Primary N2(B) vibrational distributions from excitation-transfer reactions between Kr(3p2) or Xe(3p2) atoms and N2

Primary N2(B) vibrational distributions from excitation-transfer reactions between Kr(3p2) or Xe(3p2) atoms and N2

Volume 82, number PRIMARY 1 CHEMICAL N;?(B) VIBRATIONAL BETWEEN Kr(3P2) OR Xe(3P2) PHYSICS DISTRIBUTIONS LETTERS 15 August FROM EXCITATION...

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Volume

82, number

PRIMARY

1

CHEMICAL

N;?(B) VIBRATIONAL

BETWEEN Kr(3P2)

OR Xe(3P2)

PHYSICS

DISTRIBUTIONS

LETTERS

15 August

FROM EXCITATION-TRANSFER

1981

REACTIONS

ATOlMS AND N2

N .SADEGHI Laboratoire de Spectromttrie

Physique, Umversitb Scientifique et M~dicale de Grenoble, 38041

Grenoble Cedex, France

and D.W. SETSER Chemistry Department. Received

Kansas State University. Manhattan, Kansas 66506,

13 May 1981

Low-pressure flowing afterglow tional distributions and the Nz@) The N*(C) formation rate constant teractlon mechanism favormg only

experiments with Ne carrier gas have been performed to obtam the initial Nz(B) vibraformation rate constants for Xe(3P2) and Kr(3P,) excitation-transfer reactions to Nz. for Ar(3Pz) + N2 also was measured. The results are consistent with I very specific ma few of the many available esit channels.

1 _ Introduction

The excitation-transfer reactions between metastable rare-gas atoms and N2 provide an excellent opportunity for study of product-state distributions by analysis of the resulting N9 emission spectra [l-l l] . The excitaticn transfer frcm Ar(3P0,2) yielding N,(C) has been thoroughly studied [5-l 11. Reactions of Kr(3P2) and Xe(3P2) have not been as extensively analyzed, although previous work established that N,(B, u) was the only N, product that emits in the 195-1000 nm range 1141: Xe(3P,)

+ N, + Nz(B, u’=O-5)

AH: = -7462 kQ(300 Kr(3P2) AH: Q(300

to 477 cm-1

K) = 1.9 X 10-l’

+ Xe ,

cm3 molecule-l

+ N2 + N2(B, u’ = O-12) = -20667

to -2126

K) = 0.39 X lo-l1

(1)

,

cm-l

s-l,

-t Kr , ,

(2)

cm3 molecule-l

s-1 _

In our earlier work [l] the rapid relaxation of N2(B, u), even at 1 Torr argon [ 123, was not fully realized and the previous reported N2(B) vibrational 44

USA

distributions are not the Initial distributions. In the present work we have utilized the flowing afterglow technique with low pressures of Ne carrier gas to obtain the initial vibrational distributions for reactions (1) and (2) at 300 K. Our results are in general agreement with a molecular-beam study [4] of reaction (I), which confirms that vibrational relaxation has been eliminated and that the distributions reported here are initial populations for processes (1) and (2). Translational energy is utilized for N2(B, u’ = 5) formation from the Xe(3P2) reaction [I 2,4] and the present work documents more fully the high propensity for u = 5 formation by (1). Actually u = 5 also is the most highly populated level from (Z), but the distribution is not so sharply peaked in this case. The initial N2(B) vibrational distributions provide a basis for discussion of the reaction dynamics of (1) and (2)_ In addition to the initial N,(B) vibrational distributions, the possibility of other product channels is an important question_ Although only N2(B) emission has been observed, except for N2(A 3 Zz-X 1 xl) which is a consequence of radiative cascade, several Nq states could be formed that would be difficult to observe by emission spectroscopy_ Therefore, the Nz(B) formation rate constant was measured by a

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Volume 82, number 1

CHEMICAL PHYSICS LETTERS

comparative technique [13] in which the N2(B-A) intensity was compared to the XeCl* or KrCl* emission intensity from Xe(3P2) or Kr(3P2) + Cl,, which have known rate constants and branching fractions [ 141. These measurements were also made under experimental conditions of little or no relaxation of N2(B) vibration. We also took this opportunity to reinvestrgate [S] the vexed question of the exit channels from the Ar(3P2) + N2 reaction using the comparative technique with Ar(3P2) f Kr serving as the reference reaction_ This information is also included m the formulatron of a quahtatrve model for excitation transfer of Ar, Kr and Xe metastable atoms wrth N, 2. Experimental The experimental measurements were done at Kansas State University using standard flowing afterglow techniques [ 133 _The Kr(3P2) or Xe(3P,) atoms were generated by adding small flows of Kr or Xe to either Ar or Ne carrier gas prior to passage of the gas flow through a hollow cathode discharge operated at 200 V and a few milliamperes. Commercral Ar and Ne were purified by flowing the gas at low pressure through two liqmd-nitrogen cooled molecular sieve traps. In order to achieve low pressure and mamtain an adequate concentration of me&table atoms, a high-pumping-capacity apparatus was employed for most experiments. This system had a 5 cm diameter flow tube and was pumped with a roots blo&er and 1500 Q min-l mechanical pump, the flow velocity was ~60 m s-l with >O.lS Torr Ne. This work appears to be the first to use Ne as a carrier gas for Xe(3P2), Kr(3P2) or Ar(3PZ) atoms. The merastable atom concentrations were not measured but appeared to be *i the concentrations normally obtained using Ar carrier [ 13]_ The high concentration is somewhat surprising since the Ne(3P2) atoms Penning ionize Ar, Kr and Xe; thus, all the heavy metastable atoms must be made in the discharge. Experiments were made without adding Xe, Kr or Ar to the neon flow to verify that there were no direct excitation processes leading to N2(B) from metastable neon atoms. For some applications the presence of Ar+,Kr+ orXe+ from theNe(3Pz)Penningionization reactions could be a disadvantage, but they caused no difficulty in the present work. The only apparent ad-

15 August 1981

vantage of Ne over Ar as a carrier gas is to provide a medium in which the rate of vibrational relaxation is reduced; this was of critical importance to this study. A few attempts were made to use He as a carrier gas. However the relaxation of N,(B, u) seemed to be more severe in He than Ne. An additional problem was the interference of the g(A-X) bands generated by He(2 3S) + N2 Penning ionization_ Gas handling was done in the usual way. Preprepared C12/N2 or Kr/N, mrxtures were used to record emission intensities for the rate constant determinations. Two types of mrxing nozzles were used, a concentric ring with numerous small holes and a flat rectangular inlet wrth holes on only the downstream srde. With the latter, the emission very close to the inlet (=4 mm) could be observed wrth a focusing lens. Emission spectra were recorded with either of two calibrated monochromators. The 1 m McPherson instrument was used with a 500 nm blazed grating and a cooled RCA C-3 1034 photomultiplier tube to record the Av = 2,3 and 4 Nz(B-A) sequences. The 0.3 m instrument with a 1000 nm blazed grating and cooled Hamamatsu R-632 photomultiplier tube was used to record the Au = 2,1 and 0 sequences.Bothinstruments were cahbrated with a quartz+ NBS standard lamp. The lm instrument also was used to compare the XeCl*, KrCl* and Kr* emission intensities to the N,(B) and N2(C) intensities for rate constant measurements. The response curves were reproducible to within +lO% over a -+lOO nm range. However, for more extended ranges, e.g. compansons at 300 versus 600 or 600 versus 1000 nm, the uncertainty may be ~30% despite great care m calibration. This uncertamty is thought to be associated with the dependence of the response curve upon the illumination of the grating and photocathode surfaces. The N2(B-A) bands were plammetered and converted to relative populations using the Einstein coefficients given by Lofthus and Krupenie [ 151. Good . agreement was obtained for relative populations based on different sequences.

3. Results 3. I, PrirnatyN2(3) vibrationaldistributions The Nz(B) vibrational distributions from (i> is sharply peaked at v = 5 and presents a good opportu45

Volume 82, number 1

CHEMICAL PHYSICS LETTERS

mty to establish conditions that nunimize or eliminate vibrational relaxation_ Survey experiments demonstrated that N2(B) was the only observable product state and that the previous reported vibrational distribution [l] could be reproduced at 0.5 Torr Ar. However, the dependence of the distributions upon further reduction in Ar pressure and upon the position of observation in the emission flame could be demonstrated in the large flow apparatus_ Also, exchanging Ne for Ar has a dramatic effect upon the vibrational populations and upon the appearance of the spectrum, see fig. I_ The Av = 2 sequence was chosen for systematic study of the dependence of the v = 2-S populations upon experimental variables, some of these results are summarized in table 1. Even at 0 .15 Torr Ar there is some relaxation as judged by comparison to the low-pressure Ne data. The variation of the vibrational distribtion with distance from the mixing nozzle was surpnsmg because the N,(B) radrative hfetrme IS -6 ps [ 151 and the molecules should decay before moving an appreciable distance for a flow speed of 60 m s-I _ The only plausible explanation is that N2(B) is collisionally transferred to a long-lived Nz state that does flow along the tube and this state is subsequently returned to N2(B) in 2 lower v level. Based upon timeresolved studies [ 121 of N2(B) in given vibrational levels, the intermediate state has been suggested to be N2(kV 3Ag)_ Depending upon the pressure and gas, the decay of N2(W) can be collisional and/or radrative [ 161. Even when using reaction (1) for a N2(A) source, some care [17] must be exercised to allow enough time for complete decay of the intermediate Nz state. Comparison of the data for experiments A and C shows that Xe efficiently induces relaxation between v = 4 and 5 _ Simrlar results (not shown) were found for the variation of the v = 4 and 5 populations with N2 flow. These qualitative observations are consistent with a N2(B)-N2(W) coupling rate constant of =-5 X lo-lo cm3 molecule-1 s-1 for Xe and N2_ The apparent farlure of the lower levels to increase in experiment C and the observation of a high population in v = 5 by Nguyen et al. [2], even at 0.05 Torr Xe, may be a consequence of a large quenching rate constant for some levels of N2(B) or N,(W) by xenon. By noting that further reduction of the Xe (or Kr) and N2 pressures by a factor of 2 caused no change in relative N2(B, v) populations, safe limits for N2 and Xe (or Kr) pressures of 6 X lOa and 7 X 10J Torr, respectively, were assigned for the present experiments. -The initial N2(B) distributions of (1) and (2) were Al;

15 August 1981

5-3

730

4-2

750

3-l

2-o

770

“in

Fig. l_ Emissionspectra of Ns (first positive), Au = 2 sequence obtained by Xe(3P2) excitation transfer reaction to Nz In the fast flow system (60 m s-i). (A) With 0.2 Torr argon carrier gas, observation at 4 mm downstream of the Nz mixing point; (B) with 0.2 TOITargon carrier gas, observation at 12 mm downstream of the N2 mixing point; and(C) with 0.3 Torr neon carrier gas, observation at 4 mm downstream of the N2 miuingpoint. measured in 0.15 Torr of Ne with N2 and Xe (or Kr) concentrations below the limits mentioned above. The emission was observed 4 mm from the planar mixing nozzle. Based upon comparison of experiments A and B of table 1, we believe that vrbrational relaxation was negligible under these conditions. The determination of the relative vibrational populations for v > 2 was straightforward and the signal/noise ratio was satisfactory 2s illustrated by fig. 1_This, however, was not the case for the v = 0 and 1 spectra. These populations are low and the response of the detection system is poor in the red region. In addition, the 5-6 band is very close to the O-O band, from which the population of v = 0 was determined and a correction was necessary. Thus, the uncertainty for v = 0 and 1 is larger as reflected by the larger error limits shown in table 1. The u = 2 population is larger than for u = 3 for both reactions. This is a definite conclusion and cannot be attributed to uncertainty in the data. The molecular beam study [4] of (1) also found N2 > N3.

Volume 82, number 1

CHEMICAL PHYSICS LETTERS

15 August 1981

Table 1 Variation of Nz(B, u) distrrbutron with experimental conditions -Experiment

Condition&

Population ~---_-_________-___~_

-----

-

v=2

v=3

v=4

u=S

A

0.3 Torr Ne 7 X 10m5 Torr Xe 6X lo4 TorrNZ

4mm 12 mm 36 mm

11 16 18

6 7 9

13 15 15’

70 62 58

B

0.17 Torr Ne 7 X 10d5 Torr Xe 6 X lo4 Torr N2

4mm 12 mm 36 mm

13 15 19

7 7 9

13 14 16

67 64 56

C

0.3 Torr Ne S X lo4 Torr Xe 6 X lo4 Torr Nz

4mm 12mm 36 mm

8 12 14

7 7 9

20 23 24

65 58 53

D

0.15 Torr Ar lo4 Torr Xe 6 X lo4 Torr N2

4mm 12 mm 36 mm

9 12 13

7 10 12

26 27 26

58 51 49

E

OS Torr Ar lo4 Torr Xe 6 x lo4 Torr N2

4nun 12 mm 36 mm

10 13 17

9 14 18

33 34 33

4s 39 32

a) All data are for large flow system; 4,12 200 and SO0 ps.

3.2. Rate constants

and 36 mm distance downstream of the mixmg point correspond approximately to 66,

for Nz(B J fonnation

The N2(B) formation rate constants were measured by comparing the integrated N2(B-A) and XeCl* or I&Cl* emission intensities from pre-prepared N2/C12 mixtures. The rate constant for Xe(3P2) is given by kx” N+)=

UN~(B--A)/IX~C~

~~C~2l~PJ2l)kxec1

kXeCl = 72 X 1O-11 cm3 molecule-r krC1

= 73 X lo-l1

cm3 molecule-1

s-r s-l

,

,

,

(3)

and a similar expression holds for Kr(3P2)* _The intensity measurements were done in 0.3 Torr of Ne and low N2/C12 and Xe (or Kr) flows. Observations were made 4 mm from the planar mixing point. For the Xe(3P2) reaction the Nz(B, 5-3) band was observed. This intensity was scaled to allow for other u’--u” bands, and finally by the ZZ,N,/Ns ratio to obtain &a@). This was compared to the inteorated XeCI* emission S intensity. The final result is k,,(,) = 1.7 X LO-11 cm3 * The total quenching rate constant for KrCPa) is used and the cls emission intensity (10% of KrCl) was included in the ua* emission intensity measurement.

molecule-l s-1 _This is sufficiently close to the total quenching rate constant, 1.9 X lo-l1 cm3 molecule-l s-1, that N,(B) is concluded to be the only state formed directly by Xe(3P2) + N2 _ For reaction (2) the 5-3 and 4-2 bands were measured_ Scaling these to allow for other 5-u” and 4-u” bands and for other vrbrational levels gave k$(n~ = 0.2 X lo-I1 cm3 molecule-l s-l _This can be compared KOtotal quenching rate constant measurements of 0.38 X lo-l1 [S] and 0.33 X 1O-11 cm3 molecule-’ s-l [3] _The difference is sufficiently large to suggest the possibility of other exit channels. However, our experimental error for the Kr(3P2) measurement is larger than for XecP2) because the KrCl* emission lies below 270 nm, where the detection response is falling rapidly. In fact, only the KrCl(C-A) emission was recorded and the total KrCl* intensity was obtained from the known [ 141 B-X/C-A emission intensity ratio. We estimate the reliability of k&rj to be 230%. The present measurement cannot be regarded as conclusive, but they may suggest that channels other than N2(B) may be open. The fonnation of N atoms is exoergic, but there is no definitive evidence that N atom formation comprises a sing&ant fraction of the quenching. 47

Volume 82, number 1

CHEMICAL PHYSICS LETTERS

Further work IS needed for conclusive assignment of exit channels for (2). 3.3. Rate constant for N2(C) _iiorrtlatio~z from Ar( 3P2)+ iiT2 The possrble dtrect formatton of N,(B), as well as from cascade from N,(C)_ has been the subject of consrderable controversy [5 -7 ,l 1] _ One argument m favor df N2(B) formation has been that the N,(B, u) distributron observed m flowmg afterglow experiments differed from that expected (the fVO and N, populations were higher) by radiative cascade from Nl(C). The difficulty of reliably measunng ‘VI and lVo, winch radrate in the 1000 nm region, has complicated attempts to measure I(C-B)/I(B-A) Wrth acceptance of the rapid relaxation of N& _‘) [ 121. this argument is no longer pertinent. Using the carefully cahbrated 1 m monochromator, measurements of the emission Intensity from N?/Kr mixtures were made. The Kr(5p[3/212) emission at ?60.2 nm WAScompared to the N2(C) O-O, O-l and O-2 bands at 337.1.357.6 and 380.5 nm. Usmg an Ar(3P2) f Kr rate constant of 0.60 X 10-l ’ cm3 molecule-l s-1 , the appropriate Kr* radiative branching fractions [ 181, the Einstein coefficients for N3(C) and LV~/?I~/lV? population ratio of 100/33/5 gave rate constants of 3.4,3_4 and 3.9 X lo-L1 cm3 molecule-l s--1 from the three u = 0 bands. The good reproducibility for the three bands is a test of the calibration in the 337-380 nm region_ These values conipare favorably with the total quenching rate constant of 3.6 X 10-11 cm3 molecule-1 S-L _ Attempts were made to measure the IC/IB ratto and the vibrational distribution ofN2(B) from the Ar(3P2) reaction by using He as the carrier gas. Unfortunately a large flow of Ar was required to remove He(2 3S) to prevent interference from Nt(A-X) bands, and significant vibrational relaxation of N2(B, u) resulted. The N?(B, u) distribution from Ar(3P2) + N, would not be conclusive alone, but the results are consistent with formation of N,(B) only by radiative cascade. We conclude that N2(C)is the only exit channel from Ar(sP,) quenching by N, .

4. Discussion Quenchmg of metastable 48

rare-gas atoms by N, is

15 August 1981

very specific with respect to exit channels_ For Xe(3Pz) and Ar(3P,) there is only one channel (within the 220% uncertainty of the present measurements), N2(B) and N2(C), respectively_ The data for Kr(3P2) are not so conclusive but N,(B) is most likely the dominant exit channel. Both N2(C) and N2(B) arise by promoting u-type electrons to the rr* orbitals. The remarkable specificity of the exit channels is further displayed by the sharply peaked vtbrational distribution from reaction (1) and by the rotational, spin-orbit and h doublet populations observed for N*(C) [6] _The conclusion that Ar(3PZ) +N2 gives only N7(C) is important for two general reasons. Interest now can be focused on NZ(C) formatron without concern for N2(B) or other “dark” channels_ Secondly, the Ar(3P2) f N2 reactron provrdes an excellent reference reaction [ 131 for assignment of other rate constants because the N?(C) emission is m a convement wavelength range and because N, is chemically inert, allowmg pre-prepared mixtures to be made. One caution in the use of N, as a standard 1s the presence of N,(AsZc), which is the terminus of the energy flow pathway. Unfortunately the Xe(sP?) + NZ reaction is not so suitable as a reference react& because of the strong coupling of Nz(B) and N2(W) under conditions routinely used for flowmg afterglow measurements_ The N2(C) vrbrational distributions are in moderately good agreement wrth modified Franck-Condon factor populations [l l] (Franck-Condon factors multiplied by the density of exit channel states) for excitation by Ar(3P2) but not for Ar(3P,-J [19] _The agreement between either Franck-Condon or modified Franck-Condon N,(B, u) populations for reactions (1) and (2) is very poor; see tables 2 and 3. Be1 Bruno and Krenos [4] suggest that the Xe(3P2) reaction proceeds by two components - one giving u = 04 and a second giving specifically ZJ= 5. Even rf this division is accepted, the agreement between the experimental and modrfied Franck-Condon population is poor for u = 04. Based upon the Kr(3P2), Xe(3Pz) and Ar(3Po) results we conclude that the apparent match of the N2(C) vibrationai distribution from Ar(3P2) and the modified Franck-Condon model is largely fortuitous_ The role of Franck-Condon factors and the vtbrational distribution are not necessarily governed by the equilibrium N2(X) internuclear distance. All three reactions, but especially Kr(3P2), have some

Table 2 N2(B *I$

15

CHEMICAL PHYSICS LETTERS

Volume 82, number 1

Aupst

1981

u) vibrational distribution after excitation by Xe(jPz)

work (300 K) ref. [4] a) Franck-Condon Franck-Condon x

No

Nl

N2

N3

IV4

NS

23 -

8+-2 17.4 12 5

921 14 23.0 10.4

621 7.7 11.4 5.3

12+1 6.4 17 7 1.3

63~3 63 12.3

this

7.2 7.4 --_-

p (C)b)

a) These results are normalized to our most highly populated level since the lower levels were not observed_ b) Populations are normalized to obtain C&N, = 37 as in our experImental results. Table 3 Nz(B 2+,

v) vibrational distribution after ercltation by Kr(3Pz) No

this work (300 K) ref. [3]a) (300 K) franck-Condon Franck-Condon x

,<5

p (E)

6.1 93

Nl ==s 14.8 20 0

N* _

N3

N4

NS

5 0 19.1 19.6

10 -

13 6 13.6 10.5 7.6

IV6

N7

Nl3

Ns

1vlo

Nil

N12

8 9 5 1 1 0.3

5.2 3.7 0.6 0 1

2.3 2.3 0.3 -

1.1 1.3 0.1 -

___--

-

9.7

19.5 23.0

15.1 13.1

13.4 13.3 6.6 3.9

12 12.5 3.9 1.9

10 11.1 2 2 0.8

a) These results are normalized to our most highly populated level.

reactlon channels of large exoergicity, which implies the presence of an attractive potential in the entrance channel. We suggest that the attractive potential(s) is essentially the first Rydberg state associated with the RgNf molecular ion. The ArNz and KrN$ ions are known to be bound by ~1 eV [20,2 l] and, by analogy to the Rgz ions versus the Rg2 excimers, the RgNi states will be bound by -0.5 eV. Based upon simple molecular orbital theory [20], the most strongly bound Rgl$ geometry will be collinear_ Constructing potentials from collinear Rg+ and N, shows that 2Z+ and z Ii type potentials will be expected, with 2Xt being the more strongly bound. Adding the Rydberg electron gives 1~3X and lr3 II states. For collisions involving Rg(3P2) atoms only certain components of these potentials will be sampled and labeling by the St values gives 0,l and 2 components in order of decreasing expected binding energy. In fact the RgN@2 = 2) potential may be only weakly bound by the net result of the Rg-N$ ion-induced-dipole forces and the repulsion of the Rydberg electron_ For bent Rg*-N2 configurations, the a = 0 and 1 potentials are expected to become less attractive. In summary, the entrance channel has three components with binding energies ranging from 0.5 to ==O.OeV; the

binding is expected to decrease as the geometry becomes non-collinear. Interpretation of elhstic scattering in terms of a single average potential would be very difficult, and we do not attempt to include the work of Winicur and Fraites [22] in this model. The exit channels will be more repulsive than the entrance channels, except possibly for the Rg-Ns(n = 2) potential_ Excitation transfer will occur at the crossing points of the entrance and exit channel potentials with rather low probability. Quantitative discussion of these excitation transfer processes requires knowledge of the potential curves and the collision dynamics_ However, in a qualitative sense this model is consistent with the following experimental data for the excitationtransfer reactions_ (i) The orientation-dependent Rg*-N, entrance channel potentials with small probabilities for crossing to exit channels is consistent with the small cross sections which increase with energy, but with no obvious threshold energy for reaction [lo]. (ii) The crossing of attractive entrance-channel potentials with flat or mildly repulsive exit-channel potentials facilitates formation of exoergic product states. (iii) Selective formation of N2(B 3 II,) or N2(C 311g) is consistent with collinear (end-on) attractive interac49

Volume 82, number 1

CHEMICAL

PHYSICS

tion between N, and Rg*, and promotion of electrons from o-type molecular orbrtals. (iv) The endoergic formation of N2(B, u = 5) by Xe is perhaps the result of a fortuitous curve crossing between the repulsive Xe--NT@2 = 2) potential and Xe-Nz(B_ u = 5) potential_ If so, the division of the B(3fIg) vrbrational drstr:bution into o < 4and u = 5 components is sensible. (v) The values of TN2 at the crossing points of :he entrance and exit channel potentials will determme the Franck-Condon integrals that influence the r:T vrbrational populations_ As Gislason et al. [23] noted, allowance must be made for variation of r~a during the traversal of the two legs of the trajectory_ This idea is especially important for the bound potentials since the ma distance presumably will be altered as the Rg*-N2 bonding occurs_ (vi) The specific rotational, spin state, and X doublet N2(C) populations [6] can be explained by propensity and symmetry restrictions, arising naturally from the collmear geometry, at the crossing points of attractive entrance and exit potentials. (vii) Conversron of orbital angular momentum of the entrance channel into Nz rotational angular momentum in the exit channel can be explained if the entrance and exit channel crossing points lie at relatively long distances. A somewhat indirect argument also provides insight for the exrt-channel potentials. We mentioned earlier that vibrational relaxation of N2(B, u) by Xe occurs

with a large cross section_ We assume that this relaxation invoIves coupling of other N, states, which seems to be N2(W 3A,,)_ Yet the transfer from Xe* to N, gives selectrvely N2(B) wrth a smaIl cross section_ This dilemma can be rationalized only if the exit channels for (1) differ from the Xe-NT potentrals couphng N2(B) and N2(W). We suggest that the explanation involves the Xe-N2 orientation requirement (collinear) for (I), whereas the couphng of N2(B) and N2(W) occurs without a significant dependence on orientation_

Acknowledgement This work was supported by the US Office of Naval Research under contract NOOO14-80GO346. We thank Dr. T.D. Nguyen for a discussion of angular momentum coupling in the Ar(3P2) + N, reaction,

50

LETTERS

15 August 1981

and Mr. TD_ Dreiling for his assistance in the Nz(C, B) formation rate constants measurements.

References [l]

D.H. Stedman and D_\V.Setser, J. Chem. 52 (1970)

3957. [2] T-D. Nguyen, N. Sadeghi and JC. Pebay-Peyroula, Chem. Phys. Letters 29 (1974) 242. [3] CJ.Tracy and HJ. Oskam, J. Chem. Phys. 65 (1976) 1666. t41 J. Be1 Bruno and J. Krenos, Chem. Phys Letters 74 (1980) 430. [51 D-W. Setser, D.H. Stedman and J A. Coxon, J. Chem Phys. 53 (1970) 1004; J H. Kolts, H.C. Brashears Jr. and D-W. Setser, J. Chem. Phys. 67 (1977) 2931. 161 J. Derouard, T.D. Nguyen and N. Sadeghi, J. Chem. Phys. 72 (1980) 6698. 171 M. Touzeau and D. Pagnon, Chem. Phys. Letters 53 (1978) 355. ]81 J-E. Velazco, J-H. Kolts and D.W. Setser, J. Chem. Phys 69 (1978) 4357. [91 E.R. Cutshall and EE. Muschlitz, J. Chem. Phys. 70 (1979) 3171. 1101 T-R. Parr and R-M. Martin, J. Chem. Phys. 69 (1978) 1613. [IL] J. Krenos and J. Be1 Bruno, Chem. Phys. Letters 49 (1977) 447. [I21 N. Sadeghi and D-W. Setser, Chem. Phys. Letters 77 (1981) 304. [I31 J-H. Kolts and D.W. Setser, in: Reactive intermediate in the gas phase: generation and monitoring, ed. D.W. Setser (Academic Press, New York, 1979). 1141 D-W_ Setser, T-D. Dreding, H-C. Brashears Jr. and J.H. Kolts, Faraday Discussions Chem. Sot. 67 (1969) 255. [I51 A. Lofthus and P.H. Krupenie, J. Phys. Chem. Ref.

Data 6 (1977) 113. [I61 D. Cerny, F. Roux, C. Effantin and J. dlncan, J. Mol. Spectry. 81 (1980) 216. r171 T D. Dreiling and D.\V. Setser, Chem. Phys. Letters 74 (1980) 211. 1181 L G. Piper, D.W. Setser and M A.A. Clyne, J. Chem. Phys. 63 (1975) 5018; L-G. Piper, J. Chem. Phys. 67 (1979) 1795. 1191 T.D. Nguyen and N. Sadeghi, to be submitted for publication1201 H.H. Teng and D.C_‘Conway, J. Chem. Phys. 59 (1973) 2316. 1211 M.S.B. Munson, F.H. Field and J-L. Franklin, J. Chem. Phys. 37 (1962) 1790. [221 D.H. Winicur and J.L. Fraites, J. Chem. Phys. 61 (1974) 1548. v31 E.A. Gislason, A.W. Kleyn and J. Los, Chem- Phys. Letters 67 (1979) 252.