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Fig. 1.19 Penning ionization
(PI) cross section in the pair
collisions of Rydberg
hydrogen atoms in the S , P ,
D ,and F states
Cross sections (Eqs. 1.68 , 1.69 , 1.70 ) for symmetrical collisions of two highly
excited atoms are calculated by the Monte Carlo method in McGeogh et al. ( 1988 ).
Experiments carried out in this work show that a good agreement between the
calculated and the experimental results is obtained at relative velocities of collisions
V c ¤ e ,where¤ e is the characteristic velocity of the Rydberg electron. The
calculated values at lower velocities of V c are greater than the experimental ones.
Written in atomic units, the cross section of the molecular ions (Eq. 1.69 )inthe
temperature range 10-10 3 K is equal to
¢.n;V c / D 0:703V c 0;65 n 3:35
The Penning ionization rate constants in the binary collisions of the Rydberg
hydrogen atoms, the alkali atoms, and the ions of strontium are calculated by
Bezuglov et al. ( 1995 ). The calculation is carried out using the model approach of
Katsuura-Smirnov (Galitskii et al. 1981 ) for the long-range dipole-dipole ionization
mechanism at large internuclear distances.
Autoionization widths of the intermediate quasi-molecular states are expressed
in terms of the photo-ionization cross section f ( nl ) of the Rydberg atomic nl
levels and the line strengths of the transitions between them. These quantities
are calculated in the semiclassical approximation taking into account the quantum
character of the Rydberg electron motion in the approximation of the straight line
motion of particles (see Figs. 1.19 and 1.20 ).
Features of the ionization cross section of the strontium ion were calculated in the
framework of the dipole-dipole ionization mechanism caused by the nonmonotonic
dependence of photo-ionization cross sections of the singly charged strontium ions
on n . In this case the dipole-dipole approximation of the atom-atom interaction
is best for the subthermal collisions because the interaction time increases at large
internuclear distances.
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