VIBRATIONAL LINE-SHAPES OF SOLVATED MOLECULES WITH A NORMAL-MODE APPROACH

We develop a theory of the vibrational absorption line shape of a solvated molecule. This approach is based on the instantaneous normal mode approximation, in which the fluid is taken to evolve on a harmonic potential surface whose curvature matches that of the true potential surface at the fluid's initial configuration. We apply this method to the vibrational line shape of a harmonic diatomic molecule dissolved in an atomic solvent. The line shape is related to a configurationally averaged phonon Green's function. A diagrammatic analysis of this Green's function is shown to lead to a self-consistent approximation to the line shape. The only inputs to this calculation from other theory or simulation are the pair correlation functions for two solvent particles and for a solute atom and a solvent particle. The resulting spectra are compared with calculations for a similar model by Berne et al., based on the generalized Langevin equation [J. Chem. Phys. 93, 5084 (1990)]. © 1995 American Institute of Physics.