EGAS41 - Swansea University

41 st EGAS CP 46 Gdańsk 2009
Chemiionization and spin exchange in collisions of triplet
metastable atoms in a mixture of helium isotopes
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
It is known that, in a collision of two triplet metastable atoms of helium a quasimolecule,
He ∗ 2 is formed that breaks down via the reaction:
He ∗ (2 3 S 1 ) + He ∗ (2 3 S 1 ) → He ∗ 2 → ions (1)
The quasi-molecule formed in the collision can be in one of three states with a total
spin S = 0, 1, and 2. These spin values correspond to three molecular states 1 Σ + , 3 Σ +
and 5 Σ + , which can be described by the singlet (V s ), triplet (V t ), and quintet (V q )
molecular terms. On conservation of the total spin of a system, ionization is only possible
from the states of low multiplicity, i.e., from the singlet (cross section σ s ) and triplet
(cross section σ t ) states, since in this case, the total spin is the same at the exit and
entrance of reaction. At the same time, in collision of triplet metastable atoms of helium
with atoms possessing electronic paramagnetism, process (1) is accompanied by the spinexchange
process, i.e., the colliding atoms exchange valence electrons. Such processes
accompanied by chemiionization, in particular, the interaction of triplet metastable atoms
of helium with alkali-metal atoms in the ground state, were studied both theoretically
and experimentally. Indeed, the following process occurs in the collision of two triplet
metastable atoms of helium:
He ∗ (2 3 S 1 , m 1 ) + He ∗ (2 3 S 1 , m 2 ) → He ∗ 2 → He∗ (2 3 S 1 , m ′ 1 ) + He∗ (2 3 S 1 , m ′ 2 ) (2)
Spin-exchange process can be described by three cross sections. σ 1 is governed by
interference of elastic scattering from the triplet and quintet terms, σ 2 , from the singlet
and quintet terms, and , σ 3 from all the three terms.In collision of different isotopes of
helium ( 3 He and 4 He) or in collision of two 3 He atoms, all three cross sections present
in can be observed. In the collision of two even helium isotopes ( 4 He and 4 He), only the
spin-exchange cross section σ 2 can be observed in an experiment. In addition, the identity
of colliding particles is important only at very low temperatures, and it can be taken into
account by introducing a factor of (−1) l into formulas for the cross sections. Thus, we
can introduce the cross section σ abs and σ tr which, characterize the decay of orientation
and alignment of colliding particles as a result of chemiionization and spin exchange in
the optical orientation experiment as follows:
σ tr
or = 1/3σ tr
1 + 1/3σ tr
2 + 2/3σ tr
3 (3)
σ abs
or
= 1/9σ abs
s
+ 1/3σ abs
t (4)
σ tr
al = σtr 1 + σtr 2 (5)
Cross sections of spin exchange and chemiionization in collisions of two triplet metastable
helium atoms have been calculated for different isotopes of helium. The data on the spin
exchange cross sections for the systems studied are reported for the first time.
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