Geoscience Reference
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which there are excited atoms and photons, although the fundamental difference
between multiphoton ionization of the collision step process takes place when the
characteristic lifetimes of the excited atom absorption and scattering are separated
in time. Moreover, in the case of “weak” light fields, nonlinearity in the external
radiation intensity process of the ionization by the collision of two pairs of excited
particles, for example, the resonant excited alkali metal atoms, is possible.
1.6.1
Binary Collisions of Alkali Atoms
The first series of quantitative measurements of the constants AI of alkali metal
atoms was carried out using the optical excitation of the medium in the regime of
a radiation transport, which allows obtaining a uniformly excited medium volume
of 100 cm
3
to be relatively easy (Bezuglov et al.
1994
). The photo-ionization of
excited atoms can be used as a diagnostic method in studies of chemo-ionization
processes. Recently, a method for measuring the ionization rate constants of the
optically excited atoms has been developed that can be used for measurement of
photo-ionization currents in relatively simple form. However, for atoms with a low
vapor pressure, for example, rare earth elements, such studies are hard to conduct
because it is difficult to obtain the vapor of these elements in the volume of the
reaction (Klyucharev
1993
). However, this difficulty can be eliminated by using
optical excitation of the atomic beam.
Previously it was thought that the role of collisions inside the beam is negligible.
However, as calculations show, the real atomic beam average relative velocity is
reduced by approximately three times compared to the gas-filled cell at the same
temperature (Bezuglov et al.
1987
).
For a qualitative analysis of the mechanism of AI atoms, it is necessary to know
the parameters of the terms of the molecular ions and quasi-molecules involved
in the entrance and exit channels of the reaction. For ions of alkali atoms, the
dissociation energy D
e
exceeds the dissociation energy of the molecules; therefore,
it is realized otherwise in the molecule and hydrogen molecular ions.
The AI processes from the first excited levels of the alkali metal atoms are so
specific that they should be, apparently, in a separate class of endothermic ion-
molecule reactions. These conditions are associated with dipole transitions from the
atom ground states and can be efficiently populated by the optical excitation, which
opens wide possibilities of experimental investigations of AI in binary collisions
of the resonantly excited atoms. The first quantitative results were obtained under
the optical excitation of a vapor-filled cell in the mode of the resonance radiation
transfer, which is formed by the equilibrium Maxwell distribution of energies of the
excited atoms at a controlled temperature
T
(Devdariani et al.
1978
).
In the next stage, the temperature dependence of the photo-ionization rate
constants of the reaction, occurring in collisions of resonantly excited atoms, is
found. It is determined by the activation energy and the AI cross section as a function
