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Emission spectra produced from the reactions of Ne+ and Ar+ ions with OCS at thermal energy
Volume
100. number
E&fISSION SPECTRA PRODUCED WITH
OCS
30 September 1983
CHEWCAL PHYSICS LETTERS
6
AT TiIERhfAL
FROM THE REACTIONS
OF Ne+ AND Ar+ IONS
ENERGY
Hiroshi SEKIYA. Masaharu TSUJI and Yukio NISHIMURA h’escarch Institute of Industrial Science and Department of Molecular Science mid Technology. GradtrareScImol of Engineer&g Sciences. E;rusItu Unil ersil_r,h-amga-slri.Fukuoka 816. Japan I;cccived 30 June 1983:in
find
form
15 Jul?
1983
from the Ne’+ OCS reacrion is identified in a Ne afterglow,\\‘hile The CO+(A ‘n-x 2x+ ) emission system produced the OCS’(~ ‘Il-% 2tl) emiv+ion system rcwiting from the Ar+ + OCS reaction isobserved in an Ar afterglow. An anomalous population disrriburion is found between thr ‘IlIE and ‘IlxR sp-m-orbit states of 0CS’[X2~j(OOO)].
I_ introduction A number of papers have appeared recently on the emission spectra of molecular ions resulting from charge-transfer (CT) reactions between rare-n,as ions and neutral molecules at (near) thermal energy [ 121. Ilowever. most of the observed emission spectra of molecular ions were produced from the reactions with He* ion [2] _ No study has been made by optical spectroscopy on reactions with Ne+ and Ar+ ions escept for those of Ar+ -+ Hz0 and Ne‘ + H-,0 reactions [; A]_ whereas total CT rate cons&s and branching ratios for various e-tit channels have been determined by flowing afterglow [5] and by ion cyclotron resonance (ICR) [3.4,6-9]_ In general, Ne* and Xr+ CT reactions proceed quite slowly unlike 1le+ CT reactions [9]. Accordingly, the emissions of rhe product molecular ions are expected to be either weak or hardly observable. In this communication, we describe the emission spectra resulting from the reactions of Ne’, Ar+, and He+ ions with OCS in a flo\\ing afterglow. where characteristic features are found in the OC.C?(z z 11-g ? n) emission spectrum from rhe Ar+ + OCS reaction_
2. Esperimental The flowing afterglow apparatus used is essentially 494
the same as that described previously [lo] _The flow was maintained by a 10000 Q/min mechanical booster pump_ Active species of Ne, Ar, and He were produced in a 2450 MHz microwave discharge (70 iv). In order to study the effect of ionic species, ion-collector grids were installed between the discharge section and the reaction zone_ The pressure in rhe reaction zone was measured by an IMKS Baratron gauge. The Ne and Ar pressures ranged from 600 to 700 mTorr. The sample gas pressure was controlled to 30-40 mTorr by a variable leak valve _The OCS gas (Matheson, >97_5% purity) was admixed to the Ne, Ar, and He flows at = 11 cm downstream of the discharge section. Emission spectra in the 200-750 run range were observed through a quartz window with aNippon JarrellAsh M2 monochromator equipped with a cooled HTV R376 photomultiplier.
3. Results and discussion Fk. 1 a shows a typical emission spectrum in the Ne afterglow obtained without an electrostatic potential on the grids. The spectrum consists of characterisitc doublet bands due to the CO+(A ?fI-X “Z’) system and to the OCS+(x lIl-2 ‘11) system. By application of a 20 V dc electrostatic potential to the grid, the CO+(A 2fl-X z Z+) system disappeared completely, while the OCS+(Z 211-2 ‘fl) system remain0 009-2614/83/0000-0000/S
03.00 0 1983 North-Holland
CHEMICAL
Volume 100, number 6
30 September
PHYSICS LETTERS
1983
(a)
(b)
350
400
450
500 (nm)
Fig. 1. Ilmission spectra in the 360-520 nm region obtained from the (a) Nc(~P 0,~) and NC++ OCS and (b) Ne(*Po,*) + OCS rrxtions. Lines marked with asterisks are stray NeI lines. The emission bands around 380 nm in (b) are unidentified bands, which heavily overlap with the (4.0) hand of the CO+(A-X) emission system in (a).
ed as shown in fig_ lb. This suggests that the OCS’(x 2fI-% ‘11) emission system is produced from Ne (3Po ,) Penning ionization [ 1 l] , while the CO+(A zn-X 2.IZ’) emission system is produced by the reaction between theionic species of Ne and OCS. The energy level diagram of various possible products from OCS [ 12-161 is shown in fig. 2 along with the
ocs-
I
co‘*s
CO*S(M
cs*o
CS*o(3P)
nr’
energy levels of the possible active species in the Ne, Ar, and He afterglow [ 17]_ Only Ne+ ion can produce the CO+(A 2fl-X ?I?) system as a result of the following dissociative CT reaction: Ne+ + OCS + CO+(A “II) + S + Ne _
(1)
TO our knowledge, there is no observation of an emission spectrum produced from a Ne+ CT reaction at thermal energy, except for a remark by Govers et al. [6] on the existence of an emission due to the Ne+ + Hz0 reaction in an ICR cell at (near) thermal energy.
In the reaction of He+ t OCS, the CO+(A ‘IIX 2F) emission is identified up to u’ = 11 [lo] _ On the other hand, the identified vibrational levels are limited up to u’ = 5 in the Ne+ + OCS reaction_ If we assume that the sulfur atom in the ground 3P stateis
Fii_ 2. Energy level diagram of possible products in the flowing afterglow reactions.
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CHEMICAL
PHYSICS
formed as the partner of CO+(A Zll) in reaction (l), a vibrational distribution of CO+(A ‘11) up to u’ = 9 is energetically possible_ In contrast, when the excited sulfur atom in the 1D state is produced. the vibration of CO+(A 2 Tl) can be excited up to u’ = 4. On this basis. the detection of CO+(A ZII-X ?Zi) emission from the u’ = 5 level suggests that the sulfur atom in the ground state must be involved in reaction (1). In the Ne afterglow, the CS(A 2 II-X 1 S+) and CO(a 311-X Is+) emission systems are also observed in the 190-250 run region. The CS(A 1 n--X 1Z+) emission is found to be produced from the Ne(3P0,2) + OCS reaction [ 1 I], whereas an ion-electron recombiuation process may be involved in the fomiation of the CO(a “II) state, as conlinned in the He afterglow IlO]. The reaction in the Ar afterglow gives emission composed of weak OCS+(i ?ll-R ‘11) parent ion fluorescence, as shown in fig. 3, as well as moderately strong CO(d 3&-a311).weak CO(a”C--a 311). and strong CS(A ‘II-X r S’) fluorescences due to dissociative excitation_ The observation of CO(a’ sZf, d 3A.e 35---ajil) and CS(A ill--X lEi) fluorescences is consistent with the result of Gundel et al. [IS] _The OCS+(x 2D-z ‘II) emission system has not been detected in any earlier studies. It arises from the reaction with active ionic species of Ar, because it was eliminated by an application of 20 V dc electrostatic potential to the grid. Possible candidates for the ionic species are Ar+ ion and/or AI-“” ions [19.20], which satisfy the energetic requirement. Suzuki et al. [20] reported that the existence of Ati”+ ions could be monitored
ocski -2)
350
-kOOh-N
Fig_ 3. frnieion s?ectrrl in the 320-410 nm region obtained from the Ar+~nd Ar(31’o 2) + OCS reactions. Lines marked with
496
open
cirdes
are .str& Ari lint%.
30 September
LETTERS
1983
by observing the CH(A Z~-X ‘ll) emission resulting from the reactions between fl+ ions and CH,CN; the recombination energies of Ar+ ions are insufficient to produce the CH(A?A) state. By referring to their results; we have examined the existence of the CH(AZ&-X2il) emission from CH3CN under the conditions of the occurrence of the CkS ‘(K Zn-2 2n) emission. i-lowever, no CH(A ?A-X ‘Tl) emission has been observed, which leads to the conclusion that the observed OCS+(x %---X Z TI) emission is produced from the following near-resonant CT reactions:
Ar+CZP,,2) -t OCS --f OCS+ [x ZII(OOO)] i- Ar + 0.86 eV , Ar+(‘P,,?)
(2)
+ ocs
--f OCS+ [x 2rl(OOO)] + Ar + 0.68 eV _
(3)
The OCS~(~ill-~ ‘fl) emission system was vibrationally analyzed by Horani et al. [2 l] _The absence of fluorescence from the u’ L 1 level of the xi’11 state was interpreted as predissociation of these levels [22] _ One of the remarkable features of the 0CS+(x211? ‘ll) emission spectrum, shown in fig_ 4, is an extensive enhancement of the R = l/2 band compared to the Q = 3/2 band. In the OCS+[i Zfl(OOO)R zll(OO2)] emission produced from the Ar+ + OCS reaction, the relative intensity ratio of the spin-orbit states. I(R = 1/2)//(sL= 3/2), is 1.5, while those from the He (3 S) + OCS and Ne (3Po 2) + OCS reactions are 0.5 and 0.7, respectively_ Judge and Lee [22] reported in the photoionization of OCS that the cross section for the production of fluorescence from the level was only 75% of that from the x ‘l13,z xQ,,z level, which was attributed to a larger predissociation rate of the x ZI1,,, level than tha_t of the AZn,j2 level. The lifetimes of the OCS+(A 211,-X ‘KI,; Q = l/2 and 3/2) transitions [23] are aho compatible with the above results on the photoionization of OCS. These findings suggest that the spectrum obtained from the Are + OCS reaction is remarkable, since the population distribution between the two spin-orbit states seems to be far from an apparent equilibrium_ The upper spin-orbit state is favorably produced in the Ar+ + OCS reaction despite the small separation of the two spin-orbit states (0.014 eV) [22] in comparison with the excess energies in reactions (2) and (3).
s
CHEMICAL
Volume 100. number 6
ocs+cxrrntooo,
--3i?hltoota
A 1
i
I
l-l;312 [bl
t
I
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370
1983
distance energy-sonant process and the other is a short-distance non-resonant pmcess. In the resonant process, electron jump without momentum transfer has been observed, whereas in the non-resonant process electron jump with momentum transfer leading to 1-Z eV kinetic energy release has been observed. A number of examples of the resonant processes have been shown in the He+ and Nei CT reactions by the experiments on emission spectra and kinetic energy release. However, non-resonant CT reactions have been found in the Ar+ + Hz0 [36] ~Ar+ + NH, [S], and Are f- 02 [6,7] reactions. In the Ar+ + OCS reaction, the observation of the OCS+(?i ?ll-z 211) emission system suggests that Fe 0CS’(X211) state is populated via an energetically near-resonant CT process, became the kinetic energies of the OCS* ions produced in the x ‘II state are expected to be smaller than 036eV.Thisvalue hasbeenestimated on the assumption that the excess energy of reaction (2) is converted exclusively to the kinetic energies of OCSi ion and Ar. Another prominent feature of the OCS+(x 2nj;: ‘II) emission spectrum in the Ar* + OCS reaction is the broadening of the vibronic bands as compared with those produced from the He(2 3S) Penning ionization, as shown in fig_4; the full width at halfmaximum (fwhm) of the emission bands in spectrum (a)
R=l/Z Cl=% (a)
30 September
PHYSICS LETTERS
I
360 (nm)
F&L 4. Expanded emission spectra of OCS+ [x * n~(O00)obtained from the (a) Ar* + OCS and (b) He (2 3s) + OCS reactions.
g *nn(007):i2= l/2 and 3/2]
It was shown by Marx f l] that two different processes might occur in thermal energy CT reactions of rare gas ions with small molecules. One is a long-
Table 1 Identified emission systems from OCS Emittins species
Electronic transition
Vibrational level (u’)
Excitation source
ocs co
x*n-z*n aJrI-x lx+ a’3n-a3n d5A-a3n e32.--a3n
0
Ne(3Po,2),
0.1 a), 0 b) S-11 1-7 o-4
-w3Po,2) IQ (3 PO.2 ) *(%J,,-)
co+
A*n-_X’z+ B2_v+-X2Z+
o-5c),o-lld) o--3
Ne+, He+ He+
cse)
A'll--X'S+
O-5
Ne(3Po,2),Ar(3Po,2)
Ar+, He(2 3S)
He(Z3S),He+
cs+
B*x+-__A2n
O-6
a) Observed in the Ne afterglow_ b) Observed in the He after.alow_ c) Observed in the Nr+ + OCS reaction. d) Observed in the He* + OCS reaction_ e) In the He afterglow, secondary collision processes are involved k the production
He+
of the CS(A) state.
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Volume 100, number 6
CHEhfICAL PHYSICS LETTERS
about twice as large as that in spectrum (b)_ Since the optical resolusions used for the measurements are identical for spectra (a) and (b), the broadening of the bands must be due to the rotational excitation_
is
The entission systems from OCS observed in the Ne, Ar, and He afterglows are sununatized in table 1. The emission systems identified in the He afterglow are the same as that reported previously [IO]. It is seeenfrom table 1 that the dominant emitting exit channel in the Nei + OCS and He* + OCS reactions is dissociative charge transfer, while that in the or+ + OCS reaction is charge transfer. It is possible to employ OCS as a monitof of thermal enegg Ne+’ and Ar+ ions through observation of the CO+(A ‘llX ‘E+) and OCS*(x ?Tl- 2 ‘11) emission systems, respectively.
Acknowledgement
The authors are indebted to H. Obase and &I. Endoh for valuable discussions. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education. Science and Culture.
References
111 K. Mar\. in: Kinetics of ion molecule
rextion, ed. I’. Au\looz (l’lcnum Press. Sew York, 1979) p. 103. I’1 31. Tsuji and Y. Nishimura. Bunko Kcnkyu 13. Spectrosc. SW. Japan] 32 (19SS) 77, and references therein. Goxcrs. R. Mar\. 131 R. Derai. S. Fenistrin, .\I. Gfrard.T.R. G.hlsucIsire. C-2. Profous and C. Souricsxm, Chem. l’hys. 44 (1979) 65.
49s
30 September 1983
[4] T-R. Govers, hf. G&xl, G. hfsuclaire and R. hfarx, Ber. Bunsenaes Physik. Chem. 81 (1977) 181_ [S] R.S. Hemsworth, R-C. Bolden, hfi. Shw and N.D. Twiddy,Cbem. Phys. Letters5 (1970) 337;J. Phys. B3 (1970) 45. [6] R. Marx, G. Mauclaire. T.R. Govers, hf. Girard. S. Fenistein and R. Derai. J. Chim. Phys. 76 (1979) 1077. (71 G. hfauclaire, R. Derai, S. Fenistein and R. hfarx, J_ Chem. Phys. 70 (1979) 4017. R. Derai,G. hfauclaire and R. hfarx, Chem. Phys. Letters 86 (1981) 275. J-B. Laudenslger. W-T. Huntress Jr. and M.T. Bowers, J. Chem. Phys. 61 (1974) 4600. 81. Tsuji, hf. hfatsuo and Y. Nishimura. Intern. J. hfass Spectrom. Ion Phys. 34 (1980) 373. hi _ Tsuji, hf. Endoh, l-l. Obasz and Y. Nisbimura, The Reports of Research Institute of Industrial Science, Kyusbu Universitv _ 74 (1983) 109. _ 1121K-P. Huber and G. Herzberg, Constants of diatomic molecules (Van Nostrand, Princeton, 1979). 1131 R. Frey, B. Gotchev, W.B. Peatman, H. Pollak and 11.W. Sclkg. Intern. J. hfass Spectrom. Ion Phys. 26 (1978) 137. 1141 P.H. Krupcnic, NSRDS Natl. Bur. Stand. 5 (1966). and hf.Horani.Can.J.Pbys.56 (1978)587. iiS1 D.Gauyscq 1161 N. Jonatlmn, A. hforris. hl. Okuda, KJ_ Rossand DJ. Smith. 1:amday Diwrssions Chern. Sot. 54 (1972) 48. [ 17 1 C.ll.~loor~.Atonlic~nc~y Icvcls, U.S. N.itl. Bur. Stnnd. Circuhr367 (1919). [ 161 L. Gundel. D-W. Setscr, h1.A.A. Clyne, J.A. Coxon and W. Nip. J. Chem. Phys. 64 (1976) 4390. [IV] T. hfrtsuo, N. Kob+asbi -and Y. Kaneko. J. Phys. Sot. JapJn50 (1981) 3481;51 (1982) 1558. [20] I(. Suzuki and K. Kucbitsu. Bull. Chem. Sot. Japan 50 (1977) 1905; 1. Ni\bi~am.r. Y. Ozlihi. Ki. Suzuki and K. liucbitsu. Chem. Phys. Letters 67 (1979) 258. 1211 .\l.Horsni.S.Leacl~,J.RostasandG.Berthier,J.Chim. Phys. 63 (1966) 1015. [El D.L. Judge and L.C. Lee, Intern. J. hfsss Sptctrom. Ion. Phyr. 17 (1975) 329. 1231 1’. Lrman and hf. Larsxon. Pbysica Scripta 33 (1981) 1051.
100. number
E&fISSION SPECTRA PRODUCED WITH
OCS
30 September 1983
CHEWCAL PHYSICS LETTERS
6
AT TiIERhfAL
FROM THE REACTIONS
OF Ne+ AND Ar+ IONS
ENERGY
Hiroshi SEKIYA. Masaharu TSUJI and Yukio NISHIMURA h’escarch Institute of Industrial Science and Department of Molecular Science mid Technology. GradtrareScImol of Engineer&g Sciences. E;rusItu Unil ersil_r,h-amga-slri.Fukuoka 816. Japan I;cccived 30 June 1983:in
find
form
15 Jul?
1983
from the Ne’+ OCS reacrion is identified in a Ne afterglow,\\‘hile The CO+(A ‘n-x 2x+ ) emission system produced the OCS’(~ ‘Il-% 2tl) emiv+ion system rcwiting from the Ar+ + OCS reaction isobserved in an Ar afterglow. An anomalous population disrriburion is found between thr ‘IlIE and ‘IlxR sp-m-orbit states of 0CS’[X2~j(OOO)].
I_ introduction A number of papers have appeared recently on the emission spectra of molecular ions resulting from charge-transfer (CT) reactions between rare-n,as ions and neutral molecules at (near) thermal energy [ 121. Ilowever. most of the observed emission spectra of molecular ions were produced from the reactions with He* ion [2] _ No study has been made by optical spectroscopy on reactions with Ne+ and Ar+ ions escept for those of Ar+ -+ Hz0 and Ne‘ + H-,0 reactions [; A]_ whereas total CT rate cons&s and branching ratios for various e-tit channels have been determined by flowing afterglow [5] and by ion cyclotron resonance (ICR) [3.4,6-9]_ In general, Ne* and Xr+ CT reactions proceed quite slowly unlike 1le+ CT reactions [9]. Accordingly, the emissions of rhe product molecular ions are expected to be either weak or hardly observable. In this communication, we describe the emission spectra resulting from the reactions of Ne’, Ar+, and He+ ions with OCS in a flo\\ing afterglow. where characteristic features are found in the OC.C?(z z 11-g ? n) emission spectrum from rhe Ar+ + OCS reaction_
2. Esperimental The flowing afterglow apparatus used is essentially 494
the same as that described previously [lo] _The flow was maintained by a 10000 Q/min mechanical booster pump_ Active species of Ne, Ar, and He were produced in a 2450 MHz microwave discharge (70 iv). In order to study the effect of ionic species, ion-collector grids were installed between the discharge section and the reaction zone_ The pressure in rhe reaction zone was measured by an IMKS Baratron gauge. The Ne and Ar pressures ranged from 600 to 700 mTorr. The sample gas pressure was controlled to 30-40 mTorr by a variable leak valve _The OCS gas (Matheson, >97_5% purity) was admixed to the Ne, Ar, and He flows at = 11 cm downstream of the discharge section. Emission spectra in the 200-750 run range were observed through a quartz window with aNippon JarrellAsh M2 monochromator equipped with a cooled HTV R376 photomultiplier.
3. Results and discussion Fk. 1 a shows a typical emission spectrum in the Ne afterglow obtained without an electrostatic potential on the grids. The spectrum consists of characterisitc doublet bands due to the CO+(A ?fI-X “Z’) system and to the OCS+(x lIl-2 ‘11) system. By application of a 20 V dc electrostatic potential to the grid, the CO+(A 2fl-X z Z+) system disappeared completely, while the OCS+(Z 211-2 ‘fl) system remain0 009-2614/83/0000-0000/S
03.00 0 1983 North-Holland
CHEMICAL
Volume 100, number 6
30 September
PHYSICS LETTERS
1983
(a)
(b)
350
400
450
500 (nm)
Fig. 1. Ilmission spectra in the 360-520 nm region obtained from the (a) Nc(~P 0,~) and NC++ OCS and (b) Ne(*Po,*) + OCS rrxtions. Lines marked with asterisks are stray NeI lines. The emission bands around 380 nm in (b) are unidentified bands, which heavily overlap with the (4.0) hand of the CO+(A-X) emission system in (a).
ed as shown in fig_ lb. This suggests that the OCS’(x 2fI-% ‘11) emission system is produced from Ne (3Po ,) Penning ionization [ 1 l] , while the CO+(A zn-X 2.IZ’) emission system is produced by the reaction between theionic species of Ne and OCS. The energy level diagram of various possible products from OCS [ 12-161 is shown in fig. 2 along with the
ocs-
I
co‘*s
CO*S(M
cs*o
CS*o(3P)
nr’
energy levels of the possible active species in the Ne, Ar, and He afterglow [ 17]_ Only Ne+ ion can produce the CO+(A 2fl-X ?I?) system as a result of the following dissociative CT reaction: Ne+ + OCS + CO+(A “II) + S + Ne _
(1)
TO our knowledge, there is no observation of an emission spectrum produced from a Ne+ CT reaction at thermal energy, except for a remark by Govers et al. [6] on the existence of an emission due to the Ne+ + Hz0 reaction in an ICR cell at (near) thermal energy.
In the reaction of He+ t OCS, the CO+(A ‘IIX 2F) emission is identified up to u’ = 11 [lo] _ On the other hand, the identified vibrational levels are limited up to u’ = 5 in the Ne+ + OCS reaction_ If we assume that the sulfur atom in the ground 3P stateis
Fii_ 2. Energy level diagram of possible products in the flowing afterglow reactions.
495
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100. number 6
CHEMICAL
PHYSICS
formed as the partner of CO+(A Zll) in reaction (l), a vibrational distribution of CO+(A ‘11) up to u’ = 9 is energetically possible_ In contrast, when the excited sulfur atom in the 1D state is produced. the vibration of CO+(A 2 Tl) can be excited up to u’ = 4. On this basis. the detection of CO+(A ZII-X ?Zi) emission from the u’ = 5 level suggests that the sulfur atom in the ground state must be involved in reaction (1). In the Ne afterglow, the CS(A 2 II-X 1 S+) and CO(a 311-X Is+) emission systems are also observed in the 190-250 run region. The CS(A 1 n--X 1Z+) emission is found to be produced from the Ne(3P0,2) + OCS reaction [ 1 I], whereas an ion-electron recombiuation process may be involved in the fomiation of the CO(a “II) state, as conlinned in the He afterglow IlO]. The reaction in the Ar afterglow gives emission composed of weak OCS+(i ?ll-R ‘11) parent ion fluorescence, as shown in fig. 3, as well as moderately strong CO(d 3&-a311).weak CO(a”C--a 311). and strong CS(A ‘II-X r S’) fluorescences due to dissociative excitation_ The observation of CO(a’ sZf, d 3A.e 35---ajil) and CS(A ill--X lEi) fluorescences is consistent with the result of Gundel et al. [IS] _The OCS+(x 2D-z ‘II) emission system has not been detected in any earlier studies. It arises from the reaction with active ionic species of Ar, because it was eliminated by an application of 20 V dc electrostatic potential to the grid. Possible candidates for the ionic species are Ar+ ion and/or AI-“” ions [19.20], which satisfy the energetic requirement. Suzuki et al. [20] reported that the existence of Ati”+ ions could be monitored
ocski -2)
350
-kOOh-N
Fig_ 3. frnieion s?ectrrl in the 320-410 nm region obtained from the Ar+~nd Ar(31’o 2) + OCS reactions. Lines marked with
496
open
cirdes
are .str& Ari lint%.
30 September
LETTERS
1983
by observing the CH(A Z~-X ‘ll) emission resulting from the reactions between fl+ ions and CH,CN; the recombination energies of Ar+ ions are insufficient to produce the CH(A?A) state. By referring to their results; we have examined the existence of the CH(AZ&-X2il) emission from CH3CN under the conditions of the occurrence of the CkS ‘(K Zn-2 2n) emission. i-lowever, no CH(A ?A-X ‘Tl) emission has been observed, which leads to the conclusion that the observed OCS+(x %---X Z TI) emission is produced from the following near-resonant CT reactions:
Ar+CZP,,2) -t OCS --f OCS+ [x ZII(OOO)] i- Ar + 0.86 eV , Ar+(‘P,,?)
(2)
+ ocs
--f OCS+ [x 2rl(OOO)] + Ar + 0.68 eV _
(3)
The OCS~(~ill-~ ‘fl) emission system was vibrationally analyzed by Horani et al. [2 l] _The absence of fluorescence from the u’ L 1 level of the xi’11 state was interpreted as predissociation of these levels [22] _ One of the remarkable features of the 0CS+(x211? ‘ll) emission spectrum, shown in fig_ 4, is an extensive enhancement of the R = l/2 band compared to the Q = 3/2 band. In the OCS+[i Zfl(OOO)R zll(OO2)] emission produced from the Ar+ + OCS reaction, the relative intensity ratio of the spin-orbit states. I(R = 1/2)//(sL= 3/2), is 1.5, while those from the He (3 S) + OCS and Ne (3Po 2) + OCS reactions are 0.5 and 0.7, respectively_ Judge and Lee [22] reported in the photoionization of OCS that the cross section for the production of fluorescence from the level was only 75% of that from the x ‘l13,z xQ,,z level, which was attributed to a larger predissociation rate of the x ZI1,,, level than tha_t of the AZn,j2 level. The lifetimes of the OCS+(A 211,-X ‘KI,; Q = l/2 and 3/2) transitions [23] are aho compatible with the above results on the photoionization of OCS. These findings suggest that the spectrum obtained from the Are + OCS reaction is remarkable, since the population distribution between the two spin-orbit states seems to be far from an apparent equilibrium_ The upper spin-orbit state is favorably produced in the Ar+ + OCS reaction despite the small separation of the two spin-orbit states (0.014 eV) [22] in comparison with the excess energies in reactions (2) and (3).
s
CHEMICAL
Volume 100. number 6
ocs+cxrrntooo,
--3i?hltoota
A 1
i
I
l-l;312 [bl
t
I
360
370
1983
distance energy-sonant process and the other is a short-distance non-resonant pmcess. In the resonant process, electron jump without momentum transfer has been observed, whereas in the non-resonant process electron jump with momentum transfer leading to 1-Z eV kinetic energy release has been observed. A number of examples of the resonant processes have been shown in the He+ and Nei CT reactions by the experiments on emission spectra and kinetic energy release. However, non-resonant CT reactions have been found in the Ar+ + Hz0 [36] ~Ar+ + NH, [S], and Are f- 02 [6,7] reactions. In the Ar+ + OCS reaction, the observation of the OCS+(?i ?ll-z 211) emission system suggests that Fe 0CS’(X211) state is populated via an energetically near-resonant CT process, became the kinetic energies of the OCS* ions produced in the x ‘II state are expected to be smaller than 036eV.Thisvalue hasbeenestimated on the assumption that the excess energy of reaction (2) is converted exclusively to the kinetic energies of OCSi ion and Ar. Another prominent feature of the OCS+(x 2nj;: ‘II) emission spectrum in the Ar* + OCS reaction is the broadening of the vibronic bands as compared with those produced from the He(2 3S) Penning ionization, as shown in fig_4; the full width at halfmaximum (fwhm) of the emission bands in spectrum (a)
R=l/Z Cl=% (a)
30 September
PHYSICS LETTERS
I
360 (nm)
F&L 4. Expanded emission spectra of OCS+ [x * n~(O00)obtained from the (a) Ar* + OCS and (b) He (2 3s) + OCS reactions.
g *nn(007):i2= l/2 and 3/2]
It was shown by Marx f l] that two different processes might occur in thermal energy CT reactions of rare gas ions with small molecules. One is a long-
Table 1 Identified emission systems from OCS Emittins species
Electronic transition
Vibrational level (u’)
Excitation source
ocs co
x*n-z*n aJrI-x lx+ a’3n-a3n d5A-a3n e32.--a3n
0
Ne(3Po,2),
0.1 a), 0 b) S-11 1-7 o-4
-w3Po,2) IQ (3 PO.2 ) *(%J,,-)
co+
A*n-_X’z+ B2_v+-X2Z+
o-5c),o-lld) o--3
Ne+, He+ He+
cse)
A'll--X'S+
O-5
Ne(3Po,2),Ar(3Po,2)
Ar+, He(2 3S)
He(Z3S),He+
cs+
B*x+-__A2n
O-6
a) Observed in the Ne afterglow_ b) Observed in the He after.alow_ c) Observed in the Nr+ + OCS reaction. d) Observed in the He* + OCS reaction_ e) In the He afterglow, secondary collision processes are involved k the production
He+
of the CS(A) state.
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CHEhfICAL PHYSICS LETTERS
about twice as large as that in spectrum (b)_ Since the optical resolusions used for the measurements are identical for spectra (a) and (b), the broadening of the bands must be due to the rotational excitation_
is
The entission systems from OCS observed in the Ne, Ar, and He afterglows are sununatized in table 1. The emission systems identified in the He afterglow are the same as that reported previously [IO]. It is seeenfrom table 1 that the dominant emitting exit channel in the Nei + OCS and He* + OCS reactions is dissociative charge transfer, while that in the or+ + OCS reaction is charge transfer. It is possible to employ OCS as a monitof of thermal enegg Ne+’ and Ar+ ions through observation of the CO+(A ‘llX ‘E+) and OCS*(x ?Tl- 2 ‘11) emission systems, respectively.
Acknowledgement
The authors are indebted to H. Obase and &I. Endoh for valuable discussions. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education. Science and Culture.
References
111 K. Mar\. in: Kinetics of ion molecule
rextion, ed. I’. Au\looz (l’lcnum Press. Sew York, 1979) p. 103. I’1 31. Tsuji and Y. Nishimura. Bunko Kcnkyu 13. Spectrosc. SW. Japan] 32 (19SS) 77, and references therein. Goxcrs. R. Mar\. 131 R. Derai. S. Fenistrin, .\I. Gfrard.T.R. G.hlsucIsire. C-2. Profous and C. Souricsxm, Chem. l’hys. 44 (1979) 65.
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