THRESHOLD PHENOMENA IN ULTRACOLD ATOM-MOLECULE COLLISIONS

Vibrational relaxation cross sections for an atom-molecule collision a re calculated by explicit numerical solution of the coupled equations of scattering theory and shown to follow an inverse velocity dependenc e at very low initial kinetic energies of the incident atom, in accord ance with Wigner's threshold law. Rate constants for H + H-2 (upsilon, j = 0) --> H + H-2(upsilon') < upsilon, j = 0) are calculated for all the vibrational levels of H-2 and found to vary by seven orders of ma gnitude between the deepest level upsilon = 0 and the vibrational leve l upsilon = 12. (C) 1997 Elsevier Science B.V.