Abstract

We use a semlclassical, effective-state approach to laser pulse propagation in a gas of polyatomic molecules. Our numerical model of multiple-photon excitation includes quasicontinuum transitions to calculate the effects of coherent propagation on multiple- photon absorption. The molecular model consists of a single ground state and several bands of discrete states, which are successively connected by means of electric-dipole transitions.¹ This model correctly simulates some of the major characteristics of multiple- photon absorption, such as power threshold, fluence-dependent absorption, and complete population inversion at large fluences. As the pulse propagates, the Rabi oscillations induced in the lower-level states are encoded onto the pulse and create a series of peaks and troughs similar to those found in pulse propagation in two-level systems.² Another coherent propagation effect that can change the excitation process is coherent sideband generation. Temporal distortions and the generation of new frequencies result in changes in the population distribution among the states, as well as changes in the total population transferred to the quasicontinuum. The effective increase in total population transfer by propagation effects is offset by the decrease in fluence. The resulting transfer of energy from the pulse to the molecule depends strongly on the initial characteristics of the pulse.

© 1990 Optical Society of America