Threshold phenomena in ultracold atom-molecule collisions

Vibrational relaxation cross sections for an atom-molecule collision are calculated by explicit numerical solution of the coupled equations of scattering theory and shown to follow an inverse velocity dependence at very low initial kinetic energies of the incident atom, in accordance 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 magnitude between the deepest level upsilon = 0 and the vibrational level upsilon = 12. (C) 1997 Elsevier Science B.V.