Vibrational energy relaxation of polyatomic molecules in liquids: The solvent's perspective

Vibrationally excited polyatomic molecules can relax in a variety of different ways in solution; the excess energy can be dissipated directly to the solvent, or it can be redistributed between any number of different intramolecular modes, with the liquid absorbing (or supplying) just enough energy to make the process work. What we consider here is how the solvent participates in these mechanistic choices. Using the prototypical example of a symmetric linear triatomic molecule, we compare the molecular origins of the vibrational friction for the direct vibrational cooling of the symmetric and antisymmetric stretching modes and contrast both of those with intramolecular vibrational energy transfer between these two modes. Instantaneous-normal-mode analysis reveals that a solid-statelike perspective is a plausible starting point for understanding these processes; the solvent does define a band of intermolecular vibrations, and it is only when the energy being transferred falls within that band that the solvent can easily accept energy from a solute. However, it is also possible to discern some more liquid-state-specific details. Despite their different symmetries and different kinematic requirements, all of the different relaxation pathways are apparently driven by the dynamics of the same instantaneously nearest solvents. (C) 2002 American Institute of Physics.