Impact-parameter formulation of the impulse approximation for charge exchange

An impulse approximation to the charge-transfer amplitude in an ion-atom collision is derived. When a straight-line trajectory is used for the nuclear motion the impact-parameter method provides the same charge-transfer cross section as the plane-wave treatment of nuclear motion. The impact-parameter formulation allows a simply physical interpretation to be given to various peaking approximations that have been used to simplify the impulse approximation. The full impulse approximation provides the same high-velocity limit as the second Born approximation but an additional peaking approximation only provides the correct limit when Z 2 /Z 1 to 0.