparatus
was consistent with the ICR data except for U’ = 0 and 2 1321. Then, assum-
ing that the rate constant for formation of N:(B, u’ = O-5) in our flowing
afterglow experiment coincides with that in the ICR method, the upper limit
of k(CO+, A) was estimated at 2 X lo-l1 cm3 molecule-’ s-l, and its probable
value at about 9 X lo-‘* cm3 molecule-’ s-r. Dividing the emission rate con-
stants by the relative average velocity of the reactant particles in the-rmal
equilibrium with the flowing gas, the corresponding emission cross-sections
for OCS’(2) and CO’(A) are about 0.04 and 0.70 A*, respectively.
The Penning ionization cross-section for the emitting OCS’(A) state is
relatively small. However, it is about two orders of magnitude larger than the
fluorescence cross-section of 0.65 Mb(= lO_” cm*) obtained by photoioniza-
tion at h = 637 A (19.5 eV) [ 111. In the present study, no emission from
vibrational levels higher than v’ = 1 of OCS’(2) was found, as in the case of
photoionization [ 111. The absence of fluorescence from the u’ Z 1 levels can
be attributed to predissociation into CO + S* [ll J. According to the photo-
electron-photoion coincidence study of Eland [33], only lo-20% at most
of the OCS’(?I) ions initially in the u’ = 0 level fluoresce, and the rest predis-
sociate into CO + S’. Thus, it seems that the smallness of the emission cross-
section on Penning ionization is due mainly to competition of fast predis-
sociation in OCS+(A) for vibrational levels above u’ = 0.
Charge transfer from rare-gas atoms to small molecules at thermal energies
has been studied by use of an ICR cell [34-361, a selected ion flow tube

284
[37] and a flowing afterglow apparatus 1381. Unfo~na~ly, no ~fo~ation
has been reported for the thermal charge-transfer reaction of OCS. However,
the reported product distribution for thermal charge-transfer ionization of
He’ with such triatomic molecules as Hz0 1311, CO, [ 35) and N,O [35]
have shown that t.he dominant reaction channels are dissociative ionization,
giving rise to the diatomic ions OH+, CO’ and N:. Optical spectroscopic
studies for these systems by ICR and flowing afterglow methods [5,6,31,39]
have verified that some of the diatomic-ion products are present in the emit-
ting excited states. From the present study on the He’/OCS reaction, the
dominant emitting products are also found to be diatomic ions such as
CO’(A, B) and C%*(B), produced by competitive dissociative ionization. No
evidence of He+/OCS charge-transfer ionization leading to OCS’(A) can be
detected.
Thermal charge-transfer reactions between rare-gas ions and small mole-
cules have been discussed in terms of energy resonance and Franck-Condon
factors. Laudenslager et al. [ 343 have concluded that charge-transfer ioniza-
tion is favored by the existence of resonance levels, particularly those having
favorable Franek-Condon factors. As described above, the main channels
for the thermal charge-transfer ionization of He+ with triatomic molecules
are dissociative ionization_ Dissociative ionization is more favorable than
charge-transfer to parent molecular ion states on the basis of the energy-
resonance requirement. However, the latter channels probably occur with
higher probability due to their Franck-Condon factors; most of the tri-
atomic molecules discussed here have Franck-Condon factors which are
favorable for ionization into low-lying parent molecular ion states, as seen
from photoelectron spectroscopic data 1401. The propensity for dissocia-
tive-ionization channels suggests that the energy-resonance requirement takes
precedence over the. Franck-Condon factors in reactions between thermal
He* ions and triatomic molecules. As a second requirement, Franck-Condon
factors or distorted ones may contribute to the electronic and vibrationaldistri-
bution of the diatomic ions produced through near-resonant processes. How-
ever, since little is known about the production and relaxation processes of ti-
atomic molecular ion states in the 20-25 eV range, futher information is
required to estimate the relative contribution of Franck-Condon factors.
In this paper, we report the reactions of OCS in a helium afterglow. Car-
bony1 sulfide exhibited weak photoemission from the parent molecular ion,
OCS”(A-X), and relatively intense photoemission from fragment species
such as CS(A-X, a-X), CS’(B-A), CO(a-X) and CO*(A-X, B-X). The
active species responsible were determined by a study of the dependence of
emission intensities on the grid voltage. The results obtained are summarized
in the last column of Table 1. An important finding is that He’ is a dominant
source for emission of excited products from OCS. Although it has been
pointed out that ionic species play a part in the production of excited spe-
cies in the helium afterglow, overlapping of intense photoemission due to
Penning ionization has often made the detailed analysis of the reaction pro-

285
cesses difficult. In the present study, ion/molecule reactions and Penning
ionization were separated for the first time by the use of an ion-collector
grid and synchronous photon-coun~g equipment. This new technique was
very useful for the measurement of emission rates. We hope to extend this
study to other di- and tri-atomic molecules to elucidate not only Penning
ionization but also charge-transfer ionization at thermal energies, where
beam experiments cannot be used.
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