Light-induced viscous flow originating from velocity-selective heating or cooling

An alternative mechanism is demonstrated to bring about light-induced kinetic effects in gases without the necessity of a state-dependent collision cross section. This is achieved by velocity-selective optical excitation followed by collisional deexcitation in the subsequent collision. This process may be termed velocity-selective heating and gives rise to an enhanced transport coefficient for a selected velocity class. A mean-free-path description of this phenomenon is given for the case of momentum transport in a one-component gas. For nonuniform illumination, a shear stress is found to result, which gives rise to light-induced viscous flow. Experimentally this effect is demonstrated for CH₃F, CD₃F, CO₂, and C₂{H}₄$, rovibrationally excited by a ${CO}₂$ laser. It is shown that the magnitude of the effect is determined by the rotational energy transfer. It can be of either sign (velocity-selective heating or cooling), depending on R- or P-branch excitation, and it can easily overwhelm the contribution to light-induced viscous flow arising from a state-dependent collision cross section.