The excitation function for Li+HF-->LiF+H at collision energies below 80 meV

TL;DR

This study measures how the reaction cross section of Li+HF forming LiF+H varies at very low collision energies, revealing a steep increase and supporting recent quantum calculations over traditional threshold predictions.

Contribution

It provides experimental data on the excitation function at energies below 80 meV, challenging existing theoretical threshold estimates and supporting quantum scattering predictions.

Findings

Cross section rises monotonically as energy decreases.

Reaction occurs at energies below previously predicted thresholds.

Supports quantum calculations predicting product formation at very low energies.

Abstract

We have measured the dependence of the relative integral cross section of the reaction Li+HF-->LiF+H on the collision energy using crossed molecular beams. By varying the intersection angle of the beams from 37{\deg} to 90{\deg} we covered the energy range 25 meV < E_tr < 131 meV. We observe a monotonous rise of the cross section 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.