LIGHT-INDUCED KINETIC EFFECTS IN MOLECULAR GASES

Velocity-selective excitation can be achieved by tuning a narrow-band laser within the Doppler-broadened absorption profile of a gas. If the excitation modifies the kinetic properties of the molecules (e.g., the kinetic cross-sections or the molecule-surface interactions), the Maxwell velocity distribution will be distorted. This gives rise to a new class of kinetic effects that do not require external gradients to be imposed on the system; rather, the laser intensity and its gradient can be considered as thermodynamic forces. Examples are light-induced drift, surface light-induced drift and light-induced viscous flow. A phenomenological overview is given of the various light-induced kinetic effects which can arise in pure gases or mixtures. This includes effects resulting from velocity-selective excitation immediately followed by collisional de-excitation ('velocity-selective heating'). The theoretical description of the various effects is briefly reviewed. Experiments are described which investigate these effects in molecular systems, where the excitation is rovibrational, and their results discussed. So far, data have been obtained for CH3F, NH3, CO2, OCS, CH3OH, CH3Br, SF6 and C2H4. Applications include determination of the vibrational-state dependence of intermolecular potentials, dependence of molecule-surface interactions upon the rotational state (J, M(J)), isotope separation, ortho-para conversion rates and possible astrophysical applications.