Resonance fluorescence study of electronically excited sulphur atoms: reactions of S(31D2)

Volume

74. number

CHEMICAL

1

RESONANCE REACTIONS

FLUORESCENCE

LETTLRS

STUDY OF ELECTRONICALLY

1.5 August 1980

EXCITED SULPHUR

ATOMS:

OF S(3 1 D7)

M C. ADDISON,

R J. DONOVAN

Drp~rmrem of Clwntsrry Recawd

PHYSICS

Umrers~t_~of

and C. FOTAKIS Edrrrhwgh

Edmburgh

EH9 3JJ UK

/3- hIay 1980.III find form 1-8 May 1980

The direct obserratton of S(3 ‘Dz) using tlmc-rcsolred resonance fluorescence IS reported Rate data for the removal of techniques thus cwatcd state by C& OCS and CH.1 arc presented, and compared ~1t.h data obtamed usmg absorption S(3 ‘Dz) WIS produced b> photoI>srs of CS, at 193 nm &(S tD) = 1.57) usmg an ArF laser.

1. Introduction The ttme-resolved resonance fluorescence techmque has been used extensively to study the krnetlcs ofgroutldstate atoms [l--3], but the only electrottrcal@ excited atom to be studled by this techmque is I(5 2P,,z) [4,5] The advantages of usmg fluorescence techntques for klnetlc studies. compared wth absorption techmques. are now well recogmscd and have been dlscussed prcwously [4]. It IS clear that further developments m flilorescence techniques. particularly for the study of elcctromcally esctted states. are kery deslrable The time-resolved fluorescence techruque most commonly used to study the kmettcs of ground-state atoms does mt employ a monochromator to select the appropriate atomic lines, but rehes on there bemg no excited-state species present durtng the tune perrod studled. In practice It IS not difficult to arrange For excited species to be quenched dnd For ground-state atoms to predommate The study of ewlted states IS clearly more difficult as ground-state atoms will generally be present and fluorescence signals may be observed from both species simultaneously. The mtroductlon of a monochromator mto such an experiment would severely reduce the slgnal ievel and necessitate signal averagmg For lengthy periods of time. particularly m cases where only weak atomic lines are avadable * *

In one be0 fa\ounble (51.

58

use a monochromator

has been used

In our prewous work [4] on I(S 2P,,z) we have descrtbed a simple arrangement which effectively uses a cutoff Filter to eliminate ground-state atom fluorescence and allows the ewtted state to be studted. The same prmclple can be applied to a number OF other excned states where the ewztted-state transttlon hes on the tong-wavelength side of the ground-state transrtlons. In the present work we describe a technique which can be used when the euclted-state transltlon hes on the short-wavelength side of the mam ground-state transltion. The two prmclples employed to achieve this are (I) the strong reversal encountered in atomic lamps for hnes which terminate on the ground state (the lamp used here ma~ttmses thus reversal). and (II) the use of an extreme solar-blmd photomultlpller to rhscrlmmate m Favour of shorter-wavelength (excttedstate) transitions. Despite the importance of S(3 ID,) m the development of excited-state kmetlcs [6] It has only recently been observed directly using absorption techmques [7] _The present study thus prowdes a useful evtenslon of the work on S(3 ID,) and allows an Interesting comparlson to be made between the absorptton and fluorescence techniques. In principle the technique descrtbed here could be e.xtended (with the use of a more extreme solar-blind photomultlpher) to study the analogous excited state of oxygen O(2 lD2).

Volume

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1

CHEMICAL

PHYSICS

LETTERS

15 Augsb 1980

2. Ex~~rnen~

3. Results

The experrmental techmque used for tlus work was broadly srmllar to that descnbcd previously for the study of electromcally excited rodme atoms [4]. The main drfferences lay rn the use of a pulsed monochromatrc (193 nm) ArF laser (Lambda Physrk EMC 500) as a photolysrs lamp, and an extreme solar-blind photomultipher (EMR 541) to momtor the resonance fluorescence signal. The fluorescence cell was constructed from a 500 cm3 Pyrex bulb with srde arms to mount the mput and output wmdows. The laser beam passed through the cell via two hrgb quality quartz (Spectrosil) windows mounted at the Brewster angle. The mput atomIC radratron and outgomg fluorescence passed through hlgF? wmdows. For some cxperrments a gas-filter cell was used whrch also had hlgF, wmdows. The atomrc radratron used ;o excite fluorescence was generated by flowmg a mrxture of I-J-$ (2%) m He (p-t = 250 N m-‘) through a mrcrowave powered drscharge Condrtrons were adjusted to ma\imrse the output on the 3p34s ID1 + 3p4 *D, lure (166.7 nm) Jnd produce strong r2versaI in the lines terminating on the

The most convznient source of S(3 ID,) for the present work was found to be photolysis of CS2 at 193 nm (ArF laser). The laser energy was typically 1.5 mJ (20 ns pulse duration) and a partial pressure of CS, of 10-t N rnVz (nuxed with He to a total pressure of 0.67 kN m-*) was used. Fluorescence from S(3 t&) could be observed from a single laser shot, however the srgnal-to-norse ratio was not sufficient to provide good kmetic data and signal averaging techruques were therefore employed (IO--20 shots typically). The gas mixture rn the photolysrs cell was not changed betwe+n shots as the percentage decomposition was low. ALSO, the repetition rate of the laser was restricted to
ground (3~~ 3PJ) state. Tha was done, m a separate series of experiments usmg a vacuum ultraviolet monochromator (Hrlger and Watts E766) to rsolnte the mdrvrdual atomrc lures The hght from the atomic lamp

was drrectly mcrdent on the fluorescence cell. no attempt was made to monochromate the atomic radratton. The extreme solar-blind photonlult~pller used m thus work (EhlR 542) drscrrmmates strongly (55 X) UI favour of the 166.7 nm lmes assocrated with fluorescence at 180.7-

from S(l

Dz). relative to that from S(3PJ)

183 6 nm. The use

of a gas-filter cell (1 z= 5 cm) contaming H,S (P = 100 N mm2), m front of the photomultrplr2r. provided a further means of reducmg ffuorcscence from S(3 P,). however this rs not essential and most of the experrments were done without it. The output from the photomultrplrer was fed to a transrent recorder (Datalab DL905) and signal averager (Datalab DLSOOO) combrnation. Reagents were handled usrng a conventiona gresseless vacuum lure and pressures measured wn.l~ MKS Baratron gauges (type 22 I : ranges 0. I- lo3 and IOlo5 N m-“)

59

Volume

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CHEMICAL

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PHYSICS

LETTERS

15 August

1980

Fig 1 fluorcsccna: from S(3 ‘D,) h = 166 7 nm. foilowng laser encr.g) = 0.3 mJ. s~gnnl arcwgcd owr 16 pulses)

photol)

for detectmg S(3 3PJ) allows the small growth m ground-stat2 fluorescence (a 15% when qucizchlrtg of S(3 lD,) occurs) to be neglected relative to the more sensrtrve S(l D,) fluorescence.

tecllntque

51s of CS7_ at 193 nm (PCS2 = 1 1 N m-‘,PHc

= 133 N m-‘;

The decay of S(3 lD2) 1s expected to be pseudo first order under the experimental condrt1ons used due to th2 low atomrc concentration (=I! X lo1 1 cmm3) compared with that of the quenchmg or reactant gas Thus 1s confirmed by the kinetic data (see fig. 2). Thus absolute srcond-order rate constants could be obtamed by plottmg the pseudo-frrst-order rate coefficrents versus the partial pressure of quenchmg/reactdnt gas. The slope of such plots yield the rrqurred rate constants winch are presented m table 1. together wrth previous data obtamed usmg absorption techmques.

4. Discussion

fig 2 rlrst-ordcr plot of S(3 ‘Dz.) dcca) (PCS1 = 0 1 N m-z. PHc = 0 67 LN mm2 _lajcr energy = 1 Z mJ).

60

Evcitatron of CS, at 193 nm populates the K(lB?) state which IS known to be predrssocrated by at least two repulstve states, one correlating wth S(3 3PJ) and the other wrth S(3 ID,). The fluorescence yreld IS low state lifetime 1s very short (of = 10-X) and the z(lB,) (T = 1.3 ps) showmg that predrssocratton 1s the domtnant decay channel. Yang et al. [8] have recently carried out a detaded photofragmentatron study of CS2 usmg an ArF laser.They observed the photofrag-

Volume

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CHEMICAL

15 August 1980

PHYSICS LETTERS

Table 1

Comparison of second-order rate constants and absorption [7,91 at 295 L Reactantlquenchmg

for reacttons

gas

rate constants data

Absorpnon a) XT(cm3 molecule-’ (1.2 2 0.3) (1.5 5 0.3) (1 2 L 0.3)

CH4 cs2 ocs a) Quoted rexencc

of S(3 ‘D2) obtamrd

arc based

on a -y value

of un~&.

Errors

x 10-10 x lo-‘0 x 10-10

resonance

Fluorescence k (cm3 molecule-’

ment recod energies and used laser-induced fluorescence to eyamme the wbratlonal dlstrlbutron m the CS fragment. It was concluded that a large fraction (= 80%) of the sulphur atoms are produced m the S(3 ‘D,) state, however It was pointed out that strictly their value is an upper Imut to the actual branchmg ratlo for S(3 ‘D,) productlon Thus value IS clearly mcompatlble with our value (z 15%). winch IS obtamed by a much more direct method. The conclustons of Yang et al. [8] are based on two mam pieces of evidence: firstly a change III slope of a plot of the translatIona energy dlstrlbutlon m the region where S(3 ID?) production becomes energetlcally possible and secondly, the vxbratlonal dlstrlbutton III the CS fragment. Considering first the change III slope of the translational energy dlstrlbutlon we note that the change IS gradual and that It \\ould be mipossible to make a reliable quantltatwe assessment of the branchmg ratio from such data. The data obtamed by LIF on the \lbrattonal dlstrlbutlon m the CS fragment looks more promlsmg but unfortunately the sensltlwty of the technique falls rapidly for levels above IJ” = 6, where the most crucial data hes. We conclude that our datum for the S(3 IDz) branching ratio IS currently the most rehable. A comparison of the rate data for the removal of S(3 tD7) obtained using the present techruque wth that using absorptton techniques (table l), shows the latter to be consistently lower m value This behavlour has been observed prewously wth other atomic speLIZS and has been attributed to the fadure of the sunple Beer-Lambert relationship under the conditions used for absorption studies +_There IS ml1 controversy over

fiuorescen~

(this work)

s-l)

(1.8 = 05) x lo-” (3 5 + 1.0) x lo-lo (3.0 _L1 0) x 10-10

are quoted as one standard desation

for both absorption

and

fho-

the evplanatlons that have been advanced and further work to refine the rate data obtained by both absorption and fluorescence techniques, and to extend rhe range of atomic concentrattons over which k’metic studies are made. is clearly desirable. Such studies should prowde a deeper understanding of several prevlous studies of atomic kinetics. Further work on S(3 lDz) would be parttcularly interesting as the atomic hne from the lamp should be unreversed_ This contrasts with the strong reversal generally found for lines associated with ground-state atomic species. Reaction of S(3 lD,) with the three gases studied IS clearly verl rapid and parallels the III& reactivity shown by 0(2 lD,). Reaction of S(3 ‘D,) with CHq proceeds mamly by tnsertlon to yield the mercaptan CH3SH, quenching being of only minor importance [6] _This IS analogous to the reaction of O(2 ID,) with alkanes where very detailed information on the reaction dynanucs is becoming avadable. Reaction of S(3 lD2) with OCS yields the first electronically exerted state of S, [7],

(LIF)

S(3 lD,)

+ OCS --f Sz(a ‘A_,) f CO ,

but quenchmg IS thought to account for ~20% of the total removal By contrast, removal of S(3 ‘D,) by CS2 proceeds dominantly by quenching, although reaction to produce $(a ‘Ag) IS qwte strongly exothermk The analogous mteractlon of O(2 ‘D,) with CO2 also results in quenching despite the presence of an exothermlc channel yielding CO and 02(a IA,,). This be-

* The values gILen m table 1 for the absorption data arc based on a Beer-Lambcrt coefficient (y) of umtp. A ralue of r=O would bnng the results into close ngrccment

s-‘)

by time-resolved

5

haviour can be understood in terms of theoformation of the known Intermediate CO,, which then undergoes rapid mtersystem crossmg to a triplet state which dissociates to CO, + O(2 3PI)_ The analogous species, CS,, IS also expected to be quite strongly bound and intersystem crossmg should be more rapid due to the

61

Volume 71. number

1

CHEMICAL PHYSICS LETTERS

presence of the heavier sutphur atom. This view IS reinforced by the rapld mtersystem crossing observed for CS, (cf. the prunary photochemlcal step m which the A rB, state dlssoclates manly to S(3 3PJ) f CS). In corkus~on, we have described a senslttve techmque for time-resolved resonance fiuorescence studies of S(3 ID,). Absolute rate data for removal of S(3 lDz) by CH,. OCS and CS, have been presented and should be consldered more rehable than data obtained from absorption studies.

We thank the S.R.C. for fiianclal

support

15

August

1980

References

111

hI 3. Kurylo and W. Braun. Chem. Phys Letters 37 (1976) 232, and references thercm. i J- &cf, !;‘.A. Payne and R B Klemm. 1. Chem. Phys 62 (1975) 4000. D Husain and N.K.H. Slater, .I. Chem. Sot. Faraday II 71(19?8) 1627. R-J Donovan, H M Gdlesple and R H Stnm, J Chem Sot. I araday II 72 (1977) 1553 I Arnold, F J. Comes and S. Pomteck. Chem Phys. 9 (1975) 237. O.P. Strausz and H E Gunnmg, Advan Photocbem 4 (1966) 133. M C Addison. C D Byrne and R J. Donovan Chem. Phys Letters 64 (1979) 57 S C. Yang A. Freedman, M Kawasaki and R. Bersohn, J Chem. Phys 72 (1980) 4058. bl C. Addison and R-J. D~IIOWUI,to bc pubhrhed. i; Ham and D. Phdhps, J. Chem Sot Faraday II 74 (1978) 1431