A simple theory of nearly symmetric charge transfer

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Abstract
A simplified theory is developed of charge transfer between systems whose ionisation potentials are almost equal (within about 1 eV). The model is based on adiabatic molecular states and may be regarded as a generalisation of the well known Firsov theory of symmetric charge transfer. If v, a and Delta denote the relative velocity, size of collision region and energy defect (all in atomic units) three regimes can be distinguished: (i) Delta a<v<1 where the cross section is constant with superimposed oscillations of the form sin(2 beta /v); (ii) Delta <<v< Delta a where the cross sections fall off like exp(-A/v); (iii) at very low velocities the radiative mechanism predominates, the cross section having the form B/v ( beta , A, B are constants). Results are presented for colliding pairs made up of ground-state alkali-metal, excited mercury and metastable hydrogen atoms and their ions and shown to be in reasonable agreement with available measurements.