Light-induced kinetic effects in molecular gases

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Abstract
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 ro-vibrational, 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.