Introduction

The quantum theory of molecular collisions has been extensively developed over the last three decades. As in many branches of theoretical science, the growth of this subject has been closely linked with the advances in computer technology. Powerful numerical techniques have been developed for solving Schrödinger's equation, which are well adapted to low energy, molecular collision problems, at various levels of approximation. A basic reference text in this context is Atomic, Molecular and Optical Physics Handbook [43]. The complexity of the problems that can be tackled, and the accuracy of the results that can be obtained, continue to be determined by the available computing power.

Any proper discussion of molecular collision processes involves the concept of the potential energy curve or surface. This concept drives from the Born—Oppenheimer approximation, to which we first turn.

The Born—Oppenheimer approximation

For the sake of simplicity, when discussing the basic concepts, we consider the collision between a one-electron atom, A, and a fully-stripped ion, B. The theory which pertains to this illustrative case can be generalized to collisions between many-electron atoms or to collisions between molecules.

When studying a collision problem, we are interested in the relative motions of the particles involved, and not in the motion of the centre of mass (barycentre) of the colliding system.