Stochastic theory of intramolecular energy transfer in the presence of radiation

A theory for internal energy redistribution in polyatomic molecules perturbed by strong radiation fields is constructed. Use is made of stochastic theory, which assumes that a random phase approximation is valid after appropriate time intervals. This approximation permits the Schro;die;dinger equation to be replaced with a finite-difference master equation for the probabilities of occupying the various quantum levels. The semiclassical theory of the radiation-molecule interaction is employed in this work. A laser line profile is incorporated in the formalism, and this is used to simulate the effects of rotational states and collisions. At each stochastic step the energy changes in the molecules due to radiation are estimated. Thus, energy conservation is explicitly taken into account. Model calculations for SO ₂, whose transition dipole moment matrix elements and anharmonic force constants have been determined, indicate a complex interplay of anharmonic and radiative coupling. Power density, laser linewidth, and detuning from resonance are observed to play a significant role in energy redistribution.