A symmetric impulse approximation for charge exchange

A symmetric form of the impulse approximation for charge exchange is developed. This approximation preserves the equivalence of post and prior forms of the transition amplitude. As in the usual impulse approximation the dynamics of charge transfer is seen primarily as the ionisation of a bound state. The usual impulse approximation considers ionisation of an electron from the bound state, whether initial or final, of the stronger potential. However, the symmetric form presented here includes contributions from the ionisation of an electron from both initial and final bound states.