Electron capture of the accidental resonance type

Abstract

Consideration is given to the characteristics of electron capture of the accidental resonance type exemplified by He 2+ + H(l s ) → He + (2 s or 2 p ) + H + . In contrast to what happens when the resonance is symmetrical, as in H + + H(l s ) → H(l s ) + H + , the cross-section is small at low velocities of relative motion and tends rapidly towards zero as the velocity is decreased, the behaviour being, in fact, almost adiabatic. This is of interest in several connexions: for instance, it is of interest in aeronomy where the processes H + + O( 3 P ) → H(I s ) + O + ( 4 S ) and O + ( 2 D ) + N 2 ( X 1 Σ + g v = 0) → O( 3 P ) + N + 2 ( A 2 II u v =1 have been assumed by some workers to occur rapidly in thermal encounters simply because the energy balance is almost exact; and again it is of interest in relation to the mobility of a molecular ion in its parent molecular gas.