EGAS41 - Swansea University

41 st EGAS CP 47 Gdańsk 2009
Spin-exchange frequency shift in the system triplet metastable
helium atom-ground state lithium atom.
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 well known that, apart from polarization transfer between colliding particles, spinexchange
collisions lead to a magnetic resonance frequency shift [1]. A frequency shift
occurs during collisions between atomic particles, as well as during collisions of atomic
particles with electrons. Magnetic resonance frequency shifts associated with spin exchange
produce an appreciable negative effect (in particular, in operation of quantum
electronics devices based on the optical orientation of atoms). If a spin-exchange process
is accompanied by an inelastic process, the magnetic resonance frequency shift noticeably
changes [2]. Following [2], we write the expression for the cross section of the frequency
shift in the spin-exchange process in the form
∞
σ sh = π/k 2 ∑
(2l + 1) exp(−2λ d l ) sin2(χd l − ηq l ) (1)
l=0
This expression differs significantly from the expression for the frequency shift in the
case of a ”pure” spin exchange. In our case, expression (1) contains the factor exp(−2λ d l )
associated with the ionization process. Here, η d l = χ d l + iλ d l is the scattering phase in
the doublet term and is the scattering phase in the quartet term. The presence of the
imaginary part in indicates that the doublet term may contain ionization (i.e., a decrease
in the number of particles) apart from scattering. It can be seen from formula (1) that
with increasing ionization probability (i.e., for λ d l → 0), the magnetic resonance frequency
shift tends to zero. Thus, taking into account expression (1) for the cross section of the
shift, the magnetic resonance frequency shift can be written in the form
∞
δω i = 2/3N i < S i > z π/k 2 ∑
(2l + 1) exp(−2λ d l ) sin2(χd l − ηq l ) (2)
l=0
As a result of the spin-exchange and chemiionization processes in the system formed by
a metastable helium atom and a lithium atom in the ground state, not only polarization is
transferred between the partners, but a magnetic resonance frequency shift is observed due
to the spin-exchange process. It was established that the cross section is a negative value.
A negative value of the cross section indicates that the magnetic resonance frequency is
shifted towards lower values (relative to the exact value). With increasing temperature in
the absorption chamber, the resonance frequency is shifted toward higher values (relative
to the exact value). Using the values of cross sections obtained here, we can estimate the
magnetic resonance frequency shifts due to spin exchange accompanied by chemiionization
both for metastable helium atoms and for lithium atoms in the ground state.
References
[1] W. Happer, Rev. Mod. Phys., 44, 169 (1972)
[2] V.A. Kartoshkin, Zh. Tech. Phys., 52, 123 (2007)
107