Abstract
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 Z2/Z1to 0.

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