PHYSI| AL RKVIE% A VOLUME l8, NUMBER 3 SEPTEMBER 1978

Quenching of atomic states in a low-pressure hydrogen glow discharge

A. Catherinot, 8, Dubreuil, and M. Gand-Groupe de Recherches sur l'Energetique des Milieux Ionises, Universite d'Orleans, 45045 Orleans Cedex, France

{Received 2 March 1978)

A laser perturbation method is used to determine the relaxation rates of levels n = 3, 4, 5 of atomic

hydrogen as a function of pressure and current intensity in a glow discharge. Quenching is due tospontaneous emission of radiation towards lower levels and to inelastic collisions with H2 molecules in the

ground and quasimetastable states with thermally averaged cross sections of about 1.5 X 10 "cm'.

I. INTRODUCTION

Early studies on the dissociation of H, by elec-tronic collisions'"' indicate that in the energyrange of gas discharge plasma electrons(Z, -2-10 eV), the dominant process responsiblefor' the formation of hydrogen atoms, is the dis-sociation reaction

H, (X'Z;) + e -H+ H+ e

whose efficiency is greater than that of the sec-ondary reaction

H, (X'Z')+ e -H, '+ e + e -H+ H'+ 2e . (2)

Excitation (respectively, ionization) of theseground-state atoms is then ensured by electronic in-elastic collisions. Under continuous electric excita-tion, a quasistationary equilibrium sets up inwhich excitation processes are on the wholebalanced by radiative and collisional depopulatingprocesses. These deexcitation processes may bein the case of atomic hydrogen: diffusion towardthe walls of the discharge tube, emission or ab-sorption of radiation, inelastic collisions withelectrons, and inelastic collisions with heavy par-ticles (i.e., H, molecules). Except for metastablestates, diffusion does not contribute to the des-truction of excited atomic states in low-pressureglow discharge.

II. EXPERIMENT

Qle have studied the quenching mechanisms ofthe n= 3, 4, 5 atomic hydrogen excited states inthe positive column of a l.ow-pressure glow dis-charge by a time resolved spectroscopic analysisof the population relaxations following a shortresonant laser pulse pumping. The experimentalapparatus is schematically shown on Fig. 1. Ithas been previously described in experimentsdealing with radiative and collisional processesin a helium glow discharge. 4 ~ A tunable dye laserexcited by a pulsed nitrogen laser SOPHA (pulsewidth, 4 ns, spectral width 0.2 A, energy/pulse-10pJ, repetition rate, 15 Hz) is used to induce

18

a selective and short perturbation on the popula-tion of an atomic hydrogen state by resonant op-tical pumping.

Excited states are produced in a capillary glowdischarge (inner diameter, 4 mm, length, 60 mm)After high-vacuum cleaning (10 " Torr) the dis-charge is created under continuous electrical pow-er supply with a constant flow of hydrogen high-purity gas (flow rate &1 f/h). Pressure P, mea-sured with two Pirani probes, can be adjustedfrom 0.2 to 2 Torr and current intensity i from 5to 40 mA. For each experimental situation (P, i)correspo-nding vilue of the electronic density n, ismeasured by a high-frequency cavity perturbationmethod, and value of the mean electronic kinetic en-ergy E, is only estimated in the frame of glom dis-charge theory. In pressure and current rangesunder study, we obtain, respectively, 5.10'&g,

DYE LASER

N, LASER

H, glow discharge

I

II

I

r

P. IVl. 2

1)A B

synchro.

BOXCAR PA R 162

SPECTRONIETER

f{i) = Ay8

FIG. l. I aser perturbation experimental setup.

1978 The American Physical Society

CATH V, B, lNOS. ' 8. DUBB, EUlL ANDD M. BAND 18

&5.10' cm ' and 3&E,&10 eV. Gais measured b

eV. Gas temperaturee y means of a thermo '

ta.ct with the dirmocouple in con-

t i e discharge tube (T -320 +5"ogeneous distribution in the medi-

Af ter spatial filtering, the ume pump laser beams e ischarge tube

i usion. The fluorescence li htig emitted by' n o e positive column is observed

ar erection and is imafused silica lens (f=-18 monto the sl'

0mm mmagnification =- 1'I

e s its of a 1.15 m SOPBA r( o vmg power: 50000) and then

omu iplier tube (rise time.TIme dependence of th

by a Prsneet. A eao e output si nal i

.on pplied ResearcheRI'cll Boxcar RveI'R eI'

synchronized with the u

o cha, nnels A and 8 (PARtime resolution of 5

R 164) a

signal (channel r4 i ' en o ns. The fluor

s normalized to tht typ kd etected b an ay auxiliary

r ~c annel B& sineaverager all

ince the Boxcarer ows the mea. suremement of the ratio A/8

o signals. Great care is taken tosaturation of th e electronic b~ the fl

s en to prevent

light and toe uorescence

avoid laser satura ~on of excited state populations.

cence relaxation curl curve correspondslaser shoots average.

IL

J(r j,'.vj"E [avI E

16-

15-

n=4

14- ii=3

'I 3-n=5

13-n=4

12-0=3

12-lira 2

n= 1

4»

2 u

0—0.4 0.8

I I I

1.2 1.6 2.02.0 2.4 2.8 3.2 3.6 I (A)

n= 210-

n=1

. 2. Partial energy schemes ofe and unshifted H en

correspond respecti l swiththequasim tec ive y, to collisionsaesoftheH m . a en

f 11 d 14)

ill. MEASUREMENTS

A partial dia, gram of h drolecular ener

y rogen atomic and mo-r energy states is shown in Fi . 2m ig. 2, The re-

u a Ion variations rafter„(t) for theatomic states induced b l

. e ollowing transitions:

3-

(arbitrary unit)0

)n =2-n=3 ~H liines = 6563 A dye RGG( I =, e: G+CVP),n' = 2 - n = 4 (H 1 ines =n'=2- =, ' =4861 A, dye: C30+7D4MC

n' = 2 —n = 5 (H lineA. = 4n'= — — „' e =4340 A, dye: POPOP),

have been s tudzed for various dischaIscharge conditions,e t ree transitions, the lae aser spec-

e s e width of the linesarge so that the w iole atomic ve

rom measurements of the ti,m

1

perimental results f .— . r

~A (t). Tu~ s or. I'=0.35 Torrr and 2=30 mA

~g. 3 for n= 3, 4, 5. Aft er the laser

to their equilibe, e level o ulp p ations go back

qui x r ium values with a y poepen ence. In this l.asaser-f ree relax-

o e, evel population va t'

represented b th e lawrxa sons can be

0.1-

500 10

t (na)

10 20 30 40

FIG. 3. Hel.

60 70

el.axation curves of the n= 3...-l:t'---i. '.- f. f0 or P==0,35 Torr and i =30 mA

18 QUENCHING OF ATOMIC STATES IN A 1.0%-PRESS URE. . . |099

10-FIG. 4. Variations of the

quenching rates F„(P,i)with P and i for n=4 Hatomic state.

I

0 15

P (torr)2'

IV. INTERPRETATION

Generally, in a low-pressure hydrogen-gasdischarge, the quenching rate 1'„(P,i) of the atomicexcited states may be expanded'

1'„(P,i) = P A„.„&~„+n„g R„„+n, P S„tB

In this expression, A.~„and A„,„represent, r'e-spectively, the Einstein coefficient and the opticalescape factor for the n-n' radiative transition,and R„„[vdescribing a molecular state(v= 0, U, t)] represents the rate for the reaction

H(n)+ H, H+ H *(v)+hE, (6)

and 8 is the excitation transfer rate from staten towards state m by electronic collisions

~ (f) = ~Z'e-""'"n n 7

where I'„(P, i) is the quenching rate of leveln and de-pends on the discharge pressure and current in-tensity. As an example, the I'„(P, i) experimental.values deduced from the relaxation curves throughEq. (3) are shown on Fig. 4 as a function of thetotal pressure I' in the discharge with i acting asa parairieter for n =4. From these curves, it fol-lows with a rather good approximation that in thepressure and current ranges under study, I'„(P, i)can be expressed in the form:

r„(P,i) =A„+a„(z)P, (4)

where A„ is a constant and B„(i) is a function ofi alone.

H(n)+e -H(m)+e + MFinally, n„and n, are, respectively, the molecu-lar and electronic population densities.

Under present experimental conditions, no in-duced atomic, fluorescence was observed accord-ing to reaction (7) with m & n. On the other hand,transfers with m &n were only detected for lowpressure and current intensity and correspondedonly to spontaneous radiative transitions fromlevel n to m and not to reaction (7). These two

-

experimental results clearly indicate that excita-tion transfer between n= 3, 4, 5, 6 atomic excitedstates of hydrogen by electronic collisions areriegligibly small, This observation agrees wellwith calculated values' of the rates of reaction(7) and with recent experimental data. ' Takinginto account these results, Eq. (6) acquires theapproximate form

1'„(P, i) =Q A„,„A„,„+ +nR„„.

Then, the observed variation of I"„(P,i) with ican only be explained in terms of the variationsof ri„with i, that is by the variation of the dis-sociation rate of H, with i in the discharge. In-deed, according to the dissociation process (1),the number density of H, molecules in the dis-charge n„(P, i) at total pressure P, gas tempera-ture T, and current intensity i depends on thedensity and energy of electrons, so that one canwrite

A. CATHKB, INOT, B. DUBRKUIL, AND M. GAND 18

TABLE I. Total radiative de-excitationrates of then = 3, 4, 5 levels of atomic hydrogen.

An~n An~n(s ')

(Thin plasma, Ref. 11)

A 'n An'n(sn'&, n

(Thick plasma, Ref. 11)

An(s

(This work)

9.98 x10~

4.41 x 10~

(4.42~ 0.05) x107

3.02 x107

1.74 x10~

(1.72 +0.05) x10~

1.3.53 x10

0.743 x 107

(0.90 + 0.05) x107

where n'„, is the molecular population density attotal. pressure P and gas temperature T withoutany dissociation (that is without electrical dis-charge i=0, n, =0), and f(n„E,) is a function re-lated to the dissociation rate of reaction (1) suchthatf(n, = 0, E, = 0)=1. A complete study of thisdissociation rate will be published in a forthcomingpaper, "but for the present work, one needs onlyto define f (n„E,) as the ratio of dissociated mo-leeules at pressure I' and currenti. Accordingto the relation n„' =P/KT~, one gets: n„(P, i)=Pg(n„E,) with g(n„E,) =f (n„E,)/kT„and Eq.(8) then becomes

F„(P,i) = Q A„„A~„+Pg(n„E,)Q R„„. (10)

By comparing Eq (4) and (10), one obtains

A„= Q A „A „, B„(i)=g(n„E,)QB„„. (11)

Here, A.„appears as the total radiative decay rateand B„(i) as the total collisional rate of level n.

The experimental values of the total radiativetransition rates A„are compared to accepted val-ues: Z„,&„A~„A„,„(Table I) for an optically thinplasma: A.„,„=1, and for a plasma with reabsory-tion in the resonance lines: ~„,„=1, n'&1, and

~,„=0. The A„,„are taken from Ref. 11. Com-

parison shows that in the pressure range 0.1&P&3 Torr the n=3-n'=1, Lz, and n=4-n' =1,L„ lines are completely trapped, whereas forn=5, an escape of radiation in the L, line of about40% explains the relative discrepancy with thethick plasma case. A similar effect (escape ofresonance radiation) has been studied previouslyin a He glow discharge. '

The collisional rates for reaction (6) are de-duced from expression (10) after extrapolationof the experimental quenching rate F„(P,i) tozero current intensity. In these conditions,g(n, = 0, E,= 0) = 1/k T, so that B„(0)=Z„R„„/kT,.Details of this extrapolation procedure will bereported in Ref. 10. B„(0)values and correspond-ing eollisional thermally averaged cross sectionsF„'„„-1.5 ~ 10 ' cm', are given in Table II andindicate that quenching of excited H atoms by H,moleeules is a powerful process fulfilling thequasiresonant condition ~/kT, =O. The o„" „values are compared in Table II to recent re-sults obtained by I,ewis and Williams" who mea-sured cross sections of the quenching collisionsH(n=3, 4, 5) with ground state H, after excitationand dissociation of the molecular gas by a 40-keV electron beam. For the three states an im-portant disagreement is observed. In order togive an explanation of this discrepancy, we havelooked for the molecular states (Q, v, J) for which

TABLE II. Collisional de-excitation rates B„(0)and thermally averaged collisional crosssections OH H for the &=3,4, 5 levels of atomic hydrogen.

2

n=5

B„(0){s~ Torr ~)

2

{This work)

(15.0 ~ 0.2) x10'

156 +3

(14.2+0.4) x10'

145 +4

(14.5 +0.5) x10'

(Ref. 12)76 +3 32 k3 8.9 ~ 0.7

QUENCHING OF ATOMIC STATES IN A LO%-PRESSURE. . . 1101

the following reactions:(arbitrary unit)

H(n)+ H, [X' Z'(0, 4)]-H(n'= 1)+H,(Q, v, j)+hE,

(12)

are propable, as they fulfill the condition ~«kT,and obey the usual optical selection rules. Energyvalues of H, and H states have been taken fromBefs. 11, 13, and 14. Then, following Ref. 12, wehave o"„„~Z„~A„,q„„,g„with A„, the Einsteincoefficient for the n-1 transition, q„„, theFranck-london factor for the electronic trans-ition v(X' Z', v = 0, J)- v'(0', v', J'), and g„ thefraction of H2 molecules in the state v. Summa-tion is performed on all states v, v' (typicallythree or four states) satisfying reaction (12).With the Franck-london factors given in Bef. 15,we obtain the following ratio: o'„„/o4 „ /o'„= 1/0. 2/0. 02. Experimentally, this ratio wasfound to be 1/0.9/0. 9. Results of Ref. 12 areon the other hand in better agreement with thiselementary model. In Bef 12, H, molecules weremainly in the ground state X'Z'(O, J) so that onlyreactions (12) contributed to atomic excited-statequenching as assumed in the model. But in ourexperiment, a part of H, moleeules is also inelectronic excited states. This part is generallysmall compared to the ground-state population,except for the quasimetastable group of statesc'z„and a'Z, which then may intervene as sup-plementary collisional deexcitation channels ofatomic states"

CH(n)+H, " -H(n'=2)+H, (n', v', g)~ m .

a' Z+

(13)

Tables given in Ref. 13 indicate that for everylevel n, at least 20 deexcitation channels with~&-,'kT, exist. On the other hand, measurementby absorption of a cw dye laser beam at 5947.3 Aof the population density of the a'Z'(v =O, J=2)state leads to an estimate of the total populationof quasimetastable states of about 5% of the totalmolecular population depending on the dischargeconditions (P, i), We thus can expect to obtain animportant population transfer of the atomic systemtowards the molecule according to reaction (13)which can explain the discrepancy with the resultsof Bef. 12. This has been experimentally verifiedfor the n=3 level of H in the particular transferreaction

H(n = 3)+ H, [a' Z'(v = 0, J= 4)]

-H(n=2)+H, (e'Z„'v=2, v=5) —,', ar, . (14)—

10 20 30 40

FIG. 5. Time resolved light intensities of the reso-nance atomic fluorescence (II ) and collision inducedmolecular fluorescence (molecular line) according to theexcitation transfer reaction (14).

Pumping the n= 3 atomic level with the pulsed dyelaser (X, = 6563 A) we observed at the same timean increase of the induced fluorescence lightstarting from the e' Z'„(v = 2, J= 5) state of

H, (X = 6568 Ac X,). Result of the time resolvedtransfer experiment is shown on Fig. 5 in relativeintensity scale, and exhibits clearly the efficiencyof the quenching of H atomic states by metastablemolecular states, since a great number of reac-tions similar to the one studied are allowed. Therelaxation times of the atomic and molecularstates deduced from Fig. 5 are of the same orderof magnitude, which indicates that the two levelpopulations are strongly coupled through col-lisions.

V. CONCLUSION

One thus can ascribe the quenching of atomicH states in a glow discharge to collisions with H,molecules in ground and quasimetastable states.Thermally averaged cross sections of 1.5 & 10""cm' indicate the great efficiency of the mechan-isms which strongly connect atomic and molecularstates. This coupling through atom-molecule ex-citation transfers may be for the main part re-sponsible for the anomalies observed in the mea-surements of molecular rotational temperaturesarid atomic excitation temperatures in H, glow

1102 A. CATH E RINOT, 8. DUBR E UI I. , AND M. CAN. D

discharges. The transfer process tends to de-populate atomic excited states for the benefit ofexcited molecular states, leading to an apparentincrease of H, rotational temperature and an

apparent decrease of H excitation energy. Thisis precisely the case of the discharge under studywhere T" -450'K, T, -320'K and T,„,-0.1 eV, E, -5 eV,

H. S. W. Massey and E. H. S. Burhop, Electronic andionic impact phenomena (Clarendon, Oxford, 1952).

A. A. Kruithof and L. S. Ornstein, Physica (Utr, ) 2,611 (1935).

3H. G. Poole, Proc. R. Soc. A 163, 404 (1937); 163, 415(1937); 163, 424 (1937).

4A. Catherinot, B.Dubreuil, A. Bouchoule, and P. Davy,Phys. Lett. A 56, 469 {1976).

~B. Dubreuil and A. Catheririot, Physica (Utr. ) C 93,408 (1978).

68. Dubreuil and A. Catherinot, J. Phys. D 11, 1043(1978).

Three-body collisions and associative ionization havebe neglected due to their rel.atively low rates.L. C. Johnson, Astrophys. J 174, 227 (1972).G. Himmel and F. Pinnekamp, J. Phys. B 10, 14570.977).B.Dubreuil and A. Catherinot, Laser perturbationstudy of a hydrogen glow discharge: Atomic and molec-

ular relaxations and di sociation rate J phys {1978)(to be published).

' W. L. Wiese, M. W. Smith, and B.M. Glennon, Nat.Stand. Ref. Data Ser. Nat. Bur. Stand. I, 2 (1966).J.W. L. Lewis and W. D .Williams, J.Quart. Spect-rosc. Badiat. Transfer 16, 939 (1976).

~3G. H. Dieke, The hydhoI, en molecule uavelength tablesof G. H. Dieke, edited by H. M. Crosswhite (Wiley-Interscience, New York, 1972).

4T. E. Sharp, At. Data 2, 119 (1971), and referencestherein.

~5R. J, Spindler Jr., J. Quant. Spectrosc. Radiat. Trans-fer 9, 597 (1969); 9, 627 (1969); 9, 1041 (1969).

' Bea11y only the states c3 ~„{v=0, J) are metastable.But due to the relatively long lifetime of the cs ~„(v &1, J) states and the quasiresonant collisional cou-pling with the a3 Z~ states in the discharge, the cs ~„and a Z~ group of states are highly populated.