Green Function Theory of Resonance Light-Pulse Scattering. I: Simple Application to Molecular Spectroscopy

Abstract

A thermal Green function theory of resonance long-laser-pulse scattering is developed, correctly taking account of the uncertainty-relation between the time-resolved and the energy-resolved experiments and the effect of the sample surface. The transient scattering efficiency can be calculated by folding the retarded Green's functions which are given by the thermal average of the chronological products of the pulse-current (or -density) and current operators into an uncertainty-principle broadening function. The theory is applied to resonance light scattering from a simple molecule in gas for steady and transient cases, where all the collision processes are treated quantum-statistically. At low pressure, the resonance scattering is mainly composed of Raman scattering (RS), final-state redistribution scattering (FRS), final-state transfer scattering (FTS) and collision-induced redistribution scattering (CIRS). In the steady state, it is shown that ordinary resonance Raman scattering is equivalent to RS in collisionless case, that absorption followed by emission contains RS, FRS and FTS, and that absorption followed by redistribution followed by emission is the same phenomenon as CIRS which is weighted by rotational population of excited molecules. Moreover, recent laser-pulse data by Roussea, Patterson and Williams are satisfactorily explained. Connection between this theory and other work is discussed.