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Electronic energy transfer between state selectedmetastable argon atoms and ground state krypton atoms

T.D. Dreiling, N. Sadeghi

To cite this version:T.D. Dreiling, N. Sadeghi. Electronic energy transfer between state selected metastable ar-gon atoms and ground state krypton atoms. Journal de Physique, 1983, 44 (9), pp.1007-1013.�10.1051/jphys:019830044090100700�. �jpa-00209684�

1007

Electronic energy transfer between state selected metastable argon atomsand ground state krypton atoms

T. D. Dreiling and N. Sadeghi

Laboratoire de Spectrométrie Physique, Université Scientifique et Médicale de Grenoble,B.P. n° 68, 38402 Saint-Martin d’Heres Cedex, France

(Reçu le 21 mars 1983, accepté le 26 mai 1983)

Résumé. 2014 La réaction de transfert d’energie des atomes métastables Ar (3P2) et Ar (3P0) à l’atome Kr a été étudiéedans la post-décharge d’un mélange Ar-Kr. La constante de décroissance de la densité des atomes métastables a étémesurée pour obtenir les coefficients de relaxation des atomes Ar (3PJ) par Kr. Dans des expériences séparées, leniveau Ar (3P2) a été dépeuplé dans la phase de post-décharge à l’aide d’un laser accordable continu. La comparai-son des intensités d’émission à partir des niveaux excités de Kr avec et sans depeuplement laser a été utilisée pourobtenir les coefficients de reaction pour l’excitation des niveaux de Kr par Ar (3P2) et Ar (3P0). L’excitation de Krpar des atomes Ar (3P2) est ~ 40 fois plus efficace que par des atomes Ar (3P0).Abstract 2014 The excitation transfer reactions from Ar (3P2) and Ar (3P0) to Kr were studied in the afterglow of apulsed discharge. The time decay of the metastable concentrations was monitored to obtain total quenching rateconstants for removal of Ar (3PJ) by Kr. In separate experiments, Ar (3P2) was optically depopulated in the after-glow by use of a tunable cw dye laser. Comparison of emission intensities from excited Kr levels with and withoutlaser perturbation was used to assign relative rate constants for excitation of Kr levels by Ar (3P2) and Ar (3P0).The Kr excitation by Ar (3P2) was found to be ~ 40 times more efficient than by Ar (3P0).

Tome 44 No 9 SEPTEMBRE 1983

LE JOURNAL DE PHYSIQUEJ. Physique 44 (1983) 1007-1013 SEPTEMBRE 1983, :

Classification

Physics Abstracts34.50H

1. Introduction.

In recent years, studies of the quenching reactions ofthe heavier metastable Rg (3P2) and Rg (3Po) stateshave grown rapidly due not only to fundamentalinterest in these processes but also because the excitedrare gas atoms are often the active species in lasersystems [l, 2]. Most of the experimental data pertain tothe 3P2 state; total quenching rate constants [3-5]and product channel branching (factions [6-8] havebeen determined for. a wide variety of reagents.Although some quenching rate constants have alsobeen measured for Ar (3Po) [3a], relatively little otherattention has been directed to the higher energy 3Postate, which f6r Ar (and Ne) can account for - 10 %of the total metastable density in discharge excitedsystems. Complete characterization of the productchannels from Rg (3Po) is usually hampered by inter-ference from the predominant 3P2 species.Only recently have these states been separated

and significant differences found in their relative

quenching mechanisms. Sadeghi et ale [9] examinedthe Ar(3p2,0) + N2 excitation transfer reaction andfound that the relative populations of the N2 (C, 3nu)product vibrational and rotational substates were

dependent on the Ar (’Pj) precursor. Golde andPoletti observed different atomic halogen and mole-cular argon halide electronic state distributions fromAr (3PZ) and Ar (3Po) reacting with several halogencontaining molecules [10]. Finally, the Penning ioni-zation reactions of Ne (3Po) with Ar, Kr, and Xe showsa marked propensity for formation of the Rg+ (2pll2 )ion state relative to Rg+ (2 P3,2); for Ne (3P2), theRg+ (2p 1/2’) is only slightly favoured [11].

In this paper we present a study of the excitationtransfer from Ar (3p 2,0) atoms to ground state Kratoms, which also shows large differences between thetwo metastable Ar states. Piper et ale [12] have pre-viously examined this reaction using the flowing after-glow technique and report that the only observable Krlevels excited from Ar (3P2) were Kr (2P6) and Kr (2P7)(see Fig. 1), in the ratio of - 9/1. Weak emission from

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019830044090100700

1008

Fig. 1. - Energy levels for Ar and Kr with the wavelengthsof the observed transitions. In the text the Paschen notationis used to describe the Kr (5p) states while the L-S notationis used for the Kr (5s) and Ar (4s) states. The wavelengths(branching ratios) for the Kr (5p)-Kr (5s) transitions observedin this work are given.

Kr (2P5) and an ArKr excimer also were observed butwere attributed to formation from Ar (’PO) [12].However, since the sum of the formation rate constantsfor these products was less than the Ar (3 Po) quenchingrate constant, other Kr states, presumably Kr (2P6) andKr (2P7), accounted for the main products fromquenching of Ar (’PO) [12].The present work utilizes the stationary afterglow

technique to measure total quenching rate constants,and coupled with laser depopulation of Ar (’p2) in theafterglow, to characterize the products from Ar

(3Po) + Kr. Rate constants for excitation of specific Krlevels by Ar (3Po) relative to Ar (3P2) were determinedby monitoring Kr emission intensities and Ar (3 P2,O)concentrations, with and without the laser perturba-tion. Absolute rate constants for Ar (3Po) excitationare obtained from the known values of the corres-

ponding Ar (3P2) rate constants. These experimentsindicate : i) two-body quenching of Ar (3P2) by Kris ~ 40 times faster than for Ar (3Po), ii) both Armetastable states give Kr (2P6) and Kr (2P7) as themajor (> 95 %) products, but with Kr (2P6) favouredfrom Ar (3P2) and Kr (2P7) favoured from Ar (3Po),and iii) quenching of Ar (3Po) also proceeds by a fastthree-body reaction involving Ar and Kr.

2. Experiment

The experimental apparatus and techniques have

largely been described in detail in previous publica-tions from this laboratory [9, 13]. Research grade Arand Kr (99.999 5 %) were separately flowed through a

cylindrical (D = 4 cm, L = 25 cm) Pyrex cell. A dcpulsed discharge was applied to the cell at a frequency20-200 Hz to create metastable Ar atoms. Both thedischarge pulse duration (~ 100 ps) and current

(- 5 mA) were adjusted to maximize the metastabledensities in the afterglow. The Kr product channel rateconstants were measured at low ( ~ 0.2 torr Ar)pressure in order to minimize secondary collisions ofKr* with the Ar buffer [14]; the mole fraction of Krin these experiments was 0.5 %. Measurements ofthe quenching rate constants were performed at Arpressures ranging from 1-7 torr. An MKS Baratroncapacitance manometer (model 221 A) was used for allpressure measurements.

Both Ar (’Pj) absorption signals, giving the meta-stable concentrations, and the near IR Kr* emissionsignals from the cell were detected with a 0.6 meterJobin-Yvon monochromator equipped with a Ham-matsu R 928 photomultiplier tube and by usingphoton counting and data accumulation techniques.In some experiments the Kr resonance line at 123.6 nmwas also monitored using a 1 meter VUV monochro-mator and channeltron (R.T.C.) detector. The VUVmonochromator was interfaced to one base of the cellvia a LiF window. In determining the Kr formationrate constants, absorption and emission measure-ments were made only during a 1-2 ms detection

gate located - 1 ms after the cessation of the dis-

charge. For determining the quenching rate constants,the entire metastable decay curves were accumulatedin a multichannel analyser and passed on to a computerfor later analysis. A radiofrequency powered Arlamp provided 763.5 nm and 794.8 nm line sourcesfor absorption by Ar (’p2) and Ar (3Po), respectively.Metastable concentrations were obtained from the

absorption rate measurements using conversionfactors given previously [15]. For typical operatingconditions without laser pumping, the concentrationof Ar (3P2) was - loll cm-3 with [Ar (3po)]/[Ar (3P2)] about 0.15.

Laser depopulation of Ar (3P2) in the afterglowwas achieved by use of a tunable cw dye laser (Spectra-physics, Model 375) pumped by a Kr+ ion laser

(Model 171). Oxazine 725 dye was used to producethe 696.5 nm line of Ar (2P2 +- 3P2 ). This line waschosen from several possibilities because favourablebranching fractions from Ar (2p2) [16] permit simul-taneous depopulation of Ar (3P2) and enhancementof the Ar (3Po) concentration. The dye laser outputlinewidth, 0.03 nm, was reduced to 0.003 nm bymeans of an auxiliary intracavity etalon. Fine tuningof the laser wavelength was optimized by observationof the optogalvanic signal from a separate Ar dischargecell. The laser beam was brought to the reactioncell throught an optical fibre and expanded by meansof a spherical lens before entering the cell along itsaxis. The typical laser output power used was 250 mWand, under these conditions, Ar (3P2) was depopu-lated by more than 99 %.

1009

Fig. 2. - Time decay plot of the Ar (3P2) and Ar (3Po)levels in the presence of 5.4 x 1014 atoms/cc of Kr and at1.03 torr.

3. Results.

3.1 METASTABLE QUENCHING RATE CONSTANTS. -

Following discharge excitation in the presence of

Kr, the metastable Ar (3P2,o) atoms can decay bydiffusion and several collisional processes involvingAr, or Kr or both together. At a fixed Ar pressure,the Kr induced decay frequency, kQ. [Kr], of eithermetastable state can be described by

where kQ is the effective two-body quenching rateconstant by Kr atoms at the given Ar pressure, i is themeasured decay time of the observed metastable

species in the presence of Kr and K represents the decayfrequency of the same metastable in the absence ofKr (due to diffusion and collisional deactivation byground state Ar atoms alone). Figure 2 shows thedecay of Ar (3P2) and Ar (3Po) at 1.03 torr Ar andwith a Kr density of 5.4 x 1014 atoms cm- 3. Boththese curves and others that were obtained could be

represented by a single decay constant and, as can beseen, the decay of Ar (3Po) is appreciably slower thanfor Ar (3P2). Figure 3 shows the variation of the decayconstants (l.h.s. cf. Eq. 1), obtained from curves similarto those shown in figure 2, with the Kr concentration.The slopes of the plots give directly the quenchingrate constants which were extracted from the data by aleast squares analysis. Several experiments performedat 1.00 ± 0.03 torr gave average values of 4.90 ±0.20 x 10-12 cm3 molec-1 s-1 and 2.43 ± 0.25 x10-13 CM3 molec-1 s-’ for kQ(2) and kQ(0), respecti-vely, where the error limits represent one standarddeviation from the mean. We estimate the absolute

Fig. 3. - Variation of the decay rates of Ar (3P2) andAr (3Po) as a function of Kr concentration. Note the differentscale for Ar (3 Po),

uncertainty in the kQ, which stems from uncertaintiesin the measured Kr concentration and decay timemeasurements, to be less than 15 %. The error in therelative values should be less than this as decay curvesfor the two metastables were always obtained back toback with identical Kr concentrations.The kQ measured at 1 torr represent the effective

two-body rate constants since a three body componentmay also be contributing,

In the flowing afterglow experiments, Piper et ale [12]observed emission from an ArKr excimer which theyattributed to formation from three-body quenching ofAr (3p 0). Thus reaction 3 could be an importantquenching pathway even at relatively low Ar pressures.Measurements of ko were performed at several Arpressures ranging from 1 to 7 torr. For Ar (’p2), nosignificant variation of kQ(2) was found over thelimited pressure range that could be studied, indicatingthat kQ3(2) is too small to be detected However, forAr (3Po), kQ(0) was found to increase with increasingAr pressure as shown in figure 4. The plot is linearand from the intercept and slope values of 1.14 ±0.10 x 10- 13 CM3 molec-1 s-1 and 4.35 ± 0.40 x10- 3° CM6 molec- 2 s-1 were deduced for kQ2(0) andkQ3(0), respectively. Attempts were made to extendthe plot to higher pressures but these efforts wereunsuccessful because of the rapid decay of Ar (3p 2,0)at pressures higher than 10 torr.

z

The measured rate constants are summarized intable I and can be compared to results obtained byother laboratories. For Ar (3p 2)’ most of the determina-tion support a value of N 5 x lO-12 CM3 mole-1 s-’

1010

Fig. 4. - Variation of KQ(0) as a function of Ar concentra-tion. The slope and intercept of the plot give the three-bodyand two-body quenching rate constants.

for kQ(2). The two exceptions are due to Bourene andLeCalv6 [4a] and Bochkova [17] where kQ(2) is 2-3times larger. Bourene and LeCalv6 used high energyelectron excitation of an Ar/Kr mixture along with N2as a tracer of Ar (’p2) [4a]. Possible ambiguitiesin the precursor of the tracer emission could accountfor their larger result. The discrepancy with Bochkovais more difficulty to rationalize since she used the same

experimental techniques as in this work. However,no details of her measurements were given and theresult was quoted only as an estimate. Based on thegood agreement with all the other determinations, weconclude that our measurement is reliable.Two previously reported kQ(0), both obtained at

~ 1 torr in a flowing afterglow apparatus, differ byan order of magnitude. Velazco et ale [3a] used absorp-tion to follow Ar (3Po) and found

while Golde and Poletti [ 19] derived

from monitoring the decay of tracer emission as afunction of added Kr. In the latter case onlykQ(2)/kQ(O) ( =18) could be accurately determined andkQ(0) was assigned from assuming

Our results are in agreement with Golde and Polettisince at one torr Ar pressure we obtain

and thus confirm the much lower value for kQ(o).However, we emphasize the fact than kQ(0) containsboth a two- and three-body deactivation of Ar (3p 0).

3.2 KRYPTON EXCITATION RATE CONSTANTS. - A por-tion of the Kr fluorescent spectrum excited in theafterglow is shown in figure 5a without laser pertur-bation. Our observations of the spectrum generallyresemble those of Piper et ale [ 12] in that only the Kr(2P6) and Kr (2P7) levels are strongly populated Infact, the only other emissions observed aside fromthat shown in figure 5a were the Kr lines at 819.0 nm,829.8 nm, and 123.6 nm with the first two also origi-nating from Kr (2P6,7) and the latter being a conse-quence of radiative cascade [12]. Although bothAr (3p 2) and Ar (3p 0) are present, the Kr emissionis due almost entirely ( > 99 %) to excitation byAr (3p 2) Because of its much larger concentration([Ar (3p 2)]/[Ar (3p 0)] ’" 6) and the fact that quenchingof Ar (3Po) by Kr is much slower than for Ar (3P2).Taking into account the intensity of the observed linesat 760.2 and 769.5 nm along with the branching frac-tions for these transitions [20], we find the relativeformation rate of Kr (2P6) to Kr (2p.7) to be 8.5/1 inexcellent agreement with the value of 8.6/1 obtained byPiper et ale [12]. If we consider that quenching ofAr (3P2) by Kr results only in excitation of Kr (2P6,7),our measured k,(2) can be partitioned into formationrate constants of 4.39 x lO-12 CM3 mole-1 s-1 and0.51 x 1 O-12 CM3 molec-1 s-1 for Kr (2p6) andKr (2P7), respectively. These values are given intable I and agree with those obtained by Piper et ale [12]and Bochkova [17].

Fig. 5. - Spectra of the Kr fluorescence without (a) andwith (b) laser depopulation of Ar (3P2) using the 696.5 nmlaser line. The intensity scale factor of the Kr (2P6) line in (b)is 1/20 of that in (a).

When Ar (3p 2) is depopulated by 696.5 nm laserradiation, the absolute intensities of the Kr (2P6,7)emission lines are reduced and their relative magnitudechanges as shown in figure 5b. In addition, emissionfrom Kr (2ps) at 758.7 nm, previously obscured by thestrong 760.2 nm line, is observed Since atomic

absorption gave no measurable signal from Ar (’p2),the spectrum in figure Sb is due primarily to excitation

1011

Table I. - Rate constants for Ar (3p 2) + Kr andAr (3p 0) + Kr reactions.

(") Unless otherwise noted rate constants are in units of 10-12 CM3 molec-’ s-1.(b) Ref. 3a.

(C) Ref.4a.

(d) Ref. 17.

(e) Ref. 18.

(f ) Ref. 19.

(g) Ref. 12.

(h) Effective 2-body rate constant at 1 torr for comparison with other measurements.(’) The three-body rate constants have units of 10-3° CMI molec-2 s-’.

from Ar (3p 0). Rate constants for excitation of

Kr (2P6) and Kr (2P7) by Ar (’PO) were assignedrelative to excitation from [Ar (3P2)] by a comparisontechnique. The desired ratio, ko/k2, is obtained from :

where I is the intensity of the Kr line diagnostic ofeither Kr (2P6) (760.2 nm) or Kr (2p.) (769.5 nm),[’Pj] is the concentration of Ar (3PJ) and the super-scripts refer to quantities with (w) and without (wo)laser perturbation. The average of several runs gaveR = 0.15 + 0.02 for Kr (2P7) and R = 0.008 + 0.004for Kr (2P6) which combined with the Ar (’p2) rateconstants yield 7.8 and 3.5 x 10- 14 cm 3 molec -1 S-1for ko (2p.7) and ko (2p6), respectively.Both R (2P6) and R (2P7) were derived from equa-

tion 4 with [Ar (3p 2)]W set equal to zero as no absorp-tion from Ar (3P2) was found during laser pumping.However, small absorption signals of less than 0.5 %would not have been observed with our detection

system and some residual Ar (3P2 ) could still havecontributed to the Kr excitation. Since R (2P7) is

relatively large compared to R (2p6), excitation ofKr (2P7) from Ar (3P2) is negligible compared toAr (3Po) and ko (2P7) should be reliable. On the otherhand, for Kr (2P6), the contribution from residualAr (3P2) could be significant because of the more thantwo order of magnitude difference in ko (2P6) andk2 (2P6). In fact by taking into account the atomic

absorption detection limit, it is impossible to ruleout Ar (3P2) as the sole precursor of Kr (2p6) evenduring laser pumping. Thus R (2P6) obtained fromequation 4 with [Ar (3p 2)]W = 0 must be regardedas an upper limit and ko (2p6) 3.5 x 10-14 cm3molec-’ s-’.The values for ko (2P6) and ko (2P7) are given

in table I along with ko (2p5) = 3.0 x 10-15 CM3molec-1 s-’. Formation of Kr (2P5) from Ar (3P2)is endothermic and equation 4 cannot be used todetermine ko (2p5). Instead ko (2p5) was assignedfrom ko (2P7) by comparison of the 758.7 nm and769.5 nm atomic line intensities during laser pumpingalong with the appropriate branching ratios [20].Assuming that ko (2p6) is approximately correct,the sum of the Kr excitation rate constants agreeswithin experimental error with our value of kQ(0),suggesting that all major products from two-bodyquenching of Ar (3Po) have been identified The onlyother possible products could be Kr (2p8 _ 10) and theKr (4s) levels. No emission from Kr (2p8 _ 10) wasobserved in our experiments and formation of theselevels must be negligible. Direct production of theKr (4s) levels is more difficult to rule out However,during experiments the intensity of the Kr resonanceline at 123.6 nm was also monitored The decrease inintensity of this line with laser pumping was alwayscomparable (within 10 %) to that calculated on thebasis of the corresponding decrease of the infraredlines weighted by their branching ratio for the emissionto Kr (3P1) [20]. Thus we conclude that direct produc-

1012

tion of Kr (3p 1) from Ar(3po) is negligible. SinceKr (3P1) is not formed, it is likely that other Kr (4s)states also are not produced from Ar (3p 0) as wasshown in the case of Ar (3P2) [12].

4. Discussion.

The total two-body quenching rate of Ar (3Po) by Kris approximately 40 times slower than for Ar (3p 2).Both Ar metastable states give Kr (2P6) and Kr (2P7)as the major exit channels. However, for Ar (3P 2)’Kr (2P6) is favoured by - 9/1 while for Ar (3Po),Kr (2P7) is populated at least 2 times more efficientlythan Kr (2P6). An additional exit channel fromAr (3p 0) is Kr (2p5) ; but, this product accounts forless than 5 % of the total two-body quenching ofAr (3Po).Most of these results can be qualitatively explained

by the schematic potential curves shown in figure 6 and

Fig. 6. - Schematic potential energy curves for Ar-Kr

(redrawn from Ref.12). The numbers give the S2 components.Abscissa is interatomic distance.

a simple curve-crossing model. For quenching ofAr (3P2), no symmetry restrictions exist for the

products since the entrance channel has curves withmolecular configurations of 0-, 1 and 2 while theKr (2p 6) and Kr (2P7) exit channels are 0+,1, 2 and0-, 1, respectively. Piper et al. [12] have used theLandau-Zener theory to explain the larger formationrate of Kr (2p6) relative to Kr (2P7)’ In the L-Z theorythe excitation rate constants depend on the magnitudeof the crossing point of the entrance and exit curvesalong with the probability of transfer from one curveto the other. For diabatic coupling, the transfer

probability in turn depends on the difference in theslopes of the two potentials at the crossing point.If it is assumed that the Kr (2p6)-Ar and Kr (2P7)-Arpotentials are similar in shape, the Kr (2p6)-Arpotential crosses the entrance channel at larger

intemuclear distance and with a better matching of theslopes than the Kr (2P7)-Ar curves. Thus, bothfactors favor Kr (2p6) as the preferred exit channel,as is experimentally observed. Using approximatenumerical potentials, Piper et al. [12] obtained goodagreement between predicted and observed k2 (2P6)/k2(2P7) ratio.The Ar (3Po)-Kr entrance channel contains only

a single 0- curve while the Kr (2P5) exit channel is 0+.Thus mixing of these two curves cannot take placeand only a minor amount of Kr (2ps) is observed

experimentally. On the other hand, formation of bothKr (2p6) and Kr (2P7) can be expected since thesestates have at least one curve which crosses theAr (3Po)-Kr potential and is of the proper symmetry(0- or 1). However, the intersections will be too

abrupt to allow for efficient transfer. Thus, comparedto Ar (3P2) the Kr excitation rate constants fromAr (3Po) should be much smaller, which is consistentwith our experimental results. In addition, Kr(2py)is statistically favoured since both the 0 - and 1 compo-nents can interact with the entrance channel while forKr (2p6), interaction occurs only withy = 1 compo-nent. If all components are similar in shape and crossthe entrance channel potential at roughly the sameintemuclear distance, the predicted ko (2P7)/ko (2p6)ratio is 1.5, which can be compared with experi-mental ratio of >- 2.1. The relatively good agreementis somewhat fortuitous since the repulsive part of theKr (2P6,7)-Ar potentials is not well known and thedifferent components can be expected to have differentrepulsive characters and coupling strengths with theentrance channel potential. Nevertheless, the productchannel branching fractions from quenching of Ar (3Po)are qualitatively consistent with the simple curve-crossing mechanism, although a more quantitativecomparison will have to await more accurate Ar-Krpotentials.Quenching of Ar (3Po) was also found to proceed

by a 3-body step involving Ar and Kr and we obtaineda relatively large rate constant of 4.4 x 10-30 cm6molec-2 s-1, two orders of magnitude larger than themost of the known 3-body reaction coefficient of raregas metastable atoms. In their study, Piper et al. [12]observed an ArKr continuum emission peaking at756.5 nm which they attributed to formation fromAr (3Po). From the pressure dependence of the bandintensity, they derived a 3-body collision rate constantof 2.5 x 10- 3° cm6 molec- 2 s-1, in reasonable agree-ment with our measured 3-body quenching rate

coefficient of Ar (3Po). However, there is no evidenceof the ArKr emission in the spectrum shown in

figure 5b or those taken at Ar pressures as high as2 torr. From the detection limit of our system, we canderive an upper limit to the ArKr formation rateconstant of 5 x 10 - 31 cm6 molec - 2 S-1 which is

considerably smaller than the quenching rate constant.Thus the products from three-body quenching ofAr (3Po) remain unidentified. It is possible that the

1013

three-body collision destroys the symmetry restrictionsof the two-body Ar (3Po) + Kr collision and results information of excited Kr levels. Since the Kr* fluo-

rescent spectra were obtained at low pressure (0.2 torr),the three-body contribution to the measured excitationrate constants would be negligible.

References

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[2] KING, D. L. and SETSER, D. W., Ann. Rev. Phys. Chem.27 (1976) 407.

[3] a) VELAZCO, J. E., KOLTS, J. H. and SETSER, D. W.,J. Chem. Phys. 69 (1978) 4357.

b) BROM, J. M., KOLTS, J. H. and SETSER, D. W., Chem.Phys. Lett. 55 (1978) 44.

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[8] GOLDE, M. F., Ho, Y.-S. and OGURA, H., J. Chem.Phys. 76 (1982) 3535.

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b) SADEGHI, N. and NGUYEN, T. D., J. Physique Lett.38 (1977) L-283.

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[10] GOLDE, M. F. and POLETTI, R. A., Chem. Phys. Lett.80 (1981) 23.

[11] HOTOP, H., LORENZEN, J. and ZASTROW, A., J. Elect.Spectrosc. Rel. Phen. 23 (1981) 347.

[12] PIPER, L. G., SETSER, D. W. and CLYNE, M. A. A.,J. Chem. Phys. 63 (1975) 5018.

[13] NGUYEN, T. D. and SADEGHI, N., Phys. Rev. A 18(1978) 1388.

[14] CHANG, R. S. F., HORIGUCHI, H. and SETSER, D. W.,J. Chem. Phys. 73 (1980) 77.8.

[15] SADEGHI, N., Doctorat d’Etat, Université de Grenoble(1974), unpublished. Metastable densities werecalculated by using l [3P2] = 3.2 1011 A7635and l [3P0] = 1.67 1011 A 7948 atom.cm-2. l is optical passe length, A is the absorption rate,and 3P2,0] atom densities.

[16] WIESE, W. L., SMITH, M. W. and MILES, B. M., Natl.Stand. Ref. Data 22 (1969) Vol. II.

[17] BOCHKOVA, O. P., Opt. Spectrosc. 28 (1970) 88.[18] PHELPS, A. V. and MOLNAR, J. P., Phys. Rev. 89 (1953)

1202.

[19] GOLDE, M. F. and POLETTI, R. A., Chem. Phys. Lett.80 (1981) 18.

[20] a) LILLY, R. A., J. Opt. Soc. Am. 66 (1976) 245.b) AYMAR, M. and COULOMBE, M., At. Data. Nucl. Data

Tables 21 (1978) 537.