Ion-Molecule Reactions in Gas Phase Radiation Chemistry Clive Willis Chemistry Division, National Research Council of Canada. Ottawa, Ontario. Canada In this paper we shall explore some of the aspects of the radiation chemistry of gases and, in particular. focus on the ion-molecule and iharge neutralization reactions which set study of the gas phase apart. Understanding of the radiation chemistry of gases draws heavily on inputs from other disciplines such as mass spectrometry and flash photolysis and, in turn, feeds into the comprehension of such diverse fields as auroral studies, interstellar chemistry, and the physics of eas lasers. 'I'hree examples will br used w illustrate the role of ionmolecule reactions in CRS n h ~ s eradiation chemistrv. (Table 1): oxygen, the radiolysis bf which produces a sin&e,stahle product ozone, 0 3 ; carbon dioxide which yields carbon monoxide and oxygen, CO and 0 2 ; and acetylene which gives a complex array of polyacetylenes and substituted acetvlenes up to at least c9specik k&h 3ystem has its uwn per~tli&ities, but together the three exnmplrs serve to demountrate the complex character of the ionic mechanism in producing the observed products. The ionic mechanism in oxygen is simple, involving about twentv reactions whereas that for carbon dioxidc~requiresexarn~oirtion3fwell w e r 1U(I ionic reactions to completely explain the dependence of p r ~ d u c tyield^ on the variuu, experimental punlmctrrs. Going further. ~fn e take a prncticnl problem, fur example the radiation chemistry of ambient air with tracesolwater, pollutantietc., the ionic reaction mechanism is su coxnplex it has not yet heen fully elucidated and evensynthetir mixturescontainingonly nitrogen and uxygen require quitr pou,~riulcomputer pr.,prums to plot the ieaumce of rhe ionic merhnt~iim.Hwerer. ihriwe delving into sich complex real systems let us go back and review somi of the starting points. Fundamental Aspects
The interaction of high energy radiation with a system causes changes to occur which can be described in terms of stages. Tahle 2 shows the sequence of events. The fundamental energy deposition mechanism, the physical stage, has been discussed in the previous paper and only a brief review is needed here. Interaction of hieh enerev radiation with matter involves transfer of energy From theiadiation field to the molecular system being irradiated. Energy deposition can be described in terms of quantum events almost identical to those resulting from the resonant absorption of electromagnetic radiation with a very broad spectrum extending from the infrared down to the windowless vacuum ultraviolet region. Thus the ~hotowhvsical stage . " " involves electronic excitation to radiative states, neutral dissociative states, neutral states above the ionization threshold which are able to auto-ionize (an ionization process which exactly parallels pre-dissociation when a hound electronic state crosses over to a dissociative state of the molecule), and direct ionization, i.e. excited states where charge is extremely rapid. . separation . These first two stages give rise to eiectronically excited states, neutral photofragments which can be either chemically stable species or highly reactive free radicals or atoms, and both positive and negative charged species. To a fairly good Issued as NRCC 18870. 88
Journal of Chemical Education
Table 1. Examvles for Discussion Substrate
Radiation Chemical Praducts
Table 2. Stages in Radlatlon Chemical Mechanism Physical Stage Photo-Physical Stage
Chemical Relaxation Stage
Energy absorbed from radiation fieldby system Production of
Radiative States Dissociative States ionizing States ion-moiecule reactions Charge Neutralization Free-Radical/Atomreactions
.
anoroximation. the events which occur in the ohvsical stage and the photophysical stage are the same for gasand liqu';d phase systems. However, the chemical relaxation stage, which is the last stage of real importance for radiation chemistry understanding, is quite different for the two phases. This chemical relaxation-stage comprises the mechanism of reactions whereby the reactive intermediates give rise to the chentically stable p r u d u ~ t sof the radiatiun evcnt. .Although l free radicals and atoms. chemical relaxation trf n e ~ t mspecies, is not dissimilar in liauids and eases. relaxation of ionic soecies u is very different. In gases the ionic mechanisms are often c o m ~ l e xand occur on a time scale m i t e different to that of neutral species. The extent of the ionir mechanism is contndled by thecharge tmutrulization proress which terminates the ionic reactions and give rise to uncharged species, either of stahle products or reactive intermtdintt.~,the sieldj of which show very strong dependences on experim;ntal parameters. These parameters include pressure, radiation intensity (both macroscopic [Dose Rate] and microscopic [Linear Energy Transfer, LET, of the radiation]), and the extent of reaction (Ahsorbed Dose): . . there is also astrone dependence on the less controllable parameter of gas p&ty. Since ion-molecule reactions are very fast, extremely low concentrations of species can react prior to the charge neutralization event and can therehv modifv the observed product array. Typically, ion-molecule reactions have rate coefficients in the range 10-S-lO-'O cm3 S-I (1010-1011 1mole s-I). Charge neutralization reactions, although they have even larger rate coefficients, in the range 10-5-10-7 cm3 s-', are second order in the concentration of charged species and their rate depends uDon charee densitv. Since charee densitv is a direct function c,t the radiation inten*ity, this means that the lifetime of ions will depend on radiaticm intensitv. For low inrenit\. radiation. ~ h a r g ~ d e n s i t iare e s low and t h i s neutralization lakes place s for the first lleth life for over long time periods (up to radiation intensities less than 1016eV g-' s-'1. Thus, taking the datagiven in Tahle 3, if we consider the relevant concen-
-
Table 5. IonClustering Reactions
Table 3. Extent of lonic Mechanism Controlled by Charge Neutralization I. AB++CD-A+BCD+ BCD++AC-AC++BC+O
2.
a. As+
+ XY-
b. BCD+
+ XY-
--
Neutralization Neutralization
cm3s-' cm3sc' Charge Densities at 10i%Vg-' s-' gives lo-' s for first lleth-lifefar reaction a. Therefore, reaction 1 can occur for concentrations of CD as low as 0.01 pan per million. k, = 5 X k. = 3 X
ABC ABC
Table 4.
Dinerences Between lonic Behavlor In Llquids and Gases
f
Table 8. Ionization
+
Dominant Small Contribution
ABC+ e AB+ C-
+
Neutralization
Liquids: Solvated ions in Stable Equilibrium Ions Unstable wiU? Respect to Neutralization Positive Ion Mechanism in Liquid Water
Gasas:
-+ -+
H20 H20+
H2Dt Hz0
e
HSOf
+ OH
tration of species which can undergo ion-molecule reactions prior to neutralization, we have the possibility of involving species with concentrations as low as 3 X 10" molecule which is equivalent to 0.01 part per million for atmospheric pressure gases. Before going on to discuss ionic mechanisms in gases, it is worthwhile to pause and examine the reasons for the differences in ionic mechanisms between liquids and gases. In liquids. ionization is a common and often stable situation. Ionic salts can be dissolved in polar liquids to produce stable solvated ions. Thus we are not surorised to find Na+ ions and Clions when salt is dissolved in water. Nor are we surprised to learn that DHis a mensure of the eauilibrium concentratinn of h y d r o x o k n ions, H30+, and that it can vary from pH = 0,corresponding to 1M H30+ ions, through to pH = 14 corresponding to 1M OH- ions. Following from this we should not be surorised that, for example, the positive ion mechanism in the ra&ation chemistry of water can simply be written as shown in Table 4. The formation of HaO+ is the formation of a stable. not "hiehlv reactive" snecies. whose concentration will be cbntrollelhithose equilibrium processes which control OH.This stahilitv of ionic soecies in a laree varietv of liauids arises from solva6on effects:~hese are ahsent in the gas phase, exceot for the so called ion-clusterinareactions which can be termed pseudo-solvation,and ionization is inherently unstable with resoect to charee neutralization. A special class of ion-molerule reactions which should he dealt with before wc: lmk at meciiir systems are these ionclustering reactions already referred & above. As shown in Table 5 these involve association of polarizable molecules with the charged species usually following a three-body collision and these reactions can be considered to be the gas phase equivalent of solvation. They certainly increase the stability of the ionic species and tosome extent the charge center can be considered to he delocalized over the clustered assemhlv. However, unlike solvation shells in liquids, the clustered ion can take on soecific cluster structures which have verv different reactivities and thus must he treated at the micro.&opic level. 1,et us now draw adividing line that hassoiar been a\.oided. that is between negauvc. and ~'wirivespecies. For a variety 01' detailed reasons, tKese can be-considered independently &en though similar types of reactions can be generalized across hoth. As shown in Table 6, ionization leads t o formation of a positive ion and a free electron. Although direct negative ion formation does occur, these processes have very small cross-
sections and can be ignored in a general view of the type taken here. Thus, in all gas phase systems, the primary negative species is the electron. The subsequent reactions of the electron and negative ions usually involve electron attachment and electron transfer and are simple relative to positive ionmolecule reactions but can have a profound influence on the products resulting from charge neutralization. Neutralization of a positively charged molecular species by a free electron almost always results in bond scission. When a negative ion is involved, there may he bond scission of the positive moiety, no dissociation a t all or in certain cases, bond scission in the negative moiety. This spectrum of possible events can be modified by the degree of clustering of hoth the positive and negative species. Oxygen
The gas phase radiation chemistry of oxygen ( I ) presents a very simple example of ionic mechanism. Energy deposition from the radiation field gives 0+ and Oz+ ions, in various electronic states, and electrons as shown in Table 7. A simplified ion mechanism is given in Table 8. Negative ions The first reaction of the negative species is attachment
which is followed by clustering of the negative ion
In absolutely pure oxygen, this is the limit of the negative-ion mechanism, but once very small quantities of ozone, 03,are produced, this product will intervene and give a subsequent step which has significant consequences,
Positive Ions Although the various electronic states of the ions react with different rate coefficients and, in some systems, yield different products. In the case of oxygen we can group them together,
O+ +02-
O+02+
o*++ o23 Od+ Volume 58 Number 2
February 1981
89
Table 7. Oxygen Primary Decomposition Steps Yielding Charged Species
----
o2
Yield
+
0.24 0.66 0.61 0.39 0.15
Opt ( X 2 r g ) e02+ (a%.) eOz+ (A%,) eO,+ (b42J e02+(B-state) e0+('S) 0 e-
+
+ + +
+ +
centrations of ozone or high " radiation intensities) where 0 9 or 0 4 - is involved in charge neutralization, each neutralization gives two molecules of ozone. Since the yield of ion pairs is about the same as the yield of oxygen molecules dissociated via non-ionic channels, the yield of ozone observed will vary by a factor of two depending on those ions involved in charge neutralization. This is illustrated in Table 8.
0.86
-0+(2D)+O+e-
0 3
Total =
3 3 (Gunits)
[W-valuefor Oz = 30.8 eV1
Table 8. Oxygen lonic Mechanism
Carbon Dloxlde
Although the ion-molecule reaction mechanism in oxygen produces different ohservable yields for different reaction conditions, it is relatively simple to explain and understand. For the case of carbon dioxide (2). . . a much more comnlex scenario is required to explain and understand the observed behavior which is summarized in Table 9. Carbon dioxide is remarkable for its demonstrated resistance to radiation induced decomposition. At relatively low intensities, very low concentrationsbf products are detec&hle. If extreme care is taken and initial yields are measured at very low absorbed doses, yields approaching G(C0) = 4.8 can be obtained by extrapolation to zero dose. At high intensities, decomnosition vields are much lareer " and easilv measured with G ~ C O=) f.8. Almost all these effects of dose and radiation intensitv on yield-. can Ilr nttritn~rrd rhr igmic mrvhnnim. i\ishut\ I I i n TAIe lo. a lilrw ~ ~ u m l w 01 r reiictllms rlrv ntwled I I I t x r h i n the details, b; briefly there are two major effects. ~ i r s t , considering the simplified mechanism in Table 11,at very low doses, the neutralization of COet on CzOd+ ions by the negative ions . gives CO. After some decomposition has taken place and oxygen is produced, the positive ion-molecule reactions give Ozt which does not give CO on neutralization. The net effect of this, then is very similar to the case with oxygen. Once a small amount of radiation product has been accumulated in the svstem. the contribution to observable vields from ionic reaction is lost. This is. however. onlv Dart of the storv. The vield of decomposition products ioes not become ;educed to a lower value after a small amount of decom~ositionbut is reduced es~tntiilllyt u zero as aitnviit.d 11). the nppnrenr srnbilir) irriidintiun. The reuion fur this I S more comi)lex Iltn awms ro find its origin in a chain recombination mechanism involving positive ions. The sequence of reactions producing this chain ;ecomhination is shown schematically i n ~ a h l 12-and e cannot he explained simply. Briefly CO is included progressively in the clustered positive ion which can then react with O2giving COz product and reform the initial ions. 1.2
--
+ 02- 20 + 2Oz ol+ + 032 0 2 + o3 Ozone Yield Od+
Non-ionic Yield Ionic Y i e i d ~ ~ Ionic YieldLon Total Yield
= 6.2 (G Units) = 6.6 (G Units) = 0.0 (G Units) = 6.2
- 12.8
Table 9. Carbon Dioxide Observable Yields Low Intensity Radiation eV g-' 5 - ' )
G(C0) r
High Intensity Radiation
q C 0 ) ;r 7.6
("hitia yields" G(C0) S= 4.6)
(lo2' PV (I-'s-'1
Neutralization Reactions The importance of charge density and hence ion lifetime becomes clear when the charge neutralization reactions are considered. Neutralization by an electron or by the 02-104ions involves an electron jump and the positive moiety dissociates e.g.
04++ 0 2 -
-
20 + 202
giving two oxygen atoms per neutralization or ion-pair. When the 03-ion is involved in charge neutralization the positive moiety does not dissociate to give oxygen atoms,
-
e.g. 04+ + 03- 2 0 2 + 0 3 (or 0 2 + 0) Thus, since each oxygen atom formed in the system gives an ozone molecule, Lo21 0+02+03
there is a clear and ohservable difference between the two situations. At low radiation intensities the charge density is low giving long ion lifetimes so that reaction with low concentrations of ozone becomes the dominant fate of negative ions and charge neutralization gives no net ozone product. On the other hand, for conditions (either vanishingly low con90
Journal of Chemical Education
Acetylene As is evident from the two previous cases, the complexity of the ion-molecule reaction mechanism rawidlv escalates as thtcnmpli~x~tyoithe *).stem incrtases. I n s p ~ t e d thii.ia~rly detailed descri~tionshave heen de\.eluped l u r the i l n l ~ l eInorganic gases &om HZ,HC1 etc. through to NH3 (I, $. For these systems, not only is it possible to describe the initial behavior where no decbmposition products are present, but also in most cases it has been possible to describe the behavior through various degrees of dkcomposition. For more complex systems where there is an array of nrodncts. the situation is more difficult and in most cases. it is only possible at present to give a global picture of the ionmolecule reaction mechanism. To illustrate this noint. . . acetylene (4) provides some useful features for discussion, Table 13. The primary ions formed from interaction of C2H2 with the radiation field are CZHZ+,C2Ht, CZ+,C+, and CH+. Each of the primary ions can add a first acetylene molecule to produce first stage ions, then it can add a second C2H2 - molecule to pnnluce.wr~,nd\ r u p ion.. and at, m . I)q~rndin::on t hr dt:nsity of wni~iltim,this ilddltim nf C j H 2 w nhe .ilnppi,d ill BIIV
Table 10.
Rate Constant'
Reaction
+ co + co2=GO2+ + co2 + co* + co2= GO3+ + co* + + C02 = COJf + C02 + CO* + COZ= car+ + COz cost + co + co, = C 2 0 ~ + + C02 08 + C02 + C02 = CO,+ + COP
CO+ CO+ CO+ C02+
02
+ + + +
+
+
+
02+ CO con= C03+ C02+ 0 2 = 02+ CO* CO+ con = CO1+CO
+ COI
o++o,=o,++o CO+ + o2= 02++ CO co4++ CO = COSC+ C02 C20,+
02
= 02+ C02
+ COz
+ co = co,+ + co, GO,+ + 0, + COz = 02++ C02 + CO2 co,+
cost + 0 2 = 02++ C0 + o2 GO2+ + 0 2 = 02 + co + CO C202++ CO = cot + CO + CO
+
+
C202+ C02 c02- C301+ O+ C02 = 02+ CO C+ C02 = CO+ CO
+ +
+ +
+ CO2
c++o2=c0++o
+ CO = C&+ + coz + CO = C&+ t C02 + = 02++ CO + CO* + CO + COZ= C203++ C02 + C02 + CO + CO* = C202++cop + + COf = CO4- + C02
GO.+ C20,+ C2OSf C204+ CP03+ e0 2
0 2
+
+ +
c30,+ e- = co co CO? COt+e-=C+O C02+ e- = CO 0 02+ e- = 0 0 COsf e- = C0 O2 COlf e- = C02 t 0 C04+ e- = C02 02 C04+ e- = 60. 0 0 Colt e- = CO 0 0. C202 e- = CO CO
+ + + + + +
+
+
+ +
+
+ + + +
+
Carbon Dioxlde Reacttons for tonic Mechanism
+
Table 11.
1.48E- 28 1.50E- 28 1.50E - 28 2.20E- 28 3.00E- 28 2.80E- 28 3 0 0 E - 28 1.00E- 10 1.40E- 09 2.0OE- 1 1 2.00E- 10 1.00E- 13 1.50E- 10 1.00E- 09 4.50E- 29 2.OOE- 10 1.00E- 12 2.10E- 12 1.40E- 31 3.00E- 10 1.60E- 09 1.1OE-09 2.20E- 10 2 0 0 E - 10 1.00E- 11 3 0 0 E - 28 3.00E- 28 3 3 0 E - 30 2.00E- 06 6.40E- 07 2.OOE- 07 2.10E- 07 1.00E- 06 1.00E- 06 1 0 0 E - 08 1 OOE - 06 1 0 0 E - 08 5 0 0 E - 07
Rate Constant'
Reaction
+ +
+
e- = CO 0 CO. C203+ + e - = C O + C O + O COz++e-+e-=CO+O+e COt+e-+e-=C+O+eO++e-+e-=O+eO>+e-+e-=O+O+eCt+e-+e-=C=eCOJt+e-+e-=CO+02+eCOat+e-+e-=C02+O+eCOlt+e-+e-=CO.+Oz+eCOf + e - + e - = C 0 + 0 2 + e CO?+e-te-=O+O+eC20zf 8e- = CO CO eC301t 8e- = C 0 C 0 eC&++e-+e-=CO+C02+ee- = CO CO eC2Ozt eC20at ee- = CO 0 eC30,+ COf = CO CO +con CO+ c0,- = co o2 co2 cost C04+ = COI CO2 0 2 02++Co~-=02+Co+o opt CO,- = OZ COZ o* COf + C 0 , - = C 0 + 0 2 + 0 2 c03+co,-=C02+0+02 Gor+ Cod- =con COP O2 C202+ c0,- = CO CO 02 C203+ c0,- = CO COZ o2 co,+ c0,- = co2 o2 o2 GO2+ = co 0 C+Co*=co+Co o+o+co*=o,+CO*
+ + + +
+
+ + + + + +
+ + + + + + +
+
+
+
+
+
+ + +
+
-
06 06 19 20 20 19 20 19 21 21 22 19
+ +
+
+
2.00E2.00E3.00E2.00E1.00E 300E100E3 OOE30OE3.00E3OOE3.00E-
+
+
+ +
+ + + +
+
c+o2=co+o C203++ o2+ C02 = 0*++ CO + COZ CgOzf + o2+ C02 = OZC+ CO + CO
+ + + + +
+ COI + C02 + C02 + CO2 + + CO
C03+ CO C02 = COI+ CO,+ CO t C02 = COI+ C202+ ee- = C eC202++e-=C+O+C0
Table 12.
Carbon Dioxide tonlc Mechanism
Positive ion:
Carbon Dtoxide Ionic Recombination Mechantam
G = 7.8
GO2+ C20,+ Negative ion:
+ C02 + 02
-
C&+ Oit
+X 0 2
lwei e + 02+0,-
I CO I
0,-
[Cod + co, + co4-
ION - MOLECULE CHARGE SHUTTLE IMPORTANT A T H i [COI + CATALYTIC OXIDATION- - - IMPORTANT POSSIBLY A T H i [O,] Net Result: 2CO +0,-2C0,
combination of stages to produce an array of neutral products. At high intensities the system charge neutralization takes place at early stages and a very complex array of products is formed, shown partially in Table 14. At low intensities the system moves to a quasi-equilibrium where the most stable products, benzene and an acetylene polymer, cuprene, are formed to give an apparently simple system. However, in neither case has the ionic mechanism been elucidated fully because of its complexity and lack of rate coefficient data.
Of
Rate
Of
'on-Molecu'e
As mentioned in the introductory section, the detailed description of gas phase radiation chemistry draws heavily on data provided from other disciplines. In terms of ion-molecule reactions, the most important input is the rate coefficients and, in order that this paper presents a complete picture, a brief review of this source of data will he given. Volume
58
Number 2
February 1981
91
Table 13. Acetylene First Stage (+ CzHd
Primary 10"
cit
CIHlt CH . '. C,H.+ C2Haf CIHf C,H+
C+
CJH+
C&+ CZHf
--
A block diagram showing the SIFT technique is shown in Table 15. The ion-injection section consists of a source gas, an ionizing section, and a quadrupole mass filter so that only ions of the desired mass are selected for injection into the flow tuhe. In the flow tuhe the pressure of neutral reactant is controlled and the exiting ionic products analyzed by a second quadrupole mass filter. Knowing the pressure of the neutral reactant and the residence time of ions in the flow tuhe, it is possible to determine rate coefficients. The development of such information has heen crucial to improving the understanding of gas phase radiation chemistry.
Second Stage (+ C2H2)
--
+
CHt
Table 14. Acetylene Observable Yields Low Intensity Products
High Intensity
Cuprene
Products
3 carbons
Benzene Allene Propene
Diacetylene 5 carbons
Vinyl Acetylene Butadiene Ally1 Acetylene
6 carbons
Propenyl Acetylene Trlacetylene Diacetynyl Ethylene Diacetynyl Ethane
4 carbons
Benzene
c~c~onexmene Clcloha~ene
Table 15. Rate Coenicient Measurement
t t FiowiDrift Tube
+EF
Analyzed Ions
Analyzer
+CD
r
,
PICD'
Ion-molecule reactions have been an area of intense theoretical and experimental study for the past two decades. Originally high pressure mass spectrometry was used to measure rates, hut following the pioneering work of Ferguson and Fehsenfeld (5),flowing afterglow become the preferred source of rate data hecaus'of thecareful control possible of the rate parameters. A more recent development has significantly extended the range and precision of such measnrements: the Selected Ion Flow Tube (SIFT) devised by Smith and Adams (6),which allows selection of the reactant ion and extends the temperature and pressure range of experiments.
92
Journal of Chemical Education
Ionic Mechanisms in Other Systems The main reason for setting out to understand the radiation chemistry of gases was undoubtedly related to the development of nuclear power and nuclear weapons, and this still remains the driving force today. However other regions of scientific interest draw on this understanding. The processes which take place in aurora (7) and in the ionosphere ( 8 ) are directly analogous to those discussed here. The upper atmosphere is illuminated by a very broad spectrum of electromagnetic radiation from the sun and is subjected to bombardment by high energy charged particles from both the sun and other cosmic sources. This radiation causes ionization and a subsequent ion-molecule, charge neutralization sequence of precisely the type we have been discussing. In those relatively dense accumulations of interstellar matter known as dark clouds, a large varietv of molecular ipwiis are formed and man). have been detected hy terrestrial rodit) astronomers ill).The molecules detevted ranee from diatomics, e.g. OH, through to a t least thirteen atom molecules, e.g. HCI1N, a cyano polyacetylene. I t is now uniformly agreed that the large majority of these interstellar molecules are formed by ion-molecule reactions and, for example, in the case of the polyacetylenes, the mechanism is probably exactly the same as that discussed in this paper for the radiation chemistrv of acetvlene. A finai exampie of comparable ionic mechanisms is the detailed physics of pulsed gas lasers such as the recently developed rare gas halide lasers (10).In these devices, a gas mixture containing helium, xenon and hvdroeen chloride is subjected to a high voltage volume (glow) dkcharge. This produces high energy electrons which deposit enerev -" in the gas medium-in the &me basic way a gas takes energy from a radiation field. This creates ions and excited atoms which. iliirr the ionic and neutral relaxation mechanwns have run theirwurse, giveriw~tr,rlectronicallyexcited XeCi which then deactivates by stimulated emission giving laser output a t 308 nm. Changing the nature of the rare gas and the halogen donor, gives different excited species and laser action a t different wavelengths, h u t in each case one of the predominant pumping mechanisms involves ion-molecule reactions and charge neutralization. Literature Clted 111 Willis, C..and Boyd, A. W., Int. J. Rodiat. Phya. C k m , 8.71-111 (19761. 12) Kummler, R., Lelfen.C.lm,~., ~ i c e i d l iR. , Kem,L..and wiuh.~.. J.phys. them, 81.2451-2463 (19771. 131 Armstrong, D. A.,and Willis,C.,~nr.J Rodiot. Phye. Chem., 8,221-235(1976). (41 Wlllis,C.,Rack, R. A.,and Morris,R.H., Con. J. Cham.. 55.3288-3293 (1977). ( 5 ) (a1 Fehsenfeld, F. C.,Ferguson. E. E., and Sehmeltekopt A. L., J. Chem. Phya,, 44, 3022-3024 (1966). (bl Ferzuson, E. E., Fehrenfeld. F. C., and Sehmeltekopf. A. L., Ad". A t . Mai. Phys., 5.1-56 (1969). (6) Smith, n.,and Adam. N. G,"Gas Phase Ion Chemistry"Bower8. M. T., (Editor), Academic Press. New York, 1979, Chspur I. (71 Vallame-Jones, A,, Aumra, D. Reidel Publishing Company (Boston1 1974. Chapter
".
(81 la1 Formaon,E. E.,Fehsenfeld,F.C., and ~lbriton,0. ~ . , ' ' ~ a r ~ h a100 a rchemistry." Bmera, M.T., (Editor), ~ c a d w i e ~ m INW n YOIL) ~w$.ch~~toz. (b)~ ~ ~ a r ~ M.J., and Phillips, L. F."Chemistry orthe Atmaaphere."E d w d Arnold. London. 6. 1~7s.ch~~te~ 191 Proceedings of I.A.V. symposium umber 87: ~ n t e ~ s tMe lO~I ~~ ~I ~AB~, ~ F B. ~ W H., , (EdiLor1.D. ReidslPublishingCompsny,Hollsnd,1980.
a r .
B ~ ~ ~ , C . A . , ~ ~ T ~ ~ ~ ~ ~ ~ A P ~ I ~ ~ ~ P ~ ~ ~ ~ V ~ I . ~ O Springer-Verlag, Berlin ~eidaiberg,1979. chapter 4.
