Rotational energy transfer in collisions of jet-cooled NO(X2Π1/2, v = 0) with Ar and with He: energy and angular momentum constraints

Abstract

Using a crossed molecular beam apparatus, we have examined rotational energy transfer in collisions of NO(X²Π_1/2, v = 0, j_i = 0.5 and 1.5) with Ar and with He. Relative values of the populations transferred into specific final states (²Π_1/2, v = 0, j_f) are reported for the following collision energies: NO–Ar, 265, 514, 720 and 980 cm⁻¹, and NO–He, 260, 530, 784 and 963 cm⁻¹. In all cases, the distribution over final rotational states is wide with the cross-sections decreasing with increasing Δj. The average energy transferred to rotation in NO is approximately independent of collision energy and is ca. 17% in the case of NO–Ar, and ca. 11% for NO–He. In NO–Ar collisions, transfer takes place to essentially all energetically accessible j_f. This is not so for NO–He, where the smaller range of orbital angular momentum leads to a lower limit to the highest j_f state. The results are compared with those from model calculations in which energy is transferred impulsively. The agreement between the results of the experiments and these calculations in respect of the highest rotational levels that are accessed is good, confirming that in collisions where the spin-orbit state of the NO is preserved, energy is transferred through the action of repulsive forces and that the difference between the results for NO–Ar and NO–He can be put down to the operation of momentum constraints in the latter case. More detailed comparisons suggest that in the case of NO–Ar collisions, the distribution of NO molecules over j_f states might be significantly modified by secondary encounters.