Nonimpact theory of four-wave mixing and intracollisional dynamics

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Abstract
A unified theory of coherent and incoherent resonance scattering of radiation in gases is presented. Expressions for the scattering rates in collisional redistribution and four-wave mixing (FWM) are obtained, starting from a microscopic formulation based on the Liouville–von Neumann equation. The derivation reemphasizes the point that coherent scattering depends on a product of scattering amplitudes at two separate points in the gas. The theory is extended to the nonimpact domain in which frequency detunings exceed the inverse duration of a single collision, taking into account intracollisional effects. Expressions are given in the so-called quasistatic limit (of semiclassical Franck-Condon transitions) for situations in which one or more radiative transitions occur during the collision. An extension of double-sided diagrams to collision-induced processes is introduced, and is used to illustrate the various phenomena described. Applications are suggested to the study of molecular dynamics, making use of the spectral features of FWM outside the impact domain. Investigations of long-range correlation effects, involving laser fluctuations, or cooperative excitations in dense fluids (utilizing the two-point nature of FWM), are also discussed.