The rotational sudden approximation at low energies

Abstract

The infinite‐order sudden approximation is studied at low energies for a system with widely spaced energy levels to determine the limits of its validity. It is found that even under these extreme conditions it gives highly accurate results for cross sections summed over final rotational states. The elastic cross sections are also reasonably accurate in this approximation, though, for this worst case situation, the state to state cross sections for inelastic scattering are not even qualitatively correct for high Δj transitions. The high accuracy of the cross sections summed over final rotational states commends the sudden approximation for use in reducing the complexity of carrying rotational channels in the calculation of vibrational transitions. The use of the sudden approximation in studying expansions of interaction potentials is discussed.