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SO2+(C̃−X̃) fluorescence from Penning ionization of SO2
a n d I o n P h y s i c s , 28 (1978) 257--265 ~) Elsevier Scientific Publishing C o m p a n y , .Arn.~terdam-- Printed in The Netherlands International Journal of Mass Spectrometry
SO~(C
--X) FLUORESCENCE
FROM
PENNING
IONIZATION
OF
257
SO2
M. TSUJI, H. F U K U T O M E , K. T S U J I a n d Y. N I S H I M U R A Research Institute of Industrial Science 86, Kyushd (Japan)
University, Hakozaki,
Fukuoka
812
(First received 10 J a n u a r y 1978; in f-real f o r m 20 February-1978)
ABSTRACT In a d d i t i o n t o SO(A3II--X3~ -) and S 0 2 ( A I B s - - X I A x ) emission, the C2B2(v~, O, 0 ) - - X2A~(v~. 0. 0) t r a d i t i o n o f SO~ fluorescence~ which has its origin at 3376 *- 2 ~ . h~_~ been identified in t h e reaction o f He(23S) with SO 2. T h e s y m m e t r i c a l stretching vibrational frequencies o f the X and C states o f S O ~ have been d e t e r m i n e d to be 1259 +13 and 788 ± 14 c m - l INTRODUCTION The electronically and vibrationally excited states of sulfur dioxide cation h a v e b e e n i n v e s t i g a t e d b y p h o t o e l e c t r o n s p e c t r o m e t r y a n d o p t i c a l erni.~sion s p e c t r o m e t r y . E l a n d a n d D a n b y [ 1 ] , a n d T u r n e r e t al. [ 2 ] h a v e r e p o r t e d t h e high-resolution HeI photoelectron s p e c t r a o f SO2. L l o y d a n d R o b e r t s [ 3 ] have analyzed the HeI and HeII photoelectron spectra above 17 eV. Eland et al. [ 4 ] h a v e a t t e m p t e d t o o b s e r v e SO~ e m i s s i o n e x c i t e d b y t h e H e I l i n e . A1t h o u ~ h t h e y h a v e d e t e c t e d n o erni.~sion, t h e f l u o r e s c e n c e q u a n t ~ J r n y i e l d o f S O ~ ( C + D ) h a s b e e n e s t i m a t e d t o b e l a r g e r t h a n 2 . 3 +- 1 . 7 X 1 0 -4 b y t h e photon--photoion coincidence method. Recently Wu and Yencha [5] have observed a strong, extensive emi.qsion band system from 300 to 550 nm in t h e H e ( 2 3 S ) + SO2 r e a c t i o n a n d a s s i g n e d i t t o t h e B ~ A 1 - - X 2 A I t r a n s i t i o n o f SO~. W e h a v e r e - ~ x s r n i n e d t h e e m i ~ i o n s p e c t r u m o f SO2 i n a b e l i l l m a f t e r glow from 200 to 510 nm at a medium resolution. Our measurements were more sensitive than those of Wu and Yencha, enabling a more precise analysis o f t h e c o m p l i c a t e d s p e c l ~ , m o b s e r v e d t o b e c a r r i e d o u t . I n c o m p a r i s o n with the high-resolution photoelectron spectra, a new reasonable assi~ment f o r t h e SO~(C2B2 - - X 2 A , ) e m i s s i o n h a s b e e n f o u n d [ 6 ] . F_J~ERIMENTAL T h e f l o w i n g a f t e r g l o w a p p a r a t u s u s e d is s h o w n i n F i g . 1 . T h e q u a r t z f l o w tube (12 mm in d~emeter) and the Pyrex reaction cell (35 mm in dismeter) were evacuated continuously by a 7000 1 mi,-! oil rotary p,,mp. Before
258 SOz
g
d ' ~ r g e d particle defleclor
talc
r ov¢~2ve
[
quc.rt z w i n d o w
c ~.vity
Fig. 1. S c h e m a t i c d i a g r a m o f t h e f l o w i n g a f t e r g l o w a p p a r a t u s .
entering the discharge section, heli,]m gas (>99.99% purity) was purified by passage through a liquid nitrogen trap packed with activated molecular sieve ( W a k o P u r e C h e m i c a l Incl. t y p e 5 A 1 / 1 6 ) . T h e m e t a s t a b l e H e ( 2 3 S ) a t o m s were generated by a 2450-MHz microwave discharge in the quartz tube with t h e i n p u t p o w e r a t 8 0 W. T h e e f f e c t o f c h a r g e d p a r t i c l e s p r o b a b l y i n v o l v e d in the discharge flow was examined by applying an electrostatic potential to a charged particle deflector. Sulfur dioxide (>99.9% purity) and oxygen C>99.99% purity) were used without further purification. The sample gas was introduced through a stainless steel nozzle 0.4 mm in diameter at about 15 cm downstream from the discharge section. The gas pressure was measured by a Pirnni gauge (ULVAC GP-2T) calibrated against a McLeod gauge. T h e heliT]m p r e s s u r e w a s 1 - - 2 t o r r a n d t h e s ~ m p l e g a s p r e s s u r e w a s 5 X 1 0 -3 5 X 1 0 -2 t o r t . The reaction flnme was focused by quartz lenses on the inlet slit of a scanning monochromator ( S h i r n a d z u G E - 1 0 0 ) w i t h a 1 2 0 0 l i n e m m -1 g r a t i n g b l a z e d a t 3 0 0 n r ~ ; i t s r e c i p r o c a l d i s p e r s i o n w a s 8 . 3 • m m -1. T h e p h o t o n s were detected photoelectrically with a photomultiplier (Hamamatsu TV R106UH) and an OP ~mplifier (B,irr--Brown 3523J). The wavelength was calibrated by means of a low-pressure mercury lnmp (Hnm~matsu TV L9870 3 ) ; i t s e r r o r w a s e s t i m a t e d t o b e w i t h i n -+2 A . T h e v i b r a t i o n a l a n a l y s i s o f the observed spect~lm was carried out by use of wavelength with maximum intensity. -
-
R E S U L T S AND DISCUSSION Figure 2 in a hel~,m to ~0 rim, responsible
s h o w ~ a t y p i c a l e m i ~ s i o n s p e c t r u m o f SO2 f r o m 2 4 0 t o 4 6 0 n m afterglow. Above weak continuous features extending from 265 n u m e r o u s rii~cxete b a n d s c a n b e o b s e r v e d . P o s s i b l e a c t i v e s p e c i e s for the appearance of these photoerni~ion are He(23•), He(21S),
259
-i
|
!
2~.0
Fig.
260
2. A typical
2:80
300
320
emi-~sion spect~lm
3~0
of
360
SO2
380
in a helinm
~
afterglow.
~0
~
Lines
marked
~
nm
* are
stray HeI lines.
H e +, a n d e l e c t r o n s i n t h e h e l i u m a f t e r g l o w a t 1 - - 2 t o r t . T h e c o n t r i b u t i o n f r o m H e + a n d e l e c t r o n s is n e g l i g i b l e , becaff-~? t h e i n t e h s i t i e s o f t h e ob~served spectra did not show any-appreciable change, when a d.c. voltage of 0--100 V w a s a p p l i e d t o t h e d e f l e c t o r e l e c t r o d e s . I t is k n o w n t h a t m e t a s t a b l e H e ( 2 1 8 ) atom-~ ~ p i d l y c o n v e ~ t o H e ( 2 3 S ) b y e l a s t i c colli.~ions w i t h elect~cons [7]. Therefore, the predominant energy carrier under experimental conditions here are metastable He(238) atoms with enough energy (19.8 eV) to ionize a n d dissociate sulfur dioxide. T h e energy-level diagrams o f various possible p r o d u c t s i n t h e H e ( 2 3 8 ) + SO2 r e a c t i o n a r e show~l in Fig. 3. A l t h o u g h W u a n d Y e n c h a [ 5 ] h a v e r e p o r t e d t h e d e t e c t i o n o f w e a k S O ( B a ~ - - X32~-) emi.~sion, t h i s b a n d s y s t e m c o u l d n o t b e i d e n t i f i e d i n o u r s p e c t r u m . I n s t e a d o f t h e B - - X s y s t e m , t h e ASH - - X 3 Z - t r a n s i t i o n o f S O , w h i c h w a s : a b s e n t i n t h e i r s p e c t r u m , was o b s e r v e d h e r e in t h e 2 4 0 - - 2 6 5 - n m regio n. T h e f l u o r e s c e n c e a n d p h o s p h o r e s c e n c e s p e c t r a o f SO2 h a v e b e e n s t u d i e d extensively b y U V photo~excita_tion. M e t t e e [ 1 4 ] has r e p o r t e d t h a t t h e fluo r e s c e n c e s p e c t r u m o f SO2 b e c ~ m e d i f f u s e w i t h a n i n c r e a s e , in t h e e x c i t a tion energy, and at excitation by the 265-nm line, a discrete resonance band w a s n o l o n g e r o b s e r v e d . _Wu a n d Y e n c h a [ 5 ] h a v e r e p o r t e d t h e o b s e r v a t i o n o f S O 2 ( A I B I - - X l A 1 ) e m i s s i o n i n t h e H e ( 2 S S ) + SO2 s y s t e m . H o w e v e r , t h e i r spectrum does not s h o w w h a t kind of 8 0 2 fluorescence, discrete b a n d and/ or continuous band, w a s identified. Althougil the di.~crete b a n d w h i c h w a s observed b y Mettee [14] at l o w excitation energies is not identified in our s_pect~Jm, the underlying continuous e m i ~ i o n w a s assigned to the _~IBi -~ a A ~ tran.~ition of SO2,- because its observed spectral range is in g o o d agreem e n t with his fluorescence, spectm]m at the highest excitation ~energy. ~_The absence of structure indicates that the excitation into lower ~ibmtional I e v e l s o f t h e AIB~ s t a t e is o f H t t l e i m p o r t a n c e i n t h e S e ( 2 3 8 ) + SO2 r e a c t i o n . S i n c e t h e f o r m a t i o n _ o f e x c i t e d p a r e n t s p e c i e s i n a heli~]m a_etergIow-has n o t b e e n r e p o r t e d , t h e p r e s e n t f i n d i n g o f t h e p h o t o e m i - q s i o n f r o m t h e sin_giet p a r e n t s p e c i e s is n o t e w o r t h y . "" T h e e x p a n d e d em~-~sion s p e c t r u m i n ~fJae 3 1 0 ~ i 0 - ~ m r e g i o n is s h o w n i n
260 eV 2 0 ___H~
SO
......
- -
so;
*
0°
so(x~z'},o'S-s)
SO'*
0
~--6'E,~c~J
~'B, _ _
m ~ _ - - x
I:Yl L 10
15 ~A, C~2
b -mA.~ ~;
SO
-,,"0
_ _
SO(B~.O(~)
- -
SO(A~n},O(~p)
- -
$o0~£~-oC ~)
E: ILl
SOz A T~.
Fig_ 3. T h e energy-level diagr~m~ o f possible p r o d u c t s in t h e He(23S) + SO2 r e a c t i o n . T h e diagrams were o b t a i n e d b y use o f t h e e x c i t a t i o n energies o f SO2 [ 8 ] a n d SO [ 9 ] , t h e ionization energies o f SO2 [ 1 , 3 ] . O [ 1 0 ] , a nd SO [ 1 1 ] , and t h e dissociation e n e r g y o f D(OS---O) = -5.5 eV [ 1 2 ] . SO~ was assigned b y r e f e r e n c e with t he nn~lyses o f Hillier and S a u n der s [ 1 3 ] an d o f L l o y d and R o b e r t s [ 3 ] .
Fig. 4. The most striking feature of the SO2 spectrum in the reaction with H e ( 2 3 S ) is t h e a p p e a r a n c e o f a r a t h e r s t r o n g , e x t e n s i v e b a n d s y s t e m , w h i c h has not been observed by the impact of VUV photons [4], 75--350-eV electrons [15], and 2--3-keVlab At* ions [16]. On the basis of the data in Fig. 3 , e n e r g e t i c a l l y p o s s i b l e c a n d i d a t e s f o r t h i s e m i ~ s i o n a r e SO + ( a X) and SO~(C, D, EX and C, D, E- A). The transition energies between various vibrational levels can be estimated by the high-resolution photoelectron data of SO [11] and SO2 [1--3]. We have calculated the transition energies of the Lwo c o m p o n e n t s o f S O +, a4H - - X 2 H ~/±.3t2, a s ~ ] m i n g t h e v i b r a t i o n a l i n t e r v a l s o f t h e X2 H a n d a4H s t a t e s t o b e a b o u t 1 3 6 0 a n d 8 0 0 c m -1. A l t h o u g h a n emi~ion series was identified in agreement with the calculated transition energies, irregular variations were found in the intensity rli~tribution of the Lwo c o m p o n e n t s . For the foxther cla_rifieation of the presence of the SO* (a--X) emir.sion, the emi-~ion spectr-m of 02 was measured under identical e x p e r i m e n t a l c o n d i t i o n s , b e c a u s e a n a l o g o u s i o n i c s t a t e s exi.~t i n m o l e c u l a r o x y g e n [ 1 1 ] . T h e o b s e r v e d s p e c t ~ l m f o r t h e H e ( 2 3 S ) + O2 r e a c t i o n w a s v e r y s i m i l a r t o t h a t o f R i c h a r d s o n a~ld S e t s e r [ 1 7 ] . T h e p r e d o m i n a n t emission w a s t h e A 2 I I , - - X2IIg s y s t e m o f O~. T h e s p i n - f o r b i d d e n t r a n s i t i o n a4H - X2II w a s n o t i d e n t i f i e d i n t h e s p e c t r a l r a n g e e x p e c t e d f r o m t h e p h o t o e l e c t r o n d a t a . T h e s e r e s u l t s i n d i c a t e t h a t S O + ( a - - X ) e m i s s i o n f r o m S O 2 is e x c l u d e d f r o m t h e p o s s i b l e c a n c l i d a ::es. R e m a i n i n g p o s s i b l e e m i s s i v e s p e c i e s is S O ~ f o r w h i c h s i x s t a t e s a r e k n o w , , by photoelectron spec~oscopy [1--3] : X (arlinbatic/P = 12.3 eV), A (13.0
261
t-~.o)-(vo.o}~-
~0
0
~
1
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_
3%
=%
~
3
~
~0
~o
.--
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~
~ 5
~0
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r
!
6
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,~0
7
6
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~o
7
F i g . 4 . Fluorescence spectrum o f the SO~[C2B2(v~, 0, 0)--X2Az(v~", 0, 0)] system produced by the He(23S) Penning ionization o f SO~. Lines marked * are stray HeI lines.
eV), B (13.2 eV), C (16.0 eV), D (16.3 eV), and E (16.5 eV). The ground ionic s t a t e arises f r o m t h e r e m o v a l o f an e I e c t r o n f r o m t h e s u l f u r l o n e - p a i r 8az o r b i t a l , w h e r e a s t h e A , B, C, a n d D s t a t e s r e s u l t f r o m t h e l o s s o f a n e l e c t r o n f r o m t h e l a z , 5 b 2 , 2 b ~ , a n d 7a~ o r b i t a l s , r e s p e c t i v e l y [ 1 3 1 . A t f i r s t , w e attempted to interpret the observed spectrum according to Wu and Yencha's analysis of SO~(C--X) [5]. The corresponding band system of SO~(C--X) w a s f o u n d in o u r s p e c t r u m , t h e r e f o r e , a s l m H a r D e l a n d r e s t a b l e w a s o b t a i n e d . H o w e v e r , t h e f o l l o w i n g s e r i o u s d e f e c t s , as is a l s o f o u n d in t h e i r t a b l e , c a m e OUt. ( 1 ) T h e r e a r e l a r g e d i f f e r e n c e s b e t w e e n t h e o b s e r v e d w a v e - n u m b e r o f t h e b a n d o r i g i n ( 2 9 9 6 5 - + 1 5 c m -1) a n d t h e e s t i m a t e d v a l u e s f r o m t h e photoelectron data (29730-+ 121 [1] and 29682-+ 161 [2] cm-Z). (2) The t r a n s i t i o n e n e r g i e s o f t h e ( 0 , 0 , 0 ) - - (v~, 0 , 0 ) b a n d s f o r 0 ~< v~ ~< 3 a r e s y s t e m a t i c a l l y a b o u t 1 0 0 c m -z l o w e r t h a n t h o s e e s t i m a t e d f r o m t h e o t h e r bands, n~mely, the vibrational interval derived from the difference bebween t h e ( 0 , 0, 0 ) - - ( 3 , 0, 0 ) a n d ( 0 , 0 , 0 ) " ( 4 , 0 , 0 ) b a n d s is a b o u t 1 0 0 c m -z s m a l l e r t h a n t h e o t h e r intervAl.q.
262
A
0
OlD
~
0
o
.o A
A
o f.D CO
~ (3~
w--4
v--I
T ~ .,--,. t " -
1.0 L"O
¢X) 1.0
w--I ~.,.~
~¢0
I A C~D
0
i °
!
~ U.~
~
A CO
263 Taking into account the discrepancy between the observed origin and the estimated values from photoelectron data, a second attempt was. made to analyze the observed spectrum. This time we found the calculated wavelengths to fit the observed spectrum very well, when the value of 3376 ± 2 A~ W a s a s s i g n e d t o t h e b a n d o r i g i n : T h e d e t a i l e d v i b r a t i o n a l a s s i g n m e n t is given in Fig. 3. As shown in Fig. 3, most of the prominent discrete bands in the 310 510-nm region are ascribable to the C2B 2 -- X2AI transition of 805, where the upper and lower states are represented according to the assignm e n t o f L l o y d a n d R o b e r t s [ 3 ] : I n t h e o b s e r v e d s p e c t r a l r a n g e , C2B2(v~; 0 , 0 ) - - ~K2A~(o'z", 0 , 0 ) f o r 0 ~ o~ ~ 8 w i t h 0 ~ u'~ ~ 3 C a n b e i d e n t i f i e d . T h e D e l a n d r e s t a b l e f o r t h e ~ 2B 2- - ~ X 2A ~ t r a n s i t i o n o f S O ~ is g i v e n i n T a b l e 1 . T h e o b s e r v e d b a n d o r i g i n o f 2 9 6 2 0 +- 1 8 c m -~ a g r e e s r e a s o n a b l y w e l l w i t h t h e c a l c u l e t e d v a l u e s o f 2 9 7 3 0 +- 1 2 1 [ 1 ] a n d 2 9 6 8 2 -+ 1 6 1 [ 2 ] c m - ! f r o m photoelectron data. Combining the emission photon energies with the ioni z a t i o n p o t e n t i a l o f t h e X 2 A ~ ( 0 , 0 , 0 ) l e v e l , w h i c h is r e p o r t e d t o b e 1 2 . 3 0 5 eV [1], the ionization potential~ for the vibrational levels of the X2A z and C2B2 s t a t e s a r e o b t a i n e d a n d l i s t e d i n T a b l e 2 . One of the most prominent feaL~tres i n t h e ~ m i - ~ i o n s p e c t r l ] m O f S O 2 TABLE 2 T h e i o n i z a t i o n p o t e n t i a l s f o r t h e X2A z and C.2B2 vibrational levels o b t a i n e d f r o m the emission s p e c t r u m . P h o t o e l e c t r o n d a t a are also given f o r c o m p a r i s o n Vibrational levels
I o n i z a t i o n p o t e n t i a l s (eV) l~.m;~-~ion s p e c t r u m Present work
Photoelec~-on spectra E l a n d and Danby *
L l o y d and R o b e r t s **
Turner e t al. ~**
~,2A z
(0,0,0) (1,0,0) (2,0,0) (3,0,0) (4,0,0) (5,0,0)
(6,0,0) (7,0,0)
(..8.0,0)
12.305
15.977(2) 16.075(4) 16.171(5) 16.264(5)
15.986 16.092 16.188 16.284
12.29(0)
--
13.506(1)
C2B2 (0,0,0) (1,0.0)
(2,0,0) (3,0,0) * gel
12.305 12.461(2) 12.613(2) 12.766(3) 12.919(2) 13.067(3) 13.214(2) 13.3~3(1)
i.
*'~ Ref. 3. . . . . . * * * Ref. 2 : Q u a ntities in p a r e n t h e s e s are s t a n d a r d deviations. " -
15.992(3) 16.090(3) 16.189(5) -16.286(6) -
15.97(2)
-
-
: .._r
--
264 TABLE 3 The symr~etrical stretching vibrational frequencies for t h e X and C states o f SO~ ] ~ e c t ~ ' o ~ c states
Vibrational frequencies (cm -1 ) Emission s p e c t r u m Present w o r k
Molecular ground state X2A 1 CSB2
Photoelectron
sl3ec~
Eland and Danby *
L l o y d and Roberts **
Turner e t al. ***
-816 +_ 15
-782 -+ 20
-850
1151 -~
Ref. 1; ~* ref. 3; * ~
1259 _+ 13 788 + 14 ref. 2; -~ ref. 8.
b y H e ( 2 3 S ) P e n n i n g i o n i z a t i o n is a n a p p e a r a n c e o f t h e p r o g r e s s i o n s o f t h e symmetrical stretching vibration in the ground state, which has not been obs e r v e d in t h e p h o t o e l e c t r o n s p e c t r a . F r o m a n a n a l y s i s o f t h e S O ~ ( ~ - - ~ ) emission, the symmetrical stretching vibrational frequency of the ground i o n i c s t a t e w a s d e t e r m i n e d t o b e 1 2 5 9 -+ 1 3 c m - l . M o r e o v e r , +~he s y m m e t r i c a l stretching vibrational frequency of the C2Bs state was d~t~t~ined to be 7 8 8 -+ 1 4 c m -~. T h i s v a l u e is i n g o o d a g r e e m e n t w i t h t h e p h o t o e l e c t r o n v,~Jue o f 7 8 2 +- 2 0 c m -~ [ 3 ] , b u t i t is s l i g h t l y l o w e r t h a n o t h e r e x i s t i n g v a l u e s o f 8 1 6 -+ 1 5 [ 1 ] a n d 8 5 0 [ 2 ] c m -! a s s u m m a r i z e d i n T a b l e 3 . A n i n c r e a s e i n t h e vibrational frequency of the ground ionic state compared with that of the m o l e c ~ J | 3 r g r o u n d s t a t e s h o w s a n a n t i b o n d i n g c h a r a c t e r f o r t h e s~]lfur l o n e p a i r 8a~ o r b i t a l , w h i l e a l a r g e d e c r e a s e i n t h e v i b r a t i o n a l f r e q u e n c y o f t h e e x c i t e d C 2 B z s t a t e c o r r e s p o n d s t o t h e s t r o n g l y b o n d i n g n a t u r e o f t h e 2b~ o r b i talin S--O. F r o m t h e p r e s e n t o p t i c a l s t u d y , iC c a n b e c o n c l u d e d t h a t t h e m a j o r c h a n n e l i n t h e H e ( 2 3 S ) + S O s r e a c t i o n is t h e P e n n i n g i o n i z a t i o n i n t o t h e C 2 B 2 state. Molecular excitation (SOs*) and dissociative excitation (SO') are m i n o r r e a c t i o n s . T h e p r e s e n t o b s e r v a t i o n o f ~ h e .qO~ ern~=~ion, w h i c h h a s not been detected by any other excitation tecth~iques, lead us to conclude t h a t P e ~ n i n g i o n i z a t i o n is a p r o m i s i n g w a y o f g e n e r a t i n g i o n f l u o r e s c e n c e . SUMMARY T h e c o n i ~ i o n s o f I-Ie(23S) w i t h S O s g a v e a r e l a t i v e l y s t r o n g b a n d s y s t e m i n t h e U V a n d v i s i b l e r e g i o n s , i n a d d i t i o n t o w e a k e r emi.o~sion f r o m S O ( A X) and SO2(A --X). From the analy~s on the basis of high-resolution phot~ e l e c t r o n d a t ~ . t h i s b a n d w a s a s s i g n e d t o t h e C~B=(u~, 0 , 0 ) - - ~ . 2 A , ( u ~ , 0 , 0 ) t r a n s i t i o n o f SO~. T h e D e l a n d r e s t a b l e f o r thi~ t r a n s i t i o n a n d t h e i o n i z a t i o n
265
p o t e n t J a l ~ f o r t h e ~V,2At(v~' , 0 , 0 ) a n d C 2 B 2 ( v ~ , 0 , 0 ) v i b r a t i o n a l levels are given in rabies 1 and 2. The symmetrical -~Ltetching vibrational frequencies of the X and ~ states obtained from the analysis of the SO~(~ -- X) fluorescence are summarized in Table 3. ACKNOWLEDGEMENT Financial aid by the 2 7 4 1 4 4 ) is a c k n o w l e d g e d .
Ministry
of
Education
of
Japan
(Grant-in-Aid
REFERENCES 1 J . H . D . E I a n d a n d C.J. D a n b y , I n t . J. Mass S p e c t r o m . I o n P h y s . , I ( 1 9 6 8 ) 1 1 1 . 2 D.W. T u r n e r , C. B a k e r , A . D . B a k e r a n d C . R . B r u n d l e , M o l e c u l a r P h o t o e l e c t r o n S p e c troscopy, Wi]ey--Interscience, London, 1970. 3 D . R . L l o y d a n d P.J. R o b e r t s , Mol. P h y s . , 26 ( 1 9 7 3 ) 2 2 5 . 4 j.I-I_D. E l a n d , M. D e v o r e t a n d S. L e a c h , C h e m . P h y s . L e t t . , 4 3 ( 1 9 7 6 ) 97. 5 K.T. Wu a n d A . J . Y e n c b ~ C a n . J_ P h y s . , 5 5 ( 1 9 7 7 ) 7 6 7 . 6 R e a d e r s s h o u l d n o t be c o n f u s e d b y t h e a s s i g n m e n t o f SO~ ; A l t h o u g h Wu a n d Y e n c h a [ 5 ] have assigned t h e i o n i c s t a t e w i t h t h e a d i a b a t i c / P 1 6 . 0 e V t o t h e B s t a t e , w e h a v e a s c r i b e d it t o t h e C s t a t e . 7 A.V. Phelps, Phys. Rev., 99 (1955) 1307. 8 G. H e r z b e r g , E l e c t r o n i c S p e c t r a a n d E l e c t r o n i c S t r u c t u r e o f P o l y a t o m i c M o l e c u l e s , Van Nostrand Reinhold, New York, 1966. 9 R.W.B. P e z r s e a n d A . G . G a y d o n , T h e I d e n t i f i c a t i o n o f M o l e c u l a r S p e c t r a , 4 t h e d n . , C h a p r o ~ n a n d I-I~II, L o n d o n , 1 9 7 6 . 10 G. Her-zberg, A t o m i c S p e c t r a a n d A t o m i c S t r u c t u r e , D o v e r P u b l i c a t i o n s , N e w Y o r k , 1944. 11 J.M. D y k e , L. G o l o b , N. J o n a t h a n , A . Morris, M. O k u d a a n d D.J. S m i t h , J. C h e m . S o c . , F a r a d a y Tr~n~. H , 7 0 ( 1 9 7 4 ) 1 8 1 8 . 12 C. L a l o a n d C. V e r m e i l , J. P h o t o c h e m . , 3 ( 1 9 7 4 / 7 5 ) 4 4 1 . 13 I.H. Hillier a n d V . R . S a u n d e r s , Mol. P h y s . , 2 2 ( 1 9 7 1 ) 1 9 3 . 14 H . D . llfettee, J. C h e m . P h y s . , 4 9 ( 1 9 6 8 ) 1 7 8 4 . 15 W_H. S m i t h , J. Q u a n t . S p e c t r o s c . R a d l a t . Tr~n~., 9 ( 1 9 6 9 ) 1 1 9 1 ; M. Tsuji, H. F u k u t o m e , Y. N i s h i m u r a , T . O g a w a a n d N. r~hlb~h{, t o be p u b l i s h e d . 16 H. F u k u t o m e , M. Tsuji a n d Y. N i s h i m u r a , J. P h y s . S o c . J p n . , 4 4 ( 1 9 7 8 ) 1 4 0 1 . 17 W.C. R i c h a r d s o n a n d D.W. S e t s e r , J. C h e m . P h y s . , 5 8 ( 1 9 7 3 ) 1 8 0 9 .
SO~(C
--X) FLUORESCENCE
FROM
PENNING
IONIZATION
OF
257
SO2
M. TSUJI, H. F U K U T O M E , K. T S U J I a n d Y. N I S H I M U R A Research Institute of Industrial Science 86, Kyushd (Japan)
University, Hakozaki,
Fukuoka
812
(First received 10 J a n u a r y 1978; in f-real f o r m 20 February-1978)
ABSTRACT In a d d i t i o n t o SO(A3II--X3~ -) and S 0 2 ( A I B s - - X I A x ) emission, the C2B2(v~, O, 0 ) - - X2A~(v~. 0. 0) t r a d i t i o n o f SO~ fluorescence~ which has its origin at 3376 *- 2 ~ . h~_~ been identified in t h e reaction o f He(23S) with SO 2. T h e s y m m e t r i c a l stretching vibrational frequencies o f the X and C states o f S O ~ have been d e t e r m i n e d to be 1259 +13 and 788 ± 14 c m - l INTRODUCTION The electronically and vibrationally excited states of sulfur dioxide cation h a v e b e e n i n v e s t i g a t e d b y p h o t o e l e c t r o n s p e c t r o m e t r y a n d o p t i c a l erni.~sion s p e c t r o m e t r y . E l a n d a n d D a n b y [ 1 ] , a n d T u r n e r e t al. [ 2 ] h a v e r e p o r t e d t h e high-resolution HeI photoelectron s p e c t r a o f SO2. L l o y d a n d R o b e r t s [ 3 ] have analyzed the HeI and HeII photoelectron spectra above 17 eV. Eland et al. [ 4 ] h a v e a t t e m p t e d t o o b s e r v e SO~ e m i s s i o n e x c i t e d b y t h e H e I l i n e . A1t h o u ~ h t h e y h a v e d e t e c t e d n o erni.~sion, t h e f l u o r e s c e n c e q u a n t ~ J r n y i e l d o f S O ~ ( C + D ) h a s b e e n e s t i m a t e d t o b e l a r g e r t h a n 2 . 3 +- 1 . 7 X 1 0 -4 b y t h e photon--photoion coincidence method. Recently Wu and Yencha [5] have observed a strong, extensive emi.qsion band system from 300 to 550 nm in t h e H e ( 2 3 S ) + SO2 r e a c t i o n a n d a s s i g n e d i t t o t h e B ~ A 1 - - X 2 A I t r a n s i t i o n o f SO~. W e h a v e r e - ~ x s r n i n e d t h e e m i ~ i o n s p e c t r u m o f SO2 i n a b e l i l l m a f t e r glow from 200 to 510 nm at a medium resolution. Our measurements were more sensitive than those of Wu and Yencha, enabling a more precise analysis o f t h e c o m p l i c a t e d s p e c l ~ , m o b s e r v e d t o b e c a r r i e d o u t . I n c o m p a r i s o n with the high-resolution photoelectron spectra, a new reasonable assi~ment f o r t h e SO~(C2B2 - - X 2 A , ) e m i s s i o n h a s b e e n f o u n d [ 6 ] . F_J~ERIMENTAL T h e f l o w i n g a f t e r g l o w a p p a r a t u s u s e d is s h o w n i n F i g . 1 . T h e q u a r t z f l o w tube (12 mm in d~emeter) and the Pyrex reaction cell (35 mm in dismeter) were evacuated continuously by a 7000 1 mi,-! oil rotary p,,mp. Before
258 SOz
g
d ' ~ r g e d particle defleclor
talc
r ov¢~2ve
[
quc.rt z w i n d o w
c ~.vity
Fig. 1. S c h e m a t i c d i a g r a m o f t h e f l o w i n g a f t e r g l o w a p p a r a t u s .
entering the discharge section, heli,]m gas (>99.99% purity) was purified by passage through a liquid nitrogen trap packed with activated molecular sieve ( W a k o P u r e C h e m i c a l Incl. t y p e 5 A 1 / 1 6 ) . T h e m e t a s t a b l e H e ( 2 3 S ) a t o m s were generated by a 2450-MHz microwave discharge in the quartz tube with t h e i n p u t p o w e r a t 8 0 W. T h e e f f e c t o f c h a r g e d p a r t i c l e s p r o b a b l y i n v o l v e d in the discharge flow was examined by applying an electrostatic potential to a charged particle deflector. Sulfur dioxide (>99.9% purity) and oxygen C>99.99% purity) were used without further purification. The sample gas was introduced through a stainless steel nozzle 0.4 mm in diameter at about 15 cm downstream from the discharge section. The gas pressure was measured by a Pirnni gauge (ULVAC GP-2T) calibrated against a McLeod gauge. T h e heliT]m p r e s s u r e w a s 1 - - 2 t o r r a n d t h e s ~ m p l e g a s p r e s s u r e w a s 5 X 1 0 -3 5 X 1 0 -2 t o r t . The reaction flnme was focused by quartz lenses on the inlet slit of a scanning monochromator ( S h i r n a d z u G E - 1 0 0 ) w i t h a 1 2 0 0 l i n e m m -1 g r a t i n g b l a z e d a t 3 0 0 n r ~ ; i t s r e c i p r o c a l d i s p e r s i o n w a s 8 . 3 • m m -1. T h e p h o t o n s were detected photoelectrically with a photomultiplier (Hamamatsu TV R106UH) and an OP ~mplifier (B,irr--Brown 3523J). The wavelength was calibrated by means of a low-pressure mercury lnmp (Hnm~matsu TV L9870 3 ) ; i t s e r r o r w a s e s t i m a t e d t o b e w i t h i n -+2 A . T h e v i b r a t i o n a l a n a l y s i s o f the observed spect~lm was carried out by use of wavelength with maximum intensity. -
-
R E S U L T S AND DISCUSSION Figure 2 in a hel~,m to ~0 rim, responsible
s h o w ~ a t y p i c a l e m i ~ s i o n s p e c t r u m o f SO2 f r o m 2 4 0 t o 4 6 0 n m afterglow. Above weak continuous features extending from 265 n u m e r o u s rii~cxete b a n d s c a n b e o b s e r v e d . P o s s i b l e a c t i v e s p e c i e s for the appearance of these photoerni~ion are He(23•), He(21S),
259
-i
|
!
2~.0
Fig.
260
2. A typical
2:80
300
320
emi-~sion spect~lm
3~0
of
360
SO2
380
in a helinm
~
afterglow.
~0
~
Lines
marked
~
nm
* are
stray HeI lines.
H e +, a n d e l e c t r o n s i n t h e h e l i u m a f t e r g l o w a t 1 - - 2 t o r t . T h e c o n t r i b u t i o n f r o m H e + a n d e l e c t r o n s is n e g l i g i b l e , becaff-~? t h e i n t e h s i t i e s o f t h e ob~served spectra did not show any-appreciable change, when a d.c. voltage of 0--100 V w a s a p p l i e d t o t h e d e f l e c t o r e l e c t r o d e s . I t is k n o w n t h a t m e t a s t a b l e H e ( 2 1 8 ) atom-~ ~ p i d l y c o n v e ~ t o H e ( 2 3 S ) b y e l a s t i c colli.~ions w i t h elect~cons [7]. Therefore, the predominant energy carrier under experimental conditions here are metastable He(238) atoms with enough energy (19.8 eV) to ionize a n d dissociate sulfur dioxide. T h e energy-level diagrams o f various possible p r o d u c t s i n t h e H e ( 2 3 8 ) + SO2 r e a c t i o n a r e show~l in Fig. 3. A l t h o u g h W u a n d Y e n c h a [ 5 ] h a v e r e p o r t e d t h e d e t e c t i o n o f w e a k S O ( B a ~ - - X32~-) emi.~sion, t h i s b a n d s y s t e m c o u l d n o t b e i d e n t i f i e d i n o u r s p e c t r u m . I n s t e a d o f t h e B - - X s y s t e m , t h e ASH - - X 3 Z - t r a n s i t i o n o f S O , w h i c h w a s : a b s e n t i n t h e i r s p e c t r u m , was o b s e r v e d h e r e in t h e 2 4 0 - - 2 6 5 - n m regio n. T h e f l u o r e s c e n c e a n d p h o s p h o r e s c e n c e s p e c t r a o f SO2 h a v e b e e n s t u d i e d extensively b y U V photo~excita_tion. M e t t e e [ 1 4 ] has r e p o r t e d t h a t t h e fluo r e s c e n c e s p e c t r u m o f SO2 b e c ~ m e d i f f u s e w i t h a n i n c r e a s e , in t h e e x c i t a tion energy, and at excitation by the 265-nm line, a discrete resonance band w a s n o l o n g e r o b s e r v e d . _Wu a n d Y e n c h a [ 5 ] h a v e r e p o r t e d t h e o b s e r v a t i o n o f S O 2 ( A I B I - - X l A 1 ) e m i s s i o n i n t h e H e ( 2 S S ) + SO2 s y s t e m . H o w e v e r , t h e i r spectrum does not s h o w w h a t kind of 8 0 2 fluorescence, discrete b a n d and/ or continuous band, w a s identified. Althougil the di.~crete b a n d w h i c h w a s observed b y Mettee [14] at l o w excitation energies is not identified in our s_pect~Jm, the underlying continuous e m i ~ i o n w a s assigned to the _~IBi -~ a A ~ tran.~ition of SO2,- because its observed spectral range is in g o o d agreem e n t with his fluorescence, spectm]m at the highest excitation ~energy. ~_The absence of structure indicates that the excitation into lower ~ibmtional I e v e l s o f t h e AIB~ s t a t e is o f H t t l e i m p o r t a n c e i n t h e S e ( 2 3 8 ) + SO2 r e a c t i o n . S i n c e t h e f o r m a t i o n _ o f e x c i t e d p a r e n t s p e c i e s i n a heli~]m a_etergIow-has n o t b e e n r e p o r t e d , t h e p r e s e n t f i n d i n g o f t h e p h o t o e m i - q s i o n f r o m t h e sin_giet p a r e n t s p e c i e s is n o t e w o r t h y . "" T h e e x p a n d e d em~-~sion s p e c t r u m i n ~fJae 3 1 0 ~ i 0 - ~ m r e g i o n is s h o w n i n
260 eV 2 0 ___H~
SO
......
- -
so;
*
0°
so(x~z'},o'S-s)
SO'*
0
~--6'E,~c~J
~'B, _ _
m ~ _ - - x
I:Yl L 10
15 ~A, C~2
b -mA.~ ~;
SO
-,,"0
_ _
SO(B~.O(~)
- -
SO(A~n},O(~p)
- -
$o0~£~-oC ~)
E: ILl
SOz A T~.
Fig_ 3. T h e energy-level diagr~m~ o f possible p r o d u c t s in t h e He(23S) + SO2 r e a c t i o n . T h e diagrams were o b t a i n e d b y use o f t h e e x c i t a t i o n energies o f SO2 [ 8 ] a n d SO [ 9 ] , t h e ionization energies o f SO2 [ 1 , 3 ] . O [ 1 0 ] , a nd SO [ 1 1 ] , and t h e dissociation e n e r g y o f D(OS---O) = -5.5 eV [ 1 2 ] . SO~ was assigned b y r e f e r e n c e with t he nn~lyses o f Hillier and S a u n der s [ 1 3 ] an d o f L l o y d and R o b e r t s [ 3 ] .
Fig. 4. The most striking feature of the SO2 spectrum in the reaction with H e ( 2 3 S ) is t h e a p p e a r a n c e o f a r a t h e r s t r o n g , e x t e n s i v e b a n d s y s t e m , w h i c h has not been observed by the impact of VUV photons [4], 75--350-eV electrons [15], and 2--3-keVlab At* ions [16]. On the basis of the data in Fig. 3 , e n e r g e t i c a l l y p o s s i b l e c a n d i d a t e s f o r t h i s e m i ~ s i o n a r e SO + ( a X) and SO~(C, D, EX and C, D, E- A). The transition energies between various vibrational levels can be estimated by the high-resolution photoelectron data of SO [11] and SO2 [1--3]. We have calculated the transition energies of the Lwo c o m p o n e n t s o f S O +, a4H - - X 2 H ~/±.3t2, a s ~ ] m i n g t h e v i b r a t i o n a l i n t e r v a l s o f t h e X2 H a n d a4H s t a t e s t o b e a b o u t 1 3 6 0 a n d 8 0 0 c m -1. A l t h o u g h a n emi~ion series was identified in agreement with the calculated transition energies, irregular variations were found in the intensity rli~tribution of the Lwo c o m p o n e n t s . For the foxther cla_rifieation of the presence of the SO* (a--X) emir.sion, the emi-~ion spectr-m of 02 was measured under identical e x p e r i m e n t a l c o n d i t i o n s , b e c a u s e a n a l o g o u s i o n i c s t a t e s exi.~t i n m o l e c u l a r o x y g e n [ 1 1 ] . T h e o b s e r v e d s p e c t ~ l m f o r t h e H e ( 2 3 S ) + O2 r e a c t i o n w a s v e r y s i m i l a r t o t h a t o f R i c h a r d s o n a~ld S e t s e r [ 1 7 ] . T h e p r e d o m i n a n t emission w a s t h e A 2 I I , - - X2IIg s y s t e m o f O~. T h e s p i n - f o r b i d d e n t r a n s i t i o n a4H - X2II w a s n o t i d e n t i f i e d i n t h e s p e c t r a l r a n g e e x p e c t e d f r o m t h e p h o t o e l e c t r o n d a t a . T h e s e r e s u l t s i n d i c a t e t h a t S O + ( a - - X ) e m i s s i o n f r o m S O 2 is e x c l u d e d f r o m t h e p o s s i b l e c a n c l i d a ::es. R e m a i n i n g p o s s i b l e e m i s s i v e s p e c i e s is S O ~ f o r w h i c h s i x s t a t e s a r e k n o w , , by photoelectron spec~oscopy [1--3] : X (arlinbatic/P = 12.3 eV), A (13.0
261
t-~.o)-(vo.o}~-
~0
0
~
1
~o.
_
3%
=%
~
3
~
~0
~o
.--
~o
~
~
~ 5
~0
3~
r
!
6
3~
~0
,~0
7
6
~'o
~o
7
F i g . 4 . Fluorescence spectrum o f the SO~[C2B2(v~, 0, 0)--X2Az(v~", 0, 0)] system produced by the He(23S) Penning ionization o f SO~. Lines marked * are stray HeI lines.
eV), B (13.2 eV), C (16.0 eV), D (16.3 eV), and E (16.5 eV). The ground ionic s t a t e arises f r o m t h e r e m o v a l o f an e I e c t r o n f r o m t h e s u l f u r l o n e - p a i r 8az o r b i t a l , w h e r e a s t h e A , B, C, a n d D s t a t e s r e s u l t f r o m t h e l o s s o f a n e l e c t r o n f r o m t h e l a z , 5 b 2 , 2 b ~ , a n d 7a~ o r b i t a l s , r e s p e c t i v e l y [ 1 3 1 . A t f i r s t , w e attempted to interpret the observed spectrum according to Wu and Yencha's analysis of SO~(C--X) [5]. The corresponding band system of SO~(C--X) w a s f o u n d in o u r s p e c t r u m , t h e r e f o r e , a s l m H a r D e l a n d r e s t a b l e w a s o b t a i n e d . H o w e v e r , t h e f o l l o w i n g s e r i o u s d e f e c t s , as is a l s o f o u n d in t h e i r t a b l e , c a m e OUt. ( 1 ) T h e r e a r e l a r g e d i f f e r e n c e s b e t w e e n t h e o b s e r v e d w a v e - n u m b e r o f t h e b a n d o r i g i n ( 2 9 9 6 5 - + 1 5 c m -1) a n d t h e e s t i m a t e d v a l u e s f r o m t h e photoelectron data (29730-+ 121 [1] and 29682-+ 161 [2] cm-Z). (2) The t r a n s i t i o n e n e r g i e s o f t h e ( 0 , 0 , 0 ) - - (v~, 0 , 0 ) b a n d s f o r 0 ~< v~ ~< 3 a r e s y s t e m a t i c a l l y a b o u t 1 0 0 c m -z l o w e r t h a n t h o s e e s t i m a t e d f r o m t h e o t h e r bands, n~mely, the vibrational interval derived from the difference bebween t h e ( 0 , 0, 0 ) - - ( 3 , 0, 0 ) a n d ( 0 , 0 , 0 ) " ( 4 , 0 , 0 ) b a n d s is a b o u t 1 0 0 c m -z s m a l l e r t h a n t h e o t h e r intervAl.q.
262
A
0
OlD
~
0
o
.o A
A
o f.D CO
~ (3~
w--4
v--I
T ~ .,--,. t " -
1.0 L"O
¢X) 1.0
w--I ~.,.~
~¢0
I A C~D
0
i °
!
~ U.~
~
A CO
263 Taking into account the discrepancy between the observed origin and the estimated values from photoelectron data, a second attempt was. made to analyze the observed spectrum. This time we found the calculated wavelengths to fit the observed spectrum very well, when the value of 3376 ± 2 A~ W a s a s s i g n e d t o t h e b a n d o r i g i n : T h e d e t a i l e d v i b r a t i o n a l a s s i g n m e n t is given in Fig. 3. As shown in Fig. 3, most of the prominent discrete bands in the 310 510-nm region are ascribable to the C2B 2 -- X2AI transition of 805, where the upper and lower states are represented according to the assignm e n t o f L l o y d a n d R o b e r t s [ 3 ] : I n t h e o b s e r v e d s p e c t r a l r a n g e , C2B2(v~; 0 , 0 ) - - ~K2A~(o'z", 0 , 0 ) f o r 0 ~ o~ ~ 8 w i t h 0 ~ u'~ ~ 3 C a n b e i d e n t i f i e d . T h e D e l a n d r e s t a b l e f o r t h e ~ 2B 2- - ~ X 2A ~ t r a n s i t i o n o f S O ~ is g i v e n i n T a b l e 1 . T h e o b s e r v e d b a n d o r i g i n o f 2 9 6 2 0 +- 1 8 c m -~ a g r e e s r e a s o n a b l y w e l l w i t h t h e c a l c u l e t e d v a l u e s o f 2 9 7 3 0 +- 1 2 1 [ 1 ] a n d 2 9 6 8 2 -+ 1 6 1 [ 2 ] c m - ! f r o m photoelectron data. Combining the emission photon energies with the ioni z a t i o n p o t e n t i a l o f t h e X 2 A ~ ( 0 , 0 , 0 ) l e v e l , w h i c h is r e p o r t e d t o b e 1 2 . 3 0 5 eV [1], the ionization potential~ for the vibrational levels of the X2A z and C2B2 s t a t e s a r e o b t a i n e d a n d l i s t e d i n T a b l e 2 . One of the most prominent feaL~tres i n t h e ~ m i - ~ i o n s p e c t r l ] m O f S O 2 TABLE 2 T h e i o n i z a t i o n p o t e n t i a l s f o r t h e X2A z and C.2B2 vibrational levels o b t a i n e d f r o m the emission s p e c t r u m . P h o t o e l e c t r o n d a t a are also given f o r c o m p a r i s o n Vibrational levels
I o n i z a t i o n p o t e n t i a l s (eV) l~.m;~-~ion s p e c t r u m Present work
Photoelec~-on spectra E l a n d and Danby *
L l o y d and R o b e r t s **
Turner e t al. ~**
~,2A z
(0,0,0) (1,0,0) (2,0,0) (3,0,0) (4,0,0) (5,0,0)
(6,0,0) (7,0,0)
(..8.0,0)
12.305
15.977(2) 16.075(4) 16.171(5) 16.264(5)
15.986 16.092 16.188 16.284
12.29(0)
--
13.506(1)
C2B2 (0,0,0) (1,0.0)
(2,0,0) (3,0,0) * gel
12.305 12.461(2) 12.613(2) 12.766(3) 12.919(2) 13.067(3) 13.214(2) 13.3~3(1)
i.
*'~ Ref. 3. . . . . . * * * Ref. 2 : Q u a ntities in p a r e n t h e s e s are s t a n d a r d deviations. " -
15.992(3) 16.090(3) 16.189(5) -16.286(6) -
15.97(2)
-
-
: .._r
--
264 TABLE 3 The symr~etrical stretching vibrational frequencies for t h e X and C states o f SO~ ] ~ e c t ~ ' o ~ c states
Vibrational frequencies (cm -1 ) Emission s p e c t r u m Present w o r k
Molecular ground state X2A 1 CSB2
Photoelectron
sl3ec~
Eland and Danby *
L l o y d and Roberts **
Turner e t al. ***
-816 +_ 15
-782 -+ 20
-850
1151 -~
Ref. 1; ~* ref. 3; * ~
1259 _+ 13 788 + 14 ref. 2; -~ ref. 8.
b y H e ( 2 3 S ) P e n n i n g i o n i z a t i o n is a n a p p e a r a n c e o f t h e p r o g r e s s i o n s o f t h e symmetrical stretching vibration in the ground state, which has not been obs e r v e d in t h e p h o t o e l e c t r o n s p e c t r a . F r o m a n a n a l y s i s o f t h e S O ~ ( ~ - - ~ ) emission, the symmetrical stretching vibrational frequency of the ground i o n i c s t a t e w a s d e t e r m i n e d t o b e 1 2 5 9 -+ 1 3 c m - l . M o r e o v e r , +~he s y m m e t r i c a l stretching vibrational frequency of the C2Bs state was d~t~t~ined to be 7 8 8 -+ 1 4 c m -~. T h i s v a l u e is i n g o o d a g r e e m e n t w i t h t h e p h o t o e l e c t r o n v,~Jue o f 7 8 2 +- 2 0 c m -~ [ 3 ] , b u t i t is s l i g h t l y l o w e r t h a n o t h e r e x i s t i n g v a l u e s o f 8 1 6 -+ 1 5 [ 1 ] a n d 8 5 0 [ 2 ] c m -! a s s u m m a r i z e d i n T a b l e 3 . A n i n c r e a s e i n t h e vibrational frequency of the ground ionic state compared with that of the m o l e c ~ J | 3 r g r o u n d s t a t e s h o w s a n a n t i b o n d i n g c h a r a c t e r f o r t h e s~]lfur l o n e p a i r 8a~ o r b i t a l , w h i l e a l a r g e d e c r e a s e i n t h e v i b r a t i o n a l f r e q u e n c y o f t h e e x c i t e d C 2 B z s t a t e c o r r e s p o n d s t o t h e s t r o n g l y b o n d i n g n a t u r e o f t h e 2b~ o r b i talin S--O. F r o m t h e p r e s e n t o p t i c a l s t u d y , iC c a n b e c o n c l u d e d t h a t t h e m a j o r c h a n n e l i n t h e H e ( 2 3 S ) + S O s r e a c t i o n is t h e P e n n i n g i o n i z a t i o n i n t o t h e C 2 B 2 state. Molecular excitation (SOs*) and dissociative excitation (SO') are m i n o r r e a c t i o n s . T h e p r e s e n t o b s e r v a t i o n o f ~ h e .qO~ ern~=~ion, w h i c h h a s not been detected by any other excitation tecth~iques, lead us to conclude t h a t P e ~ n i n g i o n i z a t i o n is a p r o m i s i n g w a y o f g e n e r a t i n g i o n f l u o r e s c e n c e . SUMMARY T h e c o n i ~ i o n s o f I-Ie(23S) w i t h S O s g a v e a r e l a t i v e l y s t r o n g b a n d s y s t e m i n t h e U V a n d v i s i b l e r e g i o n s , i n a d d i t i o n t o w e a k e r emi.o~sion f r o m S O ( A X) and SO2(A --X). From the analy~s on the basis of high-resolution phot~ e l e c t r o n d a t ~ . t h i s b a n d w a s a s s i g n e d t o t h e C~B=(u~, 0 , 0 ) - - ~ . 2 A , ( u ~ , 0 , 0 ) t r a n s i t i o n o f SO~. T h e D e l a n d r e s t a b l e f o r thi~ t r a n s i t i o n a n d t h e i o n i z a t i o n
265
p o t e n t J a l ~ f o r t h e ~V,2At(v~' , 0 , 0 ) a n d C 2 B 2 ( v ~ , 0 , 0 ) v i b r a t i o n a l levels are given in rabies 1 and 2. The symmetrical -~Ltetching vibrational frequencies of the X and ~ states obtained from the analysis of the SO~(~ -- X) fluorescence are summarized in Table 3. ACKNOWLEDGEMENT Financial aid by the 2 7 4 1 4 4 ) is a c k n o w l e d g e d .
Ministry
of
Education
of
Japan
(Grant-in-Aid
REFERENCES 1 J . H . D . E I a n d a n d C.J. D a n b y , I n t . J. Mass S p e c t r o m . I o n P h y s . , I ( 1 9 6 8 ) 1 1 1 . 2 D.W. T u r n e r , C. B a k e r , A . D . B a k e r a n d C . R . B r u n d l e , M o l e c u l a r P h o t o e l e c t r o n S p e c troscopy, Wi]ey--Interscience, London, 1970. 3 D . R . L l o y d a n d P.J. R o b e r t s , Mol. P h y s . , 26 ( 1 9 7 3 ) 2 2 5 . 4 j.I-I_D. E l a n d , M. D e v o r e t a n d S. L e a c h , C h e m . P h y s . L e t t . , 4 3 ( 1 9 7 6 ) 97. 5 K.T. Wu a n d A . J . Y e n c b ~ C a n . J_ P h y s . , 5 5 ( 1 9 7 7 ) 7 6 7 . 6 R e a d e r s s h o u l d n o t be c o n f u s e d b y t h e a s s i g n m e n t o f SO~ ; A l t h o u g h Wu a n d Y e n c h a [ 5 ] have assigned t h e i o n i c s t a t e w i t h t h e a d i a b a t i c / P 1 6 . 0 e V t o t h e B s t a t e , w e h a v e a s c r i b e d it t o t h e C s t a t e . 7 A.V. Phelps, Phys. Rev., 99 (1955) 1307. 8 G. H e r z b e r g , E l e c t r o n i c S p e c t r a a n d E l e c t r o n i c S t r u c t u r e o f P o l y a t o m i c M o l e c u l e s , Van Nostrand Reinhold, New York, 1966. 9 R.W.B. P e z r s e a n d A . G . G a y d o n , T h e I d e n t i f i c a t i o n o f M o l e c u l a r S p e c t r a , 4 t h e d n . , C h a p r o ~ n a n d I-I~II, L o n d o n , 1 9 7 6 . 10 G. Her-zberg, A t o m i c S p e c t r a a n d A t o m i c S t r u c t u r e , D o v e r P u b l i c a t i o n s , N e w Y o r k , 1944. 11 J.M. D y k e , L. G o l o b , N. J o n a t h a n , A . Morris, M. O k u d a a n d D.J. S m i t h , J. C h e m . S o c . , F a r a d a y Tr~n~. H , 7 0 ( 1 9 7 4 ) 1 8 1 8 . 12 C. L a l o a n d C. V e r m e i l , J. P h o t o c h e m . , 3 ( 1 9 7 4 / 7 5 ) 4 4 1 . 13 I.H. Hillier a n d V . R . S a u n d e r s , Mol. P h y s . , 2 2 ( 1 9 7 1 ) 1 9 3 . 14 H . D . llfettee, J. C h e m . P h y s . , 4 9 ( 1 9 6 8 ) 1 7 8 4 . 15 W_H. S m i t h , J. Q u a n t . S p e c t r o s c . R a d l a t . Tr~n~., 9 ( 1 9 6 9 ) 1 1 9 1 ; M. Tsuji, H. F u k u t o m e , Y. N i s h i m u r a , T . O g a w a a n d N. r~hlb~h{, t o be p u b l i s h e d . 16 H. F u k u t o m e , M. Tsuji a n d Y. N i s h i m u r a , J. P h y s . S o c . J p n . , 4 4 ( 1 9 7 8 ) 1 4 0 1 . 17 W.C. R i c h a r d s o n a n d D.W. S e t s e r , J. C h e m . P h y s . , 5 8 ( 1 9 7 3 ) 1 8 0 9 .