Theory of momentum changes in atomic translation due to irradiation with resonant light

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Abstract
A theory of the changes in momenta suffered by the translational motion of a two-level atom due to interaction near resonance within uniform light, is presented. The leading changes in the translational energy are first derived and the changes in the momenta due to the irradiation are then deduced from these in a simple and transparent manner. The derivation depends on the inclusion of a quantum interaction term whose classical origin is the Rontgen current in the electric dipole approximation. Thus the theory presented differs considerably from conventional approaches which are normally based on considerations of the transition rates and do not include the effects of the Rontgen term. The momentum changes obtained can be expressed in terms of the susceptibility and its first derivative, and are shown to be independent of the state of the atomic translation. They depend, mainly in sign, on whether the atom is in its ground or excited state. These features lead to a simple derivation of the average momentum changes induced by the irradiation of an atomic system.