The xenon sensitised radiolysis of HBrl

Received June 15. 1976

SURJIT S. NAGRA and DAVID ANTHONY ARMSTRONG. Can. J. Chem. 54, 3592 (1976). The 60C07,-radiolysis of mixtures of HBr with Xe has been investigated at room temperature.

lonisation measurenlents and hgdrogen yields provide strong eLidence for energy transfers between the two components. The small but significant decrease in ionisation yields at low HBr:Xe is independent of total pressure and consistent with a competition between HBr and Xe for the Hornbeck-hlolnar states of Xe. The increase in ionisation lields at higher HBr:Xe ( Z above 0.2) is probably due to collisionally-induced ionisation or chemi-ionisation reactions of highly excited states of HBr.

As in pre~iously reported experiments with W2S the hydrogen yields from HBr are strongly enhanced in the presence of Xe. The maximum sensitisation of 4 inolecules of H ? per 100 eV can be explained by energy transfer froni excited states of Xe (G - 2.5) and subexcitation electrons having energy between 4.3-8.3 eV (G - 1.5) in Xe. The results obtained from thermal electron scavenging with SF6 confirin previous findings that inert gases are inefficient a t moderat- ing electrons to thermal energies. At Iligh [SF6] [HBr]. SF6 interferes with hydrogen-forming reactions other than those of electrons.

SURJIT S. NAGRA et DAVID ANTHONY ARMSTRONG. Can. J. Chem. 54, 3592 (1976). O n a etuciie, h temperature de la piece, la radioiyse 7 par le60Co de nltlanges de HBr avec du

Xe. Les rnesures d'ionisation et les rendemcnts hydrogene fournissent de bons arguments en faveur d'un transfert d'energie entre les deux composants. La diminution faible niais significative dans le rendenient d'ionisation a des rapports faibles de HBr:Xe est independante de la pression totale et en accord avec les conipCtitions entre HBr et Xe pour les etats Hornbeck-hloliiar du Xe. L'augmentation des rendements d'ionisation a des rapports plus ClebCs de HBr:Xe ( Z au- dessus de 0.2) est probablement due a des ionizations induites par des collisions ou des reactions de chemi-ionisation des etats hautenient excites du HBr.

Comme dans les cas rapportes anterieuren~eilt avec HIS. les rendeineiits en hydrogkne B partir de HBr sont grandement augn~entes par la presence de Xe. On peut expliquer la sensibiii- sation maximum de quatre niolec~~les de H2 par 100 eV par un transfert d'energie B partir des etats excites du Xe (G - 2.5) et par des tlectrons subexcites ayant des energies eiitre 4.3-8.3 eV ( G -- I .5) dans le Xe. Les resultats obtenus B partir d u piegeage des Clectrons thermiques par SF6 confirn~ent les rtsultats ohtenus anterieurement B l'effet que les gaz inertes sont inefficaces pour modkrer des electroils aux energies thermiques. A des rapports de concentration d e [SF6]/[HBr] Clevis, SF6 interfere avec des rtactions produisant de l'hpdrogkne mais n'impli- quant pas des electrons.

[Traduit par le journal]

Bntroductian Experimental A recent publication from this laboratory (1) Xenon research grade (99.995';, purity) was obtained

reported on electron capture and hydrogen- from the ,Matheson Company. It was further purified in the vacuum line by trap-to-trap distillation froni 170 t o

forming reactions of HBr irradiated in mixtures 70 K , gas stored in a bulb attached to the with a number of gases. With CzFs and COz the mercury-free preparation line. Details of other materials ionisation and product yields were linear func- ~ ~ s e d , purification metliods. irradiation procedures and

tions of the power fractions of H B ~ e q u ~ p n ~ e i i t , Ion current measurementi. hgdrogeil analysis

and the added gas, Ho15e\.er. for HBr-Xe and dosin~etry determlnat~ons were essentially the same as described pre\iousl) (1).

mixtures there was a departure from this rela- tively straightforward behaviour and evidence Results for energy -transfer bet~veen excited species of Iorzisation Yields the two components was obtained. Here we The yields of ions per 100 eV of radiation present and discuss for the energy in the HBr-Xe nlixturei, G;,, mixtures in detail. are plotted in Fig. 1 as a function of 2, the

1Research supported by NRCC 3571. stopping power fraction of HBr. As used in

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NAGRA AND ARMSTRONG

0 0.2 0.4 0.6 0.8 1.6 Z

FIG. 1. Ionization yields, G;, in Xe-HBr mixtures p!otted as a function of Z, the stopping power fraction of HBr: 0. Xe 550 torr and HBr 10-550 torr: V, Xe 400 torr and HBr 25-675 torr: A, Xe 200 torr and HBr 1G480 torr; 0, Xe 100-450 torr and HBr 300 torr: m, Xe 25-200 torr and HBr 500 torr; A, Xe 10-500 torr and HBr 200 torr; @, pure components.

ref. 1 and previously - defined (2) this quantity is - equal to (1 + p ~ B r P x , / ~ H B , ) - ' . Here pFB, is the ratio of the weighted mean stopping powers of xenon and HBr taken from ref. 2, while the quantities Px, and PHBr are the partial pressures of xenon and HBr in the mixture.

The complex variation of Gf, with increasing Z in Fig. 1 is in marked contrast to the linear dependence on Z, which was observed for G&, in HBr-C02 and HBr-C2F6 mixtures with the same apparatus (1). Its interpretation in terms of interactions between highly excited states of xenon and HBr is given in the Discussion section.

Hjdrogen Yielcls The points on line A in Fig. 2 are total hydro-

gen yields plotted against Z. Since every electron produced by ionisation forms a molecule of hydrogen (3), one can obtain the yields of hydrogen from species other than e by subtract- ing the values of G& in Fig. 1 from the total hydrogen yields for the same Z. These yields are

shown by the points on line B in Fig. 2 . The significance of the other lines is explained later.

Effects of SF6

As shoun in ref. I the rate controlling step in the mechanism through which thermal electrons form hydrogen is second order in HBr. It can be represented by [I],

[ I ] e + 2HBr + H + BrHBr-

since hydrogen atoms react stoichiometrically with HBr to form HI2 and Br. The contribution G(H2),,, of thermal electrons to hydrogen produc- tion in the presence of an electron scavenger S, like SF6, is given by expression [%]

[2] G(I12),,, = Gk ( 1 $- k 3 [ S l / k , [ ~ ~ r l z J -" where Gk is the yield of thermal electrons in the mixture and k3 the rate constant of reaction 3.

[31 e + S + S -

Due to this competition G(H2) from HBr-Xe mixtures is suppressed to a plateau for [SF,j]/

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3594 CAN. J. CHEM. VOL. 54. 1976

TABLE 1. Hydrogen yields from reactions of hot and thermal electrons in mixtures of HBr with xenon

Fraction of H2 molecules per 100 eV from subexcitation

electrons [HBrl Hot Thermal (= GL)O thermalised

(molecules ~ m - ~ ) [Xe]. [HBr] ([4] e+HBr+) ([1] e+2HBr+) Total (= G&)* (= G&/ G,)

6 to 22X 1018 0 1 . 1 3 .0 4.09 0 . 73 6 X 1018 0 .6 1 .8 2 . 7 4.50 3.9X1Ol8

0 61 0 .8 1 . 5 3 0 4 47 0 67

1.6X101s 4 1 6 2 . 8 4 43 0.63

aDetermined from scavenging experiments with [SF81 /[HBr] in the range 0 to 5 X lo-'. Uncertainties are k0.2 per 100 eV.

Wncertainties are i O . 0 2 per 100 eV

FIG. 2. Hydrogen yields, G(H2) in Xe-KBr mixtures plotted as a function of Z, the stopping power fraction of HBr : A, total H2 4 ield; B, total Hz - G', : C , H z yield in HBr alone from processes other than electron capture; D, net H2 yield from sensitisstion by Xe, obtained by subtract- ing C from B.

[HBr] = 0 to 0.01 as exemplified in Fig. 3(a), the data for which are taken from ref. 1 for [HBr] and [Xe] = l.62 X 1018 and 6.48 X 10ls molecules ~ m - ~ , respectively. The net reduction is less than the total yield of electrons given by GL, because the cross section for reaction 4 is large enough that about 30y6 of the subexcita-

tion electrons with initial kinetic energy above the reaction threshold are captured before they cool to thermal energy (-0.04 eV at 298 K). In this study we investigated the effect of [Xe]/

[HBr] on this fraction. Gi, was determined from experiments with SF6, corrections being made for the removal of hot electrons by this scavenger (1). The results are presented in Table 1.

As shown by the data in Fig. 3(a) SF6 scav- enges essentially all thermal electrons for [SF61 [HBr] about 0.02 and thereafter G(H2) decreases only slowly up to [SF6] [IIBr] = 0.05. In this study higher [SF61 /[HBr] ratios aere used to determine whether reaction 4 could be sup- pressed as well as [I]. The results for a mixture with [HBr] and [Xe] equal to 1.3 X 1018 and 1.78 X 1019 molecules ~ m - ~ respectively are shown in Fig. 3(b). The upper dashed line at a C(H2) value of 5.3 molecules per 100 eV is the yield expected for complete scavenging of thermal electrons. The lower dashed line shows the yield calculated by subtracting G& from the total G(H2) and is the residual hydrogen when reactions 1 and 4 are both supprcsscd. I t is seen that SF6 was able to reduce G(H2) below 5.3 molecules per 100 eV, which implies that it does compete with [4]. Howcver, the points for [SF61 [HBr] 2 0.48 lie substantially below the lower dashed line, indicating that hydrogen forming processes other than [ I ] and 141 are also effected at these large SF6 concentrations. We shall return to this point later.

Discussion

Ionisafion Processes Ionisation in the inert gases arises mainly from

transitions of the monatomic species to ionic or auto-ionising states (4, 5). For xenon repre- sentative equations are

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FIG. 3. Effect of SF6 on hydrogen yields for m~xiures of Xe and WBr: G(W2) rs. [SF6]/[HBr] for: ( a ) 200 torr Xe and 50 torr HBr, dose rate = 2.4 X 1018 eV g-1 h-1, and (b) 40 torr HBr and 550 torr Xe at hlgher SF6 concentrations.

[71 Xea- Xe' + e

where Xea symbolises an auto-ionising state. Dimer ions can be formed in three body reac- tions

for which k8 = 3 X cm6 molecule-l s-I in xenon (6) . However, some dimer species are also produced by the Hornbeck-Molnar mechanism, involving reactions 9 and 10 (7).

For xenon [9] has a threshold of 11.16 eV (4) . The cross section for Xe*" production by elec- tron impact peaks sharply about 2 eV above this and then falls, until above 60 eV it decreases only slowly with electron energj (8) . The magni- tude of klo from mass spectrometric measure- ments ( 8 ) is 9 X I0-l4/cr for 70 eV impacting electrons and 9 X /a for 13 eV impacting electron^.^ Mere cr is gg '(us + f f 6 ) . If one assumes cr cx 0.003 and cx 0.03 for 70 eV and E maximum respectively as in argon (8) then k lo rr 3 X 10-l1 cm3 molecule-' s-l.

2Photoionisation studies indicate a Large number of Hornbeck-Molnar states with varying magnitudes of k lo (7) and excitation cross sections. The values from electron impact excitation would be weighted mean values and are more appropriate to radiation chemistry.

In the presence of HBr electrons will be cap- tured in reactions 1 and 4 and HBr+ ions will be produced both directly and by auto-ionisation processes (9). The HBri ions may also be expected from the charge transfer reaction 11, since the ionisation potentials of xenon and HBr are

[ I l l Xe- + HBr + Xe + IHBr'

respectively 12.13 ( 4 ) and 11 .62 eV (10). Since the ionisation potential of XeZT (11 11.2 eV (4)) is too small for charge transfer to HBr, these ions probably become clustered. However, if kl l is of the order of a typical ion molecule rate constant and greater than 10-lo cm3 molecule-I s-I, it can be shown that reaction 11 would dominate over [8] for all the mixtures used here. Because of this and the fact that the Hornbeck- Moinar states are quenched by HBr (see below), the ions undergoing recombination in the pres- ence of xenon under our conditions should be the same as those in pure HBr except that clustering would be less extensive.

We turn now to the quenching of the Horn- beck-Mo!nar states and other processes which may affect the total yields of ions in the system. Previous observations of decreases in G& for the other rare gases in the presence of molecular gases at low stopping power fraction have been attributed to quenching of the Hornbeck-Molnar states (1 1). Following Klots9 treatment the cor-

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3596 CAN. J. CHEM. VOL. 54, 1975

responding reactions in the xenon-HBr system are

[I21 xe* * + HBs + Ion pairs

[I31 )<eW + f Br + Neutral products

with kI3 obviously finite. Also the magnitude of G i should be given by expression [14]

which can be rearranged to [15j with GE replac- illg ( G S e Q e + GbBrcHBr).

Application of [15] to the results for Z < 8.1 in -. big. 1 necessitxtes the calculation of the effective electron fractions EX, and €HBr Klots used a "'fitting procedure" to obtain the best values of these functions, g(Xe**)kl 3(k12 + kls ) -hand k10(kl2 + k13)-' from his observations, but the behaviour of G& above Z = 0.2 is too complex for that procedure to be effective here. Two different approaches were therefore used: (a) ex, was taken as unity and E H B ~ as zero, which is not a serious approximation, since Z is 5 0.11 for the [Web [HBr] range used, and (b) eXe and ~ H B ~

were assumed to be equal to the stopping power fractions ( i - Z ) and Z respectively. Figure 4 presents the results plotted in accord with the first approach. The fact that the data for the three pressures are accommodated on the same !ine is consistent with a competition between HBr and xenon for the excited Xe** species. If for example HBr were competing against reac- tion '7 for Wea, pressure independent plots would not be expected.

The intercept in Fig. 4 leads to g(Xe**)ki3 X (kl? + k13I-I = 0.z8, while that from the plot using the second approach was 0.2,. Thus, given that ki3(k12 + lc1 3)-I 5 I , g e e * * ) is > 0.20. Since the value of a is expected to be small we may assume it to be equal to g(Xe**>/Gk,, which in fact gives the weighted mean value for the electron energy distribution in the present radiolysis. Thus from the present data a > 0.2 X 4 . 6 ' or > 0.04, which may be conlpared bith cr 2 0.27, > 0.04, and > 0.1 1 reported by Klots

FIG. 4. Reciprocal of (G&, - G',) CS. [Xe],'[HBr] for mixtures of Xe and HBr with different pressures of Xe; C. 550 torr; A, 400 torr: C. 200 torr.

for neon, argon, and krypton respectively. From the ratios of the slopes to the intercepts in Fig. 4, k10(ki2 + k13)-l was 0.1 and approach (b) gave the same value. This ratio lies in the same range (0.06 to 0.7) as those reported for quench- ing of the corresponding states of the other rare gases (1 1). Also, if one assumes klo .v 3 X lo-" cm3 molecule-I s - ~ as estimated above, then (klz + kI3) cx 3 x 10-lo cm3 s-l, which is simi1a.r to 5 X 10-lo cm3 molecule-I s-I for quenching of the Xe*(3P2) metastable by HBr (12).

The broad maximum in G:, occurring near Z = 0.5 is more difficult to interpret than the decrease in ionisation for Z in the range 0 to 0.1. Taken a t face value it suggests that a highly excited state of HBr with a yield of about 0.2 per 100 eki produces neutral products, but is capable of forming ions in the presence of xenon. This could involve either a chemi-ionisation reaction with xenon or collisionally induced ionisation of the HBr excited state. The present data do not permit further conclusions, but an investigation of the ion yields by the more precise method of total absorption of a or P particles (see, for example, ref. I l ) might reveal further details relating to these processes. Also a

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NAGRA AND ARMSTRONG 3597

mass spectrometric search for chemi-ionisation v~ould be of interest.

Hj-clroge~z Yielcls,froin Electron Cuptlrre Reactions As stated in the Results section some sub-

excitation electrons form hydrogen via reaction 4 while cooling through the 0.8 to 0.15 eV range where the cross section for resonance capture is significant. The remainder are captured in [I] after thermalisation. As seen from Table 1 the fraction of electrons G:, 6, reacting a t thermal energies does not increase with addition of xenon as it did for addition of COz, where ( G i , Gh,) rose from 0.73 for pure HBr to 0.80 for [COz], [HBr] = 4 (1). This was attributed to the additional moderating effect of CB2. The lack of evidence for a moderating effect by xenon is in keeping with the large mass and absence of degrees of freedom which the electron could excite. It is also consistent with the almost negligible electron thermaiisation rates observed (13) in the heavy inert gases.

At this poini it is important to recognize that Eo, the energy of the lowest excited state, lies at 8.31 eV in xenon (4) while in HBr it is a t about 4.3 eV (14). Thus the energy spectrum of the subexcitation electrons is expected to be different in the two gases. For - example, using the ap- proximate formula SE = Eo { 2 + E0,'I) -l (151, where I is the ionisation potential, one finds aver- age subexcitation energies of % = 2.5 eV in HBr and 3.1 eV in xenon. The shift toward higher energy in xenon should lead to a small increase in the yield per 100 eV of subexcitation electrons with energy above the 0.3 eV maximum of the cross section for reaction 3, and this is probably the reason for the fact that G;,'Gi in Table ! in fact shows a slight tendency to rlecre~~se with [Xe]![HBr].

Hylrogen Yields ,fiorn Srrbexci!ation Electrons \t.ith Initial Kinelic Energy between 4.3 and 8.3 e V nrzd,fro~~z Xerion Excited States

Line C in Fig. 2 is the yield of hydrogen expected from processes other than electron capture occurring in HBr calculated on the assurnption that the energy absorbed by the HBr molecules in the mixture is a linear function of Z. These other processes are mainly dissocia- tions of excited species (9). Curve B is the yield actually observed in the mixtures. The difference, which is shown by curve D, is the net extent of sensitisation by xenon.

Because the energy dependence of excitation cross sections I-na:, be complex and because of the significant roles of secondarl and tertiary electrons, the partitioning of energ) in a twb component mixture need not necessarily be !inear in Z calculated for the initial electron energy spectrum as for line C.3 However, the extent of sensitisation observed here is too large to be due simply to a failure of Z t o describe the energy partitioning. In cur view it must be attributed primarily to energy transfer to HBr from neutral xenon excited states and to the excitation of HBr by electrons uhich are below Eo for xenon but above it for HBr. Using the experimer;tai points from Fig. 6 of Opal, Peterson, and Beaty (16) it was estimated that roilgh!y 257 , of the electrons formed by ionisation of xenon with 500eV eIectrons \.vould have energies in that range. Thls percentage corresponds to a yield per 100eV of I.?. A higher yield of elec- trons in the 4.3 to 5.3 eV reglon may occur rn the radiolvsis due to the iresense- of lower energy secondary and tertiary electrons in the degradation spectrum (see, for example, the d i s cussion of the importance of excitations b! siouz electrons in ref. 15). However, it seerns very unlikely that this would exceed 2 per 100eV. These electrons ivould be expected to possess relatively large cross sections (cJ: data for WCl (17)) for exciting the low lying Q 3Ul, 2nd 3110+ states of HBr (18-%0),

which dissociate (18) and produce hydrogert through reaction 18 (9).

A yield of I to 2 molecules per l00eV may therefore result from [16] through [18].

A rudimentary estimate of the total number of xenon excited states formed per 100 eV may be gained from the energy balance (15). Thus, taking Gk, = 4.566, the average energy of transi- tions to ionic states (includizg those via [6]-[7] and [9]-[lo] as 13.4 eV and St = 3.1, the energy consumed in ionisations in pure xenon is 7 5 eV (= 4.566 X (13.4 + 3. l)), per total energy ex- penditure of 100 eV. Since the mean energy per

3See ref. I I and references cired therein for discussions of energy depositivrr in mixtures.

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CAN. J. CHEM. VOL. 54, 1976

XENON HBr

Meta Resonance ions Neutral ions D~ssociation Stables States States Products

- ---------- *p3/2 - W o r w k k sglolnos States 2nq/2= -- -------- VIE+ 312

- 3 '$1 and other

Po - - states kc4 3 3 P2 P,

-:- c ln (Mafly - - -I- ~ l n Predissociot~ons) -

FIG. 5. Energiec of xenon and HBr. Data for xenon from ref. 4 and for HRr from refs. 9. 14, and 18 to 20.

excitation must be near 9 eV the total number of excited states should be about 2.7 per 100 eV. This is much lower than -5.0 assumed in ref. 21, but in better agreement with 1.9 per lO0eV (g(sing1ets) e 1.5 and ~(tr iplets) -- 0.4) esti- mated by Sato, Okazaki, and Ohno using binary encounter theory (22). As shown in Fig. 5 the energies of all the neutral excited states are large enough to cause dissociation of FIBr and hydro- gen formation. The maximum sensitisation of 4 molecules of hydrogen per 100 eV seen in Fig. 2 can therefore be explained by g(sing1et) $ triplet) - 2.5 and g~u,,,, (4.3 to 8.3 eV) 2 1.5 per 100 eV in xenon, provided the energy trans- fers f ~ o m Xe* to HBr are sutficiently rapid. If the transfer cross sections for all xenon states are similar to 6 X 10-1° cm3 molecule-I s-I measured for Xe 3Po quenching by HBr (12), then the xenon excited state lifetimes at [HBr] =

3 X 1018 niolecules cm-"vould be -0.6 ns, which is about one fifth of the radiative lifetimes of the singlet states (-3.5 ns (4, 23)). On these grounds energy transfer is therefore likely to be by far the most important mechanism of Xe* loss. It will be made more efficient by radiation trapping (4, 24).

Beyond the maximum, curve D in Fig. 2 runs

linearly down to C(H2) = 0 a t Z = 1, which indicates the decreasing contribution of xenon states to the radiolysis. The overall behaviour of curve D in Fig. 2 bears a strong resemblance to that of the H2S-Xe system, uhere the maximum G(H2) from energy transfer is near 5 for both 7 (21, 25, 26) and a radiolysis (27). The tendency for a slightly higher value than in HBr may be due to a contribution from reaction 19

which is energetically feasible (25) and would lead to two molecules of hydrogen per Xe* (27). However, the general similarities of the H2S- and HBr-Xe systems imply [g(Xe*) + (4.3 to 8.3 eV)] = 4 to 5 per 100eV.

As pointed ou t in the Results section the values of G(H2) for [SF6], [HBr] > 0.48 show that SF6 is interfering with hydrogen-forming reactions other than [ l ] and [4]. Thus when it is used as an electron scavenger in the inert gas systems its concentration must be low relative to HBr (or other substrate molecules), and not just low relative to [Xe]. The present experiments d o not show whether SF6 is competing with HBr for Xe* states or for electrons in the 4.3 to 8.3 eV range. More information on their quench-

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NAGRA AND ARMSTRONG 3599

ing cross sections for Xe* states (the quench- ing cross section of HBr is about twice that of SF6 for Xe*(3P2) (12)) and electron impact spec- tra near threshold should help to resolve this point. Competitions between H2S and SF6 for Xe* have already been considered in the H2S-SF6-Xe system (25).

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