THEORETICAL CROSS-SECTIONS FOR RESONANT EXCHANGE OF ROTATIONAL ENERGYIN A STARK FIELD

Cross-sections for resonant exchange of rotational energy are obtained in closed form for the long-range limit where there is negligible def lection of the relative velocity vector during the collision. Exchange s mediated by dipole-dipole interaction or by the anisotropic part of the London potential have cross-sections similar to 10(-14) cm(2) for changing M, with values one or two orders of magnitude smaller for cha nging J and K. Exchanges mediated by dipole-quadrupole and dipole-indu ced dipole interactions have cross-sections similar to 10(-15) cm(2) f or M-changing collisions. Despite the existence of large cross-section s for resonant exchange, the decay of alignment of an ensemble of mole cules by rotationally elastic collisions involving only changes in M i s expected to be slow because cross-sections for Delta M = +1 and -1 a re equal and the loss of alignment must occur by a random walk on the lattice of M values. (C) 1998. Published by Elsevier Science B.V. All rights reserved.