Atom-atom and atom-ion collisions: I. Collisions between neutral atoms in which no excitation occurs

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Abstract
The wavelengths associated with the motion of atoms and molecules at ordinary temperatures are short compared with the effective range of interaction between them. Nevertheless, interference effects may be observed. Their detailed study, which is now possible because of developments in experimental technique, provides much information about the details of the inter-atomic forces. Although the effects arise because of the wave aspect they may be discussed to a very great extent in terms of a semi-classical approximation, in which the phases as well as the amplitudes of the waves concerned can be derived with very good accuracy from classical mechanics. The first two articles are concerned with interference effects associated with collisions in which no change of internal energy of the colliding system occurs. The present article will deal with collisions between neutral atoms. An account is given of the semi-classical theory and of the rainbow and glory interference effects, analogous to the corresponding meteorological phenomena. This is followed by a description of the experimental methods used to observe the effects and of some of the results which have been obtained. The second article will deal with collisions between ions and atoms of the same species between which charge transfer may occur without change of internal energy.