The excitation function for Li plus HF → LiF plus H at collision energies below 80 meV

We have measured the dependence of the relative integral cross section of the reaction Li + HF -> LiF + H on the collision energy (excitation function) using crossed molecular beams. By varying the intersection angle of the beams from 37 degrees to 90 degrees we covered the energy range 25 meV <= E(tr) <= 131 meV. We observe a monotonous rise of the excitation function with decreasing energy over the entire energy range indicating that a possible translational energy threshold to the reaction is significantly smaller than 25 meV. The steep rise is quantitatively recovered by a Langevin-type excitation function based on a vanishing threshold and a mean interaction potential energy proportional to R(-2.5) where R is the distance between the reactants. To date all threshold energies deduced from ab initio potentials and zero-point vibrational energies are at variance with our results, however, our findings support recent quantum scattering calculations that predict significant product formation at collision energies far below these theoretical thresholds. (C) 2011 American Institute of Physics. [doi:10.1063/1.3664303]