EGAS41 - Swansea University

41 st EGAS CP 45 Gdańsk 2009
Spin-exchange cross sections at the interaction between ground
state sodium and metastable helium atoms
S.P. Dmitriev, N.A. Dovator, V.A. Kartoshkin ∗
A.F.Ioffe Physico-Technical Institute, Russian Academy of Sciences, Polytechnical str. 26,
194021 St. Petersburg, Russia
∗ Corresponding author: victor.kart@mail.ioffe.ru
At the interaction between spin-polarized excited atom and ground state alkaline metal
atom in gas discharge elastic and inelastic processes take place simultaneously. In such
a case these two processes influence on each other giving rise to a change of the cross
section’s value for the elastic process. It means, that besides the chemiionization of the
ground state atom at the expense of the atom’s excitation energy (inelastic process), an
exchange of electrons was shown to be possible without a great depolarization (s.c. spin
exchange, or elastic process) [1].
Up to the present there were only two experimental works where the spin-exchange and
chemiionization cross sections were measured. The experiment have been done for He ∗ −
Cs and He ∗ −Rb systems [1,2]. In order to determine interesting us cross sections we have
to separate two simultaneously occurring spin-dependent processes. In the experiment
on optical polarization of the helium metastable atoms these atoms may be aligned or
oriented along a static magnetic field. It can be shown that the rates of distruction
of the orientation < S He > z and alignment < Q He > zz of metastable atoms depend on
chemiionization and spin-exchange processes as follows:
1/τ or = πδf or = N(1/3C ci + 1/2C se ), (1)
1/τ al = πδf al = N(1/3C ci + 3/2C se ), (2)
here N is the alkali metal atom concentration, C ci and C se are the chemiionization and
spin-exchange rate constants, δf or and δf al are the widths of the orientation and alignment
signals,1/τ i is the rate of the decay of the metastable atom’s orientation or alignment.
As one can see from Eqs., the contribution to the width of the magnetic resonance line
for aligned helium atoms should be different from that of oriented atoms. This difference
makes it possible to determine the rate constants of the two simultaneously occurring
processes.
In this work the experiment on optical orientation of atoms has been done for He*-Na
system. It was established that the rate constant for spin exchange (C se ) in collision
of metastable 2 3 S 1 helium atom with a Na atom in 3 2 S 1/2 ground state equals (23 ±
11)10 −10 cm 3 s −1 . The rate constant for chemiionization of rubidium atoms by metastable
helium atoms (C ci ) was determined at the same time to be (29 ± 14)10 −10 cm 3 s −1 .
References
[1] S.P. Dmitriev, N.A. Dovator, and V.A. Kartoshkin, JETP Lett.,66, 151 (1997)
[2] S.P. Dmitriev, N.A. Dovator, and V.A. Kartoshkin, JTP Lett.,634, 30 (2008)
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