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SO+(A2Π-X2Πr) emission produced from a dissociative charge-transfer reaction of He+ with SO2 at thermal energy
Volume 73, number 3
SO+ (A *II-X REACTION
1 August 1980
CHEMICAL PHYSICS LE-ITERS
*I$)
EMISSION
PRODUCED
FROM A DISSOCIATWE
OF He+ WJTH SO2 AT THERMAL
CHARGE-TRANSFER
ENERGY
Masaharu TSUJI, Ch.ikash_~ YAMAGIWA, Minoru ENDOH and Yukio NISHIMURA Research Inshtute of Industrial Sczence. Kyushu University 86, Higasht-ku. Fukuoka 812. Japan Recewed 27 May 1980
The extensive bands observed from the hehum afterglow reaction of SO* m the 250-540 mn region are assigned to the new SO+(A *rt--X*l$) system produced from the He*/S02 dissociative charge-transfer reaction at themA energy. They had been erroneously mterpreted as the SO; (c-4 system produced from He(2 3S)/S02 Penning ionization The spectroscopic constants for the SO+(A*n) and SO+(X*nJ states were determined_
1. Introduction
source is thermal-energy He+ ions involved iu the ciischarge flow.
In 1977, Wu and Yencha [l] observed a rather strong,
extensive emission band system in the 300550 run spectral range from a flowing helium afterglow reaction of SO*. The new band system has been ascribed to the EzA, -% 2A, transition of SO;, the band origin of which is located at 3339 A. According to the notation of Brion and Yee [2], this transition corresponds to the SOt(E2A1-z *AI) system. Wu and Yencha reported a long progession of symmetrical stretching vibrations (u; ,O, O)-(u'; ,0, 0) for O
incorrect. Here it is shown that the spectrum is due to the ion of sulfur monoxide, SO+, and its excitation
2. Experimental The flowing afterglow apparatus used was essentially the same as that reported elsewhere [4] _ The whole system was continuously evacuated by means of a 10000 Q/min booster pump. Active species of helium were generated by a 2450 MHz microwave discharge through a fast flow of helium. The relative concentrations of active species of helium flow, He(2 3S), He’ and He;, were estimated by separately observing $ (B-X) emission resulting from the reactions of He(2 %), He+ and He; with N2 151. Although the dommant active species at low helium pressures is He(2 %), the concentrations of He+ and He; increase with an increase in helium pressure. In order to examine the contribution of ionic species to the observed emission, an ion-collector grid was placed between the discharge section and the reaction zone. The satisfactory efficiency of the ion-collector grid was checked emission produced from the by monitoring $(C-X) He+/N2 near-resonant charge-transfer reaction. We found that the charged-particle deflector used in the previous work [3] did not have enough efficiency to trap ionic species. Target gases were injected through a nozzle (0.6 mm i-d.) 407
Volume
73, number
CHEMICAL
3
PHYSICS LETTERS
ca. 15 cm downstream from the disc-harge section. The pressures of helium gas and commercially obtamed sample gases (SO2 nortunal purity > 99.9% and SOCI, guaranteed reagent) varied from 0.4-0.6 Torr and l-5 mTorr, respecttvely, as measured by a MKS capacrtance manometer. Spectra in the 200-600 nm region were observed through a quartz window m front of the gas inlet with a JarreL-Ash 1 m spectrometer equipped with a 1200 groove/mm grating blazed at 300 MI. A HTV R376 photomultipLier and a Burr-Brown 3523J OP amp& fier were used for the photomet~c me~urements.
3. Results and discussion A typical emrssron spectrum of SO2 rn a hehum afterglow IS shown rn fig. la. In addrtion to wellknown SO(A-X) and 01 emission systems, a relatrvely strong, extensrve band system, most of which was believed to be SOS (E-2) ermssion resultmg from He(2 3S)/SO, Penning ionization, is observed m the 240-550 run regron. When active iomc species were removed from the helium flow, the dommant band
1 August 1980
completely disappeared as shown in Rg. lb. ‘ihis result indrcates that the excitation source is not He(2 3S), but a charged particle. Since this emission was observed at the heB~-pr~e range, where the domrnant charged particle was He*, and the He; concentratron was negligible, the charged particle responsible for the formation of the emitting species was concluded to be He+. The fmding that the excitation source was He+ prompted us to reexamine the previous assignme,“f of the observed emission, the formation of SO;(C) by the He+fSO, ch~ge-tr~sfer reaction is unfavorable, because the reactron is exothermic by as large as = 9 eV as is seen from energy relations (table 1). We have recently made optical spectroscopic studies of charge-transfer reactions of He+ with various triatomic molecules at thermal energy by using a flowing afterglow apparatus [4--81. Smce the dominant ermtting products are diatomic ions such as N$ (B) for hi20 [6], CO”(A,B) and CS(B) for OCS [4], CS+(B) for CSz [S] , CO* (A) for CO, [7] and OH* (A) for H,O [8], SOi emissron is probably responsrble for the observed emission resulting from the He+/SOZ charge-transfer reaction. Eland has recently predicted
(a)
250
300
350
400
450
560
500
tnmf
He’. so2 .
(b)
/
1
SOtA%-X't-)
250
300
350
Emissionspectra of SO2 in the 230450 * are stray He1 hnes
Fag. 1.
408
400
450
500
550
tnm)
nm region by colhssons with (a) He(2 ‘~3) + He+ and (b) He(2 3S)_ Lmes marked
Volume 73, number 3
CHEMICAL
1 August 1980
PHYSICS LETTERS
Table 1 Adabatic ionizatron potentials of SO and SO*, and calculated minimum energies required for formation of various SO and SO* states from dissociative ionization of SO* and SOCI~. The av;ulable energies of He(23S1 and He* are 19.82 and 24.58 eV. respectively [IO] Processes
SO+SO++e-
Electronic states of products
1029
2001
x*n3,2
10.33
to01
a4n
13.50
f 0.05
x*n1,2
A2ll
z14.4
b4X-
=19 6
!?*A
1
12.30
f 0.0,
X*A
2
13.01
f 0.05
G*B
2
13.24
+ ‘-” - 0.15
E*A 1
15.986 f 0.005
i52B
16.326 + 0.005
1
X3X-
19.26
+ 0.01
A3no
23.97 24.39
f 0.01 *- 0.01
B3z-
SO2 +SO++O+e-
C
24.44
X2nI12
15.94
f 0.02
X2n3j2
15.98
+_0.02
19.15
f 0.06
a%
A*rI
==20.1
b4z-
20.59
B*Z-
22 09
c*n
SOC12 --c SO + Cl: + e-
111,121
16 44
C2l-l
SO2 +SO+O++e-
Ref.
14 94
B*Z-
SO2 -+ SO: + e-
Energies (ev)
(2.131
[10,12.14J
[11.12.E4J
==25.3
X3X-
17.71
f 0.01
A3no
2241
f 0.01 2001
BJZ-
2284
C
22.88
D3n
24.45
X32-
13.72
f 0.01
A3no
18.43
c 0.01
B3Z:-
18.85
f 0.01-
[lO.l2,14J
[12,14J
409
Volume 73, number 3
CHEMICAL PHYSICS LE’ITERS
1 August f980
Table 1 (contmued) Energxs (eV)
Processes
Electromc states of products
SOCIZ * so + Clt; + e-
C
X2%,z
1889
D3II
X
=173/2
20 47
E%I
x
2n3/2
22 14
X 2nz,*
“G/Z
1499
2002
soclz
- So+ -)- 2Cl+ e-
SOC12 -F
SO++ Cl2
+
e-
x2fi3i2
2G,2
1503
+0.02
Wi,*
18.20
+006
A2fI
* G2
[11.12,14]
I
b4Z-
“P&
1964
B2I3”
*PO 312
21.14
C2Il
* G2
X
2%,2
X%;
X
2n3r2
m24.3 1251
to02
XlZ+ &
1255
it02
a4n
XIX+
1572
2006
A2n
XIX*
g
ml66
b4C-
g X’Z+g
B%’
X’Z;
G18.66
C2Il
x1x;
r21.8
a broad A ~f&--X~ll, transition of SO+ covering the W and virabXespectral range 193. In order to obtain information about the emitting species, the emission spectrum of thionyl chloride was measured m the helium afterglow under similar experimental conditions. We found that SOClz exhibits the identical band system produced from the He+/SOCl2 charge-transfer reaction. Since the emitting species are produced from both SO, and SOCI, by pnmary collisions with thermal-energy i-i& ions, the possible emitters are SO and SO+ formed by drssociative chargetransfer reactions. Atomic species are excluded from the candidates, because of the complicated appearance of the spectra. The ~~ energies required for the various processes considered in this work are listed in table 1. As is seen from fig. la, most bands occur in paus of subbands of almost equal intensities. The characteristic doublet splitting of the bands suggests that a *If state in Hund’s case (a) must be mvolved in the transition. 410
112.141
a4n
=I9
Ref
[11.12.141
17.16
There are three kricnvn zll states in SOi below 21 eV as shown in table 1. Among them, the X 2II state has been known to split into a doublet as a result of spinorbit interaction. Analogous ionic states exist in SO and 02, both of which have a 3Z- ground state 11 I]. Only the A 2lI(b)--X 2flr(a) emissron system has been detected among three possible transitions between the doublet states of Oz. The spectrum observed in the helium afterglow reaction of SO, is very similar to the well-known A-X band system of 0; [lS]. Thus, when the extensive bands were analyzed on the basis of the reported photoelectron data for SO summarized in table 2 with reference to the O$(A-X) emission system, aImost all bands were interpreted as the A 2iI(b)X 211r (a) system of SO+. The detailed ~bration~ ass@unent in the 250480 nm region is given in fig. 2. Although numerous bands in the 250-310 nm region could not be ident.ifIed in the previous work [3 1, almost all bands in the 250-540 nm region were ascribed to the SO+ (A ZI’I-X 24) system for 0 < v’ G I I with
1 August 1980
CHEMICAL PHYSICS LETTERS
Volume 73, number 3
Table 2 Equihbrium molecular constants for SO+(A *lI, X *%I Electronic states
T, (cm-’ ) PES b)
0~s a)
A%
(32943)
X2%/2
414 f 5
340 f 25
X2%/2
0
0
a) Tlus work.
b) Ref. [ll].
use
We (cm-‘)
c)
PES b)
OES a)
33150
805 * 5 (1323 f 3) c)
(cm-?
OES a)
-
6.4 c OS
1360 I- 30
7.8 * 0.3
c) Prehmmary results, see text.
1 G u” < Il. The vibrational numbering was made based on reported photoelectron data for the SO radical [l l] . We assumed that the adiabatic ionization potentials for the X 2111,2, X 2113,2 and A211 states are 10.29,10.33 and 1440 eV, respectively, and the vibrational frequency in the X 211r state is 1360 f 30 cm-l . Since the (0, u”) progression is expected to occur strongly by analogy with the 0; (A-X) system [ 151, the vibrational numbering for the upper A 2ll state is
probably correct. On the other hand, the viiratio& numbering for the X21iIr state is tentative because of the low accuracy of the reported adiabatic potentials for the A 211 state. Vacuum wavenumbers for the sub-band heads were tabulated in a Deslandres array [16]_ Estimates of we and w&e can be obtained assuming that the expression 1171AGv+1/2= 0, - 2oex,(u+ 1) holds for u’ < ZB and II” d 11. The equrlibrium molecular constants ob-
SO+(A’ll-
X2&)
fig. 2. The SO*(A *lI-X2$) emission system produced from dissociative charge-transfer reaction of He+ with SO2 at thermal energy. Limesmarked * are stray HeI lines.
Volume 73, number 3
CHEMICAL
PHYSICS
tamed are hsted III table 2 together with the photoelectron data. Although the vibrational frequency of the A *II state has not been determmed by photoelectron spectrometry (PES) because of overlap wth the td of the considerably stronger 4il band [ll], it was measured to be 805 -t 5 cm-l from the present optical emission spectrometry (OES). The vlbratlonal frequency of the A *lI state is smaller than that of the SO(X 3C-) state (1149 cm-’ [12]), inbcating removal of an electron from a bonding orbital. Although an isotopic study LSrequued to make the absolute vibrational numbering, providing the accurate term value of the A *ll state and the ground-state vibrational frequency, the prelimmary values of T, and we obtained in this w@rk (given in parentheses m table 2) are in reasonable agreement with the photoelectron results. The increase in the vibrational frequency of the SO+(X *II,) state in comparison with SO(X 3Z-) mdicates removal of an anti-bondmg electron The spin-orbit sphtting in the X *lIr state was found to be almost constant for all the transItIons identified III this study. It was estunated to be 414 5 5 assummg that the spin-orbit sphtting m the cm-l, upper A *ll state is negligl%le. The spin-orbit coupling constant obtamed in this work 1s Iarger than the photoelectron result (340 +. 25 cm-l) by z-70 cm-l. This discrepancy IS probably due to lower accuracy m the photoelectron spectroscopic measurements. In conclusion, the extensive bands observed From the helium afterglow reaction of SO, are assigned to the A *II-X *fir system of SO+ produced from the thermal-energy He+/SO, &ssoclative charge-transfer reaction. The molecular constants for the SO+(A *II, X *II,) states were estiated, and are summarized m table 2. We are planning an isotopic study to obtain the absolute vibrational numbermg for SO+(X *II,). The observation of a new SO+(A-X) emisnon, not detected m any other excitation sources, indicates that charge-transfer reactions of He+ with gaseous molecules at thermal energy are 8 promising way of providmg new ion-fluorescence.
412
LElTERS
1 August
1980
Acknowledgement
The authors thank Mr. Hisao Fukutome and Mrs. Kumiyo TsuJi for their cooperation at the beginning of this work, and Mr. Hiroshi Obase for discussion. Thus work was supported in part by a Grant-in-Ad for Scientific Research from the Mirustry of Education.
References
Ill K.T. 121C.E.
Wu and A.J Yencha, Can. J. Phys. 55 (1977) 767. Brion and D.S C. Yee, J. Electron Spectry. 12 (1977) 77. K. TSUJ~ and Y. Nishunur-a, t31 M. TSUJI. H. Fukutome, Intern. J. Mass Spectrom. Ion Phys. 28 (1978) 257. Intern. J. Mass [41 M. TSUJI, M. Matsuo and Y. Nistumura, Spectrom. Ion Phys. 34 (1980) 273. 151 M. TsuJi. H. Obase, M. Matsuo, M Endoh and Y. Nlshimura. Chem. Phys.50 (1980) 195. Intern. J. Mass [61 M. TSUJI, K. Tsuj; and Y. Nishunura. Spectrom Ion Phys. 30 (1979) 175. I71 M. TsuJi. H Obase, M. Matsuo and Y. Nlshlmura, Abstracts XI ICPEAC, Kyoto, 1979. p. 856. to be pubbshed. 181 M. Tsujr and Y. Nlshmura, theory, tech191 J H.D. Eland, III: Electron spectroscopy ruques and applications, Vol 3, eds. C R. Brundle and A.D. Baker (Acadermc Press, New York, 1979) p. 231. [ 101 C E. Moore, Atomic energy levels, US Nat1 Bur. Std. Cuculx 467 (1949). [ 111 J.M Dyke, L. Golob. N. Jonathan, A. Moms, M. Okuda and D.J. Snuth, J. Chem Sot Faraday Trans 1170 (1974) 1818. [ 121 K-P. Huber and G. Herzberg, Constants of diatonuc molecules Oran Nostrand, Prmceton, 1979). [ 13 1 J H.D. Eland and C.J. Danby, Intern J. Mass Spectrom. Ion Phys. 1 (1968) 111. [ 141 H. Okabe. Photochemistry of small molecules (Wiley, New York, 1978) [ 151 P.H. Krupenie, 5. Phys. Cbem. Ref. Data 1 (1972) 423; W.C hchardson and D.W. Setser, J Chem Phys. 58 (1973) 1809. [ 161 M. TSUJI,M. Endoh and Y. Nlshmmra. to be pubbshed. [ 171 G. Herzberg, Spectra of dmtomic molecules, 2nd Ed. (Van Nostrand, Pnnceton, 1950).
SO+ (A *II-X REACTION
1 August 1980
CHEMICAL PHYSICS LE-ITERS
*I$)
EMISSION
PRODUCED
FROM A DISSOCIATWE
OF He+ WJTH SO2 AT THERMAL
CHARGE-TRANSFER
ENERGY
Masaharu TSUJI, Ch.ikash_~ YAMAGIWA, Minoru ENDOH and Yukio NISHIMURA Research Inshtute of Industrial Sczence. Kyushu University 86, Higasht-ku. Fukuoka 812. Japan Recewed 27 May 1980
The extensive bands observed from the hehum afterglow reaction of SO* m the 250-540 mn region are assigned to the new SO+(A *rt--X*l$) system produced from the He*/S02 dissociative charge-transfer reaction at themA energy. They had been erroneously mterpreted as the SO; (c-4 system produced from He(2 3S)/S02 Penning ionization The spectroscopic constants for the SO+(A*n) and SO+(X*nJ states were determined_
1. Introduction
source is thermal-energy He+ ions involved iu the ciischarge flow.
In 1977, Wu and Yencha [l] observed a rather strong,
extensive emission band system in the 300550 run spectral range from a flowing helium afterglow reaction of SO*. The new band system has been ascribed to the EzA, -% 2A, transition of SO;, the band origin of which is located at 3339 A. According to the notation of Brion and Yee [2], this transition corresponds to the SOt(E2A1-z *AI) system. Wu and Yencha reported a long progession of symmetrical stretching vibrations (u; ,O, O)-(u'; ,0, 0) for O
incorrect. Here it is shown that the spectrum is due to the ion of sulfur monoxide, SO+, and its excitation
2. Experimental The flowing afterglow apparatus used was essentially the same as that reported elsewhere [4] _ The whole system was continuously evacuated by means of a 10000 Q/min booster pump. Active species of helium were generated by a 2450 MHz microwave discharge through a fast flow of helium. The relative concentrations of active species of helium flow, He(2 3S), He’ and He;, were estimated by separately observing $ (B-X) emission resulting from the reactions of He(2 %), He+ and He; with N2 151. Although the dommant active species at low helium pressures is He(2 %), the concentrations of He+ and He; increase with an increase in helium pressure. In order to examine the contribution of ionic species to the observed emission, an ion-collector grid was placed between the discharge section and the reaction zone. The satisfactory efficiency of the ion-collector grid was checked emission produced from the by monitoring $(C-X) He+/N2 near-resonant charge-transfer reaction. We found that the charged-particle deflector used in the previous work [3] did not have enough efficiency to trap ionic species. Target gases were injected through a nozzle (0.6 mm i-d.) 407
Volume
73, number
CHEMICAL
3
PHYSICS LETTERS
ca. 15 cm downstream from the disc-harge section. The pressures of helium gas and commercially obtamed sample gases (SO2 nortunal purity > 99.9% and SOCI, guaranteed reagent) varied from 0.4-0.6 Torr and l-5 mTorr, respecttvely, as measured by a MKS capacrtance manometer. Spectra in the 200-600 nm region were observed through a quartz window m front of the gas inlet with a JarreL-Ash 1 m spectrometer equipped with a 1200 groove/mm grating blazed at 300 MI. A HTV R376 photomultipLier and a Burr-Brown 3523J OP amp& fier were used for the photomet~c me~urements.
3. Results and discussion A typical emrssron spectrum of SO2 rn a hehum afterglow IS shown rn fig. la. In addrtion to wellknown SO(A-X) and 01 emission systems, a relatrvely strong, extensrve band system, most of which was believed to be SOS (E-2) ermssion resultmg from He(2 3S)/SO, Penning ionization, is observed m the 240-550 run regron. When active iomc species were removed from the helium flow, the dommant band
1 August 1980
completely disappeared as shown in Rg. lb. ‘ihis result indrcates that the excitation source is not He(2 3S), but a charged particle. Since this emission was observed at the heB~-pr~e range, where the domrnant charged particle was He*, and the He; concentratron was negligible, the charged particle responsible for the formation of the emitting species was concluded to be He+. The fmding that the excitation source was He+ prompted us to reexamine the previous assignme,“f of the observed emission, the formation of SO;(C) by the He+fSO, ch~ge-tr~sfer reaction is unfavorable, because the reactron is exothermic by as large as = 9 eV as is seen from energy relations (table 1). We have recently made optical spectroscopic studies of charge-transfer reactions of He+ with various triatomic molecules at thermal energy by using a flowing afterglow apparatus [4--81. Smce the dominant ermtting products are diatomic ions such as N$ (B) for hi20 [6], CO”(A,B) and CS(B) for OCS [4], CS+(B) for CSz [S] , CO* (A) for CO, [7] and OH* (A) for H,O [8], SOi emissron is probably responsrble for the observed emission resulting from the He+/SOZ charge-transfer reaction. Eland has recently predicted
(a)
250
300
350
400
450
560
500
tnmf
He’. so2 .
(b)
/
1
SOtA%-X't-)
250
300
350
Emissionspectra of SO2 in the 230450 * are stray He1 hnes
Fag. 1.
408
400
450
500
550
tnm)
nm region by colhssons with (a) He(2 ‘~3) + He+ and (b) He(2 3S)_ Lmes marked
Volume 73, number 3
CHEMICAL
1 August 1980
PHYSICS LETTERS
Table 1 Adabatic ionizatron potentials of SO and SO*, and calculated minimum energies required for formation of various SO and SO* states from dissociative ionization of SO* and SOCI~. The av;ulable energies of He(23S1 and He* are 19.82 and 24.58 eV. respectively [IO] Processes
SO+SO++e-
Electronic states of products
1029
2001
x*n3,2
10.33
to01
a4n
13.50
f 0.05
x*n1,2
A2ll
z14.4
b4X-
=19 6
!?*A
1
12.30
f 0.0,
X*A
2
13.01
f 0.05
G*B
2
13.24
+ ‘-” - 0.15
E*A 1
15.986 f 0.005
i52B
16.326 + 0.005
1
X3X-
19.26
+ 0.01
A3no
23.97 24.39
f 0.01 *- 0.01
B3z-
SO2 +SO++O+e-
C
24.44
X2nI12
15.94
f 0.02
X2n3j2
15.98
+_0.02
19.15
f 0.06
a%
A*rI
==20.1
b4z-
20.59
B*Z-
22 09
c*n
SOC12 --c SO + Cl: + e-
111,121
16 44
C2l-l
SO2 +SO+O++e-
Ref.
14 94
B*Z-
SO2 -+ SO: + e-
Energies (ev)
(2.131
[10,12.14J
[11.12.E4J
==25.3
X3X-
17.71
f 0.01
A3no
2241
f 0.01 2001
BJZ-
2284
C
22.88
D3n
24.45
X32-
13.72
f 0.01
A3no
18.43
c 0.01
B3Z:-
18.85
f 0.01-
[lO.l2,14J
[12,14J
409
Volume 73, number 3
CHEMICAL PHYSICS LE’ITERS
1 August f980
Table 1 (contmued) Energxs (eV)
Processes
Electromc states of products
SOCIZ * so + Clt; + e-
C
X2%,z
1889
D3II
X
=173/2
20 47
E%I
x
2n3/2
22 14
X 2nz,*
“G/Z
1499
2002
soclz
- So+ -)- 2Cl+ e-
SOC12 -F
SO++ Cl2
+
e-
x2fi3i2
2G,2
1503
+0.02
Wi,*
18.20
+006
A2fI
* G2
[11.12,14]
I
b4Z-
“P&
1964
B2I3”
*PO 312
21.14
C2Il
* G2
X
2%,2
X%;
X
2n3r2
m24.3 1251
to02
XlZ+ &
1255
it02
a4n
XIX+
1572
2006
A2n
XIX*
g
ml66
b4C-
g X’Z+g
B%’
X’Z;
G18.66
C2Il
x1x;
r21.8
a broad A ~f&--X~ll, transition of SO+ covering the W and virabXespectral range 193. In order to obtain information about the emitting species, the emission spectrum of thionyl chloride was measured m the helium afterglow under similar experimental conditions. We found that SOClz exhibits the identical band system produced from the He+/SOCl2 charge-transfer reaction. Since the emitting species are produced from both SO, and SOCI, by pnmary collisions with thermal-energy i-i& ions, the possible emitters are SO and SO+ formed by drssociative chargetransfer reactions. Atomic species are excluded from the candidates, because of the complicated appearance of the spectra. The ~~ energies required for the various processes considered in this work are listed in table 1. As is seen from fig. la, most bands occur in paus of subbands of almost equal intensities. The characteristic doublet splitting of the bands suggests that a *If state in Hund’s case (a) must be mvolved in the transition. 410
112.141
a4n
=I9
Ref
[11.12.141
17.16
There are three kricnvn zll states in SOi below 21 eV as shown in table 1. Among them, the X 2II state has been known to split into a doublet as a result of spinorbit interaction. Analogous ionic states exist in SO and 02, both of which have a 3Z- ground state 11 I]. Only the A 2lI(b)--X 2flr(a) emissron system has been detected among three possible transitions between the doublet states of Oz. The spectrum observed in the helium afterglow reaction of SO, is very similar to the well-known A-X band system of 0; [lS]. Thus, when the extensive bands were analyzed on the basis of the reported photoelectron data for SO summarized in table 2 with reference to the O$(A-X) emission system, aImost all bands were interpreted as the A 2iI(b)X 211r (a) system of SO+. The detailed ~bration~ ass@unent in the 250480 nm region is given in fig. 2. Although numerous bands in the 250-310 nm region could not be ident.ifIed in the previous work [3 1, almost all bands in the 250-540 nm region were ascribed to the SO+ (A ZI’I-X 24) system for 0 < v’ G I I with
1 August 1980
CHEMICAL PHYSICS LETTERS
Volume 73, number 3
Table 2 Equihbrium molecular constants for SO+(A *lI, X *%I Electronic states
T, (cm-’ ) PES b)
0~s a)
A%
(32943)
X2%/2
414 f 5
340 f 25
X2%/2
0
0
a) Tlus work.
b) Ref. [ll].
use
We (cm-‘)
c)
PES b)
OES a)
33150
805 * 5 (1323 f 3) c)
(cm-?
OES a)
-
6.4 c OS
1360 I- 30
7.8 * 0.3
c) Prehmmary results, see text.
1 G u” < Il. The vibrational numbering was made based on reported photoelectron data for the SO radical [l l] . We assumed that the adiabatic ionization potentials for the X 2111,2, X 2113,2 and A211 states are 10.29,10.33 and 1440 eV, respectively, and the vibrational frequency in the X 211r state is 1360 f 30 cm-l . Since the (0, u”) progression is expected to occur strongly by analogy with the 0; (A-X) system [ 151, the vibrational numbering for the upper A 2ll state is
probably correct. On the other hand, the viiratio& numbering for the X21iIr state is tentative because of the low accuracy of the reported adiabatic potentials for the A 211 state. Vacuum wavenumbers for the sub-band heads were tabulated in a Deslandres array [16]_ Estimates of we and w&e can be obtained assuming that the expression 1171AGv+1/2= 0, - 2oex,(u+ 1) holds for u’ < ZB and II” d 11. The equrlibrium molecular constants ob-
SO+(A’ll-
X2&)
fig. 2. The SO*(A *lI-X2$) emission system produced from dissociative charge-transfer reaction of He+ with SO2 at thermal energy. Limesmarked * are stray HeI lines.
Volume 73, number 3
CHEMICAL
PHYSICS
tamed are hsted III table 2 together with the photoelectron data. Although the vibrational frequency of the A *II state has not been determmed by photoelectron spectrometry (PES) because of overlap wth the td of the considerably stronger 4il band [ll], it was measured to be 805 -t 5 cm-l from the present optical emission spectrometry (OES). The vlbratlonal frequency of the A *lI state is smaller than that of the SO(X 3C-) state (1149 cm-’ [12]), inbcating removal of an electron from a bonding orbital. Although an isotopic study LSrequued to make the absolute vibrational numbering, providing the accurate term value of the A *ll state and the ground-state vibrational frequency, the prelimmary values of T, and we obtained in this w@rk (given in parentheses m table 2) are in reasonable agreement with the photoelectron results. The increase in the vibrational frequency of the SO+(X *II,) state in comparison with SO(X 3Z-) mdicates removal of an anti-bondmg electron The spin-orbit sphtting in the X *lIr state was found to be almost constant for all the transItIons identified III this study. It was estunated to be 414 5 5 assummg that the spin-orbit sphtting m the cm-l, upper A *ll state is negligl%le. The spin-orbit coupling constant obtamed in this work 1s Iarger than the photoelectron result (340 +. 25 cm-l) by z-70 cm-l. This discrepancy IS probably due to lower accuracy m the photoelectron spectroscopic measurements. In conclusion, the extensive bands observed From the helium afterglow reaction of SO, are assigned to the A *II-X *fir system of SO+ produced from the thermal-energy He+/SO, &ssoclative charge-transfer reaction. The molecular constants for the SO+(A *II, X *II,) states were estiated, and are summarized m table 2. We are planning an isotopic study to obtain the absolute vibrational numbermg for SO+(X *II,). The observation of a new SO+(A-X) emisnon, not detected m any other excitation sources, indicates that charge-transfer reactions of He+ with gaseous molecules at thermal energy are 8 promising way of providmg new ion-fluorescence.
412
LElTERS
1 August
1980
Acknowledgement
The authors thank Mr. Hisao Fukutome and Mrs. Kumiyo TsuJi for their cooperation at the beginning of this work, and Mr. Hiroshi Obase for discussion. Thus work was supported in part by a Grant-in-Ad for Scientific Research from the Mirustry of Education.
References
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