A comprehensive approach to determining molecular inelastic rate constants*

We present experimental and computer-simulated cross-sections and rate constants for rotationally inelastic processes in collisions of rare gases with laser-excited lithium dimer. The results are evaluated for their agreement as well as their adherence to microscopic reversibility and detailed balance. We use laser-induced fluorescence spectroscopy to find rate constants experimentally, and quasiclassical trajectory (QCT) and close-coupled (CC) quantum calculations to computationally determine the rate constants. Our goal is to evaluate the mutual consistency of the experimental and computational methods used in measuring and calculating level-to-level rate constants.

For low initial rotational levels, QCT fails to satisfy microscopic reversibility and detailed balance.  We investigated the cause of this failure, and we propose a binning approach that largely overcomes this issue while retaining fidelity to the CC cross-sections to the extent possible. This binning approach respects the uncertainty principle as well as the time-reversal invariance of the equations of motion; we judge the results by examining how closely QCT and CC cross sections agree.  In the process, the shortcomings of standard approaches become apparent.

We conducted a case study of Li₂ A Σ_u⁺ (v_i, j_i), for which we also obtained experimental data.  We refined our data analysis procedures to give the most accurate rate constants for the inelastic processes under consideration.  The result of this integrated approach is a set of recommended practices that result in accurate, internally consistent rate constant determinations.