Measurement of rates of charge exchange and dissociative recombination reactions in ArN2, ArH2 and ArO2 mixtures

Internotio~Journalof~~SpecbometryondfonPhysier. OE%evier8cientificPuMinhingcompany.~

25<197?)211-227 - Printed in The Netherlands

_C2<. Iz!Ul -:J _

-i --..; I OF RATES OF CHARGE EXCHANGE DISSOCIA’3XVE F&ECOMBINATION REACFIONS INAP-N,.Ar--ff,dAl?O2-

-

AND

P-

GAUCHEXEX.

Labomtois?d’A&vthennique (FirstreoeivdSNovember

and BERTRAND

ROWE

du CNRS. 4ter. route des Garde+ F 92190. Meudon

(Fence)

1976;infiiform19~1977)

easuremen~hsvebeenmadeuaingararef~p~jetforde~ Aseriesofm _ _ g ion-molecule reaction rate values. Rate values for charge exchangereacti0ns0fAr+ ~N1.~~dHpandfoardissaciativerecamhrnationofN;.~~dArH'srerrported. ThemearuredvzsJues

Ar*+H=~Ar~+H.1.25XIO-9c~lar-~ are in good agreement with other &xperimental data where available especisnY fcfr m g variations in reaction ratea 18 a filnction of exchzalge reactions. IdicatioLls CoPb tempembneca~be deducedfkomtheresultsprese-nted.

INTRODUCTION

-_

zlik

tunnel, and it is possible to perform spatial scanning of the stream usingthe spectxometer. The experimental system will be described first, as well as the measurement methods employed‘I%% wiIl be followed by the results obtained for an argon plasma into wbicb oxygen, Ilitrogen or hydrogen iI3injectedIn this study, the rates of the foIlowing reactions are determined chargeexcbange

k++Nz+flz+Ar Ar++o2+o;+Ar Ar++o+o++Ar

Ar++Ht+ArH++H dissociative recombination

Wz + e + N + N O$+e+O+O ArW+e+Ar+H

For our temperature range, generally speaking, the only available results easurements. However, as it was pointed out by arethoseofdrifttubem Lindinger et al. [2]. drift tube results must be employed warily, particularly when they are to be employed in lieu of temperature data. In spite of this fact, wbicb sometimes makes the comparison difficult. our measurements show good agreement wit31 previous measurem ents, especially for charge exchange reactions. The fact that our thermal conditions are suh6tantiaJly different from those of previous studies also constitutes a fount of additional data corxemin g this group of reactions. For two of these reactions the authos are not aware of any previous measurements.

plasma jet flowing tbrou@~ a Ourmeasu-~entsweremadeoaararefkd wind tunnel (F’ig. 1). The plasma generator is similar to that previously described [3]butitwassetonamovingcartesianaariageenablingaispIaoement of the jet with reference to the sampling probe. Thephsmais monitoredbyan~ccsrc~betweenacathodebaraadananode nozzletbro~whichnowsagjvengas(intfiepresentcase,~~),Inthe ~d~theplasmajet~arfiameterO~--0.3mandalength1--2m. Pressureinthejetiswnifixmandisintheraagel~Pa-

f

. L I i

1

Fig. 1. General

view of the instabtioa

jet as the end of tke mass spectrometer probe, and its axis is at a distance of 2 cm fYom the quadrupole axis. Finally, injection into the argon jet of a molecular gas, nitrogen, oxygen

or hydrogen, was provided tkrougk an injector consisting of seven porous alumina bars, 16 mm in diameter, 300 mm in length and spaced at intervals of 46 mm. Tkis type of injector was selected in preference to a point type injector, in order to minimize diffusion effects.

Ekctzvnic

density

and electronic

tempemture

These parameters are obtained by the Langmuir probe using Standard methods [4]. Tkeoretically, measureme nts do not raise any problems; kowever, effects suck as tke influence of surface characteristics may prove to be sources of error in certain cases. Electxonic density especially can be overestimated by tkis method. It seems very unlikely however that it can be overestimated by a factor greater than two.

Den&y

of ionized species ami kkic

tempenztwa?

as a-&r&on of various f&t& has been very and tke results of I+ investigation kave akadY been puMished[5].Agddspsteminparticular~eggPalysisoftheplasmaions’ M3ss spectrometer

~omtighly

investigated

respo&e

__

I

_---

E-ii_ 2. BncIcmam%

energy and dekrmkaion of their transh&on temperature. tie electron multiplier w8s The spectrcmeter enclosure was modified: placedperpendic~tothejetaxlssoasto~thebac~~dnoise due to the photons emitted by the arc. As for the density measurem ents of each ionic species, while the mass previously, separation effects associated with t&e flow had been investi@& I*- -tJTof the quadrupole and the electxon multiplier to the difference in different masses wzs not taken into account. Consequently, the folIowing inve&gationwasmadein order to derive an ion density fLrom the ion current. An encx~ wasco~~withtwotubesat~t~~(Fig_2).One of these tube3 conk&s the quadrnpotiprobe and the other contains an ekcthe ionizf&ion chamber trongunpo&ionedsothattieekckonbeam~ of the quadrupole, As in the anaIysis of plasma iow this standard ionization are at tie same potenti& 3osantheseeI~ chamberiskeptoutofsenke, byOYEA,wa!3deE&nedtoprovidealmogt Theelectrongun,cons&ucki monOenergetic eIe&xons of l-50 eV (150 * 0.5 ev). %y intzoducing known mxxtwes of gases and in view of i&e -eff&&iwe‘ ionization Aolls, it

tbenbecomespoedde_~~~e~oftqeinstrmnentasa . fimction ofmasslii ~tJ1e~~_~e-~ion~tt3 andtotalekctron agraitrr d~ofeachioniespecks

-

_~_F+

y

v_t”

‘=

e_

hfetmurememt of flow

vdixity

- -_ The flow velocity measurem ent method has already been discussed [S]. Briefly, its principle is the following: the plasma is locaUy seeded by a source of ions different in nature km that of the do minant ion in the plasma. This source is cut off very rapidly and the response signal of the mass spectrometer as a function of time variable source-spectrometer distances makes it possible to determine the time of flight of the ions and hence the velocity. In ref- 6. the proof of a constant velocity on the jet axis is g&ens

Genend method

for

analyzingresults

As the plasma jet can be displaced in relation t0 the mesure ment point, parameters in the it is possible to compile a chart of the different m esurable jet and then to derive the reaction rates. The foliowing hypotheses and results are valid for all the cases investigated. Diffusion is ignored, 8s experienCe shows that a zone a few cm In diameter exists at the centre of the jet, in which the radial gradients are very low. Moreover, the analysis of a pure argon jet (in which reactions are nonexistent) shows that in this case the variation in ion density along the axis of the jet is slight. As for the ion density profiles as a function of distance from the injector, scanning of the jet also shows that these profiles are strongIy affected at short distances from the injector by the Wages of the bars (widely differing profiles inside and outside the wake). However, beyond a distance equal to a

few

bar diameters, the profiles become neady identical and it can be stated that the raci&I velocity and concenfration proiYes are once again uniform in the profiles obtained for the theamtraIzoneoftbejet.Figure3iIiu&ates electronconcentrationat~e~~andata~ceof2cmfromtheaxis easurements can only ).ItiseIearthatcormctm (czzofan~ . h bemadeoutsidethewakezone. of our measuretowithintheaecuracy W~respecttot49IlpCS&um& ments, no +.&&ant variation was detected along the jet axis. In addition to th&,sincetherates oftbe~onsinvsstigstedvargonly~tlywithtemassumedtobestrictlycoll!ztant.Henceinaucases perakue,tbelakterwas the problem was de& with as a unidimensionaI problem, with uniform flow and constant tempemtnms. TRemwdynumiiz

conditiims within

the jet

Dissociatme recombination and charge exchange reactions being very fast, cannot be determined under CLTE conitisquiteclearthattheirratevahxs ditions. Now the comparison between relaxation times, deduced from rate and croaxe&on data, and hydrodynamic time (characteris& length/jet velocity) in our experiment3 shows that: can be assumed for heavy species (1) A ESoIfknann velocity distribution with a temperature T. Rotational states are also populated according to a BoItzmann ~ution with t.be same temperature T iu the case of oxygen and nitrogen ve.I~ distribution can be assumed for the electrons (2) A Boltzmann witlz a tzunperature !P,=. (3) Vibrational states of mokcuies are not populated as in L.T.E. conditions. Now molecukrr gases are injected into the plasma jet with a vibrational temperakneequaltoroomtemperatum (300 E). Then only the ground vibrational level is significantly populated and taking into account the absence of pnxesses abie to populate higher 1eveI.s (even ekctron collisions, considering the very low electron energy), the rates of charge errchange htermhed here can be taken as the rate for the ground viirational level. (4) Both states of At’, (Pm and Pan) are bound to be present in our jet. The concentration of the Ar+ (Parr) state results mainly from two processes: excitation by inelastic collisions and deexcitation by supemkstic collisions. Then taking into account the very small energy gap between the Ar* (PI& and Ar* (P& states and the plasma ekctron energyitcanbeseenthattbese twoprocessgmustbevery~.Co~~tlyitisquitedoubtrul &&the Are (PRn) density is very far from its value for L.TJZ.~conditions. Kenee, it can be considered that the Ar+ (Pan) concentration is at least 205& Concerning the other Ar* w-es, t&illg “to account their high energy, their eoncentralion c%i be Cne&gibk Thus the rates.of charge measured here foi a mixtun& of Ar* (P,,) and AS (Pxn). e=h=w=a= A main problem is the v&m ottbe argon B! U%., Pfl)_co=mka-

$337 --

metastables are strongly coupled with the radiative lev&-&), by electron coll.isions; but owing to radiation tzapping this mechanism cannot lower the metashbks’ concentzations, Now a computation of the deexcitation rate by electron collision founded on data from Lloyd et al. [ 7 3 and Van Begemorter [8] can be made. This computation shows that in the free becomes ve~p significant. Now in jettherateistoolow,sothatthisprocess a divergent nozzle, where the electron density is very high and the electron energy is low (thermal electrous) a significant part of the argon metstables can be destroyed_ On the other hand, Ar+ ions in the nozzle can be destroyed only by ternary recombination. for which the rate is very low, so that the Ar+ concentration at the nozzle exit must be very high, which agrees with measuEments. Thus it seems that the argon metashbks theresub ofour concentration can be considered much lower than the Ar* concentration. Hence the state of the species which undergo charge exchange reactions is quite well defined. This is not true for the vibrational staks of the mdecuhr ions created in charge exchange as the rate of dissociative recombination is given for a population of viirational levels which are not fairly well defined. tions. These

RESULTS Argon--nitrogen

mixtunz

The injectCon of molecular nitrogen into the argon plasma is reflected, with ing distance from the injector, by a sharp drop in electron dhtp, the appearan ce of the N; ion, and, to a very slight extent, the N+ ion. The foIlowing reactions are involved Charge exchange path of ion Ar* with N7 Ar++N*+N$+Ar followed

(1)

by dissociative

N$+e+N+N

recombination (2)

and charge exchange f&r+ Nf

+N-+

Ar N2

+N+

(3)

for (1) and (2). In effect, the wz ion, We can ignore the reverse -0118 which is destroyed by reaction (2), cannot give the reverse of reactipn’61). _ ~,theatomic~formedremainsinquantities~aretoo oaxurence of the reverse of reaction (2). Fkrthexmore, the low smaxlforthe proportion of J!4+ion formed shows t&at reaction (3) can be ignored. Under t&se conditions the e of equations m concentration vaI5ations is

(A)

k dl

=-

_.

where v and [N*] do not vaxy as a function of 2 ana assuming that temperatureis constant, kd, mustbeconstant.Thismeansthatthevariationoflog [Ai] with distance from the inject-or must be linear. The ssult is shown in Fig_ 4 and, outside the wake zone, it may be observed that the variation of log [Ar+] 8s a function of x is in kzt linear. T%is ma&s it easy to derive the rate of reaction,kdl = 7 * lo-” cm” s-I, for a edtrnndntiontemperatureoftbeio~T=3700~Itmaybenoted that 0nXy the slope of Iog [Ar+] is needed_ Thus a good absolute measurement of the electron density is not needed, but only relative measurements. The only known values for t&s reaction are those of Fehsenfeld et al. [9] at 300 K (k = 6 - 10’” cm’ s-l), Adams et al. [lo] (kd = 1 to 4.5 - ZO-*2 cm3 SC-‘) at 300 K, and Hyatt and Knewstubb [ll] (kd = 3.6 - lo-l2 cm3 6-l) at 298 K.

-y+&

-,z_

7=Tfiemeasurementsmadeby Gnekoet8l.[12]show&attheeffe&ve with energy, However, the drift tube crosse&onfortXsreactionincreases techniclueemployedbytheseauth~rshindersacomparisonwitbtherates m~underconditionsofMaxwellianequiLib~lamathightemperature. Ineffect,theionenergiesintixeir experimentsconsistofthesumofamean energp,whicfiisverywelldefined,andofarandom~ergy,the~bution ofwhkhisunknown_ Subjeizt to these reserpations,and by perfo~gtbecalculationsas thoughthedatacorrespondedtomonoenergeticio~ls(whichhappenstobe true for sufficiently highenergies),alawgiving kd,as afunctionoftemperatureisobtained -10--(T) kd, =5~10-1=~"(1+4.9 Figure5showstbeoverallresultsrelativetoreaction

(l).Thevaluemeashows good agreement with previous results and confirms a variationlaw of k,, as a function of temperature in the form given above. It should benotedthatAdamsetal_[lO]showedthatreaction(l)should sured

in our experiment

befasterwithAr'(2p,,)thanwithAr'(2p,,).Aswasseenpreviously,Ar' C=,,) -37 beP resentinahigherproportioninourcasethaninpreviaus investigations.Thisisano~erreasonforfin~a~ VahIe of kdl underour experimentalconditions.

225 By considering the equation for vahtions in the density above)itcanbeseenthatthefisttermofthe~~dmemberisequalto ud[Ar*]/dx_Thenwecanwriteeqn.(B)as

of N;

(eqn.

(B)

MN-51 dx

V-=Vd~-kd~[N;l[e]

Moreover, the density of hG measured as a function of distance from the injector rises initial&, goes through a very flat peak. and then falls slowly. Since the de of N; is always low in comparison with that of Ar+ ~ acAr’ dx

dC%l dx

Conseque&ly

aCAr”3

v

dx

wbicb

=

the variation equation

+dJGl

can be

hm

[el

z&mitten

ki= IN’23 iel -ki,

for N’* is reduced to

(using

eqn. (A) above) as

[Ar+I IN21 = 0

which the value of kdl isobtained

for T = 3700 K and T, = 4500 K_ (NIL Owing to the small &action of dissociated nitrogen, the density of the molecular nitrogen is derived from the ratio of nitrogen flow rate to the argon flow rate_ By means of the mass spectrometer and standard mktures, it was moreover confirmed that this calculation method t perfedly valid, showing that the mixture is uniform in the measurement zone.) The rate of reaction (2) was determined by different authors, but a rather Figure 6 shows ou wide disparity prevails between their measurem en& value compared to some of the previously determined values- By adopting the law suggested by Cunningham and Hobson [133. based on a model is much derived from O’Malley 1141, which assumes that recombination faster for the ground vibration level than for other levels, our results are most closely approximated by those of Mehr and Rio&i [lS], extrapolated to ourtemperahres. In all cases. our vafues are much lower than those of previous authors. Two exphnntions can be givers The first one is the accuracy of the electron density value. measured by the Langmuir probe, G
T

1s.w

IE

1161

. Fig.

QI

6. Comparison

IrJ

VabJm.

of the different

realts

for Nf + e +

N + N.

then the reaction rate is underestimated. The second reason is that the vibrational state of the wz ion formed by charge exchange is unknown, and that, at all events, tbe vibration relaxation times are far too long for it to be deexcited in vibration_ Hence it is probable, in view of the energy of reaction, that the ground state of the W2 ions formed is less densely populated than in the case of local thermodynamic equilibrium, especially if Ar’ (Pan) plays a SignScant roIeinchargeexchange_ hypothesis. easurement provides proof of O’bklley’s Intbiscase,ourm However, it should be noted that the latter has been criticized, particularly by Bardslep C171.

The injection of molecular oxygen into the argon jet is reflected by a sharp drop in electron density, with increasin g distance from the injector_ The 0; ion appeaxs as well as the 0’ ion: however, in this case, the density of the 0’ ion created~Far~~thatofthe~ionforthepreviouscase. The main reactions involved are the following Ar++O&o;+Ar

(41

Of+e+O+O

(5)

Art+o+o++&

(6)

o++o*-o~+o

(7)

Letusno~~~e~onrateforAr*+O~-+O+O+Ar(~being metastabk argon? has a quite high value, rather similar to the charge exchange rate tie; now according to us the argon mekstable density can be considered much lower than the Ar* density- This reaction is therefore not taken into a&ount_ Abasicdifferenceinthecaseofnitrogenisthattheo’ionhasa~ ionjzation potentiaI than 0,. It is also ob6enfed that the 0” ion initklly increases and then faEs under the effectof reaction (7)- The reversereactions are againignoredfor this case and, in view of the orders of magnitude of the various densities, we asume that the variation in densities of Ar* and 0; is mai& associated with reactions (4) and (5), whereas the variation for 0’ is associated with reactions (6) and (7)_ Fckze types of argument as applied to the argon-nitrogen

d

k d4=-&-&logCAr'l

kas =

CA0

co21 kd, C023iel

c0+1c021 kd"=~~+lEO,k& Figure 7 shows tfzftt the variation in log [AC] as a function of x: is deady linear beyond a certam distance from the injector- Hence k ~=6.3-10-f1cmss~fforT=~00K Acertainvohuneofdatais avaikbleforthisreactionat3OO~suchasthat of Fehsenfeld et aL [l] (1.1 -lo-'0cm3s-*),Wameck [18](1.l -lO-'"cms s-~~y;Sdand Knewstubb [ll] (9 -10 -I*cm3 8-l). Theratecoefficientwas ecreasefrom 7.05 X lo-" cm3 8-l to !Ll8 X lo-” cm” s-l over tbevesselwal.ltemperakue mngel~KbySmitbetaL[lS]The dependence of reaction rate on tempemture was also investi@ed by

Adamsetal.[ZO]whonotedadecreasewith k,= 7.8-10-'"~scm3s-1 between100and600~Moreover, Kobayashi [Zl] tive~o~o,ofthis~onbytbedrifttube~It~ d~8Sa~~~nofenergy&andthenriaee,~,a~~atabout 0.4eV_~mhis~a~~oftbef~o~famr~be~:k&=l_!a- 10-'2-'~[1+9.3-10-=P]

invest&a&d

the effii

__ -._ __ _

__.z

Pig.

7. Log

Ar+

versus

distauce

from injecbr

for Ar-02

mixture.

TheoveraIlremrl~sppresentedinFig.S.Certaindisparitiesexistamong the various authors. However, ourresultckarEyshows,asmayhavebeen foreseen~mtheresukofKobayashi,thattheiawwith~ =isonlyvalid for moderate temperatures, 1tmayalsoben0tedtJx&in2kccord8ncewitb the law of variation of k4 as a function of temperature e here, our value shows good _ agreementwiththeresuItsinrefi.l,8andQ. As for dissoclatme recombination, we obtained kds =8.2-1~cnP~-~

forT=2700KandZ’,=4OOOK

If a 300 K value of 2-1 x lo-’ cm3 s-l (Smith et al_ 119 1). which seems now established to within 1096, is used, and if following Donahue [22] a T’ dependence is assumed, our value, while being nearly half of the calculated value, is much closer to previous data than the value for N; recombination_ With respect to reaction (6), its rate is derived from that of reaction (7). Vohnninous data are availahle for reaction (7) at 300 K, and drift tube results are also available [23] from which we can deduce a rate value for our thermal conditions k d7=2-10-u

forT=2700K

which is nearly the same as the 300 K value_ Then we obtain k,

= 0.31 k,

k ab = 0.64 X lo-=

(NJ3. The density of atomic oxygen [0] is determined from the drop in plasma ionization subsequent to reaction (5), and by assumipg that t&e qqmtity of [O’] ion formed is always low compared with that of atomic oxpgen o-1 By comparing the results obtained for argon--nitrogen and argon--oxygen ~,itcanalsobeshown~attherateofthereactionAr’+N4~+Ar is neceey far lower than the rati of reaction (6)_ We failed to find eariier -values for these reactions in the literature . A~on-hytim%en

m-

Following the in&&ion of molecular hydrogen into the argon, a sharp dropisob6erved inekctxondenaitywithkmssiagdktancehPmthe injessor, together * the appearance oftheArH+ion.UnIikethetwo previous~,the~ionsoonbecomesmoneabrmdantthantbe~+ian, Dependingondif&moe&omtheinjectar,itrisesiniGlIy~grOesthrougha ~andt,henfallsaIowiy. _ _ _:_-_.~ -

_-_;y-* -_ , .<:-__I:

The mactions involved are the following

Ar++H*+ArH++H

03)

ArH++e+Ar+H

(9)

Here againwe governed by u

dma dx

V In

ignorethe

reverse

this

the variation of the plasma

is

=--kd,[~+l[Hzl

dCArH+l =kd~+lkH21 dx

However,

reactions and

case

again, the

--kd,[Arwl[el variation in log [Ar+] foregoing cases,

contrary to the

is a

in

linear function of x (Fig. view of the large quantity

9).

of

[ArE?j

ions created,

was taken

into

t&e term

account

for variations in kd, .

Hencefor kaa k ds =

1.25 - 10-scm8s-z

forT=5OOOK.

Adamsetal. [20] investi@edthedependfmce tempee~andfotmd k&

=0.6-1pcm3s-'

k da =

ofthis rate as a function of

at3OOK

10-gcm3s-1at600K

g this reaction,such asthatof MuchadditionaIdataisavailableconcernin Byanand Graham [24] (kd = 0.6 - 10m9 cm3 s-l).Fehsenfeld et al. [9] (kd = l-1 - 10" cm3 ~-~),Bowers 1251 (kd = 0.68 -lo-*' cm3s-'),Aquihmti [2?] (kd = etd.[26] (kd = 1.6 - lo-I* cm3s'l)and Stevenson and S&is&r 1.7 - 10-*1cm3s-').At~events,ourvalueindi~~thatthedependen~of isnotparticuladymarIcedathightemperathereactionrateontempemture turs. Finally,forreaction(9)weobtain k d9 =

7 -10-10cm3s-1

for T=T,=

5000K

Thisvalueisperhapsunderestima~.Therelativeratevaluewithrespectto the rate value of W2 and 0; disociativerecombinationisaccurate.This pointsoutth.&disociativerecombinationisabouttentimesdowerforArH+ ents areavailable. thanforO~orEr‘;.Toourknowledgenopreviousmeasurem CONCLUSIONS

Theresultspresentedhereahowgood~mentwithprevious~v~tions,speci&y for charge exchangereactions,aIthough comparisonsare sometimesdifficult~~owingto~elackofdatainthetempera~ rangewithwhichweaxeconcerned. Wehaved&ussedpreviou&ythepo&blerole ofexcitedstatesinthereactionsinvestigated,ItwouldalsobeinterestLngtoknowthesta~ofthereactionp~uct,Gea~,speak-ing,,itybenoticed~~ifthcelectron densityvalueisnotoverestimated,~entfie~rabinationoftbeo~a shouldbe ==mP=iedbythF?rmnl effects(riseintemperature) incasesin wfiichthe~ons~notinvah7eelectronicanyor~~o~yeJrcited

<;--I-m ? -“>

cemablethattheth~effectsaresuffjiciently~~tobeundetectable wi&inthelim& ofaccuracy ofourmeasurements.

ItisMfalrauyofplimaryinterestto examinetheradiationemittedbyour mlxtuEssoastodrawanylessonsconcerning thestate ofthereaction productandtheroleplayedbyvibrationalstatesindisociativerecombination.Ontheotherhand,ourresultsprovideindicationsconcemingvariations inratesasafun~onoftemperature,andconfirmthefewobservationsavailable on thissubject, FLEFEFLENCES

1 F-C. Fehsenfeld, AL_ Schmeltekopf.P.D.Goldan.H.I_ Schiifand E-E.Ferguson.J. ChernWys_,44(1966)4087. 2 W. Lindinger,M. McFarland, F.C. Fehsenfeld,DL. Albritton.AX._ Sehmeltekopf andE.E. Ferguson, J.Chem_Phy5.,63(1975)2175. G.~u,ThbedeDoc~orat~Sciences,Univ~tedeP~.l967_ Am&zrdam.1968. W.~~Holtgre~en,~~ostics.North-Holland, B.Rowe.Int-J.MasSpectrom. IonPhys..16(1976)209. Ph.GaucherelandB.Rowe,Int.J.MassSpectrom.IonPhys..23(l977)227. C.R.Lloyd,E.Weigold,Pd.TeubnerandS_T_Hood,J.Phys_B,5(1972)1712. H. VanRegemorter.Astrophya.J.,136(1962)906. F.C. Fehsenfeld,E-E. Ferguson and kL.Schmeltekopf,J.Chem.Ph~_.45 (1966) 404. 10 N.C.Adams._~G.DeanandD.Smi~,IntJ.~Spec~m.10~~~10(1972)63. 11 D.Hyattand P-F. Knewstubb.J.Chem.Soc Faraday'Ikans..EI,68(1972)202. 12 Y. Kaneko,N.KobayashiandLKanimata,J.Phys.SocJpn..27(l969j992. 13 A_J.C - _hamandR_B!l.Hohson,J.Phys_B.5(1972)2329. 14 TE.O%.¶aUey.Phys.Rev_.185(1969)101. 15 F.J_MehrandM.k Biondi.Phys.Rev..181(1969)264. 16 M.G.Dunn audJ.k Lordi.J.,8(1970)339_ 17 J-N- Bardsley and bLA_ Biondi.in D.R.Batea and LEstermann (Eels.). Advancesin AtomicandMo1ecuIarPhysics.Vol.6.AcademicPress.1970.pp.1.57. 18 P_Wame&J.Chem_Phy~.,46(1967)513. 19 D.Smith,C.V_ Goodan.N.G.AdamsandkG.Dean,J.Phys. B,3(1970) 34. 20 N.G. Adams. D.K. Bohme. D-B. Dunkin and F.C. Fehsenfeld,J. Chem. Phys.. 52 (1970)1951_ 21 N. Kobayrahi. J.Phys.Soc.Jpn_.36(1974)259. 22 TX Donahue. Planet.Space Sk. 14(1966)33. 23 BE_ McFmland. D.L Albritton,F.C. Febenfeld,B.& Fergson andAL.ScbmeItekopf,J.Chem.Phys.,59<1973)6620. 24 ILRRyanaudLG.Graham,J.Chem.Phpr,59(1973)426025 ~T_BoacasandD.D.Elleman,J.<=h~Pbpr.51~1969)4606. 26 V. Aquila&i, A_ GaIli,A GilardiniGuidoni and G_G.Volpi. J. Chem.Phy=.. 43 (1965)196X