Geoscience Reference
In-Depth Information
beam source and the gas cell is enriched, in the latter case, by the fast particles
as compared to the single beam. The maximum ยข(
E
), along with the threshold
behavior of the cross sections at
E
0.1 eV, was found by Wang et al. (
1985
). It
is consistent with the model (Devdariani
1979
) where the maximum dependence
(
E
) in the region 0.005
E
0.10 eV is comparable in magnitude with the cross
section at 0.3 eV. From this it follows that the process for sodium atoms AI (
1.60
)
is a multichannel one.
The cross section of Na
3
2
P
C
Na
3
2
P
!
Na
2
C
e
for the temperature
range 0.75 mK to 650 K was calculated in the semiclassical approximation by
Geltman (
1988
). This cross section of the order of magnitude coincided with the
measurement result at the point
T
D
0.75 mK (see, for instance, Huennekens and
Gallagher
1983
). However, it was by two to three orders of magnitude greater than
the experimental values in the thermal collision energy range.
The energy dependence of the cross section was measured in an experiment with
two intersecting beams for 3
2
P (Wang et al.
1987
)and3
2
P
3/2
(Kircz et al.
1982
)
states of sodium. This dependence has a minimum at
E
D
0.15 eV in the range of
0.08-0.26 eV collision energy and increases with increasing energy.
A joint analysis of the experimental data obtained in the gas cell, single, and
crossing beams conducted by Bezuglov et al. (
1987
) reduces to determining the
dependence of reaction rate on the relative velocity (energy) of the colliding
particles, which is equivalent to obtaining the temperature dependence of the rate
constant. It turns out that this approach is helpful in the case of large values of the
reaction energy threshold
U
, and at lower values of
U
, where the exponential
factor e
U
/
KT
is not dominating in the magnitude of the rate constant.
At the same time, simultaneous estimation of the autoionization width (R)
carried out in the framework of the AI model with two resonantly excited alkali
metal atoms (Devdariani
1979
) is possible in the field of
R
R
c
,where
R
c
is the
intersection point of the ion and the covalent potential curves.
Graphic solution of systems of nonlinear equations for the AI rate constants
in the cases of the gas cell Na(3
2
P
3/2,1/2
), single effusion beam Na(3
2
P
3/2
), and
two intersecting beams of Na(3
2
P
3/2
) allowed estimation of the possible values of
the process parameters
U
0.05 eV and
8
10
12
s
1
for two 3
2
P
3/2
sodium
atoms.
Kircz et al. (
1982
) found that the yield of the molecular ions during the excitation
of 3
2
P
3/2
atom Na states increased by 1.7 times at changing the light polarization
from perpendicular to parallel with respect to the beam direction. Blange et al.
(
1997
) showed that the contribution of the 3
2
P
1/2
state of sodium in the ionization
depends on the polarization of the exciting radiation, and the initial spin orientation
does not affect the yield of the molecular ions at the electron beam energy of
25 MeV. The differences of the results of experiments performed for the atomic
beam and the gas-filled cells (Huennekens and Gallagher
1983
; De Iong and van
der Valk
1979
) may be related to the polarization effects.
In the first case the polarization vector was perpendicular to the beam direction,
and in the second one the resulting polarization effect was insignificant. At the
