Nonlinear Thermal Rayleigh Scattering in Gases

A theory for stimulated thermal Rayleigh scattering in gases is presented. The dependence of the nonlinear gain on thermalization times for absorbed optical energy is derived. For gas mixtures containing N${\mathrm{O}}_2$ as the molecule which absorbs optical radiation, the nonlinear gain for antistokes-shifted back-scattered light increases with density due to the increased rate of thermalization with density. Diffusion of optically excited molecules is shown not to substantially affect the nonlinear susceptibility for N${\mathrm{O}}_2$ mixtures. In case ${\mathrm{I}}_2$ is used as the dye molecule, translational energy is released immediately in dissociation following optical absorption. This leads to little or no dependence of the nonlinear susceptibility on gas density within wide limits. The antistokes shift for the scattered radiation is also calculated. For gas mixtures containing ${\mathrm{I}}_2$, the shift is equal to that previously predicted for absorbing liquids - approximately half the laser linewidth. For mixtures containing N${\mathrm{O}}_2$, the predicted shift is somewhat less.