Chemical reactions in the limit of zero kinetic energy:: virtual states and Ramsauer minima in F+H2→HF+H

The behaviour of reactive scattering at ultracold temperatures is explored by calculating the real and imaginary parts of the scattering length for the reaction of F with a molecule composed of a pair of pseudo-hydrogen atoms of arbitrary mass. The origin of a low energy feature in the cross section for the reaction of F with H-2 and its absence for the reaction with D-2 is investigated. Close-coupling calculations of the scattering matrix show that the F-H-2 feature arises from the presence of a virtual state associated with the van der Waals well in the entrance channel and that the virtual state is responsible for the enhanced zero temperature rate coefficient of the F-H-2 reaction. For a mass of about 1.12 hydrogen masses the virtual state turns into a zero energy resonance and the corresponding zero temperature rate coefficient is I X 10(-9) cm(3) s(-1) despite an energy barrier of 300 K. Evidence in support of the virtual state is also provided by the detection of a deep Ramsauer-Townsend minimum in the elastic component of the total cross section for F-H-2 which the present calculations predict to occur at low energies.