Theoretical cross-sections for resonant exchange of rotational energy in 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 deflection of the relative velocity vector during the collision. Exchanges mediated by dipole–dipole interaction or by the anisotropic part of the London potential have cross-sections ∼10−14 cm2 for changing M, with values one or two orders of magnitude smaller for changing J and K. Exchanges mediated by dipole–quadrupole and dipole-induced dipole interactions have cross-sections ∼10−15 cm2 for M-changing collisions. Despite the existence of large cross-sections for resonant exchange, the decay of alignment of an ensemble of molecules by rotationally elastic collisions involving only changes in M is expected to be slow because cross-sections for ΔM=+1 and −1 are equal and the loss of alignment must occur by a random walk on the lattice of M values.