Detailed Balance as an Internal Consistency Check for Molecular Collision Studies

We present experimental and computer simulation results on microscopic reversibility and detailed balance for rotationally inelastic processes in laser-excited molecular states (Li₂-X), aiming to evaluate the internal consistency of the experimental and computational methods used in measuring and calculating level-to-level rate constants. We use laser-induced fluorescence (LIF) spectroscopy to find rate constants experimentally, and quasiclassical trajectory (QCT) or close-coupled quantum calculations to computationally determine the rate constants. We observe that for low initial rotational levels, QCT fails to satisfy microscopic reversibility and detailed balance, a consequence of the artificial quantization imposed through binning of the trajectories. We compare experimental detailed balance ratios for Li₂-Li and Li₂-Xe. Our ultimate goal is to perform a comprehensive determination of detailed balance and microscopic reversibility across all three methodologies: experimental results along with quasiclassical and quantum simulations. This integrated approach serves as an internal consistency check of the rate constants, thereby validating our experimental and computational methods by checking the principle of time-reversal invariance, a fundamental aspect of rate processes in molecular dynamics.