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

Abstract

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 H2 and its absence for the reaction with D2 is investigated. Close-coupling calculations of the scattering matrix show that the F–H2 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–H2 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 1 × 10−9 cm3 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–H2 which the present calculations predict to occur at low energies.