Excitation in the radiation chemistry of inorganic gases

Int. J. Radiat. Phys. Chem. 1976, Vol. $, pp. 71-111. Pergamon Preu. Printed in Great Britain

EXCITATION IN THE RADIATION CHEMISTRY OF INORGANIC GASESt C. WILLIS Division of ChemistrY, National ~ C o u n ~ Canada, Ottawa, Ontario, Canada K1A 0R6

and A. W. BoYv Chalk River Nuclear Laboratories, Atomic Energy of Canada Limited, Chalk River, Ontario, Canada K0J l J0

(Received 9 April 1974; in revised/brm 15 October 1974) Abatract--Gas phase radication chemistry yield data and electron impact cross-section data are used to derive excitation mechanisms and to discuss the role of excited states in the radiation

chemistry of O., N,, NO, CO, COt, HiS, I-ItO and N ~ . For each of these systems available cross-sections for ionization and neutral excitation are listed, together with relevant reaction rate data and a summary of the radiatim! chemistry studies at both high and low dose rates. In genera~ fairly complete mechanisms are derived and further tested by energy balance calculations. In order to present as complete a picture as ~ i b l e , a summary of rates and products of ionneutralization reactions is given at the end of the paper.

INTRODUCTION

RA~ than being a strict review of our own work, the object of this paper also is to explore how much data from electronoiml~ct c r o l ~ - s e ~ n studies arc useful in deriving information on excitation and excited state formation in the radiolysis of inorganic gases. Yields of primary processes calculated from electron-impact crosssections are compared with those obtained from radiation chemistry data. Our aim in doing this is to test the validity of the measurements and, at the same time, the reliability of the mechanisms. Taking this global approach also allows us to point to areas where the information is incomplete or weak. Most of the radiation chemistry data considered are stable product yields. Emphasis is placed on mechanisms obtained in our own work using single, high intensity electron pulses from Febetrons. We have not discussed the HCI and HBr systems as they have been recently reviewed¢1) and are further covered elsewhere in this issue(z).

Energy balance calculations In order to test further the validity of the calculated yields and deduced mechanisms, energy balance calculations are presented for each system. If a complete mechanism is understood it should be possible to account for each 100 eV of energy absorbed by a system in terms of the G,values ( = number of events/100 eV)multiplied by the energy required for that event (in eV/event). Two types of approach can be taken in such calculations. ? Issued as N.R.C.C. no. 14717 and A.E.C.L. no. 5155. 71

72

C. WILLISand A. W. BoYD

The opticad approximation(a) predicts that the major part of the energy transferred to a gaseous sample by a beam of fast electrons will be due to "optical transitions". These can be considered due to the creation of virtual photons and lead to Franck-Condon ("vertical") transitions which conform to spectroscopic selection rules. Only a relatively small amount of excitation occurs by optically forbidden processes, ~ , l y excitation by "slow" electrons at energies below those where the Born approximation is valid. Although excitation mechanisms are complex, in principle it should be possible to compute exactly energy losses and to predict the yields of the various ionic and excited neutral states formed. This computation requires a knowledge of the transition probabilities, differential oscillator strengths or equivalent information, for the production of the various states together with a detailed knowledge of the spectrum of "secondary" electrons produced in the sample (the degradation spectrum). Transition probability data are gradually being accumulated from inelastic electron scattering, photoionization mass spectrometry, ~ o n beam mass spectrometry and from vaziable energy ~ a m excitation spectroscopy. Our e ~ m e n t a l knowledge of the degradation spectra, however, is sparse (s) and probably not very useful. Apart from the atomic number and mean excitation potential of the target, degradation spectra will depend on sample pressure and much more importantly on specific sample geometry, container material and boundary conditions. The "continuous slowing down approximation" (CSDA), used by Green et al. m and others calculates degradation spectra by fitting transition probabilities to semiempirical expressions and calculating energy losses of an electron as it slows down. At the present time it is difficult to place a realistic estimate on the overall reliability of CSDA c ~ a t i o n s ; however, it does appear that such calculations tend to overemphasize p r ~ whose cross-sections peak at low electron energies, i.e. spin and symmetry forbidden processesm. Tbe tec~ique we have used in the present work is very much simpler. We have used relative cro,,~-seetions for ]00eV ~ and calculated yields for the various processes based on W-values (ener~ ~ to form an ion pair). (1)

Yield of process, G(process) --

......

%rooeee

x

100 eV

¢Ytot&lionis&tlon

W

We have chosen to use a fixed electron energy rather than attempting an integration of an a s s ~ ~dation spectrum, The choice of 100 eV e ~ o n energy gives some weight to "forbidden" process and, as we have previously discussed {6), relative cross-sections for major processes do not vary much above this energy. Where cross-section data are not available we have used standard 70 eV mass spectral abundances to derive the yields of the primary ionic processes. (2)

~proee~

G(process) -= y, Abundance x

100 eV W "

In these cases, yields of neutral processes must be derived from radiation chemistry mechanisms. The "probable energies" for the processes are derived from detailed potential energy curves and are the energies required for Franck-Cendon excitation from ground state molecules. Where detailed potential energy curves are not available,

Excitation in the radiation chemistry of inorganic gases

73

probable energies are taken arbitrarily to be "threshold energies" (or appearance potentials) plus 2 eV. The energy balance is then (3)

Total energy required = Y, Gproce~ × Eproee~ + ~-~sub-exeit&tion"

This in theory should sum to 100 eV. The sub-excitation energy is the amount of energy lost by electrons with energies below the lowest excitation level of the target system: most of this energy goes t o vibrational and rotational excitation of electronically ground state molecules(e). Because of the "wedge-like" shape of degradation spectra at relatively low energies (s) we have chosen to estimate subexcitation losses to be (4)

~-~subexcitation = Lowest dissociation energy× 100 eV 3 W

This is not very different from other workers' methods of estimation (x,v.s). Mechanisms In our radiation chemistry work we measure stable product yields from single high intensity electron pulse irradiations and combine these yields with fragmentation data from mass spectrometry ~ d electron impact studies, with W-value data from ionization chamber work, with rate data for ion-molecule reactions, charge neutralization reactions and with quenching rate data for atoms, radicals and excited molecules. Putting all this information together we try to produce a coherent more or less detailed mechanism of the radiation excitation event. This is the approach we have used in the present paper to discuss the excitation mechanisms of inorganic gases. Data are collected in the form of tables. For each system we present a table containing electron impact cross-sections or mass-spectral abundances and calculate yields of primary processes. The behaviour of the primary species is discussed on the basis of relevant rate constant data collected in a second table and in the fight of a summary of the results of radiation chemistry studies at both high and low dose rates which is given in a third table. Finally, in a fourth table, comparisons of results from various sources are made and the results of the energy balance calculation given for the system under discussion. In deriving these detailed mechanisms we need to know the products and reaction rates of ion-neutralization reactions. Although there are reliable data for the rates of such reactions (s), these studies are normally carried out under conditions where it is virtually impossible to measure the products formed. Radiolysis of gases itself seems to hold the most promise for generating this information and, for the reasons we have already pointed out (1°), reliable data can be obtained from single pulse, very high intensity studies. SFs and some fluorinated hydrocarbons, e.g. cyclo-C4Fs, can attach thermal electrons very rapidly (u) forming $Fs- , etc. As we have discussed at some length in several of our papers, charge neutralization involving SF s- normally does not lead to dissociation of the originally positively charged moiety. Thus by comparing yields from the pure system with those from mixtures containing small amounts (0.01-0.1 Yo) of SFs, etc., it is often possible to deduce precisely the product yields of neutralization reactions. The requirement for single pulse, high intensity radiolysis

74

C. WILLm and A. W. BOYD

arises from the need to minimize ion-molecule reactions between primary ions and products or impurities ¢1°). A summary of ion-neutralization reaction products is presented at the end of the present paper.

Experimental techniques The experimental techaique of radiolysis with very high intensity, single pulses is basically simple. C o n v e m t i ~ ~ are used for p~paration and analysis of samples. The samples are irradiated with p ~ from either a Febetron705 ( ~ 6 0 n s , I-1.7 Mev, ~3000A) or a Febetron 706 ( ~ 3 ns, 0-6 MeV, ~ 10,000 A) at dose =rates of 1 ~ 1 0 m eV g-1 s-X giving absorbed dose8 per pulse of u p t o 10m eVig -x. Dose per pulse is very reproducible, -+1-2 per cent, and the overall ~ p r e c ~ o n ~ d b e better ~ 4 per cent. Dosimetry is very precise either absolutely using thin metal disc adiabatic calorimeters a~-l') or relatively to a calibrated gas such as NsO [G(N2)= 12.4_+0-2 at 1 0 ~ e V g - l s -xc~,x*~, or O2 [G(O2) --12.8 _+0.6 at 10~ eV g-1 s-1 (s, ~-1~]. 1. OXYGEN

Oxygen, a homonuclear ~ m i c molecule, forms a single stable product on irradiation, ozone. However, the details of the excitation mechanisms are complex.

Primary process yields T a b l e I ( A ) 8ives the yields o f p r i m a r y processes. F o r t h e Oa + states, b e c a u s e they a r e m o r e complete, we have used calculated, semi-empirical cross-sections cxg). TAm,B I(A). OXYGEN: ]PRIMARY PROCESS YIEI.JDS W = 30.8 eV: 3-28 ion palrstl00 eVus~ Reactions

Yield (G units) ~t', m)

Threshold energy (eV) ~'j

Probable Energy used energy (eV) ~m~ (eV/100 eV)

12.08

12.4

3.0

16.11

7"8 3.4 7.6 3.0 5.9 1.3 3.2 0.5 17.2 8.4

I o ~ processes Oi -+ Ol+(Xff'/,)+e -

(d)

0-24

(a)

0.OO

Ol+(a'/-/,) + e -

(d)

0.47 0.21 0-43 0.18 0.32 0.07 0.13 0-02 0.86

18.8 or 20-55

16.7 16.2 17.6 16.9 18.3 18.2 24.6 24"6 20-6

0.37

22.0

22.5

O, -+

(a)

O= ~ o,+(a ff/D+e-

(d) (a)

o, -,,- O.+(b'Z',-)+ e01 -~ Os+(B state)+e-

[B = (c'Z',-. c '~',-)1 o~ --,. oPP or ~D)

+ O+('S)+e

(d) (a) (d) (a)

16.82 18.17 24.56

-

OI ~ O ~ P ) + O + ( I D ) + e -

Total energy for ioniT•tion ~ / 1 0 0 Neutral excitation Os .+ Osfa 1,4) O, -, O,(b 12.~+) O, --*Oi(A s2~.+) and (C sJ.) o, -~ O,(B*,V.-) (9.7 eV loss process) Os "-" O + O *

eV absorbed:

(20) 0"012 0.002 0-063

(22) 1"0

(22) 1-0

1"6

1"7

4"3

5"0

1.82

5"2

9.7

,~0.18

,,,12

61.3 eV 0-012 0.'014 0.31 17-7 2"2

(diss. ex. to give O above 1Slevel) Total enerlly for neutral excitation processes: (d) Direct ionization. (a) Autoionization.

20.2 eV

Excitation in t h e radiation chemistry o f inorganic gases

75

However, they compare well with experimentally measured absolute values css). The cross-sections were combined with the cross-section for dissociative ionization~S°~§ and W-value for oxygen¢lSk Threshold energies are based on detailed °potential energy curves(Uk There are some uncertainties in Table I(A). For example, the energies of the states which give rise to autoionization are not known, and, although it is known that there is a large yield of O+~D) ~ss~, the state of the O atoms formed by dissociative ionization is not known. Mechanism Table IfB) gives relevant rate constant data and Table I(C) summarizes radiation chemistry results. As is obvious from the discussion below, each O atom formed in

the radiolysis of oxygen yields one molecule of ozone, at least at relatively low conversions. TABLE I(B). OXYGEN: RELEVANTRATE COI'q$TANTDATA Reaction

Rate constant?

Ref.

Ion-molecule reactions O + ( ' S ) + Os ~ Os+(X * ~ ) + O(aP) O+(ID) + O, ~ Os+(A */-/~ a n d / o r a 4/'I.)

2 x 10 - n 2 x 10 -x°

(26) (25a)

~-i,~ 3.6 h 2"8 x 10-ae~ ~ 5 x 10"4e~ 1.5 x 10 -s < 10 - u 2 . 0 x 10"*e~: 10 -is 3 x 10 -ax 7 x 10 - u 2"4× 10 -7 7 × 10 - n

(27) (26) (28) (29) (29) (30) (31) (26) (29) (32) (29)

Neutral reactions O(aP) + O(sP) + Os ~ 2Os O(Sp) + 2 0 s -4. Oa + Os O ( s P ) + O, -* 2 0 ,

7"2 × 10-u~ 3"6 × 10-st~ 7.5 × 10 -xs

(33) (33) (33)

o(~z>) -~ O(*~')+hv O~D) + Os -,- op/') + Os

~, ffi 110 s 4 x lO-u

(27) 04)

O f t D ) + Os ~ 2Os OtaS) -+ O(aD) + hv O(1S)+Os ~ 20,

3 x 10 -xe ¢I =ffi0"74 s 6 x 10 -x°

(35) (27) (36)

= 45 s

07)

+ope) O+(aD) ~ O + ( 4 S ) + h v O s + + 2 0 , -~ O , + + O , O4++ O j + He -* O e + + H e O+(~S)+ SFs -* S F s + + O F O,+(XST/~)+SFs -* S F s + + F O s e - + 2 O s -* O , - + O s e-+Os ~ O,-+hv Os-+2Om ~ O,-+Os O s - + S F s -* S F s - + O s e - + S F s -* SFsS F s - + Os -* O a - + S F s

ch(a ~A,) -~ O,(XSZ,-)+hv O,(a O.(a Os(b Os(b

xAg)+Oa ~ 2Os aAg)+ Os ~ 2Os_+ O ( s e ) 1~_,+) ..,. O s ( X S £ , - ) + h v x~+)+O, ~ 20,

O,(b x27o+)+ Os -* 2Os + O(SP)

.r~,

2x 3x ¢it" Ix

10 - ~ 10 -xs 12 s 10 -*s

(38) (38) (34) (38)

7 × 10- u

(3S)

t Rate constants have units c m s molecule -~ s - L 2~ Three body reactions, units cms molecule-* s -1. § The relative yield of dissociative i o n i ~ t i o n reported here from cross-section measurements at 100eV, Os + ffi 1 - 0 0 : O + ffi 0.59, is very different from that reported in standard mass spectrometry data tables ~u~, Os + - 1-00 : O + ffi 0~3-0"05, which we have previously usedCSL The difference arises because the O + ions .are formed in a n energetic state a n d discriminated again.~t in narrow geometry instruments.

76

C. Wn,tas and A. W. BoYD

T.mt~ I(C). OXYG,N: ~ m * ~ o N c~mJ~srRgDATA Dose rate

(eV g-x s -1)

Additive

Yield G(Os)

Ref.

Comments 195--283 K. Yield corrected for ,toichiomotry of analysis technique [see Ref. (5)1 90 and 200 K. Yield corrected for analysis and dosimetry [see Ref. (5)1 Initial yield at 195-283 K 77 K

10*e-10*8

None

5,9

(39)

10~

None

6.3

(40)

10'~-10*~ 10*4

None None

6 6.2

(41) (5)

10~ ~-6 x I0u 10~-2 x 10~

None None None

10'6-10 xe - 10~ 10N-2 x 10m 10u-2 x 10~ 10N-2 x I(F

Electron scavengers ~-6 SF, (1-3%) < 8.8 SF, (0-2-3yo) 6.3 + 0.6 cyclo-C4F, (0.2-3~o) 6-3 + 0.6 NnO (3-5%) 11-7_+0.3

(41) (17) (5) (5) (42)

10u-2 x 10~

N,O (3-5%) + SFe (2~o)

(42)

12-9 (17) 13.8 _+0-6 (16) 12.8 + 0.6 (5)

5.9

Corrected for dosimetrym~ 195 and 298 K Plateau yield not established

fen-molecule reactions 0 + ions. All O + ions, whether in a (ZD) or (4S) st~t_e, will rapidly undergo charge transfer with O2 to give O2 +ma,u); reaction (5): (5)

0++02

> 02++0.

02 + ions. Even at very high dose rates, above a few torr pressure, all 02 + ions will form O4+(N); reaction (6): (6) 02 + + 202 , 04 + + O2, and at lower dose rates further clustering will occur (~). Oa + ions. Significant reaction cross-sections have been reported from mass spectral studies (~) for 02+(A2IIu), and perhaps 02+(a4IIu) in excited vibrational levels, to give Oa+; reaction (7): (7) O2+(ASH,, and/or a*Fl~)+02 , O2++O(8P) although there is considerable doubt as to the magnitude o f the cross-section ~¢). It has been suggested (n) that in the radiolysis o f oxygen, O8+ will undergo charge transfer to give 02+; reaction (8):

(8)

02++02

> 0 , + 0 2 +.

We contider significam participation o f O, + in oxygen radiolysis most improbable. Firstly, reaction (8) would f o r m ozone prior to charge neutralization. N o rapid formation o f ozone is observed in pulsed experiments ~le,x7,44~. Secondly, the reaction sequence

(7)

02+(a211,,/a4FI~)+02

(8) (9)

0,++02 O, + + 02-

, O,++OPP), • 0,+0,% , 2 0 + 02

Excitation in the radiation chemistry of inorganic gases

77

gives four ozone molecules per Ol+(A 2II,/a' 17) formed. Accepting the yields given in Table I(A), the above mechanism would predict that almost the entire yield of ozone arises from ionic processes and that the yield o f dissoc/ative neutral excitation must be almost zero. This is contrary to predictions regardifig the yield of dissociative excitation. The alternative fate of Os + is that it survives to undergo charge neutralization; reaction (10): (10)

O8++O~ -

> 2Oz+O.

This does not accord with the observed effects of SFe addition. As shown in Table I(C) (and discussed below), addition of SF e, in the Febetron radiolysis of oxygen, reduces the ozone yield by an amount exactly equivalent to twice the ion pair yield. The reaction sequence (7) and (10) would require a reduction of the ozone yield by less than twice the ion-pair yield less 2.2G units, i.e. by 4-4 instead of 6.6. On the basis of the above arguments we must conclude that the amount of 08 + formed in oxygen radiolysis is very small and that the major reaction channel for O,+(A *17da~17u) is deactivation to lower states of O, +. 0 , - ions. At both low and high dose rates virtually all free electrons will react to form O2- prior to neutralization (11)

e-+20,

> 0 2 - + 0 ~,

and, at low dose rates, this ion may become clustered (m. 08- ions. At Febetron dose rates the lifetime of 0 8 - with respect to neutralization will be short compared to its reactive lifetime to form 08-(5); reaction (12):

(12)

o -+o8

,

%+0,-.

At low dose rates (lOre-101. eVg-ls-1), however, the lifetime of ions with respect to neutra, Tation is long "°) so that as soon as a very small amount of ozone has been formed (<10 u moleculescm-~), all O~- will react to give 08-.

Neutralization reactions High dose rates. At Febetron dose rates neutraHz~on will be(") between O~+ and O~- (Table IB); reaction (13): (13)

04++02 -

> 20,+20.

Addition of SF e or cyclo-CaFs suppresses dissociative neutralization, reaction (14), and reduces the yield of O atoms and hence of Os accordingly (Fable IC). (14)

O,++ SFe-

> 2 0 , ( + SFe).

The reduction in ozone yield, AG(Oa) = 6.6,

(15)

AG(OJ = C(Os)p ,. o , - C,(Oa)o,+sr, =

12.8--6-2 = 6.6

is very close to twice the ion pair yield and leads to the conclusion that neutralization is dissociative, yielding two O atoms as shown in reaction (13). There is evidence that a significant portion of O atoms formed by reaction (13) are in a (aS) state *hu) and that the ratio of triplet to singiet O atoms is close to unity~*".

78

C. W m m and A. W. goYv

Low dose rates. Here neutral~tion will almost certainly involve 04 + and On-. Since electron scavensers have no effect on the ozone yield at low dose rates (Table 1C) we must suppose that neutralization leads to no net ozone formation: (12)

(16)

02-+03

> 0 3 - + 0~,

O~+(clmtex~)+O8 - ,

> 202+Oa or 3 0 2 + 0 .

The low dose rate yield would be expected, then, to be the same as that observed at high dose rates in the presence of electron scavengers, as is observed (Table IC). Neutral processes 0 atoms. In pure Oj, all O atoms will form ozone (Table IB). O(1D) and O(XS) may lead to O s formation rat~, which are quite different to that due to O(SP), and yield formation rates which apparently are not simple first-order rates (at fixed O s pressure) cte). There is good evidence that ozone is formed vibrationally "hot ' ' ~ ) . This is based on observation of an initially broad absorption spectrum which relaxes to a normal ozone absorption spectrum with time. Because of this time dependence of the ozone spectrum it has been very difficult to determine the yields of triplet and singlet atoms using the above technique. However, we have shown by N=O scavenging techniques that a significant yield of O(xS) is formed both by ion-neutralization and by direct excitation ~ . Excited O z molecules. Som¢: Ot(aXzlg) and Ot(b x2:b+) excited molecules are formed by primary processes (Table IA) and more may be formed by various reactions occurring. Both of these metastable states can decompose ozoneCas~; reaction (17): (17)

On* + O n

> 2Oe+O,

but since this yields one O atom per O s decomposed no net ozone loss results. This process, however, may further complkate the time dependence of ozone formation. Comparisons and energy balance The medumism di~___u~d~above relates the yield of ozone to the yield of primary processes. At high dose rates

(18)

G(Os)Him = 2Y-'G(O=+) + 4Y'G(O +) +

2G(neutral dissociation).

Accepting the yields of primary processes in Table I(A), this gives G(Os)nxm = 2(2~7) +4(1-23) + 2(2.00) = 13-06.

Alternatively, accepting the yield of ozone determined by us, G(Oa)= 12.8 + 0.6 ~s), and accepting the yields of primary ionic processes, which are the more reliable, a yield of G ( ~ t i v e excitation)---~(12.8-9.06)_~1.8s is obtained. Clearly the agreement is within experimental errors. The yield at low dose rates or in the presence of SF e is

(19)

G ( O ~ D R ----2~G(O +) + 2G(neutral dissociation) = 6.3+0.2.

Excitation in the t'adiation chemistry of inorganic gases

79

This is based on the yields in Table I(A) and again agrees with experimental observations, Table I(C). These comparisons are collected in Table I(D). It is slightly disquieting that the energy balance is not better. The energy sum reported in Table I(D) predicts an ozone yield o f G(Os) -- 14.0 which is outside our T,VSLEI(D). Co~a,,~moss AND E~ROY BALANCe Species

Cross-section data (Table IA) Radiation chemistry data (Table IC)

O+

G = 3.28

G = t(12-8-6"3)

Dissociative excitation, Ot

G -- 1.82-2.00

G -- ½ [ G ( O , ) - 2 G ( O s + ) - 4 O ( O + ) ] ffi j ( 1 2 . 8 - 9 . 0 6 ) -- 1"87

Energy balance Ionization Neutral excitation Sub-excitation

ffi 3 . 2 5

61.3 20.2 7.8 Total: 89.3eV cf. 100 eV

experimental range. However, a considerable portion o f the ionization is via autoionization or dissociative ionization processes
Primary process yields Table H(A) gives the yields o f primary ionic and neutral excitation procemes calculated using cross-section data. F o r the Ns+ ~ t e s we have again used calculated semi-empirical relative cross-section (a) and combined these with dissociative ionization cross-sections ca°~ and the W-valueCXSL The yield of neutral excitation processes are calculated from experimental cross-sections ca°~ combined with the total ionization cross-section ~°~ and the W-value. Probable energies are f r o m detailed potential energy curves Cu). ? It is important here to distinguish between lum/neace~cz yields and crms-socfion data obtained using luminescence detection. Cross-section meuurements are made at low target gas pre~u~ where ion-molecule and H u ~ n reactions, etc. do not occur in the ob~rvation region. Radiation luminescence yields are total luminescence yields and are measured at pre~ures where such reactions should occur.

80

C. WnJm and A. W. BoYD

TAetz II(A). l',lrrstmsN: l,mm~Y i,Roczm~ W -- 34.5 eV: 2,9 ion pairs/100 eV cls) Yield Threshold Probable Energy used (G units) ~e, ,0~ energy (eV) m~ energy (eV)~n~ (eV/100eV)

Reaction Ionic processes Ns -~ Ns+(XS270+)+e-

(d)

1.55

(a)

0.00

(d) (a) ~ Ns+(B~'~+)+e - (d) (a) ~ Ns+(C ~'u +) + e- (d) (a) -~ Ns+(D IH~) + e- (d) (a) -~ N(*S)+N+(3P) + e - )

0.34 0.02 0.10 0.02 0.10 0-01 0.10 0.03

Nt -+ Ns+(A IJ-J'u)+ eNt Ns Ns Ns

15.59

15.59

24.1

16.87

5.7 0.4 1.9 0.4 2.5 0.2 2.5 0,7

24.32.)

16.95 21.00 18-75 21.00 25.05 23.6 24.7 22.0 26-3)

26.29

26.4

18.75 23.86 22.00

0-69 Ns -~ N(*S)+N+(XD)+e -

:18.2

Total energy for ionization processes/100 eV absorbed:

56.6 eV

Neutral excitation processes Nt ~ N ( s S ) + I ~ D ) Ns ~ N(tS) + N(tP) Ns ~ N(tD)+ N(SD) Hi -¢"Nt(A s27u+) N~ ~ Nt(B * ~ ) Ns ~ Na(C s//u) Ns -~ Na(E a~+) Ns --, Nt(a tFfj)

2-36 0"02 0~)2 0-06 0.02 0.17

12,22 13.31 14.50 6'22 7'39 11 "05 11-87 8.55

12.2 13.4 14.5 7"8 7"9 11-1 11.9 9.2

Total enersy for neutral ¢mcitation processes:

31.9 0"16 0"16 0"67 0-24 1-56 34-7 eV

(d) Direct iom~zation, (a) Autoionization.

Mechanism o f the ~ t y of d~$ a radiation c h e m ~ system as such for nitrogen the discussion o f mechanism will be centred on the fate of the initially formed species. Because much radiation information is derived f r o m additive systems, particularly, N2--Os mixtures, the expected reactions pertinent to these will be included. Discussion o f the radiation chemistry data will be postponed until potential reactions have ~ dealt with. Reactions and rate constants relevant to the nitrosen system a r e g i v e n in Table IlOI).

Ion--molecule reactknts N + iofts. In p a t e nitrogen, N + ions will f o r m Ns+ ions; reaction (20)~2e;: (20)

N + + 2N 2

~ Ns + + Nz.

A t high concentrations o f Or, reaction (21) will compete cse~, reducing Na + formation: (21a) (21b)

N++Oz

, ~ N O + + O(tS)], • ½[O2++N].

Excitation in the radiation chemistry o f inorganic gases

81

TAm~ If(B). NrrltooeN: m~ACTI(~ RATS DATA Reaction I o n - m o l o M e reactions N + 0 S ) -~ N + O D ) + h v N+(XD) -~ N + ( t p ) + h v N + ( a p ) + 2 N , -~ N . + + N . N++SFe ~ SF6++NF N , + + 2 N . -~ N , + + N ,

Rate c o n s t a n t t

Ref.

*t = 0.9 s

(27)

~'i = 4-13 rain

(27)

1.8 x 1 0 4 ~ 1.4 x 1 0 - ' 8 x lO-Sa~

(26) (29) (26)

] × ] 0 -xe

-~ N O + + N O N t + + O, -~ O r + + 2 N t N t + + S F , -~ S F , + + F + N , N++O, -, NO++ O0S) -~ O , + + N O + ( t S ) + N t -~ N O + + N

< 2 x 10 -1' 4 x 10 -1° 1.3 X 1 0 - ' 5~) x 10 -m 5-0 x 10 -~e 2 x 10 -1'

(26) (26) (50) (29) (26) (26) (26)

O+(~D)+N, -+ N.++O

1"0 x 1 0 - '

(26)

5 × 10~*$ I x 10 -1. 2.2 x 10 -1t 7-7 x 10 -1* 5"9 X 10 -1.

(51) 02) (53) (54) (54)

N.++O. -, O.++Nt

Neutral reaeaions N('S) + N('S) + N. -+ 2N. N ( ' S ) + O = -+ N O + O(aP) N(*S) + N O --, N . + O(sP) N(tS)+NOs ~ NtO+O NO+NO -, N ~ ÷ O s -+ N . + 2 0 N('S)+ O8 -~ N O + O r N e D ) - , N(,S)+hv N(~D)+Os -~ N O + 0 ( s p ) N O D ) + N, -* N('$)+ IK~ NOD) + N O -~ N ( ' S ) + N O -~ N a + O P P ) N(IP) ~ N(~S) + hv N O P ) + N8 -~ q u e ~ h S N,(~ *2:.*) -, N,(X,2:o+) + hv

N.(A '2~.+)+ ~ ~ quoncM + O r -. quench§ N,(A ,~v.+) + NdA *~.+) -~ NI(C'II__.)+N, N . ( C ' 'H,,) + N . --* Nt(B ' I / g ) + N . Nt(B ' / I o) -~ Na(A ' 2 7 . + ) + h v N,(B'/-/,)+ N. -* quenoh§, NO+O, -, NO.+%

N % + O . -, NO.+ O. NO+NO. ~ NO,+NO. NO,+NO. ~ NO+NO, GO, N O t + NOa -~ N,Os N,06 ~ NO, + NO. 2NO + O. ~ 2NO~ N O + OOP) -~ N + O , N O + O + M -~ N O , + M N O , + O -~ N O + O . NO,+O+M-~ NO,+M

1.8 x 10 - a

(54)

2,3 x 10 -1' 3"7 x 10 - a ~'t ~ffi26 h 5 x 10 -x* 6 x !0 - ~

(54) (53) (27) (55) (55)

1-8 x 10 - ' °

(55)

*'t = 12 s 3 x 10 -m

(27) (56)

~i = 2 s

(34)

< 10 - a 5 x 10 -xt

(57) (57)

1"4 x 10 -°

(58)

¢t = 4 x 10 -4 s

(59)

2.5 x 10 - u

(57b)

1.6 x 10 -14 8 x 10 -17

(53) (60)

9 x 10 -xl 4 x 10 -1' 3 x 10 -xt = 0"24 s -1 5 x 10-u~: 5 × 10 -x6 exp ( - 39-1/RT) 7 x 10-n$ 9 x 10 -is

(61) (61) (62) (62) (62) (63) (64) (65)

8 x 10-**:1:

(66)

t Rate constants have units cm s molecule -a s -I. Three body rate constants, units cm e m o l e c u l e - ' s -I. § These ~ involve quenching of the excited species but produO~ are undetermiaed. Moot probably reaction involves simple energy transfer but chemical c h a n p cannot be excluded.

82

C. W ~ J ~ and A. W, B o ~

Ns+ ions. In pure Ns, Nl + ions will form N, + by reaction (22)~e~: (22)

Ns++ 2N~

> N~+ + Ns.

But, in the presence of fairly small concentrations of 02, both N~+ and N~+ will rapidly undergo charge exchange to give O~+; reactions (23) and (24)m, ee~: (23)

N~++O~

• O~++N~,

(24)

N, + +Os

> Os + + 2N~.

Electrons. Free electrons will not attach to N ~ and thus in pure Ns they will neutralize N, + and N~+. However, electron attachment to O~, N O and NOs (possible impurities or products of:radiolysis) is rapid (Tables IB and HB) and the ionic mechanism then becomes e~remely complex. Only if reaction conditions are precisely specif~! can this be resolved. Neutralization reactions In pure nitrogen charge neutralization occurs between Ns+, N4+ and free electrons. From the results obtaiaed in Na--Oj mixtures at high dose rates~u~, it is believed that dissociative neutralization occurs; reaction (25): (25)

Nt+ + e-

> 2N + N a.

Formation of N s ( C S H . ) b y charge neutralization was inferred by Sebac~er~u~ and later clearly d e m o ~ by Sauer and Mulac c7°~. Since neutral.._ization of N4+ in a (X z 2~g÷) state cannot thermodynamically give two N atoms p l ~ N s ( C s/-/w),the yield of Nt(CsHu) must either be small or originate from neutralization ofNs + ions where it can be formed in conjunction with one N atom; reaction (20"): (26)

Na + + e-

• N + Ns(C s//~).

The yield of Ns(C sH.) observed in total luminescence e x ~ n t s (Table HC) is small, virtually that p ~ from cross-sections for ~ processes. This would suggest that the yield of N~(CalIu) from ion-neutralization is small, being much less than either G(Ns+) or G(N+) but further qua~titati~ studies are required to establish this point. TAeLmH(C a). N r n g o o ~ : ~ o ~ Radiation 50 kV electrons 850 V elegrons 15 kV X-rays Heavy ions.

CHEmSTRYDATA. L

~

~ Ref.

Comments

0.04

(71)

85-770 torr

0.08~ 0"045 0.072~

(72)

2-20 tort

Yield

G(N.C aN,) G(N,+B I ~ ) t G[Na(C I/'I~)] G[Ns(B s/I~)]

ffi

YIELDSIN ~tme NITROGEN

? Ns(CaH,) Inminesces to form Ns(Ba~rg).

Hence, G[Nm(B*]'[,)] =

~[NdB * H , ) ~ + 6[N,(C *7/J], which gives ~[N,(8 *H,)]~t~ ffi 043. 5 Yields calculated relative t o t o t a l ionization cross-section, 2.53× 10-16 cm ~ c--~ using a ~ l u t e cross-secti0n for 391.4 nm enusmon (N I first positive 0,0 band) = 1.5 × I0"17 cms cn~. -

•

c"

*

§ Temp0racy n~afive ion fm~m~on can be ~ to have occurred in the transition state i n ~ t ~ o n ot e l ~ t r o m tv~. However, this is a f o ~ and does not require inclusion in the present discussion.

Excitation in the radiation chemistry of inorganic gases

83

TAeI~ H(C b). I~rol, E eXCH.~OI YmZ.VSG(exch.)(AT I0t* eV g-t s-X) Yield

Additive

Ref.

Comments 300-307 K equimolar aeNs, BNs

G(exch.) ,= 7.30

None

(74)

G(exch.) -- 9-50:I:0.5 G(exch.) independent o f temperature G(~Ns) -- 2.56+0.13 G(mNs) ffi 2-57 G(exch.) -- 4.3 G(exch.) ffi 4-48

None None

(75) (76)

1.6~o, x4NO

(74) (75b) (74) (74)

1"6~, ~ N O

2.4~e O, 3.3~ NsO

300-1300 K 98~0 8°N, SeN,natural abundance

Yields derived from Na-O, radiolysls (at 2 × 10~ eV g-x s-X) G(neutral dissociative excitation) -- 2.9-1-0-2t cu~ I" This is a calculated yield for pure Nt using data from Nt--~ mixtures, essentially extrapolating yields to zero Os pre~ure, It differs slightly from the value given in Ref. (68) as it takes into account the yield of dissociative ionization which was not originally comidared. Equivaknt data are not extractable from the numerous studies of the system at low dose rates because of the very complex ion-molecule, ion-neutralization mechanism applicable at non-zero doses (te).

Neutral reactions N atoms. In pure Ns, both N(4S) and Nt~D) will eventually recombine to give N s. However, this process is slow for N(4S) (ra) and even slower f o r N ( t D ) as quenching m) must presumably precede recombination. The long lifetime o f N atoms with respect to recombination makes t h e system very sensitive to impurities (see reactions in Table HB). Excited~Ns molecules. The possible role o f these species is not a t present clear. Although extensive data on quenching rates are available (Sv,r~) it is not always obvious whether this quenching just involves energy transfer or whether chemical change is involved.

Isotope exchange mechanism A summary o f radiation c h e ~ s t r y studies relating to the radiation chemistry o f nitrogen is given in Table II(C). The details o f the mechanism o f radiation induced isotope exchange in m N s - ~ l s mixtures are not entirely clear. It seems generally accepted that exchange is due to atom recombination (74) a n d that no chain mechanism is involved (~e). However, assuming dissociative neutralization o f N4 + and taking G(dissociafive excitation) -- 2.9 (see Table IIC), the observed exchange yields are too high. The effects o f additives are also difficult to explain on the basis o f known rate constants. The exchange yield then cannot be explained in terms o f simple atom recombination and, allowing for the insensitivity o f the yield to traces o f oxygen ~7.), the involvement o f exchange during neutralization o f N 4+ seems an unfikely source o f the excess yield and further studies are called for.

Comparisons and energy balance Various comparisons of data are given in Table H(D) together with the energy balance calculation for the nitrogen system. Within reasonable experimental error, 7

84

C. Wn~Hsand A. W. Bo~

TA~L~ H(D). H ~ :

Species

OO~AltmONS AND BNEItGY9ALANCE

Cross-section data

Ns+ ~ N+ .J Dissociative excitation, Ns* Ns(C *H~) Ns(B s]'/l) Ns+(B "~',,+)

G = 2-91 G~ 2-51 0.06 0'02 0q2

Energy balance Ionization Neutral excitation Sub-aeRation Total:

Radiationchemistrydata G ffi 34)+0.2 [gel (68)] G~ 2-94-0-2 0.04 0"03 0"08

56.6 34-7

(56-6) 39.2

6.9

(6.9)

98.2 eV

105.5 eV

the total luminescence yields agree with those predicted to arise from primary excitation (Table liD). The yield of dissociative excitation predicted from crosssection data is somewhat lower than that required by the mechanism of N 2 - O 2 rad-'_~on chemistry, However, the agreement is satisfactory considering that the detenninatieu of the crees.sections invetved adsmption on a tungsten surface for detection rout ~ the ~ n i s m of ~ ~ i s ~ complex. Using emexection data t h e ~ balau~ is 2:eV low; ruing radiation c h e m i s t r y data it is 5 eV high (Table IID). Given the uncertainties in the probable energy for dissociative eateitation~ this agreement is adeqmate to demonstrate a fair understanding of the primary ~ o n proceues in nitrogen.

3. NITROUS OXIDE N i t r o ~ ,oxide is a very c o ~ system and alOtg.¢gh considerable effo~ has been devotedto its radiation chemistry only recently have ~ become available allowing Primary ~e!ds tO ~ derivedcTv~. The Febetron yi¢lds taken in conjunction with the data in Tables 1H(A) and (B) give a fairly compiete and consistent mechanism for high dose ~ . However, as will be d i s ¢ ~ , it is not yetpossible to derive a complete mechanism for low dose rates. This is principally because of lack of knowledge Of the p~oducts resulting at't~ electron captm~ by NsO. This reaction does not ~compete at high dose rates with electron neutralization.

Primary proce~ yields Yields of primtry ionic processes are given in Table III(A). Although very extensive reaction rate data are available, no electron-impact cross-section data have been reported. Therefore, the ionic process yields presented are from mass-spectral tables ~u~. The ratio O+(4S)[O+(ZD)is from Hughes and Tieman meoj. Since no detailed potential energy surfaces for N ~ + states are available probable energies were estimated as disoussedin the Introduction.

Excitation in the radiation chemistry o f inorganic gases

85

TASL~ HI(A). N r r a o u s oxn>e. P t m a R y IOmC W ---- 32-8 eV: 3"05 ion pah;sfl00 e W x.~

Prooms NjO NsO NIO NtO NIO NtO NjO NsO

Yield

Threshold energy

Probable energy

(~ units)

(eV)

(eV)

~ N I O + ( X I ~ ) + e - "~ ~ NIO+(Xl//t)

+e-

L

"~ NtO+(12~+)+e -~ NtO+(C)+e J -~ N O + + N ( 4 S ) + e - "~ -~ N O + + N ( S D ) + e - J ~ N+(SS) + N O + e -+ Ns + O+(*S) + e-~ N s + O+(SD) + e N s O -~ N s + + O + e -

12.89 12.93 16"39 20"10 15"3 ~ 16"4 20-0 18"0 21 "3 17.4

1"90 0"59 0-25 0'07 0"03 0.21

Total energy for ionization p r ~ / I 0 0

Energy used

(eV/100eV)

17.0

32-3

18"0

10"6

22.0 20.0 23 19.0

5"5 1"4 0"7 4"0

eV absorbed:

54"5 eV

TAeI~ IH(B). NITROUS OXIDE: m~AcrION RATE DATA Reaction Ion-molecule reactions$ N++NaO ~ NO++Ns O++NsO ~ NaO++O N.++ NsO -. N . O + + Na NO++ Os "+ N O s + + O s N O + + S F s -~ S F s + + N O F

NO++2H~

~ I~O+.(tl.O)+ H N O .

e - + 2NsO ~ NIO- + N s O NsO-+Oj ~ Os-+Ns O , - + N s O --> O , - + N s

Ref.

5"5 x 10 -so 6.3 × l 0 se 7 x 10 -le Very slow < 1 x 10 -zs • Fairly fast 6 x 10-m|

(79) (79) (26) (80) (29) (81) (82) (83) (26) (84) (85)

10 -sz

10 - n 7 x 10 -so 4 x 10 -sl

o , - + NO, --,-O, + N O , e - + N O , ( + M) --~ NO,-(+ M) Neutral reactions O~P)+N,O ~ NO+NO T -~ Nz + Os ~ O(ZD)+NsO ~ Ns+Os NO+NO N + NOt O(*P) + N , O O(SS)+N~O ~ products N(tD)+N20 ~ Ns+NO Ns(.4 s~'~,+) + NaO -~ quench Ns(B sT/u) + N , O ~ 2Ns + O ~ P ) and/or Ns + N + N O

Rate c o n s t a n t t

"~ 3

Very slow at 300 K 1 x l 0 -s* "~ 1 × 1 0 -le < 5 x 10 -ss < 5 x 10 - l j 1.5 x 10 -zx 3 × 10 -sl 8 × 10 -ul 1.6 x 10 -11

(8G)~ (57b, 87) (57a, 88) (55, 57a) (57b)

? Rate constant units cm a molecule -s s -x. ~/ Recent data regarding clustering o f ions has been reported Cn~. § Three body rate constant, units cm 6 m o l e o ~ -s s -z.

(57b)

86

C. WILLISand A. W. lloYD

No data for n e u t r a l ~ n processes are available from cross-section measurements. The yields derived from radiation chemistry studies together with threshold energies, etc. are presented in Table III(F). Mechanism

The rate constants for reactions relevant to the nitrous oxide system are given in Table HI(B). Ion-molecular reactions

The net result of reactions of the primary ionic fragments is to form either NO + or NsO+; reactions (27)-(30): (27)

N + + N~3

> NO+ + N~,

(28)

O+pS) + NsO

~ NjO + + O,

(29)

O+(SD)+NsO

, NsO++O,

(30)

Ns++ NzO

~- NsO++ N~.

Using these reactions in conjunction with the yields given in Table Ill(A), we can deduce the yields of species which result from primary ionic processes. These are given in Table Ill(C). TABLE

lII(C). NITROUSOXIDE:

Species Yield (G)

e3.05

YIELDS OF SPECIES RESULTING FROM PRIMARY IONIC PROCESSES

NtO + 2.12

NO + 0.93

N~ 0-56

NO 0.25

O 0.31

N 0.68

Neutral reactions

O(3P). I n pure nitrous oxide, ground state oxygen atoms ~1t react with either NO ~s~) or O~ ss~, produced by rapid ionic processes (31)

O(sP) + N O + M

(32)

OfSP) + Os+ M

> NOs+ M, • O3+M.

The products of these reactions, NOs and Oa, can further react but by complex mechanisms ~eej. In the ~ s e n c e of oxygen, however, ozone is produced and provided it is measured on a millisecond time scale, before it can react with any NO produced, its yield is equivalent to the primary O~P) yield(.2). O(XD), O(aS). Singlet oxygen atoms react very rapidly with NsO to give NO, N~ and Os (5'b,s'~ (33a)

O(tD) + NsO

• Ns + 02,

(33b)

OtaD) + NsO

> NO + NO.

N(*S). Ground state nitrogen atoms are essentially unreactive toward NsO and will react with NO produced by other processescos) (34)

N('S)+NO

> N s + O ~ P ).

Excitation in the radiation chemistry of inorganic gases

87

N(SD). Excited nitrogen atoms react with NsO to form N s and NO ~r~s~a~ (35) N(SD) + NsO , N s + NO. Ns(BS/'/). From our work on nitrous oxide radiolysis we were led to postulate that Ns(B s//) reacts with NsO to form 2N s and O~P) ~a) at the same rate at which it is quenched (szb) (36)

Ns(B s/-/) + NsO

> 2Ns + o(ae).

Neutralization reactions Using the primary yields given in Table III(A) combined with the effect of SF e on the observed product yields, measured at Febetron dose rates it is possible to deduce ion-neutralization yields as we have discussed (tj~. These are given in Table HI(D) together with estimated yields of primary neutral processes ~s). T A B L E III(D). N r r R o u s o x m E : Y m L D S OF I O N - N m U T R A I J Z A ~ O N R ~ C ' n O N S A N D N E U T R A L EXCITATION PROCI~J~

Gkm~ Neutralization NO + + e-

ffi 3"05~) reactions

N(aD)+ O(*P)

No++e- -~ NeS)+ OeD)

G 0.72

0.20

NsO++e - -~ NI(X127)+ O(ID) NsO++e - -~ Ns(B'~r)+ OPP) NaO++e ": ~ Na(A s~')+O~P) N , O + + e - ~ NpD)+NO

1.20 0.55 0"37 ~0

Neutral excitation processes

G

N,O

N,(X'27) + O(1D)

N.o : N,(A ' ~ + Oee)

lgaO -+ NpD)+NO(XS//)

4.0 0.81 0.05

Comparisons Since there are no cross-section data and we have explicitly used the mass-spectral and W-value data in deriving a mechanism, there are no comparisons which can be made. It is even rather difficult to inter-compare the radiation chemistry data (given in Table IIID), as the mechanisms are so very different at different dose rates. As we have previously discussed ~*tm~, at Febetron dose rates, the lifetime of electrons with respect to charge neutralization is very short compared to that with respect to attachment to nitrous oxide(U>; reaction (37): (37) e - + 2NsO ) N ~ O - + NsO. At low dose rates (lOre-10m e V g - l s -x) the reverse is true and it is reasonable to suppose that a very complex negative ion-molecule reaction is involved ~a). One point which should be mentioned is the fact that the Na yields in the presence of SF e are little affected by dose, dose rate or temperature (Table IIIE). This can be accounted for by the inertness of Na and the fact that it is formed by the rapid reactions of singlet O atoms, N(aD) or N s ( / P / / ) with NsO. Thus the N s does not take part in the back reactions and its precursors are not easily scavenged by radiolysis products or impurities.

88

C, Wu.us and A . W. BoYD

:

T,.,mas 3(i~-)~ NITROII$O~Di~" RADIATION C ~ R Y

Dose rate g-X S-I)

Additives

G(N 0

G(NO)

G(O0

Ref.

None

I0-0 + 0-2

3"4 + 0.3

4-0 + 0"4

(89)

None 0.1-0.2~ SF 6

10.1 8.0

5,1 3.3

3.8 3.2

(90) (90)

(eV

1-25 × 10t~ to 1 x 10ts 8-3 x 10~ ~ 3 x 10t6 1 - 2 x l O t*

- .

~5~

and/or CIH ., etc.

10~ 2 x 10m IOw

DATA

None None

Reactions NtO -~ H i + O * -* N t O + + e -* N 2 + O

G ,-,4

CgNO)

¢<0O

5.6

4.8

4'6 4.3 1.9

2"9

12.4+0"3 12.3 +0.3 8'1 ff(~extr. --G(~. ----

O~5*/e SFQ

O, O, + SFQ

(91)

3"0 <0-5

(42, 92, 93) (14) (42)

Energy balance T h e energy b a l a n c e c a l c u l a t i o n i s given i n , T a b l e III(F).

This energy b al an ce

appears very good but there is a good ~ of ~ u t y in its estimation. We do not know details of the ~ n t i a l ~ ~ for NsO + ions nor do we know the relative yields of O(tD) and O(IS) f o r ~ by noutral excitation. TAmE re(F). NITROOSOXIDE: eNIgOY aAt,ANCE Yield (G units)(a)

Process

~ e a e t ~ (eV) clt)

Probable e~ergy (eV) (N)

Energy used (eV/100 eV)

Neutral excitation yields and energies

NsO -~ NI(XXZ)+OOD)'~

3.64

t. NtO -~ Ns(X ~)_+.00S) f (pro~tm~yv~. ",~"smte)J

4.00

NiO ~ 1 ~ ; 4 ~ O ( * P ) NtO -~ Nf~D) ÷NO(J(!Jff)

0-81 0,05

7.0 "~

5.86

I0.8

7-9 7.3

10-2 10-5

Total energy for neutral excitation processes/100 eV absorbed: Energy balance Ionization 54-5 Neutral excitation 40.8 Sub-excitation 4.4 Total:

8.3 0.5 40,8 eV

99.7 eV

4. CARBON MONOXIDE

Although we have some understanding o f the ion-molecule reactions occurring in carbon r o d e radiotysis, as digazued below, our understanding of the system from the point of view of neutral species is speculative at best. Dissociation does not

Excitation in the,x,adiation chemistry of inorganic gases

89

appear to be very significant, possibly because o f the exceptionally strong CO. bond, D o , . o Z 11.16 eV ('e. However, a variety o ( e x c i t e d ~ ~ f o r m e d a n d some o f the chemistry o f the system can be explained:in terms o f their reactions,

Primary process yields

•

{

Yields o f p r i m a r y processes are given in Table IV(A). F o r the ionic states o f C O we have used the calculated semi-empirical cross-section data (~) together with experimental values f o r some states (m) a n d for dissociative ionization (a°~. The ratio

TASLZIV(A). CAgaON MONOXmE:PP.D~AnY PaocEss

ym~.rJs

W -- 32-2 eV: 3.11 ion pairs/100 eV (*e) Yield

Procvss Ionic proce~ms CO -~CO+(XS~'+)+e -

(Gunits) t'~,') (d) (a)

CO CO CO CO

~ CO+(A s//~)+e~ CO+(B s.~r'.+)+e~ C++O(aP)+e -~ O+ + C(*P) + e:

Neutral excitation processes CO -~ CO(a 8/'/) CO ~ CO(A 1/']) CO "-~ CO* ("13"SeV Ices") CO -~ ~CO[CX~'+(n = 3)l CO -* COlE :ll(n = 3)] CO - ~ O I C 127+(n -- 4)] CO ~ ~'~)[EZl-[(n --- 4)] CO ~ C O [ B x~'+(n -- 4)] CO -* CO[b 8~,+] CO ~ CO[B l~+(n = 3)] CO -* CO[FSlT(n ffi 3)] CO ~ CII (1325 A emission)+ O

0.81 0~6 0-87 0"62 0.55 0.20

Threshold energy(eV)m)

,Probable

Energy used

energy(eV) (u)

(eV/100eV)

14.01

14.0

12.2

17.5 20.1 24 27

15"2 12"5 13.2 5.4

16-54 19-67 20.89 24.77

"

Total energy for ionization pro~u~/100eV absorbed:

58.5 eV

0.13 0.33 0"33

6.3 8.5 13.5

0"82 2"81 4.46

0-13 0.10 0-08 0"05 0.05 04)2 0-03 0"02 0"05

11.8 :t2.0 12.5 13"0 12.4 10.6 11.4 12.8 30"0

1-53 1.20 1.00 0.65 0.62 0.21 0.34 0.26 1.50

Total energy for neutral excitation processes: 15.4 eV (d) Direct ionization. (a) Autoionization.

o f C + to O + was assumed to be that reported in standard mass spectral tables(UL A small percentage o f autoionization is predicted t~) and we have assumed this yields g r o u n d state C O ÷ ions. The situation as regards yields o f neutral excit#d states is not clear. Those given in Table IV(A) are derived f r o m the calculated data o f Sawada et al. (97). These must be considered very tentative at the present time as there are few experimental estimates except for CO(aS/'/) fm'mation where the data are inconsistent ~u~.

90

C. W n . u s a n d A. W. B o ~

Mee~m o n r ~ on ~ ~ states o f carbon monoxide is incomplete, a ~ Consistent with the observed ~ yields can be derived and is given below. Rate constant data relevant to reactions in carbon monoxide are given in Table IV(B) and a summary of radiation chemistry results is in Table IV(C). TABLE [V(B), CARSON MONOXII~: RELEVANTI~ATECONSTANT DATA Reaction

Rate c o n s t a n t t

ReL

¢I = 2 ¢I = 6 1"4 2

(59) (59) (99) (99)

Ion-molecule reactions

CO+(a ' H ) -~ CO+(X'~÷)+ hv c o + ( a , ~ +) -~ c o + ( x , ~ + ) + hv CO+(X'27+)+2CO -. C=O.++Co CsO. + + CO -~ CO + + 2(~O CO+(A * / I a n d / o r B * Z ' + ) + CO -~ C O . + C + -~ CO, + + C

(2,0++0 C + + C O ( + M) ~ C,t O + ( + M) _ O + ( 4 S ) + C O -~ C O + p 2 : + ) + OpP)§

x × x x

1 0 -4 s 10 .4 s 1 0 - = +* 1 0 -11

? Slow Very slow Assumed

(10o) (1O0, 101) (100, 101)

(lO2)

Neutral reactions O ~ P ) + 2 C O -* C O t + CO

OeD)+CO -* OpP)+ c o o 0 s ) + c o -* o ( W ) + c o CO(a a l l ) ~ C O ( X t 2 ~ + ) + h v CO(a s H ) + CO -~ C O + C O -~ C O s + C CO(a s / ' l ) + o = -+ quench

5 x IO-N~ 2 × 10 -11 I x 10-" ~rt = 10-* s 1"2 x 10 - t '

(103) (104) (105) (59) 006)

1"9 x 1 0 - l *

006)

2x 2x ~ri~9 x ~'t--- 2 x •i~ 5x

CO(a * H ) + C O . ~ quench CO(A t / / ) .~ CO(Xt2~+) + by CO(b *27+) -~ C O + b y CO(dtA) ~ CO+hv CO(d =A)+ 1 ~ ~ quench C _ ~ P ) + C O + M ~ C,sO(Xa27) + M c(=P) + o , ~ C O + o p e )

10 -x° 10 -11 10-' s 10-' s 10 -s s

6 x 1 0 -11

Fast at I atm. 3"3 x 10 -11 < 5 x 10 -1= 5 x 10 -la 4 x 10 -14 I x 10 -1°

CpD)+O~ -* c o + o ( ? ) OPP)+CsO= ~ 3CO CO=+ C10

C~O(XS~)+o~e) ~ 2 C 0

(107) (107) (108) (59) 09) (109)

(110) (110) (110) (111) (lll) (112)

1" Rate c,~lstant units c m s molecule -x s-L ~ body rate commmt, units cms molecule-* s -~. § This reaction h a S a threshold at 0 - 4 e V above thermal energies. Below this it is not clear what happens to 0 %

Low dose rate mechanism lon-.molccule reaetiosa and neutralization: (38) (39) (40)

(41)

C O + ( X 2 2~+) + 2 C O CO+(A =//and/or C + + 2CO

B s ~'u +) + C O > C=Os +

C,02++e -

CtOs + + CO, > COs + C +, -" p o l y m e r , > 2CO.

Excitation in the radiation chemistry of inorganic gases

91

TAeL~ IV(C). CAttm~¢tmNoxn~: aAVL~T~¢cnv.tasray DATA Dose rate (eV g-i s-x)

Yield

Ref.

G(CO0 -----2.0+0.2 "~ G(-CO) 8+1 ~ G(COs) =~ 0"6±0"2 G(polymer)t = 1-5 ± 0-05 G(CO0 ==0.7±0.1 ~ c 4 o 0 < o~2 . G[OPP)I = 1.41 ±0.22 G[O(tP)] < 1,6 G[O~P)I ffi 1.4

10~e-10~8

> 10tx 2 × 10~ ~ 10~' ~ 10~ 10~

(113-115) (116) (117) (117) (118, 119) (17) (120)

t Polymer has general formula (C1.5~0.sO)nand leads to an overan stoichimmm~ of 4CO-* COi÷CsOs.

Neutral reactions: (42)

CO*+CO

(43)

C2Op2;)+ C O ( + M)

(44)

O ~ P ) + C O ( + M)

> C~O(X*,~)+O~P), ) C,O,

,"polymer,

> COs(+ M).

High dose rate mechanism The ionic mechanism remains essentially unchanged but the reactions of C10 and O p P ) change to become

(45)

c s o ( x 3~) + opP)

,2co

which will dominate first-order processes at sufficiently high dose rates.

Discussion The formation o f CO2 by reaction of CO + in an electronically excited state, reaction (39), has been discussed in mass spectrometric studiesa~lol~ and seems the most reasonable explanation for formation of the COl yield which is unaffected by dose rate m~. Although some posaibifity did exist that this was due to O(tD) being the precursor o f COs reactions (46), more recent work on the system cm-lu~ (46)

O(aD) + C O ( + CO)

, C O l ( + CO)

seems clearly to eliminate this possibility. N o studies have been reported for reactions o f thermal C + ions in CO. Therefore, the fate o f C + remains uncertain but it probably reacts to form carbon suboxide polymer by reactions o f the type discussed by Schildcrout and Franklin~l~L It seems probable that neutralization o f CIOI +, reaction (41), does not lead to C - - O bond bmakageCtU~x~TL The most reasonable explanation for this es~ntially non-dissociative neutralization is that since the ionization potential o f CO [I.P.co+ ffi 14-1 eV cm] i s only 2-9 eV greater than t h e bond dissociation energy [D(C--O) ffi 11-16 eV¢~], the energy released o n charge neutralization is partitioned between the two CO molecules as electronic or vibrational excitation.

92

C. Wu.us and A. W. BoYv

Direct observation ~ havedemonstrated-formation of both C~O(X a 27)a~6~ and C(xS) ~xm,l=) in the radiolysis of pure CO and the C_~Oformed from this has been fairly convincingly demonstrated aM) to originate from reaction of excited CO molecules, reaction (42). A small amount of CO s will be formed CO(aSH) by reaction (47)a°s): (47)

CO(aSH) + CO .....

• COs+C

but the yield of this reaction willbe sn~dl, <0-02G units. It may account, however, for the small yield o f CsO! observed ~ l d g h d o s e rates(uT); reaction (48): (48)

C + 2 C 0 ( + M) ....

~ C~Os(+ M).

Comparisons and energy balance It is possible to derive on!y:, ~ . ~ , process yields :from the radiation density data as the mechanism is not yet firmly established. It is possible that the excited ion yield required to explain the high dose rate yield of COs can be identified either with the (ASH) or the (BS2~u+) state, (Table IVD). Beyond this, however, c o m ~ ~am!:~tive. F~.u~y, the energy balance cannot be expected to be anything better than poor hut is given for the sake of completeness. T~L~ IV(D). C ~ O N MONQmDS:CO~r~O.gSONSASV ~]~tOY SALANCE Species

Cross-section data (G)

CO+(A Lr/) CO+(B s2~+) CO+*

Radiation chemical data (G)

O.87 0.62 0"7±0.1

Energy balance Ionization Neutral excitation Sub-excitation

58.5 15.4 11.7

Total: 85.6 eV

5. CAIU$ON DIOXIDE

The amount of data available on cross-sections for COffi is rapidly increasing because these are required for laser-power calculations (m). At the present time our information on cross-sections for ionic species is fairly good but that on neutral excitati~ processes ~ p o o r . U~fike CO, howevex, f o r COa w e d o have reliable radiation ~ data. Primary process yie~da Yields of prima~ ~ are givm in Table V(A). The crossqtevtions used for COs + states a r e a ~ ~ o f oalculat0d s e m i ~ ~ctions (Im~ and egpefimental vakws(t~mL The ,protmbl¢ energies are those given in the paper of Sawada et a/.am~ which ale in turn ~ on both experimental data and calculated fitting procedures.

Excitation in the radiation chemistry of inorganic gases

93

TAI~LeV(A). C~mmONDtOJm)B:Pr.JU~t*~Ypnocess YIELDS W ffi 32"9 eV: 3.04 ton pairs/100 eV (Is) Yield Threshold Probable Energy used (G units) (~B-lse) energy (eV)(m) energy (eV)cm) (eV/100eV)

Process

Ionic processes CO, CO,+(X J/~rA,)+ e- (d)

co, -~ C O s + ( X * H : , , ) + e

(a)

-

CO, --* CO s+(,4 *//,,) + e-

CO, -, CO, +(B JZ,+)+eCO, ~ C O , + ( C * ~ , + ) + e -

CO, -+ C + + 2 0 + e C O s ~ CO++OQP)+e CO# -+ CO++O(tD)+e CO, -* O + p S ) + C O + e - t CO, -+ O+(~D)+ CO + e - t CO, -* O+(sP)+ CO+e -

"~

f

0.79 0.26 0.64 0.46 0.09 0-07 0-51 0"19 0-02 0.00

13-77 } 13.79 17.31 18.08 19-38 27.9 20.5 23.3 } 19.2 22.4 24.6

13-8

14"5

17-8 18.1

11 "4 8"3 1 "7

19.4 28 .0 22 20 23

2.0 11.2 3.8 0.5

Total eliergy for ionization processes/100 eV absorbed: 53.4 eV Neutral excitation processes C02 -+ COs* [10"5 eV "loss" probably c o ( x 1//) + OpD)I [Refs. (131) and (132)] CO, -* CO(A ]/7)+ O(XD) [Ref. (I33)]

4.34

10-5

0"I7

•

16

Total energy for neutral excitation processes:

45.6 2"7 48.3 eV

t" Ratio O+(*S)/O+QD)from Ref. (25c). (d) Direct ionization. (a) Autoionization. The cross-sections for neutral excitation present a problem. The values extracted by Sawada et al.(~s) do not agree at all with the experimental values of H a k e and Phelps (m). Because o f the difllculties in the methods used b y S a w a d a eta/. {lu) we have taken the overall cross-section for 10.5 eV loss based on experiment. Furthermore, we have assumed, f o r the comparison in Table V(D), that this energy loss corresponds to dissociative excitation.

Mechanism Rate constants o f reactions relevant to the carbon dioxide system are given in Table V(B) and the radiation chemistry is summarized in Table V(C).

High dose rates The production o f CO and Os at high dose rates can be explained in terms o f a very simple mechanism (xa). The fragment ions all react yielding COs + and product. The COs + (in the clustered form, CsOt+) undergoes dissociative neutralization by reacting with the free electron (49)

CaO, + + e -

> CO + O + COl.

The products o f this reaction are the same as those from neutral dissociative excitation, C O and O. The O atoms eventually dimerize to form Oz.

94

C. WILLISand A. W. B o ~

TAnLBV(B). C~dUtONDIOXIDe:lU~C'rloN gATEDATA Reaction

Rate ¢onstant~f

Ion-molecule reactions C° + +COs "" C O + CO= + O+(*S) + CO= ~--,(3=+ + CO C++CO= ~ C O + C O + COs++ O, -+ O=++CO= CO=+ + 2CO= -~ C.aO,+ + CO=

e - + C O t + M ~ COa-+M O=-+CO= -+ CO,-+O= "L

-+ C O = - + O .f O = - + C O = + O t --, C O , - + O t (3,- + 2COt ~ CO,- + COt O,-+ C0t CO,-+O= O8- + CO= -~ C O s - + O= O s - + C O -~ C O = + O t + e CO=-+CO ~ 2COt+e-

O - + C O t + M -~ C O t - + M

O - + C O -+ COs+e-

Ref.

1"1 x 70-' 1"2 x 10-' 1"9 x 70-* 1"0 x 10-l° 3"0 x 10-=:1: < 6 x 10-u++ Very

(26) (26) (26) (26) (26) (134)

slow

see

2 x 10-=~ 1.2 x 10-=++ 4"3 x 70-1° 5"5 x I 0 -~e < 10-la

(135) (26) (135) (26)

< 10-is

(736) (735) (736) (136)

Low pre~mr¢ limit 1.1 x 70-=~:~ Hish preDur¢

(136)

limit 2.7 x 70-1° 7"3 x 10-1°

(136)

3 x 10-1° 4 x 10 -18 8"4 x 10 -=+

(137) (138) (139)

Neutral reactions O Q D ) + C O s ~ O(=P)+CO= O ~ S ) + CO= - , O p D ) + COt OPP)+O(aP)+CO= ~ O r + C O t

Rate oomlam units era* molecule-1 s -1. ** Three body reaction rate, units cm= molecule-= s-L T~a~LEV(C). C,UtaONDIOXIDe:RADIATION cneMt~m¥ DATA Dose rate (eV g-1 s-~) ]0~e-]0m

System

Yield from CO=

Pure CO= Electron scavengm~

G(CO) = 4.5±0-5 G(CO) = 4.5 ± 0.5

Ref.

Comments

(140, 141) 1 (140, 141) ~Initial yields

NO=, N=O, etc.

~10 u

Pure CO=

o.o4-5% SF. 2 x l O ~n

Pure CO= 0-2-1% SFe

2 x 1 0 =a

COfOs + 20/. SF, Pure CO=

~ C O ) = 7.4±0.4 ~CO)=5.2±0.4

(17) (17)

G(CO) = 7"8 +(k3 C,(OJ = 3"8 +_0"5 G(CO) = 4"8 ± 0"2 G(O=) = 2-4 + 0"2 G ( ~ = 4.5:1:0.2 G(CO) = 7-7+0-75

(142)

I These values have been L. corrected for [- dosimetry, see J Ref. (742)

(742)

(143) (14)

Low dose rates A l t h o u g h the reactions o f n e u t r a l species are the same as at high dose rates the ionic mochanilm ~ . i ~ t h p o d t i v e , i o n a n d nvgative-ion m e c h a n i s m s are very sensitive to extremely small ~ n t s o f oxygen.

Excitation in the radiation chemistryof inorganic gases

Positive ion: (50)

CO2÷+ 2CO2

(51)

95

, CsO4++ CO2,

O2++2CO2

C2O4++OI

~

e-+O2+M

, O l - + M,

Negative ion: (52) (53)

O~-+CO2+M . . . . . • CO4-+ M,

(54)

CO,- + COl(+ M )

Neutralization: (55)

> CO,-(clustered).

02 ÷ + 02-

(56)

02 + + CO~-

(57)

Os + + CO4-(clustered)

> 20 + 02, • CO + O + 202, • nCOI + 302.

We have attempted to observe these three possible neutralization reactions by irradiation of COi-Os mixtures at high dose rates (m~. Of significance is the fact that, at a given Oz concentration, clustering reactions of type (54) decrease the yield of dissociative neutralization. It is reasonable to suppose, then, that at low dose rates where the lifetime of ions with respect to neutralization is very much longer~x°~, all neutralization will be non-dissociative (57). Thus, it is to be expected that the low dose rate yield of products should be the same as that observed at high dose rates in the presence of electron scavengers and that electron scavengers should have no effect at low dose rates. As can be seen in Table V(C), both of these effects are observed. Thus, we can rationalize the situation as regards initial yields. At low dose rates, however, after quite low absorbed doses, no further decomposition ofthe system is observed (14e)and, as the stationary state concentrations of CO and O 2 are low, this stability has to be explained in terms of a very rapid back reaction (14°). Various suggestions of the nature of this back reaction have been made, most recently by Parkes (~), but the situation still remains uncertain.

Comparisons and energy balance The comparisons made in Table V(D) are not very revealing. The "10-5 eV loss" cross-section, even including the cross-section for CO(A 1/7) production, is not a TABLEV(D). CARSONDIOXIDE"

COMPARISONS AND ENERGY BALANCE

Cross-section yield C,(ion pair) G(neutral dissociative excitation) Energy balance Ionization Neutral excitation Sub-excitation

Radiationchemicalyield

3~4 4-34

3.0 3.94

Cross-section data 53.4 48.3 6.6

Radiationchemistrydata (53.4) 41-4 (6.6)

Total: 108-3eV

101.4 eV

96

C. Wn.us-and A, W.

gross contradiction of the radiation chemistry ~ . But, because of the sparsity o f specific detail, it is ~ t o attribute 4great sinifwamce to the agreement. However, the energy ~ using the yield of diuocia'tire excitation obtained from radiation chemistry data is very good and indicative Of our good understanding of the radiation chemistry of the system.

6. HYDROGEN SULFIDE Our understanding o f the mechanism o f the radiation chemistry of this system appears to he very satisfactory. However, data required for calculation of an energy balance are either unavailable or less reliable than desired.

Primary process yields The yields o f p r i m a ~ ionic processes, b a g ~ solely on mas~spectral data, are given in Table VI(A). It is difficult to assess the validity of these abundances when TAnt.z VI(A). Hx,DitOo~ ia~taqDg: ~ ¥

IoNIc t,~c~,e-.J~

W]a,s = 25~eV: 4,0izalmits/tO0eV ~m) Ionic processes ~- HsS÷ +eHaS -* S H + + H + e HsS ~ S++2H+e HsS -+ H++SH+e -

Yield (G units) ~u' m~

~ energy (eV)m, xu)

Probable energy (eV)

1.74 1.32 0"81 0-12

10,43 14.27 17.9 17.5

12 16 20 20

Eaet~ used (eVil00 eV) 20.1 21.1 16.2 2.3

TotaleaerlD, for ionization ln~ea~s/100,eV absorbed: 59.7eV

applied to radiation cbemi~ry. Ions with :kinetic energies in excess o f thermal energies are discriminated against in narrow geometry instruments. In the case of hydrogen sulfide, particularly the yield o f H + is suspectt 'as it will have high translational energy frmn any equi_partition of excess emrgy with other fragments.

Mechanism The relevant reaction rate data are given in Table VI(B) and a summary of the radiation chemistry results in Table VI(C). The radiation chemistry mechanism required to explain the observed yields is not simple. The H i yields observed at 10m eVg -x s -x from pure HaS a~-~very close to the yield observed at low dose rates with added HCI, G(H 0 = 11.0+0-5. The hydrogen yield observed in the presence of SF e at all dose rates is the same, G(H08F. = 7-5 + 0.5. The primary ionic yields and reaction rates given in Tables VI(A) and (B), along with a simple neutralization reaction (58)

[H(HaS),~+I]+ + e -

, H+(n+I)Has

I' In ELaO,H + r ~ 20% of the ions formed, whereas in HiS with a much lower W-value, it represents only 3% (Section 7).

Excitation in the radiation chemistry of inorganic gases

97

TAme VI(B), HYDItO~N ~ J , IDt*: IUBL~ANTIAT~ DATA Reaction

Rate comtant*

Ref.

Ion-molecule reactions I-IsS++ HtS ~ HsS++SH HsS++nHtS -, [H(I-IsS)m+x]+ SH++I-IsS -, H.S++S SH++I-I,S ~ HSm++Hs S + + H # -~ I-I~S++S S++HsS ~ Ht+Ss + SI++2HtS ~ HtSs++HtS H++I-IsS ~ H.S + HSs++ HtS -~ Ss + I-laS+

7"5 x 10-1' 9"1 x 10-1* 9 x 10-l° 2"4 x 10-to 4-2 × 10-to 3"2 × 10-it 1-8 × 10-xe 1"6 x 10-xe 4"2 × 10-1*

(146-149) (146-149) 046-149) (146-149) (146-149) (146-149) (146-149) (146-149) (146-149)

Radical reactions H + I-I# -, H t + S H H + S H -* H t + S S H + S H -* HtS+S S~+Ss+M -* S , + M

8 x 10- a 4 x 10- u 3 x 10- u 2.5 x 10"-m¢ *

(150) (150) (151) (152)

t Rate constant units cms molecule-x s -~. ,* Three body rate constant, units cm" molecule-s s~1. TABLE VI(C). HYDROGEN SULFIDe: RADIATION C3FIEMI~I~YDATA

Hydrogen yield G(H0t Dose rate (eV g-1 s-l)

Additive

HsS

HsS + 1~o SFe

Ref.

7-8+0-2 7.9+0-3 8.0+0-5

(153) (154) (154)

2 × 10m 2 x IIF 2 x 10~ (high absorbed dose) 2 x 10t* I x 10m 2.3 x 10a 2xl(F 2.3 × 10n

None None None

13-1 +0-4 12.0+0-4 8-0+0.5

None None None S 1-35~ HCI (0.-100"/o)

7.4 × 10xt

1,3-Butadiene

11.5+0-5 11.0:1:0-2 7-04-0-2 7.34-0.3 10-6 Extrapolated 0.5 + 0-3 Extrapolated

8"04.0.2 7"1 7.3+0-3 7-0 to pure HaS

(151) (153) 044, 155) (153) (156) (157)

to pure HaS

t Foryt and co-workers have also done considerable work at low dose rates (m) and their results are in ~ compatible with throe of Huyton ami Woodward clu, as-aT). a n d accepting that S F e c a n suppress H a t o m f o r m a t i o n i n allow the following yields to be derived (a) G ( H 0 f r o m all ionic processes:~ (b) G(Hs) f r o m n e u t r a l i z a t i o n (c) G ( H 0 from H a t o m s f o r m e d b y n e u t r a l excitation (d) G(HI) f r o m H i formed b y n e u t r a l excitation T o t a l H s yield: H s yield with SFe: That is, primary ion formation and ion-molecule reactions.

this n e u t r a l i z a t i o n step, 3.6 3.6 3,3 0-5 11"0 7"4 ( = 11"0-3-6)

98

C. WttxmandA. W. BOVD

Within reasonable experimental error, these are in agreement with the observed yields given in Table VI(C). The change in yield in going from 10~ to 10m eVg-t s-1, G(HI) = 11.5-13-0 (1~, is most reasonably explained in terms of a competition between neutralization and clustering (59)

H(HsS)a++ e-

• 2 H + S H + ( a - I ) nzs,

(60)

H(HIS)b + + e-

> H + b H~S,

where b > a. One interesting aspect of the hydrogen sulfide system is the product which suppresses dissociative neutralization causing the yields to be unaffected by addition of SFe at low dose rates or at high dose rates. The most obvious is sulfur which has been shown to scavenge electrons very efficiently(lr~, reaction (61): (61)

e-+S,~

~ S,C

or

Sin+Sin-.

However, at room temperature, sulfur d ~ not have ahigh enough vapour pressure and we have suggested~xum,th~ ~ (HsS~) ~ also ~ in this way and that they are probable products at both high and low dose rates cls4) (62)

H2S,~+e-

• HsSn-

or

HzS + S,~-; n > 2.

Energy balance Although our understanding of the HsS system is good and almost all experimental results c a n b e explained in terms of the mechanism given, irrespective of dose rate, there are no meaningful comparisons to be made. The W-value is consistent with the observed radiation chemistry behaviour but the experimental uncertainties would probably mask aaything but a gross misfit. The energy ~ s i v e n in Table VI(D) is clearly satisfactory taking into account our lack of knowledge of the energetics of the system.

TAeL~ VI(D). HYDROGENSULFIDE:ENERGYBALANCE

Yield? (G units)

Process

Neutral excitation processes H2S -* H + SH 3.3 ~_ HzS ~ H I + S 0.5

Probableenergy (eV)

Energyused (eV/100 eV)

7.0

26"6

Energy balance

Ionization Neutral excitation Sub-excitation Total,

59.7 26.6 8.0 94.3 eV

? Derivedfrom radiation chemistrydata given in Table VI(C).

Excitation in the radiation chem~l~3,~ofinorganic gases

99

7. WATER VAPOUR A review o f water vapour radiation chemistry was given by Dixon in 1970 ~lra~. Since that time relevant rate constant data and high dose rate radiation chemistry data have appeared. However, since Dixon's review was ~ complete our present discussion can be relatively brief.

Primary process yields The ionic primary process yields given in Table VII(A) are based o n cross-section data c~°~. Some semi-empirical calculations have been made for neutral excitation processes clsl~ but the data are not readily useful for deriving neutral diss&ziation cross-sections. TABLEVII(A). WATmtvArotm: P ~ y

IONIC~ROO~_~_~__

WH.O = 30.0 eV: 3.33 ion pairs/100 eV ~lwj Yield (G units) ~se'ue~

Ionic processes HsO -* HsO++e HaO ~ O H ÷ + H + e H20 -* H++OH+e HsO ~ Hs+ + O + eHsO ~ H s + O + ( * S ) + e H20

~

Hs+O+(SD)+e

-

Threshold Probable Energy used energy (eV)~n~ enerlD,,(eV) (eV/100eV)

1.99 0.57 0-67 0.01

12.6 18.1 19.6 20.5

14.6 20-1 21.6 22,5

29.1 11.5 11.5 0.3

0-00 0.06

-22.4

-24"4

1.4

Total energy for ionization/100 eV absorbed: 56"8eV

Mechanism Rate constant data relevant to reactions in water vapor are given in Table VII(B) and a summary of radiation chemistry data is given in Table VII(C). lonic. From the data presented in Tables VH(A) and (B) we can deduce the yields of species from primary ionic processes prior t o neutralization. These are given in Table VII(D). Taking these in conjunction with the radiolytic yields given in Table VII(C) the neutralization ~reaction must be o f the form (63)

HsO+(HsO)n+e - (or Os- )

> H+(n+I)H20(+O2).

Yields o f neutral excitation processes c a n then b e obtained and are shown in Table VII(D).

Neutral species H atoms and O H radicals formed in the radiolysis o f water are easily scavenged as they do not react with water. In pure water they Can initially react only with each other but since they react rapidly with H s or HsO s it is not possible to obtain a meaningful yield o f these products at low dose rates. In Febetron irradiations the yield o f H I is independent o f dose up to 2 × 10m eVg-X showing that the back reactions, H + I-ltOs and O H + ~ are sappressed. This behaviour is consistent with the rate constants listed in Table VII(B). 8

100

C. W n . u s s a d A. W. B o ~

TABLE VII(B). W A T m t VAI,OUR: RI~LEVANT RF.ACTION RATE DATA Reaction

Rate constant*f

l o n - ~ ~ions I-IsO++]-IsO -+ H s O + + O H DsO++DsO ~ DsO++OD O H + + I ' I s O -~ H e O + + O OD++DsO ~ DtO++O H + + 2HsO -~ H , O + + H s O H s O ~ H t O ) , + H t O + M --* HsO+(HsO),L+I + M n=0-3 O + ( * S ) + H s O -~ H t O + + O

1.2 x 10-* 1.5 x I0-*

1-5 x 10 -* 3-0 x 10 -*

Radical reactions H + H + H s O -+ H i + I-IsO H + O H + H s O ~ 2I-~O O H + O H + H s O -~ I-lLtOs+ I-IsO O ( * P ) + O H -~ O , + H H+Os+HtO ~ HOs+HsO H+HOs ~ H~+Os H + H O t -~ O H + O H H + H O s -* ~ + O H O s + HOs -* H s O s + Ot H+O+HsO ~ OH+HaO H + H s O a -~ H a O + O H O H + I-ItOs -~ H I O + H O ~ O H + H a -~ H s O + H O H + O H -* I'ItO+ O O + O + H s O -~ O s + H~O O + H s -~ H + O H

Ref.

(162, 163) (162, 164) (162) (165)

3 x I0-~

(166)

2-3 x 10 -0

(167)

2 x 10-a~++

1x 1x 5x 4x

10-ae$ 10-st+, !0 -n

10-a~

~ 1 x 10 - n

~ 2 x 10 -~s ~ 1 x 10 -~1 3"6 x 10 -11 2"5 x 10-n+* 1 x 10 -la 2 × 10 -~s 9 x 10 -1. 2-3 x 10 -xs 2.7 x 1 0 - ~ , 3 x 10-1.

(168) (168) (168) (169) (170, 171) (172, 173) (172, 173) (172, 173) (174) (175) (I76) (177) (178) (178) (86, 179) (80, 181)

t Rate constant units cm ~ molecule -~ s -1. +* Three body rate constant, units cm* molecule -s s -x. TABLE VII(C). WATER VAPOUR" RADIATION CHEMISTRYDATA Yields G(H0

Dose rate

(eV g-Z s-l) 10m-10 m

Additives None

lOt'-lOtS

None

10~

None

10~

sF,

~ 1 0 ~e

101. 10~ ,~ 101.

C H I O H , COxT1OH cyclo-C, HI,, C~I'18 RH+. SFe CID,, CsHlo HCI, HBr HCI, I-IBr + S F .

Ht(D,O)

< 0~6

Ref. (182)

1-2 (~ particles)

083)

< 0"16 (fission fragments) 1.65

(184) (168)

0-90

068)

7-9

(185)

5-6 0"5 7.9 4.6 G(HD) = 13.1

(185) (185) (168) (168) (186)

The ~ enis.ma in water vapour radiolysis is the difference between the H atom yield derived from ~ t i v e s such as pro~J~, methanol and HCI and the exchange y i ~ : i n E[s.-DiO or ~ - P q , O ~ t u ~ [me Table ~ I ( C ) a n d Ref: (t68)]. The agreement between the Febetron and the low dose rate values is good evidence that no

Excitation in the radiation chemistry of inorganic gases

TAnLe VII(D). WATER VAPOUR: ym_w.nS

o r s p a ' m s mLOM x o m c CHARGE N E ~ T I O N

Species Yield (G)

e3"3

I-IsO+(I-IsO)~ 3"3

OH 2.73

101

Pgn~L~Y P R o c ~

H 0"57

O 0-63

~ t

To

Ha 0.06

WATER VAPOUR" YIELDS OF PRIMAI~Y N E ~ ID~CITATION PltOCF..qSES DERIVED FROM RADIATION CHEMmTRY DATA

Neutral excitation process H.O -* H + O H HsO -* Fit + O

Yield (G units) 3-5 0.45

more than one H atom is formed by neutralization in water vapour whether scavengers (other than SFe, etc.) are present or not. We have suggested on the basis of isotope effects in Hs formation from water vapour as~) that the high exchange yields may be due to isotope exchange in a complex of H atoms and water molecules. The value of G(H0 -- 1.2 from the a particle irradiation of waterCm~ is not consistent with the yields from fissioe.fragment irradiationttm. However, the latter were done at much higher temperatures where the reaction OH + Hi is faster and there was an unavoidable y-ray contribution to the absorbed dose.

Energy balance The energy balance, given in Table VH(E) is very good. Its calculation is of course based on the estimation of H atoms by scavengers ae~ ~m~not on the exchange yields~xss~and is further evidence for the correctness of the former. TABLE VII(E). WATERVAPOU-R:ENEltOYBALANCE Yield (G units)

Process

Threshold energy (eV) m~

Probable energy (eV) cm~

5.1 7-0

8"8t 9.0

Neutral excitation processes HsO -~ H + O H 3"58 HsO -* H a + O 0.45

Energy used (eV/100 eV)

Total energy for neutral excitation: Energy balance IoniTation Neutral excitation Sub-excitation Total:

31.7 4-1 35.8 eV

56"8 35.8 6-6 99.2 eV

t We have taken the probable energies to be the mean of the dissociation continua discussed in Ref. (161). 8. A M MO N I A

Although the radiation chemistry of ammonia bears many resemblances to that of water vapour, it is more complex because a larger variety of possible species are involved. The present discussion can be brief as recent reviews of ammonia radiolysis have been published oss. tso~.

102

C. Wn&m and A, W. BoYv

Primary process yields Yields o f p r i m a r y ionic pro¢esses are given i n T a b l e VIII(A). These are based o n s t a n d a r d mass spectral tables~u~. T ~ e p r o b a b l e energies are estimated. TABLEVIH(A). AMMONIA:PRIMARYIONIC PROCFaSES Yields (G units){N}

Ionic processes NH, -~ NHs++e NHs -* N H z + + H + e NH. NH++ H ' + e NHa = N H ÷ + 2 H + e -

}

Threshold energy (eV) ~tt}

2.00 1.60 0.17

Probable energy (eV)

10.2 16-0 17.1} 21.6

Energy used (eV/100 eV)

12.2 18.0 19

24.4 28-8 3"2

Total energy for ionization processes/100 eV:

56"4eV

Mechanism Reaction rate ~ relevant t o ~ are given in T a b l e VIII(B) a n d a summary of radiation cl~mistry ~ d s g i V e a i n T a b l e VIII(C). T h e ionic ~ m i d ne-atml ~ a t / v ¢ pfw~mses i n N H a ( T a b l e V H I B ) are very similar to those i n watt= v a p o u r . T h e n u l i c a l ~ n s at~ a t least as complex T~L~ VIII(B). A~tMONL~:~LEVANTRATECONSTANTDATA Reaction

Rate constantt

Ion-molecule re.~timm NHs + + NH, -~ NI'I4 + + NHt NHt + + NHt ~ N ~ + + NHt NHt++NHt ~ NI~++NH NH++NI-lt .+ N ~ + + I . l s NHt+ ÷ NHt-I- M -*-NI~+- NHt + M

1-6+0.2 x 10-':~ I x I0-'§ 3 x 10- u 1.8 × 10-t'i[¶

(190, 191) (191) (191) (190) (192)

1 x 10-t°

(193)

8 × 10-x°*

Neutral reactions NHt + NH! -~ NtH~ -~ N H s + N H N t H I + Ht (250-1520 totr N ~ H + H + N H s -~ H I + N H s H + N l I ~ -~ Hi + NtH, NH + N H ( + M) ~ NsHs or N s + H2( + M) H+NH, ~ HI+NHI

1"4 x 10"*x¶ 2"1 x 10-it 1 x 10-' < 2 x 10- ~ at 25°C E ~ 10 kcal mo1-1

H + NHs(+ M) ~ NHa(+ M) H+NtHa -+ N t H s + H t

N I l + N I l , -+ 2NH, N H + N I I ~ -* NsHs+NH, H + NtH~(+ M) -~ N , I ~ + M) (2) N H s + N t I ~ ~ N H t + N t H (?) t Rate constant units ~ ~ - ~ § ~

Ref.

s-I.

energy of cation. 0.3-8 eV.

]l M ffi O,.

¶ Three body rate constant, units cms molecule-2 s-t.

(194) (19S) (190 (197)

Excitation in the radiation chemistry of inorganic gases

T A m ~ VIII(C). AMMONIA: ~ J Z ~ O N D o s e rate

(eV g-i s-0 ~10 ~

(or particles) 2.3 × 10xs 2.3 x 10 u 2.3 x 10TM 2"3×10 xs 4.7 x 10xe

10~

Temk~xqture (°C)

Additives

' 25

None

25 300 25 25 25t 300t 20

None

20 150 22 20 300 20

2 x 10 ~

25

2 × 10m

400 25 400

Y

G(Ha) 6

4.8

None

15

C~I-I, N,I~

0-75 ~12

DATA

Yields G(N0

G(NzI-lt)

2

Call , CaI-Is + SFe ~

8 3.9 6 15 5.8~

C~He

1.6 5

(199)

-~8

4"4 15

c y c l o . ~ H a , C~H, C H , O H , iso-C, HTOH J is0-CsHvOH+ SF, --

(200) (2o~)

2 5

(202)

3.8~

--

Ref. (198)

0.6 3

can,, sF. 3 x 10~

~

103

1"45 5

(203)

10"8-11.4 7"9 3"6

1~

6"0 3.6 7"5

1"3 1-0 1"3

0"58 1 "9 0"58 3.0

(2O4) (205)

t Flow system. Extrapolated to zero concentration of CsH,.

but the back reactions are less efficient in NHa and the decomposition yield increases markedly with temperature. These differences probably reflect the relative thermal stabilities of H~O and NHa. Ionic. Both NH8 + and NI'!,+ form (blHs+.nNHa) before n ¢ ~ f i o n both at low dose rates and Febetron dose rate (se~'. T h e radiation chemistry yield measurements do not distinguish between NHt and NH production fin ion-molecule reactions and do not give the relative contributions of r e a s o n s (64) and (65): (64)

NHI+ + NH z

• NH~+ + NH,

(65)

NHm++ NHs

> NHs++NHs.

The neutralization reaction must give essentially only NH s and H as the reduction in the low dose rate radiolytic yield of the latter on adding SFe is very nearly equal to the ion pair yield~m~. The yield o f H s which is unafk'eted by H atom scavengers is essentially the same in the presence and absence of an electron scavengerc ~ . This suggests that no H s is formed by charge neutralization reactions. The amount of Hz that can be formed by primary ionic processes and ion-molecule reactions is uncertain but could be as much as Gm --0.34. Neutral species Rate constant values are not available for a large number of the reactions shown in Table VIII(B). Also, akh0ugh the second-order disappearance rate constants of

104

C. WtLL~ and A. W. B o ~

N H a and N H appear to b e well ¢ s t ~ ~ products of these reactions are not certain. For example, NaHI, diimide, may be an important intermediate in the radiation chemistry o f N H 8. This compound has recently bccn Shown to be relatively stable in both gas and liquid phases ~s°v~. Because o f these uncertainties it is not possible, given the primary yields of H, N H a and N H , to predict the radiolytic yields of H s, N z or NzH 4. Low dose rates. The differences in the low dose rateyields in pure ammonia at room temperature can be attributed to the differences in dose rates. At temperatures above ~ 150°C the yields are in good agreement and it appears that all the H atoms are reacting as in (66): (66)

H + NH 8

> H 2 + N H 2.

The low yields in NHa-CsH s mixtures ~2°x,~°s~ have been sug~sted to be due to scavenging o f H atoms by propene. High dose rates. At Febetton dose rates even at 400°C the H atoms are reacting by both (66) and (67) (67)

H+NHz+M

> NHs+M.

The yields of NsH 4 from Febetron irradiations are independent o f dose and are believed to arise from competition between reactions (68), (69) and (70).t (68)

NHz + N H 2

> N2H 4 > N H a+ N H > N 2 H z + H~

(69)

H + NH~(+ NH~)

(70)

H+H+NHa

> 2NH a • H s + N H a.

Primary neutral yields The most reliable values of the p r i ~ yields in NHs appear to be those of Eyre and S m i ~ L ~ s e were obtained ~ a variety of H atom scavengers at low doses t o avoid s e c o ~ ~ n s . The yields, ~ v e n in Table VIII{D), in conjunction with the data in Table' Nfflt(A) a n d 0$), give the yield o f neutralization and the primary neutral excitation yields as (71a)

NH~+nNHs+e -

• (n+I)NHs+H,

G = 3-4,

(71b)

NHa +

• NHa÷H,

G = 5-4,

(71c)

NHs+

> NH+H~,

G - 0.7.

T~LE VIH(D). AMMON~:~ Y

~s

DF~.IV-~FROMLOWDO~ gATERADiATiON

CHEMmTgY DATAtuB}

Yield (G)

Hs 0,70+0-20

e3.4+0-6

H NI-~+ NH 7.0:1:0.6 11,9+0.1

t We have suilff~ted tin4, m~ that the obur~od yield of G(NsH~) = 0.6 is larsely the result of reactions (68) and (70) ~ . Taki~ ~ , G R ~ 1 1 , then kxo~10ku. The formation of ground stato NI-II by ~ o n of H plus g r ~ state NI-I~is a symmetry forbidden proc~s and than ~ ~ Ntls fccmation,

involvtfqgNlq~ ~

'

~

ti ~

of ~ o u s

the net result of which is

Excitation in the radiation chemittty of iaorganic gases

105

Energy balance

The energy balance, shown in Table VIII(E), serves only to demonstrate that the proposed mechanism is not unreasonable and that the primary yields o f Eyre and Smithies cm~ cannot be grossly in error. TABLE VIH(E). AMMONIA: ZNmOY SALANCZ

Threshold Process

Yields (G units)

energy (eV)

Probable energy (eV)

5.4 0"7

5-7 6"6

6"0 7-0

Neutral excitation processes N H s ~ NI-I*(A s / / ) + H N H a -~ N H + H s

Energy used (eV/100 eV) 32.4 49

Total energy for neutral excitation: Energy balance Ionization Neutral excitation Sub-excitation

37.3 eV

56.4 37.3 7-6 Total:

101.3 eV

Ammonia is a reasonably important chemical system and it is very surprising that so many reactions relevant to it remain unstudied or unresolved. Clearly this is where future effort should be directed. 9. I O N - N E U T R A L I Z A T I O N R E A C T I O N S

As we mentioned in the Introduction radiation chemistry data for gases, particularly at high dose rates, have made a significant contribution to our knowledge of the products of ion-neutralization reactions. The available information on both products and rates of neutralization reactions for the systems we have discussed is collected in Table IX. TABLE IX. ION-N~UTRALIZATIONRID.CTIONS: ]PRODUCTSAND RATES Refet~q~es System

Ot

Reaction

os++e - --, o O s ) + o e e ) -~ ooze) + o f ' e ) o,++ o , - -~ o o s ) + o o P ) -~ OeD)+Oee)

Rater

z0"A'L s~e/.j 2s'A[ 75~j

O. + + cyclo-C.Fs- -+ 2 0 , ( + cyclo-CtFe) Ot++CO. - ~ 20,+CO+O

2 x 10 -7 2 + 0"2 x 10-.~ ,,, 1 x 10-7§

(45)

(208)

(5, 42)

(209)

(5)

N s + + e - ~ 2N

2"5 x 10 -7

Ni++e - ~ 2N+Nt

2 x 10"4 1 2.4 × 10-.[I [.

2-6 x 10 4 [~

See N t di~muion (Section 2)

NO++e - -*- NOD)+O(aP) -, N('S)+OPD)

75~0~. 25%J

4"1 x 10 -7

(208) [ " (208)

(68)

3 ± 0-6 x 10"4 3 NO

(120)

(143) (143)

O, + + COc(¢lustered) -~ 3Or + CO. Ns

Rate

Products

~

(209) (210) (70)

(42)

(208)

106

C. W - j a s and A . W , Bo,m

TABLE IX (cont.) References System

Reaction

NiO

NiO++ e - -~ NI(X*2~)÷O(~D) 57~'}

--, N,(A,2:)+OpP)

Is%L

-* Ni(-'H)+ ocn,) -+ NpD)+ NO

~0y/.o

CO

CsOI + + e - -, 2CO

CO~

C%++e --,C0(X~27)+0(?) 44%') --,.

~,/'~÷o

55%L

0.8%f 0-04% 3

H ( H I S ) a + + e - -+ 2 H + S H

+ ( a , 1)H,S H(HsS)~++e - ~ H+bI-I~S (b > a) H20

Products

1"8 x 10 -6

(42)

~ 2 x 10 -6

-, CO(d a/-/) + O CO(a' sH) + 0 C204 + -~ C O + O + C O I HiS

Rate'["

HsO++e - ~ HI+O+H

HaO+(clustered)+ e - -+ H I O + H

3.8 x 10-'¶

~ t s cm* ~ - 1

600-i~

t o n , 298

K.

(211)

See CO Assumed discussion (Section 4)

(212)

(208)

4 × 10 -e (100 torr)

(143)

(210)

5 x 10-6

(153)

Assumed

1"3 x 10 - e

(213) (168)

(208) (208)

(2O3)

(214) (210)

"~

~ 2 ' 7 x 10-4(?)tt (1200 torr) 1.1 × 10 -4 (100 tort)

t ~

Rate

s-1.

§ This rate with perlku3rucyclobutane ion is 1/20th of the rate with Os-. A similar reduction in rate has been observed for SF6- neutralizing tert-C4D t +(m), tert-C, D t ÷ + e > products; rate constant -- 1.92+0.2 × 10 -e, trI-C~D,++SFe ) products; rate constant <4.0+_0.4×10 -7, and also for benzene++ SFs-csxe), benzene++SFe > products; rate constant -- 1.2-1.8 × 10 -7. I[ Nz contalnin8 10 ppm Oz. ¶ The authors only report yields of CO(a *]-/), CO(d sA) and CO(a' a~/). Weassume remainder is grotmd state CO. ~ftThere is some evidence for a very much faster recombination rate m'). V e r y little c o m m e n t a r y is r ~ l u i r e d for this table. T h e p r o d u c t s o f the ionneutralization ~ n s p r e s e n t e d in the table are all derived f r o m stable p r o d u c t yields o f radiotysis ¢ ~ c e p t for O s a n d C O s where luminescence yield d a t a have also c o n t r i b u t e d . T h e rate c o n s t a n t d a t a a r e f r o m a variety o f kinetic m e a s u r e m e n t s o f i o n o r elcctxon decay. Stable p r o d u c t measu,-~ment h a s been successfully used in organic systems t o d e t e r m i n e a b s o l u t e rate c o n s t a n t s b y a c o m p e t i t i v e m e t h o d (zls) a n d this technique h o l d s p r o m i s e for s o m e inorgatflc systems.

Excitation in the radiation chemistry of iaorganic gases

107

CONCLUSIONS In the majority o f the+ systems considered, good agreement can be obtained between the radiation ehemietry data and t h e primary i o n yields derived from mass spectra and cross-section measurements. This is due to both precise values for W and the extensive rate data for ion reactions. The cross-secti0n data for neutral processes are not extensive and we axe forced to obtaia most o f our knowledge o f these by combining radiation chemistry data with the cross-section data for ionic processes. Typically, ionic processes account for 60 per cent o f the energy, with 10 per cent for snbexcitation processes leaving 30 per cent, or 3 0 e V / t 0 0 e V excitation processes. The technique we have used in t l ~ review, that is calculation o f yields based on 100 eV cross-sections, has been shown to produce fairly consistent results. Ideally CSDA calculations should produce very much better results but because o f the problems o f estimating degradation spectra pertinent to real experimental assembfies, CSDA may well have only limited application to radiation chemistry. The somewhat simplistic "optical electron" nature o f our calculations, while giving reasonable energy balances, will o f course be gromlyin error for prediction o f yields o f forbidden states. These will be mainly produmd by "non-optical electrons", i.e. those with energies below the applicability o f the Born approximation. Although energy usage in their production is certainly small their yields are probably significantly higher than those we predict. We have not considered negative ion formation in the primary ionization processes. Cross-sections for sUch processes are small [typically 0-1 per cent o f the total ionization cross-section (ms)] and consequently contribute very little to the overall excitation mechanism. Another aspect o f excitation mechanisms we have omitted is laser or superradiant emission. This has been recently reviewed by Rhodes mm and a farther c o n t r i ~ t i o n by us would be inappropriate. REFERENC~ l. D. E. WIT.SONand D. A~Ammcmc~qo,Radiat. Res. Rev. 1970, 2; 297. 2. D. A. Am~STRONOand C. WnJA~, Int../. Rad/at. Phys. Chem. 1976, t 221. 3. (a) R. L. PLATZMX~,hi ~ n ' R e a e a r e k ; e t t i t e d by G. Snm~, Nortil-Holland, Amsterdam, 1967, p. 20; Co) L. G. CmusTolN~ltOU, Atomic and Moleeul~ Radiation Physics, Wiley, New York, 1971. 4. L. It. PwrmtsoN and A. E. S. Gtmm% J. Phys. J~. 1968, l, 1131 and further references cited below. 5. C. WnJJ~, A. W. llOYD, M. J. Yotmo and D. A. Am~ntoNo, Can. J. Chem. 1970, 415,1505. 6. K. TAKAYANAGIand Y. ITIKAWA,Space Set. Rev. t970, 11, 380. 7. C. E. KLOTS, Fundamental Processes in Radiation Chemistry, edited by P. A ~ , Wiley, New York, 1968, p. 1. 8. I. SANTARand J. BEVNA/t,Collect. Czech. Chem. Comm. 1969, 34, 1. 9. S. G. LL~Sand P. AUSLOO$,lon:-MOfecale Reactions and their Role in Radiation Chemistry. ACS-USAEC Monograph in Rad/a6on Chemistry, in press. 10. C. Wn.Lt~ and A. W. B o ~ , Advances in Radtatiolt Rese,ar~. Proc. 4th Int. Conf. Radiat. Res., Evian, 1970, Gordon & Breach, London, 1973. 11. B. H. MM-D,Nand C. E. Y o u ~ , J. chem. Phys. 1966, 44, 2192. 12. C. Wnus, O. A. MILLER,A . E . R+(YrtlWZLLand A. W. ]~o~, Rtw~t. Res. 196~, 35, 428. 13. C. WheLm,O. A. MwL~R, A." E. ROTHWELLand A. W. ]~tD, Ade. Chem. Set. 1968, 81, 539. 14. C. Wn+Lm+A. W. BoYDand O. A. Mnt s-., Rad~.. ReAr.197], 4~, 428. 15. A. W. BoYD, C. WILLIS,R. CYR and D. A. ~ O N O , Can. J. L'Vunn.1969, 47, 4715.

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C. W~LUmand A. W. ~oYv

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and T. W~ Wom~w~,m, ~ .

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~o ~m~O~.G. , ]k.~cx aad T. o . ~ ~. 5s. G. ~ I 4 ~ t n d H . J. ~ ! . ~/~ys. 59. R. ~ N , 60. K. ~ ,

At~ ~ .

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Excitation in the radiation chemistry of inorganic gases

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