Molecular dynamics simulations of vibrational cooling and heating in isotopically substituted molecular clusters

Molecular dynamics simulations of clusters containing hundreds of naphthalene molecules were used to investigate vibrational cooling and vibrational heating. The effects of isotopic substitution, modeled by changing the masses of the extended-atom C-H groups, were also studied. In vibrational cooling, a hotter molecule (300 K) is allowed to interact with a cold cluster (10 K). Pure clusters of normal, light, and heavy naphthalene molecules were cooled with roughly the same time constant (∼50 ps). However, in mixed clusters containing a normal molecule in an isotopically substituted heavy or light cluster, the normal molecule cooled much more slowly, indicating the dominant cooling mechanism in pure clusters is resonant intermolecular vibrational energy transfer. In vibrational heating studies, a cold molecule (10 K) is allowed to interact with a cluster which is much hotter (300 K) than in the vibrational cooling studies (10 K). Normal molecules in pure or mixed clusters were heated at about the same rates and those rates were about what was seen in vibrational cooling simulations. At the higher temperatures of the vibrational heating simulation, phonon-assisted intermolecular vibrational energy transfer between unlike molecules in mixed clusters occurs at rates similar to resonant transfer processes between like molecules in pure clusters.