Is there resonance enhancement of the cross section in very low energy collision-induced vibrational relaxation?

Abstract

The molecular vibrational relaxation which occurs in free jet expansions at very low translational temperatures has been attributed previously to an orbiting resonance mechanism, with the assumption that ‘‘ordinary’’ mechanisms for vibrational relaxation have vanishingly small rates at low temperatures. This assumption is examined from three perspectives: (1) The kinetic energy dependence of the v=1→0 cross section for I2(X 1Σg+) + He collisions, as derived from the experimentally determined v=0→1 cross section and microscopic reversibility; (2) a semiclassical correspondence-principle description of the deexcitation process at zero kinetic energy; and (3) the quantum-mechanical threshold laws for exoergic collisions of two neutral particles. It is concluded that nonresonance mechanisms may plausibly result in relaxation rates which are comparable to those which have been attributed to the resonance mechanism, even at very low translational temperatures, and that therefore the question posed in the title should still be considered an open one.