Rotational excitation of symmetric top molecules by collisions with atoms. II - Infinite order sudden approximation

The infinite order sudden (IOS) approximation is extended to rotational excitation of symmetric tops by collisions with atoms. After development of a formalism for 'primitive' or 'one-ended' tops, proper parity-adapted linear combinations describing real rotors are considered and modifications needed for asymmetric rigid rotors are noted. The generalized spectroscopic relaxation cross sections are discussed. IOS calculations for NH3-He and H2CO-He are performed and compared with more accurate calculations, and the IOS approximation is found to provide a reasonably accurate description.