The excitation function for Li + HF → LiF + 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° to 90° 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 ∝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.