Relative integral cross sections for rotational excitation of CO in collisions with He were measured at energies of 72 and 89 meV. The cross sections are sensitive to anisotropy in the repulsive wall of the He-CO interaction. The experiments were done in crossed molecular beams with resonance enhanced multiphoton ionization detection. The observed cross sections display interference structure at low ⌬ j, despite the average over the initial CO rotational distribution. At higher ⌬ j, the cross sections decrease smoothly. The results are compared with cross sections calculated from two high quality potential energy surfaces for the He-CO interaction.

Measurements of state-to-state integral cross sections for rotational excitation of CO by collisions with Ne are reported. The measurements were performed in crossed molecular beams with resonance enhanced multiphoton detection at collision energies of 711 and 797 cm Ϫ1 . The cross sections display strong interference structure, with a propensity for odd ⌬ j below ⌬ jϭ10. Predictions of the ab initio potential surface of Moszynski et al. ͓J. Phys. Chem. A 101, 4690 ͑1997͔͒ and the new ab initio surface of McBane and Cybulski ͓J. Chem. Phys. 110, 11734 ͑1999͒, preceding paper͔ are compared to the data. The new surface agrees more closely with the observed interference structure, although significant disagreements remain.

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Collisions between argon and nitric oxide were studied in crossed supersonic beams. Resonance-enhanced multiphoton ionization was used to determine postcollision relative densities of many rotational, spin−orbit, and Λ doublet states. A double-beam reference technique was used to compensate for possible saturation in the REMPI probe. A preference for the production of the Π(A‘ ‘) Λ doublet component was observed in both final spin−orbit states. Quantum scattering calculations on the potential surfaces of Alexander (J. Chem. Phys. 1993, 99, 7725) agree well with the Λ doublet propensity but underestimate the observed probability for spin−orbit-changing collisions.