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energy (excitation energy) of the colliding particles, and the energy of relative
motion plays a minor role in energy balance. In other words, the energy of relative
motion of atoms on the characteristic values of the inelastic transition distances
in these reactions is much smaller than the ionization potential of the individual
atoms partners. In connection with the development of new physical and technical
trends in modern physics, such as the physics of active laser media, the interaction
of radiation with matter is laser synthesis of chemical substances. Interest in these
processes emerging in the past 60 years is markedly increased. However, available
literature on the processes of ionization in slow atom-atom collisions in general
relates to the reactions involving metastable atoms. The use of tunable dye lasers
makes possible to implement a resonant optical excitation of atoms in the vast
majority of elements, which markedly stimulated the experimental study of such
processes. In this case, from energy considerations it follows that the ionization
processes in slow symmetrical collisions are possible only if the pair collisions
of resonantly excited atoms take place. At the same time two possibilities can
be realized: (1) the doubled excitation energy is greater than the atom ionization
potential, and (2) this energy is lower than that potential. The first case (1)
takes place, for example, at the ionization of hydrogen and inert gases, halogens,
nitrogen, and oxygen. The second one (2) is the ionization of alkali atoms of rare
earth elements, uranium, most of the metal atoms, the ionization in collisions of
metastable atoms of heavy inert gases with hydrogen.
As a result of the ionization process, in slow symmetrical and asymmetrical
atom-atom collisions can be formed the following molecular and atomic ions:
X.X / C X ! X 2 C e
Symmetrical collision
(1.2)
X.X / C Y ! XY C C e
Asymmetrical collision
X X C X
! X C C X C e
Symmetrical collision
(1.3)
X X C X
! X C C X C e
Asymmetrical collision
or a pair of positive and negative ions
X X C X ! X C C X
Symmetrical collision
(1.4)
X X C Y ! X C C Y
Asymmetrical collision
We do not consider here the processes of dissociative ionization and the
ionization accompanied by a rearrangement of the particles, which can occur in
atom-molecule collisions. Reactions ( 1.2 , 1.3 ,and 1.4 ) differ by the process mech-
anisms. Reaction ( 1.2 ) is called associative ionization (AI). Note that reaction ( 1.3 )
in the cases where one partner is a metastable atom Y * with the excitation energy
exceeding the ionization potential of atom X is called the Penning ionization (PI).
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