On the fixed-nuclei approximation as applied to rotational excitation of molecules by atoms

The applicability of the fixed-nuclei approximation to the rotational excitation of a diatomic molecule by an atom is investigated. The approximation is shown to predict accurate quantum cross sections for the model system H2 + N2 at thermal collision energies. A quasi-classical Monte-Carlo study of the same problem is also performed, and the success of the fixed-nuclei approximation is interpreted by investigating in detail a number of coplanar classical trajectories. (Auth.)