Vibrational Energy Relaxation in Solid Carbon Monoxide

Theory predicts that vibrational energy relaxation (VER) in a dense medium exhibits an exponential dependence on the order of the multiphonon process (energy gap law). Simply put, the vibrational energy decay rate (τ^–1) has an exponential dependence on the difference between the excited and accepting frequencies Δν (frequency-gap law). Additionally, the vibrational density of states (VDOS) of the “bath” of low-frequency modes, into which vibrational energy is being dissipated, plays a role in the VER. Although analytical studies at the quantum mechanical level for model systems have provided great insights, quantification of VER dynamics for systems described by realistic interaction potentials is still scarce. Here, we focus on a simple diatomic molecule (carbon monoxide) to exclusively probe the intermolecular VER without contributions from intramolecular vibrational energy redistribution. Using classical nonequilibrium molecular dynamics (NEMD) simulations, we study VER within amorphous and crystalline clusters for mixtures of four different carbon monoxide isotopologues all described by two different interaction potentials. We also present a novel method for extracting τ(Δν) for trajectory ensembles of such NEMD simulations of weakly coupled molecules that have slow dissipation rates. For amorphous clusters, τ(Δν) is best described by a biexponential fit, whereas the situation is more complicated for crystalline clusters. In both cases, we find links to the VDOS. Although the energy transfer occurs continuously in our classical simulations, further analysis of our trajectory ensembles suggests very interesting analogies to quantum mechanical descriptions of nonresonant and resonant vibrational to vibrational (V–V) energy transfer.