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Fluorescence from excitation of CH4, CH3OH and CH3SH by extreme vacuum ultraviolet radiation
J Quant Specrrosc Radror Tran.fer Vol 44 No 4. pp 379-391, 1990
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00224073/90
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FLUORESCENCE FROM EXCITATION OF CH,, CH,OH AND CH,SH BY EXTREME VACUUM ULTRAVIOLET RADIATION GUANG MA, MASAKO SUTO, and L C Ltzt Molecular Engmeenng Laboratory, Department of Electrical and Computer Engmeenng, San Dlego State Umverslty, San Dlego, CA 92182. U S A (Recewed 22 March
Abstract-The were measured
1990)
photoabsorptton and fluorescence cross sectlons of CH4, CH,OH, and CHISH tn the wavelength regons of 52-106,48-106, and 48-106 nm, respecttvely The
fluorescence spectra were dtspexsed to Identify the emttmg species Emtsstons from the excited species of H+ and CH+ are commonly observed for all three molecules Emtsston from the excited CH: IS observed from CH,, OH* from CH,OH, and CS* from CH,SH The photoexatatton processes that may produce the observed ermsston bands are dmcussed
INTRODUCTION Fluorescence from excltatlon of molecules by vacuum ultraviolet (vuv) radtatton has received conttnuous study m our laboratory. and the results obtatned from Cl-i,. CH,OH, and CH,SH are reported m thts paper The absorptton cross secttons have been measured extenstvely for CH,, moderately for CHjOH, but very Me for CH3SH Fluorescence from vuv excitation of CH, has been observed I-5An excltatlon band that produces the u v -vlmble fluorescence has been observed’ m the 75-103 nm regton The fluorescence was dispersed’ and the enusslon systems were Identified to be CH(A*A, B*Z+X*II), H(n > 3-n = 2) and CH,(F ‘&+a’ ‘A,) The fluorescence cross sectlon m the 1 > 106 nm regon measured m an earlier expenment’ was much smaller than the later data ’ At J < 106 nm, the earlier values* are also much smaller than the current measurement This difference arouses interest for remvestlgatlon of CH, fluorescence In contrast to CH,, the fluorescence from vuv excltatlon of CH30H has not been studied extensively OH(A *Z+ +X ‘II,) emlsslon has been observed m the cxcltatlon wavelength repon of 1 > 87 nm,67 but the emlsslons from the excited species H* and CH* have not been reported previously For CH,SH, only CH,S* emlsslon IS reported m the 130-175 nm region ’ H* and CH* emlsslons were expected from photoexcltatlon of this molecule and were observed m the current expenments An unexpected CS(A ‘n-+X ‘Z+) emlssIon was also observed from thrs mokcule The fluorescence &f& %% *W&X +Ui ‘nfW&fgd~, fiJTai&zzW&i ~pkIzz~.~r~, i9inzXs Quantltatlve vuv spectroscopic data are needed for the study of interstellar photochermstry, because CH,, CHjOH, and CH3SH molecules exist m the Interstellar medium EXPERIMENTAL The photoabsorplton an& Buorescence cross se&tons were measured ustn_e. syrtchro~rcm radlab~n r 4=6-rwi- WKcxm~Jfl. Tk e%yflma.cal WqT k4.s W&‘&Z. . . wJv,s-2,v--9 ..a ..( produced by the &Z&W, %iPJWgZ been described III prr~ous papers “’ Thm-film wmdows of Al. Sn, and In were used for measurements in the regions of 48-60, 54-76, and 74-106 nm, respectively These wmdows were used to separate the high-vacuum monochromator from the gas cell (3 5 cm dla and 40 9 cm long), as well as to cut ofi htgh-order Itght The sample gas was slowly pumped by a sozptlon pump The fluorescence was observed simultaneously with the absorption measurement m a direction peqendlcular to the hght path tTo whom all correslponbence tnodrb QSRT U&A
be z&mm& 379
GANG
380
MA et al
Thee apoaralus useb SOTine’huorescence-bl~~~Taon expenmem’nas’veen ber>lneb m a DrevIous paper ‘@In brtef, the hght source was a condensed-capillary drscharge lamp that was discharged by a iI0 kV d c power supply pulsed by a hydrogen thyratron The dtscharge medtum was Ar or N, vvtth a trace of O2 The atomtc ton emtsslon hnes m the 45-200 nm region were selected by a l-m vacuum monochromaror There were no wmdows m the hghr path The fluorescence was dtspersed by an optIcal multIchannel analyzer with vanable trme delay and gate duration CH, was supphed by Matheson wtth a stated purtty better than 99 999/o CH,SH was supphed by M &W”G,Ta& A:L&w+zJP,+ SUEZ.& P~~,~~~~~*~ ‘bv~~p*. Z&W‘S 5??lr T:w US3 @+ J~iipw .vas ,&am-i from methanol lrqutd supphed by Ftsher Sctenttfic wtth a stated purny better than 99 9O80
RESULTS
CH, The absorptton and fluorescence cross secttons of CH, m the 52-106 nm region are shown m Ftgs I(a) and (b), respecttvely The quantum yreld, whrch ISdetermmed by the ratto of fluorescence to absorptton cross secttons, IS shown m Ftg I(c) In our measurements. the uncertamty ISgenerally - -n’ /(r fi!iY
-_ t
(a)
Absorption
CH,
l-
o,,,,,,.,,,,,,,,, (b)
2 a-
3 F
Fluorescence
r\ I
x10
1
r 60
(cl
Yield ‘\\ ‘T i
60 Wavelength
FIN I CH,
Absorpnon m the 52-106
“\_
/
04-
too (nm)
cross sect1011 (a) fluorescence cross secuon (b). and fluorescence quantum yeld (c) ol nm re@on The cross secuon IS rn umts of Mb (IO-‘* cm:) and the quantum yeld IS In percent
Fluorescence from excltatlon of CH,. CH,OH
I
(a)
99
and CH,SH
381
CH,
1 nm
CH&-ii) 1
00 (b)
83 4nm
CH(A-XI
20 CH(B-XI
(c)
78 5nm
2
H(3-2)
300
700
500 Wavelength
(nm)
FIN 2 Fluorescence spectra produced by photoexcItatIon of CH, at the excltabon wavelengths of 99 l-55 I nm The ldentlficatlons for the enusslon bands are mdlcated
The fluorescence cross sectlons m the 80-106 nm region are generally larger than the previous data’ by about one order of magmtude The value of about 3 7 x lo-” cm2 at 106 nm IS, however, consistent with the later measurement s The earher data2 were measured at gas pressures of + I50 mtorr, which are much higher than those employed m the current expenments Because of the high gas pressures used, the earlier data may be dlstorted by preabsorptlon and quenching effects that were not considered m the expenments ’ Fluorescence spectra from photoexcltatlon of CH, were observed at several excltatlon wavelengths m the 45 6-103 7 nm region The spectra observed at 99 I, 83 4, 76 5, and 55 1 nm are shown m Figs 2(a)-(d), respectively The fluorescence consists of the CH(A, f?+X), CH2(6-G), and H( n - 2) systems The CH(A -X) and CH(B-+X) systems start to appear at 99 I and 95 5 nm, respectively, and they become stronger at the shorter excltatlon wavelengths The CH,(F+ii) system starts at 132 nm and reaches a peak at about 106 nm 4 The emlsslon becomes very weak at 87 8 nm In Fig 3, the spectra at 99 I, 95 5, 92 3, and 87 8 nm are amplified to show the weak
GUANGMA et al
382
40
0 40
0
I
j
1 (c)
jl
92 3nm
2oo:‘iJ’ 04
’
CH(B-X)
-
I
I
(d)
1
I
I
I
87 7nm
I CH(A-X)
A
4ody1 0
d (0.1)
400
Wavelength
600 (nm)
Fig 3 Amphfied fluorescence spectra of CH, that show the weak emlsslon sqsrems of CH( 4 + Y) (0 I ) CH(B+X) (0.0). and CH,(~Y?)
CH(A -A’) (0, I), CH(B+X) (0,O) and CH2(b-G) systems The fluorescence mtenslty observed at 68 5 and 63 7 nm IS too weak to be dlstmgulshed from the noise level This weak mtenslty IS conslstent with the small fluorescence cross sectlon in the 62-76 nm region as shown III Fig I(b) The fluorescence cross sections m the 50-62 nm regions are smaller than those m the 8CklOOnm
t
CH(B-X)ICH(A-X)
60
Wavelength
(nm)
FIN 4 Ratros of the emlsslon mtensmes of the H( n = 3-2). CH(B +X) and CH,( 6-G) systems relative to the CH(A -X) (0 0) system measured at various wavelengths The data pomts are JOIned bj hnes for eyegulde
Fluorescence from exatatlon of
CH,, CH,OH and CH,SH
383
Table I Energy and wavelength thresholds for processes of CH, relevant to theobserved emthog photofragmente Bn0r.g
Procrm
(aV)
Umolo~th
CH2(;)+ H2
5 10
243 1
CH2(6)+ H2
5 98
207 3
9 19
134 9
CH2(6)+ 2H
10 50
118.1
CH(A) + H2 + H
12 06
102 a
CH(B) + H2 + H
12 41
99 9
CH(A) + 3H
16 50
74 8
CH(B) + 3H
16 93
73 2
CH
+H2+H
CH3
+ H(3)
16 64
74 5
CH3
+ H(4)
17 30
71 7
CH3
+ H(5)
17 61
7w4
(M)
region by more than one order of magrutude The atomic H(2+1) ermsslon was observed m the 50-62 nm region by Wu and Judge 4 The intensity ratios of the CH,(E-G), CH(B+X), and H(n = 3+2) emlsslons to CH(A +X) are shown m Rg 4 The ratios provtde the mformatlon for the emlsslon thresholds, that IS, the CH2( b-G) emlsslon ends at about 80 nm, and the H(3+2) emlsslon starts at about 62 nm The fluorescence data are useful for studymg the photodlssoclatlon process of CH, The energy and wavelength thresholds for the productlon of the emitting photofragments are hsted m Table I The thresholds are calculated from the heats of formulation” and the electromc excltatlon energes of CH(A, f3),14 CH,(F),15 and H(n) I6 The CH,(!?-G) ermsslon observed at wavelengths longer than 107 nm IS produced by the CH2( s) + H2 process, of wtnch the threshold 1s calculated to be at 207 3 nm The CH2( 6) + 2H process, whch has a threshold wavelength of I I8 I nm, may have a contnbutlon m the 80-100 nm region The CH(A +X) ermsslon ISobserved at 99 I nm, but not at 103 7 nm Since the threshold (102 8 nm) for the process of CH(A) + H2 + H process IS m between these two wavelengths, this process IS hkely responsible for the ermsslon The CH(B+X) emlsslon that starts to appear at 99 I nm IS likely produced by the CH(B) + Hz + H process Hrlth a threshold at 99 9 nm The CH(A, B+X) emlsslons observed m the 78-100 nm excltatlon band are definitely not produced by the CH(A, B) + 3H process, because the threshold energes of these processes are higher than the excitation photon energes The CH(A, B) + 3H processes may be responsible for the emlsslon observed m the 52-63 nm excltatlon band The H(n 2 3+2) ermsslons observed m this excltatlon band are produced by the CHj + H(n 2 3) process
CH,OH The absorption cross sectlon, fluorescence cross section, and fluorescence quantum yield of CH,OH are shown m Figs S(a)-(c), respectively The absorption cross section measured before 1979 has been summanzed by Berkowitz ” The current data are about the mean of the exlstmg data that are quite scattered In contrast to CH,, the fluorescence of CH,OH ISnot well studled Vmagradov and Vllesov6 observed fluorescence from photoexcttatlon of this molecule m the 86-143 nm repon, and they attnbuted the fluorescence to the OH(A +X) system wth a maximum yield of 0 7% at 88 nm The current fluorescence yield IS close to the earher data” at 86-98 nm, but larger by about a factor of 2 at 98-l06nm Because of the high expenmental uncertainty Inherent m the measurement of small fluorescence yleld, the agreement 1s considered to be satisfactory
GUANG
384
t
(a)
MA etal
Absorption
CH,OH
60
60 Wavelength
FIN 5
Same as
FIN 1,except
for
(nm)
CH,OH
at
48-106nm
The fluorescence cross section shows two excltatlon bands m the 50-68 and 68-100 nm regions The fluorescence spectra at 103 7, 99 I, 92 3. 83 4, and 76 5 nm are shown m Figs 6(a)+e). respectively The emlsslon consists of the OH(A -X) and CH(A, B-+X) systems The fluorescence spectra at 68 5. 63 7, 55 I. and 45 7 nm are shown m Figs 7(a)-(d), respectively Relative to the CH(A. B-+X) emlsnon, the OH(A -X) emtsslon system becomes weak at these wavelengths The H( n > 3-2) emlsslon starts to appear at 63 7 nm, and the Intensity becomes stronger at the shorter wavelength The energy and wavelength thresholds for the productlon of the emitting species are listed In Table 2 The threshold energtes are calculated from the heats of formation” ” I8and the electromc energies of the excited species ” ” The OH(A -X) emlsslon starts to appear at 147 nrnT and continues to the shorter wavelengths as shown m Figs 6 and 7 The OH(A -X) intensity decreases with decreasing excltatlon wavelength The CH(A -X) emtsslon starts to appear at 99 I nm This emlsslon can be produced by the dlssoclatlve excltatlon processes of CH(A) + H + H,O and CH(A) + Hz + OH with thresholds of II3 2 and 106 9 nm. respectively By comparison with the photoexcltatlon process of CH,, the CH(A ) + Hz + OH process IS more favorable The CH(A -X) emlsslon mtenslty Increases with decreasing excltatlon wavelength The Intensity ratlo of the CH(A +X)/OH(A -X) emlsslons IS shown m Fig 8 The ratio Increases largely toward the short wavelength, Indicating that the excltatlon band m the 70-90 nm regton mainly consists of the CH(A -X) emlsslon As shown m Fig 7. the H( n = 3+2) emtsslon appears at 63 7 nm. but not at 68 5 nm The H(n = 3) can be produced by the process of CH,OH + H(3) and CH,O + H(3). of which the threshold wavelengths are 76 7 and 70 7 nm respectively The H(n 2 3+2) systems habe emlsslon mtensltles comparable wtth CH(A -X) The intensity of H(3+2)/CH(A -X) IS shown m Fig 8 The fluorescence spectra at 63 7 and 45 7 nm show a background emlsslon m the 380-520 nm
Fluorescence from excttatlon of CH,, CH,OH
(a)
0
I
and CH,SH
385
103 7nm
(c)
A 0 3 ii
20
0
10
0 300
500 Wavelength
700 (nm)
Fig 6 Fluorescence spectra by photoexcltatlon of CH,OH at 103 7-76 5 nm The emwon CH(A, B-rXI and OH(A -X) are mdlcated
bands of
region, which could be the CO+(B+X) system This emlsslon may be produced by the process of CO+(A ) + 2Hz and CO ‘(A ) + 2H + Hz, of which the thresholds are 70 6 and 56 1 nm, respectrvely The CH+, CHO +, CH;, CH20+, and CH,OH + ions have been observed” from photolomzatlon of CH,OH m the 60-l IO nm region The photon energy m the current measurement IS sufficient to produce these Ions m the excited states that may subsequently emlt The possible emlsslons from these Ions are not observed m the current experiment, mdlcatmg that these Ions may not emlt Electron-impact excltatlon” 2’ of CH,OH also produces emlsslons from the excited OH(A), CH(A, B), H(n) and CO+(A, B), but not excited Ions Two-photon excltatlon of CH,OH by two ArF (193 nm) laser photons produces only the CH(A, B+X) emlsslons 22
CHJH The absorption cross sectlon, fluorescence cross sectlon, and quantum yield of CH, SH are shown m Figs 9(a)-(c), respectively The fluorescence cross sectlon IS very small, thus, the uncertamty
68
55
(a)
(c)
lnm
5nm
CH(A-X)
(nm)
__-___----
Wavelenglh
, CH,OH
Enerp
11 60
11 95 12 75
C"(A) + H2 + OH C"(B) t H2 + OH W(X)
17 18 17 53
+ "(4) + H(3) + H(4)
+ H + H tH
CH30 CH20 CH20 cn20
-
I6 84
CH2OH t H(4)
--
_- -
22 08
co+(A) + 2" + H2
---
17 57
18 50
CO+(A) + 2H2
+ H(5)
16 51
+ H(3)
CH30
18 20
16 la
CH20" + H(3)
C"(A) t ZH + OH
t H2 + OH(A)
16 11
11 30
+
C"(B) + H + H20
C”(A) 10 95
9 95
H + "20
OH
8 13
+ OH(A)
(aV)
4 08
Energy
relevant
_____-_-.--
56 1
70 6
67 0
68 1
IO 7
72 2
73 6
75 1
76 7
76 9
91 3
103 8
106 9
109 8
113 2
124 6
152 4
303 a
Wavelength(nm)
processes of CH ,OH
photofragmentr
lhresholds for photoexcllatlon
in the observed emutmg
and wvelenglh
+ OH
~"~(6) + H +
CH3
C"3
Pz-ocess
Table 2
- ..-
Fluorescence from excItabon of CH,, CH,OH and CH,SH
387
CH,OH l-
-
60
60 Wavelength
10
100
(nm)
FIN 8 lntenslry ratios of the H(n = 3+2)/CH(A +X) and the CH(A -+X)/OH(A -X) emwon
I
CH$H
(a)
Absorption
(c)
Yield
80
60 Wavelength FIN
9 Same as FIN I,
except for
(nm)
CH,SH at 48-106nm
systems
GLIANG MA
388
et al
(a)
99
CH,SH
1 nm
08
=
Fz 0,
‘:
- 1
o_
E,
h
Wavelength 10
Fluorescence
spectra
(c)
83 4 nm
(d)
76 5 nm
500
300
FIN
I
,
I
700
(nm)
b) pholoexcltatlon of CH,SH al 99 l-76 5 nm CH( 4 B-- \ I and CS( 4 +S) are mdlcaled
The emwion
bands ol
could be as high ds d factor of 2 To the authors knowledge, there are no exlstlng data to compare v+~th the current results Slmllar to CH, and CH,OH. there are two fluorescence excltatlon bands m the 50-72 and 72-100 nm regions The fluorescence spectra produced at 99 I. 92 3. 83 4. and 76 5 nm are shown In Figs IO(a)-(d) respectIveI) The spectra consist of the CH(.4. B+X) and CS(A -X) systems The CS( 4 +X) emlsslon IS confirmed by comparison with the fluorescence spectra from the LI.I~ eucltatlon” of CS: and OCS that Here observed by the same detection system The fluorescence spectra produced at 70 0. 63 7. 55 I dnd 45 7 nm are shown In Figs I I(aHd). respectively In addltlon to the CH* and CS* emlsslon, the H(n > 3 +2) hnes are also observed at these excltatlon wavelengths The energy and wavelength thresholds for the photoexcltatlon processes that produce the observed emlsslons are listed in Table 3. which are calculated from the heats of forrnatlon” ” ” dnd electromc energies” I6 of the excited photofragments The CH(.4 -X) emlsslon appears at 99 I nm. but not at 103 7 nm This emlsslon IS most likely produced by the dtssoclatlve excitation process of CH(A) + Hz + SH and or CH(A) + H + H?S. of which the thresholds are I I5 8 and 109 6 nm. respectively The CH(B+X) emlsslon IS produced by the same process The CS(,4 -X) emlsslon starts to appear at 99 I nm This emlsslon IS likely produced by the CS(A ) + 2H + Hz process, of which the threshold IS 99 4 nm The CS(A -A’) emlsslon Intensity Increases with decreasing
Fluorescence from excltahon of CH,, CH,OH and CH,SH
389
(a) 70 0 nm
1
300
500 Wavelength
700 (nm)
Fig 1I Fluorescence spectra by photoexatatlon
of CH,SH at 70 O-45 7 nm
Table 3 Energy and wavelength thresholds for photoexcltatlon processes of CH,SH relevant to the observed emttmg photofragments Energy
Process CH3
+
SH(A)
(eV)
Umolcngth
6 99
177 3
CH(A) +
H2 + SH
10 71
115 a
CH(B)
H2 +
11 06
112 1
+
SH
CH(A) +
H
+ H2S
11 31
109 6
CH(B)
+
H
+ H2S
11 66
106 3
CS(A)
+
H2 +
12 47
99 4
CH3S
+
HO)
16 00
77 5
CH3S
+
H(4)
16 66
74 4
CS(A)
+
4H
16 99
73 0
+
SH +
H(3)
20 04
61 9
+
SH + H(4)
20 70
59 9
CH2 CH2
2H
(run)
GUANG
390
MA et al
CH,SH
100
60 Wavelength FIN
I2
Intensity
ratios of H(n
(nm)
= 3+2)/CH(A
-X)
and CS(A +X)/CH(A
-X)
excnatlon wavelength The mtenstty rattos of CS(A +X)/CH(A -X) at various wavelengths are shown m Fig 12 The H(n b 3+2) emtsslons start to appear at 70 0 nm The H(3+2) emwslon IS likely produced by the CH>S + H(3) process, of whrch the threshold IS 77 5 nm The Intensity ratios of H(n = 3+2)/CH(A -X) are shown m Ftg 12 Emtsstons from the exctted H(n). CH(A, B), and CS(A) have been observed from the electron-Impact excttatron’4 of CH,SH The CH,S(A +X) emrsston m the 380-520 nm region has been observed from photoexcttatton of CH3SH m the 13&175 nm regron *Z However. the cross section for this emtssron IS so small (estimated to be < 10-20cm2) that tt IS not nottceable In the fluorescence spectrum The SH photofragment could be excited m the A state, but tt decays mamly through predlssoclatton’6 so that the expected SH(A -X) emtssron [srmrlar to OH(A +X)] IS not observed Photoexcltatton of CH,SH In the current wavelength regton could produce tons*’ (such ds CH,SH +, CH,S+. etc ) m the excned states, however, emrssrons from these tons were not observed CONCLUSIONS
The photoabsorptlon and fluorescence cross secttons of CH4, CH,OH, and CH,SH were measured m the extreme u v region All three molecules show two fluorescence excnatlon bands-one at 50-65 nm and the other at 75-100 nm The H(n -2) and CH(A, B+X) emtsstons are commonly observed n-r all three molecules In addltlon, the CH,(b-G) emwon IS observed from CH,. OH(A -X) from CH,OH, and CS(A -X) from CH,SH The CH and CS emlsslons are produced by dlssoctatlon of multiple bonds, further study of these photoexcttatton processes is of interest ~cktlowlvdgcmenrs--This report IS based on the work at the Uruversltv of Wlsconsm IS supported by NSF
supported
by NASA
and NSF
REFERENCES I P H Metzger and G R Cook, J Chem Phn 41, 642 (1964) 2 A R Welch and D L Judge. J Chem Phvs 57, 286 (1972) 3 L C Lee, E PhIllIps, and D L Judge, J Chem Php 67, 1237 (1977) 4 C Y R Wu and D L Judge, .I Chem fhrs 75, 172 (1981) 5 L C Lee and C C Chlang. J Chem Phys 78, 688 (1983) 6 I P Vlnogradov and F I Vllesov, Khlm Vys Energ 11, 25 (1977)
The synchrotron
radtatton
faclht\
Fluorescence from excltatlon of CH,, CH,OH and CH,SH
391
.J B Nee, M Suto, and L C Lee, Chem Phys 98, 147 (1985) I Tokue, A Hlraya, and K Shobatake, Chem Phys 116,449(1987) L C Lee and M Suto, Chem Phys 110,161(1986).L C Lee, J Chem Phys 72, 4334 (1980) L C Lee, J C Han, C Ye, and M Suto, J Chem Phys 92, 133 (1990) I 1 J Berkowtz, Photoabsorpttott, Phototontzatron. and Photoelectron Spectroscopy, Acadenuc Press, New
7 8 9 10
York, NY (1979) 12 J A R Samson and G Haddad, Data pubhshed m the review paper by J W Gallagher, C E Bnon, J A R Samson, and P W Langoff, J Phys Chem Ref Data 17, 9 (1988) 13 M W Chase, Jr, C A Davies, J R Dwoney. Jr, D J Frunp, R A McDonald, and A N Syverud, J Phys Chem Ref Data 14, Suppl No 1 (1985) 14 K P Huber and G Herzberg, Constants of Dtatomtc Molecules, Van Nostrand-Remhold, New York, NY (1979) I5 G He&erg and J W C Johns, Proc R Sot (Lottd ) A295, 107 (1966) 16 C E Moore, “Atonuc Energy Levels,” NSRDS-NBS 35, NatIonal Bureau Standards, Washmgton, DC (1971) 17 S W Benson, Thermochemtcal Ktnettcs, Wiley, New York, NY (1976) 18 H Okabe, Photochemtstry of Smalf Molecules. Wiley, New York, NY (1978) 19 J Berkowitz, J Chem Phys 69, 3044 (1978) 20 D E Donahue, J A Schlavone, and R S Freund, J Chem Phys 67, 769 (1977) 21 T Ogawa, S Ishlbashl, and H Kawazurm, J Phys Chem 88, 1662 (1984) 22 C Fotakq M Martm. K P Lawley, and R J Donavan, C’hem Phys Lert 67, I (1979) 23 L C Lee and D L Judge, J Chem Phys 63, 2782 (1975) 24 M Toyoda, T Ogawa , and N Ishbashl, Bull Chem Sot Japan 47, 95 (1974) 25 K Ohbayash, H Aklmoto, and I Tanaka, Chem Phys Lett 52, 47 (1977) 26 L C Lee, X Wang, and M Suto, J Chem Phys 86,4353 (1987) 27 R E Kutma, A K Edwards, G L Goodman, and J Berkowitz, J Chem Phys 77, 5508 (1982)
Pnnled MIGrcal
00224073/90
Bnuun All nghtsreserved
$3 00 + 0 00
Copyngh11Q1990krgamon PreWplc
FLUORESCENCE FROM EXCITATION OF CH,, CH,OH AND CH,SH BY EXTREME VACUUM ULTRAVIOLET RADIATION GUANG MA, MASAKO SUTO, and L C Ltzt Molecular Engmeenng Laboratory, Department of Electrical and Computer Engmeenng, San Dlego State Umverslty, San Dlego, CA 92182. U S A (Recewed 22 March
Abstract-The were measured
1990)
photoabsorptton and fluorescence cross sectlons of CH4, CH,OH, and CHISH tn the wavelength regons of 52-106,48-106, and 48-106 nm, respecttvely The
fluorescence spectra were dtspexsed to Identify the emttmg species Emtsstons from the excited species of H+ and CH+ are commonly observed for all three molecules Emtsston from the excited CH: IS observed from CH,, OH* from CH,OH, and CS* from CH,SH The photoexatatton processes that may produce the observed ermsston bands are dmcussed
INTRODUCTION Fluorescence from excltatlon of molecules by vacuum ultraviolet (vuv) radtatton has received conttnuous study m our laboratory. and the results obtatned from Cl-i,. CH,OH, and CH,SH are reported m thts paper The absorptton cross secttons have been measured extenstvely for CH,, moderately for CHjOH, but very Me for CH3SH Fluorescence from vuv excitation of CH, has been observed I-5An excltatlon band that produces the u v -vlmble fluorescence has been observed’ m the 75-103 nm regton The fluorescence was dispersed’ and the enusslon systems were Identified to be CH(A*A, B*Z+X*II), H(n > 3-n = 2) and CH,(F ‘&+a’ ‘A,) The fluorescence cross sectlon m the 1 > 106 nm regon measured m an earlier expenment’ was much smaller than the later data ’ At J < 106 nm, the earlier values* are also much smaller than the current measurement This difference arouses interest for remvestlgatlon of CH, fluorescence In contrast to CH,, the fluorescence from vuv excltatlon of CH30H has not been studied extensively OH(A *Z+ +X ‘II,) emlsslon has been observed m the cxcltatlon wavelength repon of 1 > 87 nm,67 but the emlsslons from the excited species H* and CH* have not been reported previously For CH,SH, only CH,S* emlsslon IS reported m the 130-175 nm region ’ H* and CH* emlsslons were expected from photoexcltatlon of this molecule and were observed m the current expenments An unexpected CS(A ‘n-+X ‘Z+) emlssIon was also observed from thrs mokcule The fluorescence &f& %% *W&X +Ui ‘nfW&fgd~, fiJTai&zzW&i ~pkIzz~.~r~, i9inzXs Quantltatlve vuv spectroscopic data are needed for the study of interstellar photochermstry, because CH,, CHjOH, and CH3SH molecules exist m the Interstellar medium EXPERIMENTAL The photoabsorplton an& Buorescence cross se&tons were measured ustn_e. syrtchro~rcm radlab~n r 4=6-rwi- WKcxm~Jfl. Tk e%yflma.cal WqT k4.s W&‘&Z. . . wJv,s-2,v--9 ..a ..( produced by the &Z&W, %iPJWgZ been described III prr~ous papers “’ Thm-film wmdows of Al. Sn, and In were used for measurements in the regions of 48-60, 54-76, and 74-106 nm, respectively These wmdows were used to separate the high-vacuum monochromator from the gas cell (3 5 cm dla and 40 9 cm long), as well as to cut ofi htgh-order Itght The sample gas was slowly pumped by a sozptlon pump The fluorescence was observed simultaneously with the absorption measurement m a direction peqendlcular to the hght path tTo whom all correslponbence tnodrb QSRT U&A
be z&mm& 379
GANG
380
MA et al
Thee apoaralus useb SOTine’huorescence-bl~~~Taon expenmem’nas’veen ber>lneb m a DrevIous paper ‘@In brtef, the hght source was a condensed-capillary drscharge lamp that was discharged by a iI0 kV d c power supply pulsed by a hydrogen thyratron The dtscharge medtum was Ar or N, vvtth a trace of O2 The atomtc ton emtsslon hnes m the 45-200 nm region were selected by a l-m vacuum monochromaror There were no wmdows m the hghr path The fluorescence was dtspersed by an optIcal multIchannel analyzer with vanable trme delay and gate duration CH, was supphed by Matheson wtth a stated purtty better than 99 999/o CH,SH was supphed by M &W”G,Ta& A:L&w+zJP,+ SUEZ.& P~~,~~~~~*~ ‘bv~~p*. Z&W‘S 5??lr T:w US3 @+ J~iipw .vas ,&am-i from methanol lrqutd supphed by Ftsher Sctenttfic wtth a stated purny better than 99 9O80
RESULTS
CH, The absorptton and fluorescence cross secttons of CH, m the 52-106 nm region are shown m Ftgs I(a) and (b), respecttvely The quantum yreld, whrch ISdetermmed by the ratto of fluorescence to absorptton cross secttons, IS shown m Ftg I(c) In our measurements. the uncertamty ISgenerally - -n’ /(r fi!iY
-_ t
(a)
Absorption
CH,
l-
o,,,,,,.,,,,,,,,, (b)
2 a-
3 F
Fluorescence
r\ I
x10
1
r 60
(cl
Yield ‘\\ ‘T i
60 Wavelength
FIN I CH,
Absorpnon m the 52-106
“\_
/
04-
too (nm)
cross sect1011 (a) fluorescence cross secuon (b). and fluorescence quantum yeld (c) ol nm re@on The cross secuon IS rn umts of Mb (IO-‘* cm:) and the quantum yeld IS In percent
Fluorescence from excltatlon of CH,. CH,OH
I
(a)
99
and CH,SH
381
CH,
1 nm
CH&-ii) 1
00 (b)
83 4nm
CH(A-XI
20 CH(B-XI
(c)
78 5nm
2
H(3-2)
300
700
500 Wavelength
(nm)
FIN 2 Fluorescence spectra produced by photoexcItatIon of CH, at the excltabon wavelengths of 99 l-55 I nm The ldentlficatlons for the enusslon bands are mdlcated
The fluorescence cross sectlons m the 80-106 nm region are generally larger than the previous data’ by about one order of magmtude The value of about 3 7 x lo-” cm2 at 106 nm IS, however, consistent with the later measurement s The earher data2 were measured at gas pressures of + I50 mtorr, which are much higher than those employed m the current expenments Because of the high gas pressures used, the earlier data may be dlstorted by preabsorptlon and quenching effects that were not considered m the expenments ’ Fluorescence spectra from photoexcltatlon of CH, were observed at several excltatlon wavelengths m the 45 6-103 7 nm region The spectra observed at 99 I, 83 4, 76 5, and 55 1 nm are shown m Figs 2(a)-(d), respectively The fluorescence consists of the CH(A, f?+X), CH2(6-G), and H( n - 2) systems The CH(A -X) and CH(B-+X) systems start to appear at 99 I and 95 5 nm, respectively, and they become stronger at the shorter excltatlon wavelengths The CH,(F+ii) system starts at 132 nm and reaches a peak at about 106 nm 4 The emlsslon becomes very weak at 87 8 nm In Fig 3, the spectra at 99 I, 95 5, 92 3, and 87 8 nm are amplified to show the weak
GUANGMA et al
382
40
0 40
0
I
j
1 (c)
jl
92 3nm
2oo:‘iJ’ 04
’
CH(B-X)
-
I
I
(d)
1
I
I
I
87 7nm
I CH(A-X)
A
4ody1 0
d (0.1)
400
Wavelength
600 (nm)
Fig 3 Amphfied fluorescence spectra of CH, that show the weak emlsslon sqsrems of CH( 4 + Y) (0 I ) CH(B+X) (0.0). and CH,(~Y?)
CH(A -A’) (0, I), CH(B+X) (0,O) and CH2(b-G) systems The fluorescence mtenslty observed at 68 5 and 63 7 nm IS too weak to be dlstmgulshed from the noise level This weak mtenslty IS conslstent with the small fluorescence cross sectlon in the 62-76 nm region as shown III Fig I(b) The fluorescence cross sections m the 50-62 nm regions are smaller than those m the 8CklOOnm
t
CH(B-X)ICH(A-X)
60
Wavelength
(nm)
FIN 4 Ratros of the emlsslon mtensmes of the H( n = 3-2). CH(B +X) and CH,( 6-G) systems relative to the CH(A -X) (0 0) system measured at various wavelengths The data pomts are JOIned bj hnes for eyegulde
Fluorescence from exatatlon of
CH,, CH,OH and CH,SH
383
Table I Energy and wavelength thresholds for processes of CH, relevant to theobserved emthog photofragmente Bn0r.g
Procrm
(aV)
Umolo~th
CH2(;)+ H2
5 10
243 1
CH2(6)+ H2
5 98
207 3
9 19
134 9
CH2(6)+ 2H
10 50
118.1
CH(A) + H2 + H
12 06
102 a
CH(B) + H2 + H
12 41
99 9
CH(A) + 3H
16 50
74 8
CH(B) + 3H
16 93
73 2
CH
+H2+H
CH3
+ H(3)
16 64
74 5
CH3
+ H(4)
17 30
71 7
CH3
+ H(5)
17 61
7w4
(M)
region by more than one order of magrutude The atomic H(2+1) ermsslon was observed m the 50-62 nm region by Wu and Judge 4 The intensity ratios of the CH,(E-G), CH(B+X), and H(n = 3+2) emlsslons to CH(A +X) are shown m Rg 4 The ratios provtde the mformatlon for the emlsslon thresholds, that IS, the CH2( b-G) emlsslon ends at about 80 nm, and the H(3+2) emlsslon starts at about 62 nm The fluorescence data are useful for studymg the photodlssoclatlon process of CH, The energy and wavelength thresholds for the productlon of the emitting photofragments are hsted m Table I The thresholds are calculated from the heats of formulation” and the electromc excltatlon energes of CH(A, f3),14 CH,(F),15 and H(n) I6 The CH,(!?-G) ermsslon observed at wavelengths longer than 107 nm IS produced by the CH2( s) + H2 process, of wtnch the threshold 1s calculated to be at 207 3 nm The CH2( 6) + 2H process, whch has a threshold wavelength of I I8 I nm, may have a contnbutlon m the 80-100 nm region The CH(A +X) ermsslon ISobserved at 99 I nm, but not at 103 7 nm Since the threshold (102 8 nm) for the process of CH(A) + H2 + H process IS m between these two wavelengths, this process IS hkely responsible for the ermsslon The CH(B+X) emlsslon that starts to appear at 99 I nm IS likely produced by the CH(B) + Hz + H process Hrlth a threshold at 99 9 nm The CH(A, B+X) emlsslons observed m the 78-100 nm excltatlon band are definitely not produced by the CH(A, B) + 3H process, because the threshold energes of these processes are higher than the excitation photon energes The CH(A, B) + 3H processes may be responsible for the emlsslon observed m the 52-63 nm excltatlon band The H(n 2 3+2) ermsslons observed m this excltatlon band are produced by the CHj + H(n 2 3) process
CH,OH The absorption cross sectlon, fluorescence cross section, and fluorescence quantum yield of CH,OH are shown m Figs S(a)-(c), respectively The absorption cross section measured before 1979 has been summanzed by Berkowitz ” The current data are about the mean of the exlstmg data that are quite scattered In contrast to CH,, the fluorescence of CH,OH ISnot well studled Vmagradov and Vllesov6 observed fluorescence from photoexcttatlon of this molecule m the 86-143 nm repon, and they attnbuted the fluorescence to the OH(A +X) system wth a maximum yield of 0 7% at 88 nm The current fluorescence yield IS close to the earher data” at 86-98 nm, but larger by about a factor of 2 at 98-l06nm Because of the high expenmental uncertainty Inherent m the measurement of small fluorescence yleld, the agreement 1s considered to be satisfactory
GUANG
384
t
(a)
MA etal
Absorption
CH,OH
60
60 Wavelength
FIN 5
Same as
FIN 1,except
for
(nm)
CH,OH
at
48-106nm
The fluorescence cross section shows two excltatlon bands m the 50-68 and 68-100 nm regions The fluorescence spectra at 103 7, 99 I, 92 3. 83 4, and 76 5 nm are shown m Figs 6(a)+e). respectively The emlsslon consists of the OH(A -X) and CH(A, B-+X) systems The fluorescence spectra at 68 5. 63 7, 55 I. and 45 7 nm are shown m Figs 7(a)-(d), respectively Relative to the CH(A. B-+X) emlsnon, the OH(A -X) emtsslon system becomes weak at these wavelengths The H( n > 3-2) emlsslon starts to appear at 63 7 nm, and the Intensity becomes stronger at the shorter wavelength The energy and wavelength thresholds for the productlon of the emitting species are listed In Table 2 The threshold energtes are calculated from the heats of formation” ” I8and the electromc energies of the excited species ” ” The OH(A -X) emlsslon starts to appear at 147 nrnT and continues to the shorter wavelengths as shown m Figs 6 and 7 The OH(A -X) intensity decreases with decreasing excltatlon wavelength The CH(A -X) emtsslon starts to appear at 99 I nm This emlsslon can be produced by the dlssoclatlve excltatlon processes of CH(A) + H + H,O and CH(A) + Hz + OH with thresholds of II3 2 and 106 9 nm. respectively By comparison with the photoexcltatlon process of CH,, the CH(A ) + Hz + OH process IS more favorable The CH(A -X) emlsslon mtenslty Increases with decreasing excltatlon wavelength The Intensity ratlo of the CH(A +X)/OH(A -X) emlsslons IS shown m Fig 8 The ratio Increases largely toward the short wavelength, Indicating that the excltatlon band m the 70-90 nm regton mainly consists of the CH(A -X) emlsslon As shown m Fig 7. the H( n = 3+2) emtsslon appears at 63 7 nm. but not at 68 5 nm The H(n = 3) can be produced by the process of CH,OH + H(3) and CH,O + H(3). of which the threshold wavelengths are 76 7 and 70 7 nm respectively The H(n 2 3+2) systems habe emlsslon mtensltles comparable wtth CH(A -X) The intensity of H(3+2)/CH(A -X) IS shown m Fig 8 The fluorescence spectra at 63 7 and 45 7 nm show a background emlsslon m the 380-520 nm
Fluorescence from excttatlon of CH,, CH,OH
(a)
0
I
and CH,SH
385
103 7nm
(c)
A 0 3 ii
20
0
10
0 300
500 Wavelength
700 (nm)
Fig 6 Fluorescence spectra by photoexcltatlon of CH,OH at 103 7-76 5 nm The emwon CH(A, B-rXI and OH(A -X) are mdlcated
bands of
region, which could be the CO+(B+X) system This emlsslon may be produced by the process of CO+(A ) + 2Hz and CO ‘(A ) + 2H + Hz, of which the thresholds are 70 6 and 56 1 nm, respectrvely The CH+, CHO +, CH;, CH20+, and CH,OH + ions have been observed” from photolomzatlon of CH,OH m the 60-l IO nm region The photon energy m the current measurement IS sufficient to produce these Ions m the excited states that may subsequently emlt The possible emlsslons from these Ions are not observed m the current experiment, mdlcatmg that these Ions may not emlt Electron-impact excltatlon” 2’ of CH,OH also produces emlsslons from the excited OH(A), CH(A, B), H(n) and CO+(A, B), but not excited Ions Two-photon excltatlon of CH,OH by two ArF (193 nm) laser photons produces only the CH(A, B+X) emlsslons 22
CHJH The absorption cross sectlon, fluorescence cross sectlon, and quantum yield of CH, SH are shown m Figs 9(a)-(c), respectively The fluorescence cross sectlon IS very small, thus, the uncertamty
68
55
(a)
(c)
lnm
5nm
CH(A-X)
(nm)
__-___----
Wavelenglh
, CH,OH
Enerp
11 60
11 95 12 75
C"(A) + H2 + OH C"(B) t H2 + OH W(X)
17 18 17 53
+ "(4) + H(3) + H(4)
+ H + H tH
CH30 CH20 CH20 cn20
-
I6 84
CH2OH t H(4)
--
_- -
22 08
co+(A) + 2" + H2
---
17 57
18 50
CO+(A) + 2H2
+ H(5)
16 51
+ H(3)
CH30
18 20
16 la
CH20" + H(3)
C"(A) t ZH + OH
t H2 + OH(A)
16 11
11 30
+
C"(B) + H + H20
C”(A) 10 95
9 95
H + "20
OH
8 13
+ OH(A)
(aV)
4 08
Energy
relevant
_____-_-.--
56 1
70 6
67 0
68 1
IO 7
72 2
73 6
75 1
76 7
76 9
91 3
103 8
106 9
109 8
113 2
124 6
152 4
303 a
Wavelength(nm)
processes of CH ,OH
photofragmentr
lhresholds for photoexcllatlon
in the observed emutmg
and wvelenglh
+ OH
~"~(6) + H +
CH3
C"3
Pz-ocess
Table 2
- ..-
Fluorescence from excItabon of CH,, CH,OH and CH,SH
387
CH,OH l-
-
60
60 Wavelength
10
100
(nm)
FIN 8 lntenslry ratios of the H(n = 3+2)/CH(A +X) and the CH(A -+X)/OH(A -X) emwon
I
CH$H
(a)
Absorption
(c)
Yield
80
60 Wavelength FIN
9 Same as FIN I,
except for
(nm)
CH,SH at 48-106nm
systems
GLIANG MA
388
et al
(a)
99
CH,SH
1 nm
08
=
Fz 0,
‘:
- 1
o_
E,
h
Wavelength 10
Fluorescence
spectra
(c)
83 4 nm
(d)
76 5 nm
500
300
FIN
I
,
I
700
(nm)
b) pholoexcltatlon of CH,SH al 99 l-76 5 nm CH( 4 B-- \ I and CS( 4 +S) are mdlcaled
The emwion
bands ol
could be as high ds d factor of 2 To the authors knowledge, there are no exlstlng data to compare v+~th the current results Slmllar to CH, and CH,OH. there are two fluorescence excltatlon bands m the 50-72 and 72-100 nm regions The fluorescence spectra produced at 99 I. 92 3. 83 4. and 76 5 nm are shown In Figs IO(a)-(d) respectIveI) The spectra consist of the CH(.4. B+X) and CS(A -X) systems The CS( 4 +X) emlsslon IS confirmed by comparison with the fluorescence spectra from the LI.I~ eucltatlon” of CS: and OCS that Here observed by the same detection system The fluorescence spectra produced at 70 0. 63 7. 55 I dnd 45 7 nm are shown In Figs I I(aHd). respectively In addltlon to the CH* and CS* emlsslon, the H(n > 3 +2) hnes are also observed at these excltatlon wavelengths The energy and wavelength thresholds for the photoexcltatlon processes that produce the observed emlsslons are listed in Table 3. which are calculated from the heats of forrnatlon” ” ” dnd electromc energies” I6 of the excited photofragments The CH(.4 -X) emlsslon appears at 99 I nm. but not at 103 7 nm This emlsslon IS most likely produced by the dtssoclatlve excitation process of CH(A) + Hz + SH and or CH(A) + H + H?S. of which the thresholds are I I5 8 and 109 6 nm. respectively The CH(B+X) emlsslon IS produced by the same process The CS(,4 -X) emlsslon starts to appear at 99 I nm This emlsslon IS likely produced by the CS(A ) + 2H + Hz process, of which the threshold IS 99 4 nm The CS(A -A’) emlsslon Intensity Increases with decreasing
Fluorescence from excltahon of CH,, CH,OH and CH,SH
389
(a) 70 0 nm
1
300
500 Wavelength
700 (nm)
Fig 1I Fluorescence spectra by photoexatatlon
of CH,SH at 70 O-45 7 nm
Table 3 Energy and wavelength thresholds for photoexcltatlon processes of CH,SH relevant to the observed emttmg photofragments Energy
Process CH3
+
SH(A)
(eV)
Umolcngth
6 99
177 3
CH(A) +
H2 + SH
10 71
115 a
CH(B)
H2 +
11 06
112 1
+
SH
CH(A) +
H
+ H2S
11 31
109 6
CH(B)
+
H
+ H2S
11 66
106 3
CS(A)
+
H2 +
12 47
99 4
CH3S
+
HO)
16 00
77 5
CH3S
+
H(4)
16 66
74 4
CS(A)
+
4H
16 99
73 0
+
SH +
H(3)
20 04
61 9
+
SH + H(4)
20 70
59 9
CH2 CH2
2H
(run)
GUANG
390
MA et al
CH,SH
100
60 Wavelength FIN
I2
Intensity
ratios of H(n
(nm)
= 3+2)/CH(A
-X)
and CS(A +X)/CH(A
-X)
excnatlon wavelength The mtenstty rattos of CS(A +X)/CH(A -X) at various wavelengths are shown m Fig 12 The H(n b 3+2) emtsslons start to appear at 70 0 nm The H(3+2) emwslon IS likely produced by the CH>S + H(3) process, of whrch the threshold IS 77 5 nm The Intensity ratios of H(n = 3+2)/CH(A -X) are shown m Ftg 12 Emtsstons from the exctted H(n). CH(A, B), and CS(A) have been observed from the electron-Impact excttatron’4 of CH,SH The CH,S(A +X) emrsston m the 380-520 nm region has been observed from photoexcttatton of CH3SH m the 13&175 nm regron *Z However. the cross section for this emtssron IS so small (estimated to be < 10-20cm2) that tt IS not nottceable In the fluorescence spectrum The SH photofragment could be excited m the A state, but tt decays mamly through predlssoclatton’6 so that the expected SH(A -X) emtssron [srmrlar to OH(A +X)] IS not observed Photoexcltatton of CH,SH In the current wavelength regton could produce tons*’ (such ds CH,SH +, CH,S+. etc ) m the excned states, however, emrssrons from these tons were not observed CONCLUSIONS
The photoabsorptlon and fluorescence cross secttons of CH4, CH,OH, and CH,SH were measured m the extreme u v region All three molecules show two fluorescence excnatlon bands-one at 50-65 nm and the other at 75-100 nm The H(n -2) and CH(A, B+X) emtsstons are commonly observed n-r all three molecules In addltlon, the CH,(b-G) emwon IS observed from CH,. OH(A -X) from CH,OH, and CS(A -X) from CH,SH The CH and CS emlsslons are produced by dlssoctatlon of multiple bonds, further study of these photoexcttatton processes is of interest ~cktlowlvdgcmenrs--This report IS based on the work at the Uruversltv of Wlsconsm IS supported by NSF
supported
by NASA
and NSF
REFERENCES I P H Metzger and G R Cook, J Chem Phn 41, 642 (1964) 2 A R Welch and D L Judge. J Chem Phvs 57, 286 (1972) 3 L C Lee, E PhIllIps, and D L Judge, J Chem Php 67, 1237 (1977) 4 C Y R Wu and D L Judge, .I Chem fhrs 75, 172 (1981) 5 L C Lee and C C Chlang. J Chem Phys 78, 688 (1983) 6 I P Vlnogradov and F I Vllesov, Khlm Vys Energ 11, 25 (1977)
The synchrotron
radtatton
faclht\
Fluorescence from excltatlon of CH,, CH,OH and CH,SH
391
.J B Nee, M Suto, and L C Lee, Chem Phys 98, 147 (1985) I Tokue, A Hlraya, and K Shobatake, Chem Phys 116,449(1987) L C Lee and M Suto, Chem Phys 110,161(1986).L C Lee, J Chem Phys 72, 4334 (1980) L C Lee, J C Han, C Ye, and M Suto, J Chem Phys 92, 133 (1990) I 1 J Berkowtz, Photoabsorpttott, Phototontzatron. and Photoelectron Spectroscopy, Acadenuc Press, New
7 8 9 10
York, NY (1979) 12 J A R Samson and G Haddad, Data pubhshed m the review paper by J W Gallagher, C E Bnon, J A R Samson, and P W Langoff, J Phys Chem Ref Data 17, 9 (1988) 13 M W Chase, Jr, C A Davies, J R Dwoney. Jr, D J Frunp, R A McDonald, and A N Syverud, J Phys Chem Ref Data 14, Suppl No 1 (1985) 14 K P Huber and G Herzberg, Constants of Dtatomtc Molecules, Van Nostrand-Remhold, New York, NY (1979) I5 G He&erg and J W C Johns, Proc R Sot (Lottd ) A295, 107 (1966) 16 C E Moore, “Atonuc Energy Levels,” NSRDS-NBS 35, NatIonal Bureau Standards, Washmgton, DC (1971) 17 S W Benson, Thermochemtcal Ktnettcs, Wiley, New York, NY (1976) 18 H Okabe, Photochemtstry of Smalf Molecules. Wiley, New York, NY (1978) 19 J Berkowitz, J Chem Phys 69, 3044 (1978) 20 D E Donahue, J A Schlavone, and R S Freund, J Chem Phys 67, 769 (1977) 21 T Ogawa, S Ishlbashl, and H Kawazurm, J Phys Chem 88, 1662 (1984) 22 C Fotakq M Martm. K P Lawley, and R J Donavan, C’hem Phys Lert 67, I (1979) 23 L C Lee and D L Judge, J Chem Phys 63, 2782 (1975) 24 M Toyoda, T Ogawa , and N Ishbashl, Bull Chem Sot Japan 47, 95 (1974) 25 K Ohbayash, H Aklmoto, and I Tanaka, Chem Phys Lett 52, 47 (1977) 26 L C Lee, X Wang, and M Suto, J Chem Phys 86,4353 (1987) 27 R E Kutma, A K Edwards, G L Goodman, and J Berkowitz, J Chem Phys 77, 5508 (1982)