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 NO2 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 NO2 mixtures. In case I2 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 I2, the shift is equal to that previously predicted for absorbing liquids - approximately half the laser linewidth. For mixtures containing NO2, the predicted shift is somewhat less. © 1969 The American Physical Society.