State to state He–CO rotationally inelastic scattering

Abstract

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.

State to state Ne–CO rotationally inelastic scattering

“…Our detection arrangement is less sensitive to such molecules than to unaligned ones, 5 so we have probably underestimated the densities at high j. In the limit of perfect negative alignment, we would report a density only 0.6 of the correct value; more likely errors are 10%-20% for jϷ15 and less at lower j.…”

“…Detailed descriptions of our crossed molecular beam apparatus and experimental procedure appeared in an earlier paper. 5 The CO was seeded in argon ͑5% CO/95% Ar͒ to help cool its rotational distribution before the collision. In our apparatus, the angle between the CO and Ne beams, and therefore the center of mass collision energy, can be varied by moving one of the molecular beam sources.…”

“…Experimentally, these interference effects have been observed in collisions of NO, [26][27][28][29] CN, [30][31][32][33] and CO. 5 The densities predicted by the SAPT Ne-CO surface show maxima at jϭ4, 6, 9, and 11 for both collision energies. The propensity at low ⌬ j is exactly opposite the experimental one.…”

“…In our earlier He-CO work, 5 we speculated that the difference between the distances of closest approach at the two ends of CO might serve as an indicator of the overall ''oddness'' of the potential and be reflected in the even-odd propensity. That speculation was clearly too simplistic; exactly the opposite case appears here.…”

“…5 One surface was purely ab initio and the other was an ab initio/empirical hybrid. Interference structure in the post-collision rotational distribution proved very sensitive to details of the potential surface.…”