ROTATIONAL ENERGY RELAXATION OF SYMMETRICAL-TOP MOLECULES BY COLLISIONS WITH ATOMS

The infinite-order sudden approximation has been used to calculate the effective cross sections for R-T energy transfer (sigma(E)) of symmet ric top molecules by collisions with atoms. In a final form sigma(E) i s expressed via [Delta E(2)] (the mean-square change in rotational ene rgy), which is analytically expressed through the partial phase shifts . A quadratic dependence of sigma(E) on phase shifts derived in the pr esent study indicates a very high sensitivity of sigma(E) to the detai ls of the anisotropic potential. Within the developed formalism, spher ical and linear molecules are treated as the limiting cases. Detailed comparisons between the IOSA values and semiclassical sudden calculati ons are carried out for rare-gas-SF6 mixtures.