EXCITATION TRANSFER AND QUENCHING OF THE N = 3 EXCITED STATES OF HELIUM IN A LOW-PRESSURE GLOW DISCHARGE

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EXCITATION TRANSFER AND QUENCHING OF THE N = 3 EXCITED STATES OF HELIUM IN A

LOW-PRESSURE GLOW DISCHARGE

B. Dubreuil, A. Catherinot

To cite this version:

B. Dubreuil, A. Catherinot. EXCITATION TRANSFER AND QUENCHING OF THE N = 3 EX-

CITED STATES OF HELIUM IN A LOW-PRESSURE GLOW DISCHARGE. Journal de Physique

Colloques, 1979, 40 (C7), pp.C7-5-C7-6. �10.1051/jphyscol:1979702�. �jpa-00219061�

JOURNAL DE PHYSIQUE ColZoque C7, suppZGment au n07, Tome 40, JuiZZet ^1979, nape C7- 5

EXCITATION TRANSFER AND QUENCHING OF THE N = 3 EXCITED STATES ff HEtRIM IN A LOW-PRESSURE GLOW DISCHARGE

5. Dubreuil, A. Catherinot.

Groupe de Reckerckes sur ZrEnerg6tique des Milieux IonisBs UniversitS drOrZdans, 45045 GrZeans Cedex, France.

I - INTRODUCTION

After spatial filtering, the pump laser beam Excitation transfers between excited atomic

states play an important part in the formation of quasi-stationary atomic excited state populations in non-L.T.E. plasmas such as low pressure and low current glow discharges. Excitation transfers by inelastic atomic or molecular collisions are respon- sible for lasing action in a great number of gas lasers, or on the contrary are limiting factors to the population inversion process as for the 3 1 P-3'~

(95.8 p) transition observed in a helium gow dis- charge (1).

In this paper we report a study of the quenching

traverses the discharge tube. The fluorescence light emitted by a cross-section of the positive column is observed in a perpendicular direction and is imaged onto the slits of a spectrometer (resolving power

% 50000) and then onto a photomultiplier tube. Time dependence of the output signal is analyzed by a Boxcar averager synchronized with the pulsed laser giving on both channels A and B a time resolution of 5 ns. The fluorescence signal (channel A) is nor- malized to the pump laser intensity peak (channel B) Each fluorescence relaxation curve corresponds to

1.5 x 10 laser shoots average. 4

and excitation transfer mechanisms for the n = 3

helium sublevels in the positive column of a low-

1 I I - MEASUREMENTS

pressure glow discharge. The experimental method is

A diagram of the n = 3, 2 helium energy states based on a time resolved spectroscopic analysis of

is shown on Fig. I . The population the population relaxations following a short reso- a

E tcm-'I

nant laser pulse pumping.

11 - EXPERIMENT

(2), (313 ( 4 )

A tunable dye laser excited by a pulsed nitro-

0

gen laser (pulse width 4 ns, spectral width 0.2 A, energyfpulse % 10 pJ, repetition rate 15 Hz) is used to induce a selective and short perturbation on the population of an helium excited state by

.-

resonant optical pumping. 2 'P

The n = 3 states are populated in a capillary

I Fig. 1

glow discharge (inner diameter 4 mm, length 60 mm).

This discharge is created under continuous electri- cal power supply with a constant flow of helium gas

(flow rate < 1 l/h). Pressure P can be adjusted from 0.2 to 7 Torr and current intensity i from 10 to 40 mA. For each experimental situation (P, i), corresponding value of the electronic density n is measured by a high-frequency cavity perturbation method (5.10 9 < ne < 5.10" ) and value of the mean electronic kinetic energy Ee is only estimated in the frame of glow discharge theory

(3 < Em < 15 eV). Gas temperature is measured by

I

of the li> = 3 L S (S = 1 , 3 ; L = S, P, D) states induced by laser optical pumping of the transition<

drawn on fig. ₁ have been studied for various dis- charge conditions (P, i). For each pumped transition

ANi(t) are deduced from measurements of the time variations of the spectrally integrated resonance and sensitized fluorescence light intensities. After the laser pulse has ceased, the perturbated popula- tions go back to their stationary values. In this laser-free relaxation mode, ANi(t) are solutions of

5

means of a thermocouple in contact with the dis- the population rate equations : charge tube (T % 325

+

^{5 K).}

g

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979702

AN. (to) = AN;' i = I,

...,

ⁿ

n is the number of sublevels coupled by collisional or radiative excitation transfers. aii is the quen- ching coefficient of the

1

i> level (depopulating coefficient) whereas aij is the excitation transfer coefficient from level

1

j> to level l i ~ (populating coefficient). Generally, a.. writes : aij = 4ij

+

P i lJ P

Bij nHe + Bij ne (2) where

Bij

(resp. Bij ) is the rate of the excitation transfer reaction from level lj> to level ti> by collisions with ground state atoms (resp. electrons), and aij is the spon- taneous radiative coefficient. aii is a negative

n

term and satisfies the relation aii = -1 a -a. (3) k=l ki ei where a is a coefficient taking into account

ei

transfer of population of the ti> level. outside the n sublevels involved in eq. (I).

For the six different pumping experiments, and in the pressure range investigated, we observed the following transfers :

1 1 1

2 S-3 P pumped : 3 ' ~ f 3 D excitation transfer

~ ' P - ~ I D pumped : 3 ' ~

2

3 1 D excitation transfer 2 P-3's pumped I : no transfer

2 3 ~ - 3 3 ~ pumped : no transfer

3 3 3

2 P-3 D pumped : 3 3 ~ f 3 P excitation transfer

~ ~ p pumped - 3 ~: no transfer ~

Then the problem arises how to determine the quenching and excitation transfer coefficients a..

13 intervening in eq. (1) from the experimental rela- xation curves AN exp(t).

i

I V - DATA

ANALYSIS

AND

RESULTS ( 4 )

The relaxation matrix A = faij) was determined from

AN^^^^(^)

so as to minimize the difference bet- ween the experimental values and those calculated from the model. This method recently developped in numerical analysis and named "identification pro- blem" (5) is equivalent to that of finding the mini-

mum : inf J(A) = inf.'; I ~ ' I A N ~ ( ~ , A ) - & N ~ ~ ~ ~ ( ~ ) ~ ~ ~ ~ A 1=1 to

where AN.(t,A) is the value corresponding to

AN^^^^(^)

calculated from equation (1) with the matrix A.

This problem can be solved by an iterative algo- (PI

-

a(P-l.)-k(P-I)aJo rithm (gradient method) : aij

-

ij aa..

i, j = 1,

.,. . ^,

n. where p is the iteration number1' and k('-') is a convergence coefficient. The func- tional derivative is obtained from solution of the differential adjoint problem.

An example of the identification procedure is 1 I

shown on fig. 2 for the 3 P

2

3 D excitation trans- fer processes.

1 3

For the 3 S or 3 S state showing no excitation transfer, solution of eq.,(l) is given by

AN (unite arbitraire)

Fig. 2

identification

'- 109 ns

A N ~ ( ~ ) = A N ~ ~ ~ ~ ~ ~ ~ whose comparison with

AN.^^'(^)

leads to the quenching coefficient. Determination of the a. .'s for different %e and n values allows

3.3

us to obtain the cc.. and B.. coefficients from eq.

1 J 1-3

(2) and (3) by least square linear regression. In fact no current dependence was found so that quen- ching and excitation transfer can be attributed to the atomic collisional processes : He(3 L)+He S +

He(3 LV)+He+bE) to the associative ionization mecha S nism : He(3 L)+He S -+ H ~ ~ + + ~ - + A E and to spontaneous radiative transition : ~e(3'~)+He(n S L+l)+hv.

The radiative coefficients as well as the thermally averaged cross-sections 5 . . deduced from B are

1 J i j

reported below :

radiative coef. coll. quenchin cross- ( 108s-1) section

(f2)

excitation transfer cross-section (i2)

associative ionization cross-section

(i2)

ion ion

U 3 l S < 3.7 033S < 0 . 3

u 3 1 p : ion 1 + I Q33p ion < 5

U31D ion : 15 C- 4 a33D ^ion : 2 . 4 t 0.5

REZEREHCES

1 . J . S . LEVINE, A. JAVAN,Appl.Phys.Lett.14(1969)348 2. B. DUBREUIL, A. CATHERINOT,Physica Cg(1978) 408 3. A. CATHERTNOT, B. DUBREUIL, M. GAND, Phys. Rev.A

E,

(1978) 1097.

4. B. DUBREUIL, ThPse (Universitl OrlBans, 1979).

5. J. DELFORGE, Appl. Math. Comp.

2

(1976) 311.