Mechanism of S, Chemiluminescence in the Reaction of Hydrogen Atoms with Hydrogen Sulphide

BY R. W. FAIR AND B. A. THRUSH

Dept. of Physical Chemistry, Lensfield Road, Cambridge

Received 27th September, 1968

Ground-state sulphur atoms were generated in a discharge flow system by the rapid reaction

H+H2S = H2+HS (1) H+HS = H2+S (2)

The resulting chemiluminescence by the B3X; state of S2 is shown to arise (a) by two-body combina- tion of S atoms into level v' = 10 ; (6) by three-body recombination into lower levels :

between €3 and H2S :

S(3P)+ S(3P)+ M = S*, (B3C3< 9) -1- M (3) S; = S2(X3Z;)+hv (4)

S*,+M = S2+M ( 5 ) with k3 = (1*0+0.2)~ lo1' em6 mole-' sec-l and k.+/k, = 20x lo-' mole ~ r n - ~ for M = Ar at 298°K ; also D(S-S) = 101.7 f0.4 kcal/mole. The mechanism of the sulphur-catalyzed hetero- geneous recombination of hydrogen atoms is studied and discussed.

The chemical behaviour of sulphur atoms has been studied mainly through the photolysis of sulphides. In this work we have produced sulphur atoms in a discharge- flow system by the reaction of hydrogen atoms with hydrogen sulphide. We report here observations on the chemiluminescent recombination of sulphur atoms which yield D(S-S) = 101.7+0*4 kcal/mole and also studies of the sulphur-catalyzed recombination of hydrogen atoms.

EXPERIMENTAL Hydrogen atoms were produced in a fast flow system by passing mixtures of hydrogen

(1 -5 %) and argon through an electrodeless radio-frequency discharge. Power was supplied by a 17 Mc/sec generator. The external electrodes consisted of two strips of aluminium foil wrapped round the discharge tube (10 mm int. diam. quartz) and spaced about 2 cni apart. The flow tube was constructed from 30 mm int. diam. Pyrex tubing and provided with four inlet jets at 20cm intervals for the introduction of reactant gases. A fifth jet was placed a further 5 cm downstream. The pressure in the flow tube was measured with a silicone oil manometer or a McLeod gauge. All gas flows were controlled by needIe valves and measured with conventional capillary flowmeters.

To obtain satisfactory hydrogen atom concentrations it was found to be necessary to poison the surface against hydrogen atom recombination. This was achieved by coating the surface with sulphuric acid, following the procedure given by 0gryzlo.l Excess acid was removed by heating the flow tube to about 80°C with Ar flowing at 1 mm pressure.

Photoelectric measurements were made with an E.M.I. 9558B photomultiplier tube operated from a stabilized power source at a potential of 1250V. Photocurrents were displayed on a Pye Scalamp galvanometer. The photomultiplier was mounted in a move- able housing which fitted closely around the flow tube. Filters were inserted into a slot in the housing to isolate various spectral regions of the emissions, Three interference filters

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R . W . FAIR AND B . A . T H R U S H 1209

were most frequently employed, having maximum transmissions at 3660 A (for S2 emission and for the sulphur dioxide afterglow), 5480 8, (for the air afterglow) and 7600 A (for HNO emission).

Emission spectra were photographed through the quartz end-window of the flow tube on a Hilger E518 f/4 spectrograph with interchangeable glass and quartz optics. Ilford Selochrome plates were used.

The argon carrier gas was passed through a phosphorus pentoxide trap and mixed with a stream of hydrogen after the flows of both gases had been measured. Oxygen impurities in the argon were catalytically converted to water by passing the mixture through a DeOxo unit, and the water was removed by a packed trap maintained at - 7 8 ° C Under these conditions hydrogen atom flows of about 1 pmole/sec (about 1 % of the total gas flow) were obtained, while the oxygen atom flow was about 1000 times less than this. Oxygen atom flows of 0.3 pmole/sec were produced by discharging mixtures of 1-2 of oxygen and argon.

Hydrogen atom concentrations were determined by the HNO emission technique,2 using the 7600 A filter to select the most intense band in the HNO spectrum. The observed photocurrent included a contribution from the air afterglow due to the presence of the small concentration of oxygen atoms in the discharge products, and a correction was determined by measuring the corresponding photocurrent with the 5480 A filter, which transmits air afterglow but not HNO emission. The ratio of photocurrents produced by a pure air after- glow in argon using the 5480 and 7600 a filters was measured and found to be constant under all the conditions used.

Concentrations of atomic oxygen were obtained from the intensity of the air afterglow, and the photomultiplier was calibrated by nitrogen dioxide t i t ra t i~n .~ Sulphur dioxide afterglows were set up by the reaction of atomic oxygen with carbonyl sulphide, and the method of Halstead and Thrush was used to calibrate the photomultiplier.

Hydrogen, argon and oxygen were obtained from B.O.C. cylinders. Nitric oxide, hydrogen sulphide and carbonyl sulphide from Matheson cylinders were purified by passage through phosphorus pentoxide traps followed by distillation. Nitrogen dioxide was pre- pared by reaction of excess oxygen with nitric oxide, and purified by distillation.

RESULTS

Addition of small flows of hydrogen sulphide to a stream of hydrogen atoms gave rise to a pale blue chemiluminescence. The spectrum of the emission showed that it was due to the B3Z;-+X3Eg transition of the Sz molecule. Vibrational levels of the B state up to v’ = 10 were populated and the most outstanding feature of the spectrum was the strong emission from levels u’ = 9, 8, 7, 6. Gaydon and Wolf- hard observed a similar intensity distribution in the reaction of hydrogen atoms with carbon disulphide, but it is quite different from that found in flames, where emission from v‘ = 0 is normally the most intense.6* The emission observed by

TABLE 1 .-RELATIVE POPULATIONS OF THE VIBRATIONAL LEVELS OF S2(B3Z;) U‘ 0 1 2 3 4 5 6 7 8 9 population 17 41 33 34 18 23 46 54 76 79

Pannetier et aZ.* from level v’ = 1 1 of the S2 B state and from the (0,O) band of SH in the reaction of hydrogen atoms with hydrogen sulphide could not be detected on our plates. This difference could be due to the low concentration of hydrogen sulphide used in the present work. The intensity distribution of the S, emission was determined by comparison with spectra of the sulphur dioxide afterglow, the intensity distribution of which is known.4* From this distribution, and using the Franck-Condon factors calculated by Herman and Felenbok,l* the relative popula- tions of the vibrational levels in the emitting state were estimated (table 1). The higher levels were clearly the most densely populated.

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1210 Sz CHEMILUMINESCENCE

/"

1022[H2S]g (mole2 cm-6)

FIG. 1.-Dependence of S2 emission on [H2S]i at zero reaction time. a, [MI = 0 . 5 8 6 ~ mole C M - ~ ; 0, [MI = 1.301 x mole cm-*.

5 10 15

10-"/[M] (cm3 mole-')

FIG. 2.-Pressure dependence of Sz emission at zero reaction time.

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R. W . F A I R AND B . A . T H R U S H 1211

Measurements with the 3660A interference filter showed that the S, emission intensity decayed downstream from the point of addition of hydrogen sulphide, the decay rate increasing with increasing hydrogen sulphide flow. The size of the photomultiplier housing made it impossible for observations to be made at distances less than 6 cm downstream from the inlet jet, so intensities in this region were esti- mated by extrapolation (log I against time) of the readings taken further downstream. This procedure was accurate for hydrogen sulphide concentrations up to 10-l1 mole ~ m - ~ but could not be used above 2 x 10-l1 mole ~ m - ~ where the intensity measured at the first point was only 60 In the “ low flow’’ region the S, emission intensities I at the point of addition of hydrogen sulpl~de (i-e., at zero reaction time) obeyed the relation

of the extrapolated value.

I = roCH2sl& (0 where [H,S], was the concentration of hydrogen sulphide added (fig. 1). The quantity I , was unaffected by decreasing the hydrogen atom conccntratioii by a factor of 2, but 1 /Io varied linearly with 1 /[MI (fig. 2). Hence hydrogen atoms are not involved in a rate-determining step in the emission process, and the enhancement of the emission intensity with increasing total pressure indicates the presence of a third- order chemiluminescent reaction.

The simplest mechanism consistent with these results involves rapid formation of ground state (”) sulphur atoms

H + H,S-+H, + HS (1) H +HS-+H, + S, (2)

S + S+M-+ST(B3C,)+M (3) Sz+S2(X3Ei)+ hv (4)

( 5 )

followed by chemiluminescent three-body combination

S z + M-+S2(X3C,) + M. Application of the steady-state hypothesis to the B state of S, predicts that

Hence I = k3k4[M][SJ2/(kq +k,[M]) = Jo[SI2. (ii)

(iii)

Eqn. (i) and (ii) are equivalent since there could be no decay of the S atoms for zero reaction time, and so [S] = [H,S], providing S atoms are produced stoichiometrically.

Fig. 2 shows that this relationship is accurately obeyed; the slope of the line combined with the absolute intensity of the emission yields k,. This was determined by comparison with the sulphur dioxide afterglow, using the measurements of Halstead and T h r ~ s h . ~ The value of k3 calculated from the emission up to the long wave- length cut-off of the Selochrome plates (about 4800 A) was 8 x 1014 cm6 mole-2 sec-l. Consideration of the Franck-Condon factors showed that over 80 % of the emission was detected; the total value of k3 is therefore

This can be compared with rate constants for the recombination of other atoms with argon as third body which are 0.6 x lo’* cm6 m o k 2 sec-l for oxygen atoms and 1.4 x lo1’ cm6 sec-l for nitrogen atoms.12 The value obtained for k3 is therefore reasonable if a large proportion of the three-body sulphur atom

k3 = (1.0-f:O-2) x 1015 cm6 sec-l.

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1212 sa CHEMILUMINESCENCE

combinations populate the B state; for nitrogen atoms 50 % of the recombinations can populate the B31T, state of N2.12

When the total pressure in the system was raised the intensity of the (10,O) band decreased relative to the (9,O) band. Level u’ = 10 is absent from S2 emission spectra in low-pressure discharges l3 and is slightly diffuse in absorption 1 3 9 l4 ; it must therefore lie entirely above the predissociation limit. It follows that Sz(B,u’ = 10) molecules predissociate much more rapidly than they radiate, and in the present system they must be formed by a two-body inverse predissociation

s + s +S2(B,U’ = 10) S?(U’ = 10)+S2 + hv.

The intensity of the emission from level v’ = 10 is given by

I = - - - - k4k6 [SI2 = K,k4[SI2, k ,+k- ,

since k4 4 k-6 and any collisional quenching of S z can therefore be neglected. The pseudo-equilibrium constant & can be calculated if it is assumed that the

energy difference AE; between S + S at infinite separation and the lowest rotational level of Sz(B,u‘ = 10) is zero. Using constants from Herzberg l 5 and from the JANAF tables,16 K6 = 0.39 cm3 mole-’ at 298°K is obtained. The radiative lifetime of S;(B) has not been measured, but an estimate can be made from the published extinction coefficient in the (13,O) band.17 This band is diffuse, and if it is assumed to have a width of 5 and a Franck-Condon factor of 0.07, a value of f ~ 5 x is obtained, yielding k 4 x 3 x lo7 sec-l, and hence K6k4w 1-2 x lo7 cm3 mole-l sec-l.

The strongest transitions from level v 1 = 10, which gives two-body emission, are around 2900A and at roughly twice that wavelength, and so it is necessary to weight the Franck-Condon factors for the frequency dependence of the radiative transition probability. Taking the Franck-Condon factor for the (10,O) band to be 0.05, it will contribute 13 % of the radiation from level 0’ = 10. The intensity of the (10,O) band is therefore calculated to be Z(10,O)z 15 x 1O5[SI2 while the observed intensity is 5 x 1O5[SI2, both in mole ~ m - ~ sec-‘ units.

This is excellent agreement since there are two factors which could reduce the calculated intensity: (a) the Boltzmann factor due to any energy of level u’ = 10 above that of separated S + S (700 cal/mole would give a factor of 3) ; (b) the possibility that not all rotational levels are populated freely in a two-body association ; however, this is improbable because all rotational states are predjssociated.

Our results show clearly that the dissociation limit must lie between levels u’ = 9 and 10 of the B3Z; state, and that there can be no significant barrier to predissocia- tion. This implies that the predissociating state correlates with the lowest multiplet component of the ground state of S(3P2 + 3P2). The energies of the higher multiplet components (3P1, 3P0) would be sufficient to reduce the intensity of two-body S2 cheduminescence by a factor of 10 or 100 if the predissociating state correlated with one or two of these excited multiplets. The dissociation energy is therefore lOl.7+0-4 kcal/mole. This is in exact agreement with the value of 4.41 k0.02 eV deduced by Rosen, Desirant and Duchesne from the breaking-off in the rotational structure of levels v‘ = 8 and 9. Their value was criticized by Herzberg and Mundie l4

because it was based on calculated rotational constants for levels which are consider- ably perturbed and which were not analyzed ; but a partial analysis l 9 gives an energy of 35,590 cm-’ (= 101.7 kcal/mole) for the sudden decrease in the intensity of rotational structure in u’ = 9.

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R . W . F A I R AND B . A . T H R U S H 121 3

Effusion measurements 2o give D(S-S) = 101.7+2-9 kca.l/mole, and an un- published value of 101 -2 & 0.4 kcal/mole from photo-ionization experiments by Berkowitz and Chupka has been quoted.20 These independent measurements agree well with the spectroscopic value quoted above. We therefore conclude that the pre&ssociation of the B3E; state of S, at v’ = 10 is caused by a state which correlates with 2S(3P,) and is case Ib, i.e., the state responsible has a potential minimum.

Barrow and du Parcq 21 suggest that the perturbations in the lower levels of the B state of S2, its strong predissociation at u’> 18 and the weaker predissociation at v’a 1 1 are all caused by the B”311, state of S 2 with the electron configuration (~~,3p)~(cn~3p)~(cn83p)(~~~3p) which correlates with 2S(3P).

The evidence on the first two points is good and the strong predissociation at v ’ 3 18 presumably corresponds to the B” state crossing the inner limb of the B state. As the B“ state must presumably lie within the B state curve from this point down to the predissociation limit, it is hard to see why the probability of predissociation should fall to a minimuin at v’ = 16, rise to a maximum where the line width corres- ponds to a life of about sec at v’ = 13 and then drop again at lower energies. A possible explanation of this subsidiary maximum i s relatively large values of the Franck-Condon factors without overlap of the terminal loops of the eigenfunctions of the states, thus resembling subsidiary Condon parabolae which can be encountered with diatomic spectra involving large changes in internuclear distance. This appears more plausible than Lochte-Holtgreven’s suggestion 2 2 of a forbidden predissociation by a lE; state, since the evidence of pressure broadening on which this was based is dubious.

The lower levels of the B state cannot be populated solely by vibrational quench- ing of S;(v‘ = lo),

S ~ ( U ‘ = lo)+ M+S;(d < 10) + M, (7)

as this would require K6k7 = k3, yielding a value of k , = 2.6 x loi5 cm3 mole-’ sec-I which is greater than the bimolecular collision frequency. We therefore consider that k3 gives the effective rate of population of the B3E; state by three-body recombination into the B”31T, state followed by rapid collision-induced crossing of excited S, between these states.

The intercept of fig. 2 yields a half quenching-pressure for argon of k4 /k5 = 2 x lo-, mole ~ r n - ~ at 29SOK. This may be compared with Durand’s value 23 of k4/k5 = 7.3 x lo-’ mole ~ m - ~ at 873°K for the quenching by argon of the fluorescence of S2 excited to level v‘ = 8. Our value is based on a long extrapolation, and could be in error, particularly by being too low; on the other hand, Durand’s data show much more scatter. The radiative life (l/k4) for S2 deduced here combined with our value for k4/k5 yields k5 = 1.5 x I O l 4 cm3 mole-l sec-l for electronic quenching of S2 by argon. This is close to unit collision efficiency, which is high but not impossibly so.

Further discussion of the mode of formation of sulphur atoms is given in a subsequent paper, where it is shown that 85 5 15 % of the hydrogen sulphide is initially converted to sulphur atoms rather than to S,. At the lower limit this could increase the observed values of k3 and K6k4 by a factor of 2.

Crone 24 suggested that the B state of S, could be populated by the reaction

and this mechanism receives some support from the work of Sugden and Demer- dache 25 on S, emission from flames. Sufficient energy is available from the

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1214 s2 C H E M I L U M I N E S C E N C E

recombination of hydrogen atoms to excite ground state S2(v" = 0) up to the levels of the B state which we have observed.

If reaction (8) occurs in our system it should become the more important source of chemiluminescence at longer reaction times where a considerable proportion of the sulphur atoms have recombined. Fig. 3 shows the results of an experiment in which the hydrogen sulphide flow was maintained constant and the hydrogen atom concentration was varied by altering the power output of the discharge unit; the S2 emission intensity at a fixed point downstream increased as the hydrogen atom concentration was raised, but the results fitted a linear relationship better than the quadratic relationship predicted by (8). The Sz intensity and the hydrogen atom concentration were followed further downstream ; they decreased to immeasurably low values at about the same point of the flow tube. These observations are, at least qualitatively, consistent with the occurrence of reaction (8) at long reaction times. However, the experiments described below suggest that the enhancement of the S 2 emission with increasing hydrogen atom concentration is due to a decreased decay rate for sulphur atoms.

107[M] (mole c1n-3)

0.73 1 1.178 1 -648 1-175 1.178

t 101O[Hjo

0.2 14 4.08 0-350 4.27 0.482 4-19

TMLE 2 t l o - " x - ~ ~

(set) (~1113 mole-* sec-1)

0.207 1 -03 0,208 1 -06 0.209 1 -00 0.149 0.8 1 0.332 1.04

TABLE 3 10-11k~~ 1010[H]o 1O-l'kH 11

1.53 9.19 1 -22 - 0.28 2-22 8.69 1 *93 - 0.20 2.76 8.67 2-05 - 0.41

Units : t sec ; [HIo mole c n r 3 ; k~ cm3 mole-' sec-'.

Measurements of the hydrogen atom decay jn thc presence of hydrogen sulphide showed considerably inore hydrogen atom removal than was predicted by reactions (1) and (2), indicating catalysis by sulphur atonis, and possibly by higher sulphur species. Tlis additional catalytic decay was analyzed by the method of Clyne and Tlzrush 26 in which various flows of H2S were added at different points and the hydrogen atom concelrtration measured at a fixed point downstream. This process was approximately first order in [HI and added hydrogen sulphide ([H,S],) as shown in fig. 4. Its rate constant was therefore defined by

-d[H]/dt = kH[H][H2S]O.

Table 2 shows that kH is independent of total pressure for constant hydrogen atom concentration, while table 3 shows that k , decreases with increasing initial hydrogen atom concentration, this being expressed as /cH K [H]:. These observations establish that the hydrogen atom decay is heterogeneous ; in fact, impossibly high rate constants as large as k , = 5 x 10l8 cm6 mole-2 sec-l would be required to account for the observed decays in terms of

H + S + M = HS+M. (9)

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R. W. F A I R A N D B . A . T H R U S H 1215

1O1O[H] (mole ~ r n - ~ )

FIG. 3.-Dependence of S2 emission on [HI at reaction time of 0.194 sec. [H,S], = 2.10 x lo-" mole c n ~ - ~ .

0

- 0.1

n - z 3: -0.2

u 1 - Y W

0 3

bD 3

- 0.3

- 0.4 0 1 2 3 4

101l[HzS]o (mole ciiir3)

FIG. 4.-First-order plot of hydrogen atom decay in the presence of H,S.

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1216 s2 C H E M I L U M I N E S C E N C E

A visible sulphur deposit rapidly removes hydrogen atoms ; this is accompanied by a blue glow in the gas phase due to the chemiluminescent recombination of S atoms. The mechanism of the H atom decay is therefore probably

H+S, = HS H+HS = H2+S

s 4 , . The decay of sulphur atonis was at least as rapid as for hydrogen atoms; it

must also be predominantly heterogeneous since the homogeneous reaction (3a)

S + S + M S,+M (30) would require a rate constant k3,-2 x lo1* cm6 mole-2 sec-I. The heterogeneous removal of S atoms could be expressed by the empirical equation

- d[S]/dt = ks[S][H,S],.

ks and k, decreased together for increasing values of [HI,; for instance, ks varied as under conditions where the exponent n for k , was -0.28 (table 3). Neglecting any contribution from higher S species, thc recombination process for S a t o m js therefore presumably

s+s, = s2 (11) s-+s,.

The apparent negative order of kH and ks with respect to [HI can be explained as competition between H and S for the same sites, the processes

H+H, = Hz S+H, = HS

being slower than (10) or (1 1). This explanation is consistent with the suggestion of Linnett et aZ.27 that on Pyrex surfaces the same sites are active for both 0 and H recombination. The relative rates of the recombination processes (lo)-( 13) suggest that H atoms are significantly more strongly adsorbed than S atoms.

The results shown in fig. 3 can, to a first approximation, be accounted for wholly by the heterogeneous decays, and there is no need to postulate that the bulk of the S, emission at long reaction times arises from population of the B state of S, by reaction (8). This conclusion was further tested by investigating the pressure dependence of the S, emjssion downstream from the hydrogen sulphide inlet jet. As the atom decays are mainly heterogeneous, the concentrations of H, S and S, will have little dependence on total pressure. Fig. 5 shows that the S, emission intensity at a fixed reaction time increased when the pressure was raised. This is consistent with formation of Si(B) by reaction (3) , due to the pressure dependelice of I. (eqn. (iii)), whereas, if reaction (8) were the main formation mechanism the opposite effect would be expected due to quenching of the emission (reaction (5)). These results can be explained on the basis of reaction (8) only if the surface decay of the sulphur atoms does not produce S,. We then have

and, assuming no decay of S2, Is = k4ks[HI2[S2l/(k4 +MMI)

CSJ = J ~ , , c s I ~ c M I ~ ~ , 0

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R. W . FAIR AND B . A . T H R U S H

where k3= is the total third-order rate constant for the recombination atoms. Hence, putting k3 = ak3,,

1217

of sulphur

(ix)

This expression predicts the correct pressure dependence and under typical conditions, using a generous estimate of the value of the integral, gives k8 % 2 x 1018cc cm6 mole-’ sec-l. The third-order rate constants for the recombination of hydrogen atoms have values 28 (in units of cm6 sec-l) of 2.3 x (M = Ar), 3.4 x lo1’ (M = H2)? and less than 2-5 x 10l6 for the very efficient third-body H20. The value required

1 O1 l[H2S], (mole ~ r n - ~ ) FIG. 5.--Pressure dependence of SZ emission at reaction time of 0-208 sec.

107[M] (mole ~ r n - ~ ) : (>, 0.731 ; 0, 1.178 ; 0, 1.648.

for k8 therefore is impossibly high unless the fraction a of the three-body sulphur aton1 recombinations which populate the B state is below 1 % which seems improbably small by comparison with other atomic recombinations.12 If a = 0.01 then the value of kSa = 1 x lo1’ cm6 sec-l is large. Therefore it is concluded that reaction (8) is not a major source of Sz(B) in the present system.

DISCUSSION

The pressure dependence of the Sz chemiluminescence from levels u‘<9 of the B state provides good evidence that it is associated with the three-body recombina- tion of sulphur atoms, the electronically excited S2 being removed partly by radiation and partly by quenching. Although it was not possible to establish the exact form of the pressure dependence of the v’ = 10 emission, the fact that its intensity decreased relative to the lower levels as the pressure was raised showed that it must have a different mechanism of population from the lower levels, since levels v’> 10 axe removed predominantly by predissociation which is independent of pressure in the range considered here.14 An obvious explanation is that level v’ = 10 i s populated by two-body association, i.e., inverse predissociation*

Sugden and Demerdache 25 have suggested that the emission from levels 0 < v’ d 15 in hydrogen rich flames arises from the process

H + OH+ S2+H20+ S z . 39

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1218 Sz CHEMILUMINESCENCE

We have presented evidence that the bulk of the S2 emission does not arise from the analogous process,

However, level u’ = 10 has an energy of 102-1 kcal/mole and could therefore be excited by the energy released in hydrogen atom recombination. Such a process would involve the recombining hydrogen atoms in making a vertical transition to the lowest vibrational level of the ground state of HZ, and there is evidence that chemiluminescent recombinations in which a third body is excited are more efficient when only a fraction of the energy available from recombination is t r an~fe r red .~~ This would be expected to favour population of the lower levels of the B state of S2. From the intensity distribution of S, emission from discharges at low pressure,13 levels u’ < 9 would then give strong emission whereas levels v’ 3 10 would be undetect- able due to predissociation. In our system, the process populating level u‘ = 10 must therefore be more rapid than the processes populating u‘G9, supporting ow contention that these are respectively two- and three-body recombinations of S at oms.

We therefore suggest that the emissjon from levels of the B state up to uf = 15 observed by Sugden and Demerdache arises by two-body association of S atoms, the population of levels up to 5 kcal/mole above the predissociation limit being consistent with the greater thermal energies available in flames. The presence of this emission in hydrogen rich rather than oxygen rich flames is attributed to the ease with which S is oxidized to such stable species as SO and SO2.

H +H+ Sz-+H2 + S z . (8)

R. W. F. thanks the Science Research Council for a maintenance award.

’ E. A. Ogryzlo, Can. J. Cliem., 1961, 39, 2556. M. A. A. Clyne and B. A. Thrush, Trans. Faratlay Soc., 1961, 57, 1305. F. Kaufman, Proc. Roy. Sac. A , 1958,247, 123. C. J. Halstead and B. A. Thrush, Proc. Roy. Soc. A , 1966, 295, 363. A. G. Gaydon and H. G. Wolfhard, Proc. Roy. Soc. A , 1952,213,366. A. G. Gaydon and G. Whittingham, Proc. Roy. Soc. A , 1947,198, 313. A. Fowler and W. M. Vaidya, Proc. Roy. SOC. A , 1931,132, 310. G. Pannetier, P. Goudrnand. 0. Dessaux and N. Tavernier, J. Chim. Phys., 1964, 61, 395. L. Herman, J. Akriche and H. Grenat, J. Quant. Spectr. Rud. Transfer, 1962, 2, 215.

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