Semiclassical study of sudden approximations for rotationally inelastic scattering of Ar + HF

"Exact" quantum close coupled (CC) and infinite-order sudden approximation (IOSA) calculations have been carried out for the Ar + HF system on an accurate potential energy surface. The differential cross sections from the IOSA calculations show marked differences from the CC results. We find that the energy sudden component of the IOSA breaks down in a different manner for small and large impact parameters, significantly shifting and distorting the state-to-state differential cross sections. We also find that the centrifugal sudden approximation alone reproduces all features of the full semiclassical calculations quite faithfully. Through the use of several rotational sudden approximations, including a semiclassical version of the IOSA, we identify how the differences between the CC and IOSA results arise.