A uniform semiclassical sudden approximation for rotationally inelastic scattering Invoking a semiclassical approach to state-to-state rotationally inelastic scattering, this study seeks to determine the origin of several novel features observed in earlier quantum mechanical calculations [Rawluk et al., ehem. Phys. Lett. 202, 291 (1993)]. These features were absent from comparable classical trajectory calculations. The semiclassical (classical path) method used here treats the relative motion of Ar and HF classically, while the HF rotation is treated quantum mechanically by expansion in a rigid rotor basis set. This semiclassical approach reproduces the exact quantum results very well. The time dependence of the classical path allows a detailed study of the role played by the potential energy surface governing the dynamics. In particular, the behavior of the expansion coefficients (Y jm l'l1(t» in the complex plane is very revealing. From this analysis, it is clear that the quantum effects stem from a balance between the attractive and repUlsive parts of the potential. 304

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, 985 (1994)."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.

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Rotationally inelastic collisions of HF are studied using a crossed molecular beams apparatus. The incident HF is rotationally "cold", with > 85% in the lowest three states. Differential cross sections are measured for scattering into a single HF rotational state by Ar. The rotational state is detected by using a line-tunable cw chemical laser operating at 2.61um, driven by the F + 112 -HF + H reaction. The laser beam rotates about the HF+Ar collision zone in tandem with a cryogenic bolometer detector, which is sensitive to the additional vibrational energy conveyed by HF molecules that are excited by the laser.We observe a rotational rainbow effect for highly excited rotational states, corresponding to strongly repulsive scattering of Ar off the repulsive core of the HF. In addition, we observe a novel "shoulder" for scattering into the rotationless state. "Exact" quantum close-coupling calculations, fully averaged over instrumental parameters, succesfully reproduce this feature, whereas classical trajectories fail completely. Semi-classical calculations qualitatively reproduce the quantum results and suggest that the shoulder is due to a delicate balancing of attractive and repulsive contributions to the scattering.