Volume 75, number 1 CHEMICAL PHYSICS LE-ITERS 1 October 1980

SPECTRA AND LIFETIME OF THE HSO RADICAL (i *A’-% *A”)

Masahiro KAWASAKI, Kazuo KASATANI and Hrroyasu SAT0 Cl~errusrrv Departwent of Resources, Faculrv of Engmeetmg. bh e Um~ers~t_v h~amrizarna-Clro Tsar. 514 Japan

Received 28 May 1980. m final form 11 July 1980

Chemdummcscence. laser-mduced tkoresccnce spectra and the fluorescence hfetlme of HSO (x 2A’-%2A”) rddlcals XC measured m the g.~s phase III the wawlcngth range 570-700 nm The progresslon of the SO strctchmg mode (u3) IS observed m cmlsslon and cxc~tat~on spectra The zero-pressure hfetune cxtrapolatcd from the Stern-Volmer plots IS 11 3:: “7 PS for the x *A’ state.

1. Introduction

The HSO radrcal 1s a monohydrlde radical which absorbs and emits vlslble hght. Chemllummescence studies have been carried out by Becker et al. [1,21. The electronically and vibrationally excited HSO radical has been generated as a product in the reaction between HZS and O3 m thz presence of atonuc oxy-

gen_ Emission from HSO (A “A’) IS observed up to IJ; = 7. SIagIe et al. [3 ] suggested the formation of HSO @ 2A”) III the reaction of various mercaptans with atomic oxygen. High-resolution spectroscopy of the HSO ra&cal has been reported by Kakimoto et al. [4] and Ohati et al. [5], gving precise molecular constants.

Because of the forbidden (n-n*) character of the x-z transition, the ralahve hfetime of x 1s expected to be fatly long. The half-hfe for t.h~s transition IS cal- culated to be = 40 /JS [6]. No experimental work on this property has however been reported. in tlus paper, we report the measurement of cherrulurninescence, laser-induced fluorescenuz_e spectra, and the fluores- cence lifetune of HSO (A).

2. Experimental

The expenmental arrangement was sun&u to that described in refs. [ 1,2,4,5]. In brief, HSO radicals

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were generated by reactlon of H2S and discharge products of oxygen molecules_ The fluorescence cell was made of a Pyrex glass tube (35 mm diameter, 600 mm long) with two light baffles at each end. The molecules were pumped out from the cell by a rotary pump (950 Q/nun) and a liquid N, trap. Hydrogen sulfide or ethyl mercaptan was mixed coaxially wth the products of a 2450 MHz rmcrowave lscharge in a flowmg O,/Ar mixture at total pressure lo-120 mTorr. Argon was added m order to keep the dis- charge stable even at low pressure. Because the pres- sure was measured by a plrr& gauge calibrated for au, absolute pressure was corrected for the gas rmxture for whch it was calculated to be less sensitive than a~ by a factor 0.9. The pressure and flow rates were optunized with respect to the mtensity of the spec- trum under study.

For chermlummescence study, the typical flow ra- tio of H,S/O,/Ar was l/l/l. Total flow rate was 2 mmol/min at the total pressure of 110 mTorr. In the system of H,S/O, = l/l, chermlurninescence was stronger than in the H,S/O,/Ar system. Spectra were taken with a 1 mm slit width (Ah = 3 MI) of a mono- chromator (N&on G-250, 1200 lines/mm blazed at 500 run) and a Hamamatsu R928 photomultlplier with a cut-off filter (Toshiba Y-45).

For laser-induced fluorescence (LIF) study, the flow ratio of H2S was increased in order to ehminate the chemiluminescence, typically H,S/O,/Ar = 2/1/l.

Volume 75, number 1 CHEMICAL PHYSICS LETTERS 1 October 1980

CzH,SH/02/Ar was also used because there was no interference by che~ummescence_ Two types of dye lasers w&h rhodamme 6G (RBG) or luton red S (KRS) dyes were used, One was a cw type (Spectra- Physics DL-375) which was pumped by the 5 W out- put from an argon ion laser (Spectra-Physics 165). LIF spectra were taken with tIus laser. The laser beam was mecharucalIy chopped at 15 Hz and the signal from the photom~ltip~er w&h four cut-off falters (Toshiba R-62 for R6G dye and R-64 for KRS dye) detected by a Iock-m amphfier (IZvans Associates 41 lo/41 14). In order to reduce the scattered laser I&t, the fluorescence IQht was collected through the waII of the celi w&h a lens (focal length 50 mm,f= I .4). The other Iaser used was a Bash-Iamp-pumped dye laser. The laser pulse duration was 0.8 ~.ts. The output from the photom~tipher was terminated by a 1 kS_L resistance and fed mto a multichannel anrdyser (Kawasaki Electronica M-50E/TMC-400, 100 ns/chan- nel) for accumulation.

3. Results and discussion

Fig. I shows a typical example of the chemi.htmines-

cence spectrum of HSO (x2A’ -+ z2A”) with the nurture of H,S/O, it is essentially the same as that obtained by Becker et al. [I ,2]. The mixture H$/ O,/Ar gave weaker signal mtenslty because of smaller

I 600 650 700 750

Wavelength I nm

Fxg. 1. HSO chemdummescence spectrum (“A*A’ - ?7*A”) tn the system 0/03/HzS Total pressure 40 mTorr.

ozone formation, in accordance with the proposed che~ummescence reaction [ 1,3], SH f O3 + HSO (x 2A’) f 0,. Usmg reported spectroscopic constants (Te = 14691 cm-*, ui = 672cm-1,w~_ul, =4cm-1, WE = 1026 cm-l, wz _v~ = 6 cm-1 for v3) [S], the bands are assrgned to the SO stretching mode (v-& Two other series of bands of bending (uz) and SH stretching (vl) modes were not observed_ This agrees with the theoretical calculation, that is, the SO bond length IS Ionger in the x “A’ state, wNe the other two coordmates change less on excitation [6]_

The chemllummescence mtensity increased with the totaI pressure up to 50 mTorr, while at higher pressures the intensity decreased_ The relative popu- lation of uj = I, 2,3 and 4, whtch is given by the chemrlurnmescence mtensity drvlded by Franck- Condon factors, wavenumber cubed, and the detector response, was 0 -71 I/O _6/0.2. This ratio did not change over the total pressure range 20-140 mTorr. This ratlo 1s close to what Schurath et al. [2] obtained at 1 2 Torr. The eiectromc quenching must be faster than the ~bration~ relaxation in the v3 m~fold of the ‘A’ state.

In DSO collisIonal popuIation of the u> IeveI by addrtion of water is observed probably because the ZJ~ Ieveis fall shghtly below the ~j levels [Z]. In HSQ sunpie cafculation in the Morse approximation (w: = 828 cm-l, wk.; = 13 cm-l, we” = 1063 cm-i, c&r~ = 21 cm-l for ~2) predicts thar the vz bands merge into the mtense nel~bo~g u3 bands *_

3 2. Laser-ulduced jluorescer~ce spectra

The fluorescence excitation spectrum of the HSO ra&ca.I IS shown m figs. 2 and 3. The transltro.ns of v3 (4,0), (3,0) and (2,0) are observed_ The relative intensity of the (4,O) and (3,0) transitions in fig_ 2 IS reversed compared to that in the ~hern~u~~s~eRce spectrum of fig. 1. ‘This is because fig. 2 is not cor- rected for the Iaser intensity. After correction for the laser power the ratios of the Franck-Condon factor of ~4,0)/(3,0)/(2,0) are obtained to be l/1.0/0.71 in good agreement with the calculated value of L/&97/

* For the ~2 viirational mode, anharmontcity constants WCEP: calculated using an equation for w5/4De in she Morse ap proxunation. For the VZJ mode, thffe values are obtained from table 1 of ref. [I].

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Volume 75. number 1 CHEMICAL PHYSICS LETTERS 1 October 1980

Wavelength I nm

Fig. 2. Laser-induced fluorescence spectrum of HSO corre- qondmg to the transltion between the z2A” (000) level and A2A’ (003) or (004) level. H2S/02/Ar nuxture 1s used In- tensity is not corrected for R6C dye laser power marked as o.

0.71 assuming the diatormc approximation of the Morse function for P-E = 1.494 A and r: = 1.661 8.

The relative intensity of the (2,0)/(3,0) transltlon did not change within the pressure range of ethyl mercaptan (S-30 mTorr) or by the presence of water (O-100 mTorr). There 1s slight dependence of reactlon rates on vibrational levels, u; = 2 and 3.

Although the v2 (2,0) transitlon IS expected at 619 run, slight absorption could be detected. Isoelec- tronic HO, molecules have smular absorption and emission spectra which are characterized by a sequence in the u3 mode because the bond angles for the ground and excited states of HO, are very sirmlar [7]. In con- trast to HSO and HO,, isoelectromc HNF spectrum 1s dominated by a progression in the v2 bending mode [8]. Woodman [9] has noted that the first excited state of these molecules corresponds to a promotion

610 620 630 6CO Wavelength I nm

Fig. 3. Laser-mduced fluorescence spectrum of HSO corre- zonding to the transition between the % 2A” (000) level and A’A’ (002) and (003) level. QHsSHj02/Ar mixture is used. Intennty LS not corrected for KRS dye laser power marked aso.

from the 5a’ to the 2a” orbital. In the linear contigura- tion, the ground electronic state and the excited state correlate with the 2KI state because of the degeneracy of these two orbitals. ‘l-he (a’)-(n) orbital is expected to strongly favor the bent configuration whereas the (a”)-(n) orbital vanes only slightly with bond angles. In HNF tis stabdization of a’ is much more pro- nounced 111 comparison with both the a’ orbltais for HSO and HO,. It 1s this shght stabdization which leads to the small bond angle changes (A0 = 5-l lo) for the x 2A’-% ZA” transition of HSO.

3.3. Fluoresceme lifetime

The fluorescence decays were measured at various pressures_ The flow rate ratio of H3S/02/Ar was kept constant during these measurements. This ratio was very cntlcal to obtain good S/N ratio. First we opti- mized tis flow rate ratro observing the signal mten- sity with the Ar+ laser pumped dye laser, then, the pulsed dye laser was used for the lifetune measure- ment. Fluorescence decay curves were obtained by accumulating =16000 shots. Rg. 4 shows a typlcal fluorescence decay excited at 583 nm corresponding to the v3 (4,0) transition. The Stern-Volmer plots are shown in fig. 5 where the fluorescence hfetlmes were obtained in the total pressure range of lo-120 mTorr. The zero-pressure limit of hfetune extrapolated from fig. 5 1s 11_3’::“7 j_ts. The fluorescence hfetime is calculated to be -40 PS usmg an ab irutlo SCF Cl method [6]. The HNO transition correspondmg to that in HSO has a lifetune of ~8 m [lo]. These long lifetimes are attributed to the forbidden character of the (n--71*) transition.

2 4 !s lb 1; * I

20 25 30 Time I us

Fig. 4. Typical decay curves of laser-induced fluorescence signal of HSO (x2A’) exated at 583 run correspondmg to the u: = 4 + u; = 0 transition.

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Volume 75, number 1 CHEMICAL PHYSICS LETTERS 1 October 1980

-0 iI

IJ

0

0 50 100 150 Total Pressure I mTorr

Fig 5 Stern-Volmer plots of l/r for the HSO (x*A’) (004) level Pressure IS mdlcated for the mature of H2S/02/Ar

(2/1/l)

The rate constant for quenching by the gas nux- ture of HZS/02/Ar (2/1/l) IS (4 C 1) X IO-” cm3/ mzlecule s for the u; = 4 vibromc level. In the HNO (A) state the electronic quenching IS enhanced by HNO bendmr mod% This IS also expected for HSO because the A and X states degenerate into the *IT state in the hnear configuratron. With DSO we will be able to measure the effect of vrbrational modes on quenching rates because the v2 band would appear in thrs wavelength region wrthout merging into u3 bands.

and Mr. T. Matsumura for his help in experiment. This work was supported partly by a Grant-in-Aid for Scientrfic Research from the Mimstry of Education, Science and Culture.

References

[II

121

I31

[41

is1

161

[71

F31

PI [IO1

K.H. Becker, hI A. lnoci%c~o and U. Schurath. Intern. J Chem. Kmetlcs 1s (1975) 205. U. Schurath, hf. Wcber and K.H Becker, J. Chem. Phys. 67 (1977) 119. I R. Slagle. R.E. Graham and D. Gutman, Intern. J. Chem. Rmetrcs 8 (1976) 451. M. Rakrmoto, S Saito and E Huota, J MoL Spectry., to be pubhshed. N Oh&u, M Rakimoto, S. Saito and E. Hrrota, J. MoL Spectry., to be pubbhed A B. Sann~grahi, K H. Thunemann, S.D. Peyerunhoff and R J. Buenker, Chcm Phys. 20 (1977) 25. H.E. Hunzlker and H F. Wendt, J. Chem. Fhys. 60 (1974) 4622, ICH. Becker, E.H. Fmk, P. Lanzen and U. Schurath, J Chem. Phys 60 (1974) 4623. D.hl. Lindsay, J.L Cole and J.R. Lombardr,Chem. Phys. 37 (1979) 333. CM. Woodman, J. hloL Spectry. 33 (1970) 3 11. F. Yamada, T. lshruwata, hl. ffiwxak~, K. Obiand I. Tanaka, Chem. Phys Letters 61 (1979) 518.

Acknowledgement

The authors wrsh to thank Dr. S. Saito, Institute for Molecular Science, for various helpful discussions

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