Quasiclassical trajectory (QCT) and quantum mechanical (QM) close-coupling calculations have been used to study the state-resolved rotationally inelastic scattering of NO(X(2)Pi(1/2),v = 0,j = 1/2,e/f) by He on the most recent ab initio potential energy surface of J. Kłos et al. [J. Chem. Phys. 2000, 112, 2195.]. Opacity functions, and integral and differential cross sections are reported at collision energies of 63 and 147 meV and compared with previous theoretical calculations and experimental measurements on this and other systems. The existence of double peaks in the QCT and QM differential cross sections is examined in detail. While at a collision energy of 147 meV two rotational peaks appear in both the QCT and open-shell QM results, only a single peak is found in the QM calculations at the lower collision energy. The double peaks in the quantum-state-resolved differential cross sections (DCS) are found to be closely related to structure found in the corresponding state-resolved opacity functions. The structure in the QCT and QM DCSs is attributed to a flattening of the potential energy surface for sideways approach of He to the near-symmetric NO(X) molecule, and in both sets of calculations, it is shown to arise from a specific odd term in the expansion of the intermolecular potential. Although significant differences are found between the QCT and QM data in the forward scattered direction, and for higher final rotational levels, reflecting differences in the nature of the rotational rainbows observed in these two methods, in general, the QCT calculations are shown to give similar results to quantum theory. Furthermore, they provide valuable clues as to the mechanism of rotational energy transfer in this system.

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State-resolved differential cross sections for the rotationally inelastic scattering of the Ar+NO system have been derived from quasiclassical trajectories and quantum close-coupling calculations on a recent ab initio potential energy surface at the collision energy of a recent high resolution experiment (66 meV). Globally good agreement is obtained between the theoretical predictions and experimental results, although some of the experimental details are not reproduced in the classical calculation. The role of attractive and repulsive interactions in the observed dynamical features is examined.

The stereodynamics of the Ar þ NO (j ¼ 0) rotational inelastic excitation has been investigated at 66 meV by means of quasiclassical trajectories on a recent ab initio potential energy surface. A marked correlation between the preferred sense of rotation of NO and the scattering plane is obtained for the highest rotational levels accessible, which are excited in strong repulsive collisions. This result is in qualitative agreement with recent quantum mechanical calculations and experimental measurements. For the lower rotational levels, where the interactions are not so repulsive, the preferred sense of rotation is found to oscillate with scattering angle, but the intensity of the oscillations is small and their angular range is not entirely coincident with those from quantum mechanics and experiment. Classical dynamics, even including attractive interactions, cannot account properly for the mentioned oscillatory behaviour. Secondary encounters between the outgoing Ar atom and NO molecule, giving rise to 'chattering', are found to be relatively frequent, leading to a decrease in the final rotational energy of NO with respect to that attained in the first encounter. Chattering trajectories are defined and their mechanism is characterized. w Electronic supplementary information (ESI) available: Fig. 3 in colour. See

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