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QUENCHING OF n = 3, 4 SINGLET HeI EXCITED STATES BY MOLECULAR NITROGEN IN A
LOW-PRESSURE GLOW DISCHARGE
A. Catherinot, B. Dubreuil
To cite this version:
A. Catherinot, B. Dubreuil. QUENCHING OF n = 3, 4 SINGLET HeI EXCITED STATES BY
MOLECULAR NITROGEN IN A LOW-PRESSURE GLOW DISCHARGE. Journal de Physique
Colloques, 1979, 40 (C7), pp.C7-85-C7-86. �10.1051/jphyscol:1979742�. �jpa-00219421�
JOURNAL DE PHYSIQUE CoZZoque C7, suppZQment au n07, Tome 40, ~ u i Z Z e t 1979, Page C7- 85
QUENCHING OF n = 3,4 SWGLET He1 EXCITED STATES BY MOLECULAR NITROGEN I N A LOW-PRESSURE GLOW DISCHARGE
A. Catherinot and B. Dubreuil.
Groupe de Recherche sur ZrEnergQtique des Milieux Ionise's, Universite' drOrZQans 45045 OrZ6ans CQdex France.
1 - INTRODUCTION
rescence light emitted by a cross-section of the positive column is observed in a perpendicular direc . -Electrical pumping can create rare gases excited
tion and is imaged onto the slits of a spectrometer electronic states with a good efficiency and trans-
(resolving power - . s 100.000) and then onto a photo- fer of this electronic energy to atomic and molecu-
multiplier tube. Time dependence of the output si- lar radiators is a promising way to developp a wide
gnal is analysed by a Boxcar averager (temporal reso range of laser sources in the visible and near ultra
lution % 5 ns) and each fluorescence relaxation cur- violet. Observation of intense laser emission in the 4
ve corresponds to 1.510 laser shoot average.
N, first negative system at 4278 (2) in (He, N,)
L
gaseous mixture pumped by the reaction :
L
111 - MEASUREMENTS
+ 2 +
( I ) He; + N2 + N 2 (B CU ) + 2 He, The population variation ANi(t) of the li> He1 state
1 1 consecutive of laser optical pumping of the 2 S-3 P, has stimulated numerous studies on excitation trans-
2'~-4'p, 2'~-3'~, 2lp-3'~, ZIP-4's and 2Ip-4'~ ra- fers from excited helium to N2 molecules. Measure-
diative transitions have been studied for fixed va- ments are generally performed in afterglow and con-
cern only reactions involving "long life-time" He1 states (metastable and ionized atomic and molecular states (2, 3, 4, 5). On the other hand, an efficient process for the production of ~ e ; ions is associa- . tive ionization of "short life-time" (radiative) He1 excited states. Consequently these states may play an important part in laser emission in the N2 first negative system.
In this paper, we present some preliminary experi- mental results concerning collisional destruction of the n = 3, 4 singlet He1 excited states by mole- cular N2 in a low-pressure glow discharge.
11 - EXPERIMENT
( 6 , 7, 8 )A tunable dye laser excited by a pulsed nitrogen
0
laser (pulse width 4 ns, spectral width 0,2 A, ener- gy per pulse
*
10 uJ, repetition rate 15 Hz) is used to induced a selective and short perturbation on an helium excited state by resonant optical pumping.lues of the partial pressure P and various values He
of partial pressure P
.
For each pumped transition N2(J+i), ANi(t) is deduced from measurements of the time variations of the spectrally integrated reso- nance fluorescence light intensity. After the laser pulse has ceased, the perturbated populations go back to their,stationnary values. In this laser-free relaxation mode AN.(t) is solution of the population rate equation : (91
dANi dt (t) = a.. 11 ~ii~(t) +
Jgi
aiJAN^(^)
ANi(t=O> = ANiO
,
J = 1,...,
nn is 'the number of sublevels coupled by collisional or radiative excitation transfers with li>. aii is the quenching coefficient of the
1
i> level (total depopulating coefficient) whereas aiJ is the excita- tion transfer coefficient fromI
J > to li, (popula- ting coefficient). However, one may be note that action of aiJ coefficients on ANi(t) arise only af- ter two successive reactions and so in the first Excited states are populated in a capillary glowphase of the relaxation and for a judicious choice discharge (inner diameter 4 m y length 60 mm). This
of PHe, we can write in a first approximation : discharge is created under continuous electrical
power supply with a constant flow of (He, N ) ga-
2 dt = aii
AN^(^)
seous mixture (flow rate < 1 llh). Partial pres-
where : a. 11
.
= -(Ai+nHe Ge+nN2 R~ N2 ) (2) sures P and P can be independently adjustedHe N2 i
from 10-3 to Torr and current intensity from 10mA A. is the total radiative de-excitation rate,
1 2 R ~ e
to 40 mA. G~~ temperature is measured by means of a and RL are the total de-excitation rates by colli- N2
therdocouple in contact with the discharge tube sions of the li> state respectively with helium (Tg
*
325 5 K). After spatial filtering, the pump atoms (number density % e ) and with nitrogen mole- laser beam traverses the discharge tube. The flue- cules (number density nN ) .2
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979742
For each state li> (li> =
IS,
3 ' ~ ,ID,
4'5, 4lp, 4 D) we have measured the aii coefficient as a func I tion of P for a fixed value of PHe. Results are shown on Flg. N? 1 and Fig. 2.Fig. 1
Fig. 2
RESULTS AND DISCUSSION
All measured destruction coefficients aii depend likearly on nitrogen partial pressure P in the
W2
nitrogen and helium partial pressure ranges we have studied. Quenching rates RN i are deduced from rela-
2
tion (2) and are reported below with corresponding
Q, i
velocity average cross-sections a N2'
level P
Hemrr %A
G2
cm3/s$k2
cm23Is 0.86 30 1.15 10-9 8.2 10-15 3 l ~ 1.00 35 4.4 10-9 3.2
lo-14
3 ID 0.83 30 5.7 10-9 4.1 10-14 4Is 0.83 30 5.7 10-9 4.110-~4 4 ' ~ 0.53 35 3.4 10-9 2.5 10-14 4 ' ~ 0.51 35 1.35 10-8 9.6 10-14 The extrapolation values of the aii at zero nitro- gen partial pressure are in excellent agreement with results obtained in a pure He1 glow discharge
( 9 , 1 5 , 1 1 ) .
On the other hand, we observe that the
4''
quen-2 ching rate is clearly lower than the other ones, perhaps due to non resonant excitation transfers towards N molecule, or to the low associative
2 1
ionization probability of this 3 S state ( 9 , 1 2 1 , in connection with the reaction (1).
Previous studies ( 1 0 ) of a pure helium glow-dis- charge have shown that for pressure greater than
1 1
1 Torr, 4 D and 4 F sublevels are in collisional equilibrium. Present results performed for
1 1
PHe = 1,16 Torr, when the 2 P-4 D transition is optically pumped (Fig. 2), indicate that the 4'F quenching rate is of the same order of magnitude than the 4 D one. 1
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