Electronichyphen;translational energy transfer processes have minute cross sections for atoms with thermally attainable kinetic energies, unless the amount of energy converted from electronic to translational form (or vice versa if the threshold permits) is rather less than 1 ev. The transfer processes of interest to chemical kinetics, therefore, involve highly restricted sets of energy levels. Two situations are commonly encountered: (1) The ``twohyphen;state'' case, in which two energy levels of the diatomic system cross or approach closely in energy, so that a respectable probability of resonance transfer is achieved; cases of this type almost always involve transfer of most of the excitation from one atom to the other. (2) Transitions among the multiplet levels of a given Russellhyphen;Saunders term of an atom, induced by collision with an ``inert'' atom. Spinhyphen;orbit coupling is the electronic interaction splitting the levels. The present work is concerned with the latter case. The ``highhyphen;energy approximation,'' a necessary modification of the Born approximation, and various ``modified adiabatic approximations,'' which take account of the details of angular momentum coupling during collision, are described, and practical methods of calculation considered.
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