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Fig. 1.5 The near-threshold
AI cross-section behavior of
the N. 2 D/
O. 3 P/
e reaction. The
circles show the dependence
of the experimental cross
section on the collision
energy (Ringer and Gentry
1979 ), the broken curve gives
the theoretical results
obtained in (Nielsen and
Dahler 1979 ), and the full
curve presents the result
obtained from ( 1.35 )
the intersection point of the terms (Lee 1977 ). A comparison with the experiment of
Ringer and Gentry ( 1979 ) shows that satisfactory agreement is observed only near
the threshold (at E 0.2 eV). It should be noted that the data used in the calculation
are not accurate enough, but the possibilities of experiments to measure the absolute
reaction cross sections for associative ionization are limited (for example, the
reaction systematic error is estimated as 35% (Ringer and Gentry 1979 )). However,
the experiment detects the “fine” structure of the cross section resulting from the
contribution of the Rydberg states. The first step is near the reaction threshold;
it corresponds to the contribution of the autoionizing states with n D 7and v D 1.
In addition, the linear approximation of the cross section near the threshold is fair
here in the broad range of temperatures from 4,500 to 7,500 K ı with accuracy
up to 15%. In particular, at the temperature T D 4,500 K ı formula ( 1.37 )gives
k AI 3:6 10 13 cm 3 =s. Obtained at the same temperature dependence is in good
agreement with the experiment.
Thus, the theory of MQD allows you to set the near-threshold dependence of the
cross sections and reaction rate constants of associative ionization of atoms with the
quantum effects and the contribution of the Rydberg states of the intermediate auto-
decay complex XY ** for endothermic (weakly exothermic) processes. In this case,
the threshold behavior of cross sections of the endothermic reaction strictly follows
the linear law AI E , in contrast to the predictions of the semiclassical theory
(Nielsen and Dahler 1979 ). It also demonstrates the emergence of some important
features of the “thin” sections of the structure (steps from the contribution of the
Rydberg resonances) and the conditions of their manifestation in the temperature
dependences of the rate constants.
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