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.

Relative state-to-state rotationally inelastic cross sections for excitation of carbon monoxide by hydrogen were measured in a crossed molecular beam experiment at collision energies 795, 860, and 991 cm Ϫ1 . The results are compared to predictions of a recent ab initio potential energy surface ͓J. Chem. Phys. 108, 3554 ͑1998͔͒. The agreement is very good. A comparison with older data on thermally averaged total depopulation cross sections ͓Chem. Phys. 53, 165 ͑1980͔͒ indicates that the absolute magnitudes of the cross sections predicted by the surface are too high. The CO excitation is dominated by collisions that are elastic in H 2 rotation, and the collision dynamics are very similar for different rotational levels of hydrogen.

No abstract