DETERMINATION OF THE COEFFICIENTS OF COLLISIONAL TRANSFER BETWEEN THE 2p LEVELS OF NEON

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DETERMINATION OF THE COEFFICIENTS OF COLLISIONAL TRANSFER BETWEEN THE 2p

LEVELS OF NEON

N. van Schaik, L. Steenhuijsen, P. van Bommel, F.H.P. Verspaget

To cite this version:

N. van Schaik, L. Steenhuijsen, P. van Bommel, F.H.P. Verspaget. DETERMINATION OF THE

COEFFICIENTS OF COLLISIONAL TRANSFER BETWEEN THE 2p LEVELS OF NEON. Journal

de Physique Colloques, 1979, 40 (C7), pp.C7-97-C7-98. �10.1051/jphyscol:1979748�. �jpa-00219454�

JOURNAL DE PHYSIQUE CoZZoque C7, suppzdment au n07, Tome 40,. JuiZZet 1979, maeC7- 97

DETERMINATlON OF THE COEFFICIENTS OF COLLISIONAL TRANSFER BETWEEN THE 2p LEVELS OF NEON

N.

Van Schaik,

L.W.G.

Steenhuijsen,

P.J.M.

Van Bommel, F.H.P. Verspaget.

Eindhoven University of !TechnoZogy, Eindhoven, The Netherlands.

Introduction. Transfer reactions between the various 2p levels are caused by collisions of 2p atoms with ground state atoms. The number of trans- fer reactions from level 2p. to level 2p in unit

j time interval is:

Here n and pi are the densities of the levels 2pi g

(i= 1....10) and the ground state level respect- ively; K. is the coefficient of collisional trans-

.j

fer between the levels pi and p j '

Measurements on collisional transfer reactions in neon have been reported by Grandin I), Coolen 2) and Smits 3'. Except Coolen they have made use of a continuous discharge. Coolen used a plasma which was generated by a proton beam. We have measured the coupling coefficients K in the afterglow by

i j

means of selective excitation of the 2p levels from the Is levels by irradiation with a dye laser beam.

Our plasma is characterised by a) a low electron temperature, b) the population of the 2p levels is produced by dissociative recombination (no excit- ation from the Is levels), c) the energy of the 2p atoms is quite large (% 1 eV), because the dis- sociative recombination is a radiationless process 4s5'6); the high atomic energy may have an in- fluence on the atomic coupling directly after the recombination.

It is clear that measurements of atomic coupling in 'an afterglow have the advantage of the absence of electron excitation. Irradiation of the plasma with a laser beam does not influence the studied process.

We performed our measurements in the afterglow period of a neon discharge (d = 31 mm) at gas pressures of 5,20,50 and 100 torr; the discharge current was 22 mA.

~alkulation of the transfer coefficients. The decay rate of a 2p. level in the undisturbed afterglow

(no laser irradiation) may be described as 2,3):

p. is the density of the 2pi level; is the partial recombination coefficient; n is the elec- tron density;A% is the total transistion probability for the 2p. level; N is the neutral. ?tom density,

g

Kij is the coefficient for collisional transfer of excitation from the 2p. to the 2p level; for mathe-- i

1 j

matical convenience we define K = I for all j=l,..](

j j

The right hand terms of ( 2 ) represent respectively the dissociative recombination, the spontaneous ra- diation, the collisional transfer from level 2pi t~

level 2p and the collisional transfer of 2p to 2p

j j i'

We define p: as the density of level 2pi in ease level 2pk is populated extra by direct excitation of

1s atoms due to laser irradiation. The laser

irradiation has no influence on the dissociative re- combination which is the populating process for the 2p levels. So in the disturbed afterglow the decay of level 2pi can be described as

k 10 10

a p i = a n~

- -

ⁱ K.. n pk + &

-

^{K.. n k}

at 2i e j=I 13 g i _{j P 1 J I g} Pj

From our measurements we know that

1- -

aei pi at and

1

^ap? << A:. This means in equation (2) and (3) the

~ 7 f

left terms can be taken as zero. We define hek =

,,

k i

p.-p.(i,k= ]...lo); then subtraction of (2) from (3) 1 1

and subsequent dividing by Ap gives: k k

When is the intensity of the 2p -Is transition

j 1 j 2

in case level 2pk is irradiated and Ijl is the in- tensity of the same transition when there is no laser irradiation then is our measured fluorescence signal given by Ijl-Ijli Prom these signals we car k determine the factor Ap./Ap: as follows:

3

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979748

with Ijl = njlAjlPj, with

n

is the detection j 1

efficiency for the transition 2pj + Isl.

Relation 4 represents a system of 100 linear equations,in which the K. are the unknown. This

lj

system has been solved with the B7700 digital com- puter of EUT.

Because we did not have at our disposal a laser wave- length above 660 nm, we could not excite a Is level to the 2p10 level. However the energy gap between the 2p10 and 2pg level is so large that collisional transfer from 2p to 2pg (or a higher 2p level) may be neglected. So we presume K = 0 for j = 2.. .9;

10 10

for the same reason we take (Api0)/ApI0) = 0.

j

Experimental realisation. The experimental set up is described in detail in lit. 7). We have measured the fluorescence radiation A I ~ _jl. during 3 time-intervils

,

with 10 us length each; these intervals were centered at t = 40, 60 and 80 us. Every 2p level

!except level 2piC) was populated extra by irradiat- ing the plasma with the dye laser beam. Every turn the complete fluorescence was detected (with except- ion of the lines h = 540 nm, 613 nm, 665 nm, 702 nm and 808 nm, which ore too weak). In this way we mea- sured the respective factors

apk

(j=I..

. .

^{10; k=l.. . 9 )}

For solution of (4) we need Ap '/Ap:. To be able to j

measure and

np:

simultaneously we have set up j

two detection systems, at both sides of the discharge tube. The monochromator of one detection system is adjusted to one of the spectrallines which originate from the excited 2p level (preferably notithe laser

k

wavelength). The other detection system measured the fluorescence spectrum. In this way all fluctuations in the laser beam power and/or wavelength can be

k k taken into account in Ap./Apk.

3

Experimental results. In table 1 the experimentally obtained values of Kij are given. The given values are found as the average of the results at gas pressures 5, 20, 50 and 100 torr all at a discharge current of 22 mA. In general a dependence of Kij on the gas pressure could not be determined within the accuracy boundaries. Table 2 gives the values for those coefficients in dependence of the pressure.

In table 1 we give also the influence of variations

k k

in the value of Ap. / apk on the values of K. The

J ~ j '

results are in good agreement with the results ob- tained by Snits 3, and Coolen 2)

.

References

1) J.P. Grandin et al., J. de Phys. 36, 787 (1975).

2) F.C.M. Coolen et al., Physica 93C, 131 (1978).

3) R.M.M. Smits, thesis EUT (1977) M. Prins et al, proc. ICPIG 13 11977)-

4) A. Rosers et al., Phys. Rev. 134, A1215 (1964).

5) T. Conner et al., Phys. Rev. 140, A778, (1965).

6) L. Fromhold et al., Phys. Rev. 185, 244 (1969).

7) L.W.G. Steenhuijsen, thesis EUT (1979).

1 I O U 5 0 2 0 5 torr