Importance of acoustic solvent mode and solute-solvent radial distributionfunctions in solvation dynamics: Studied by RISM theory

We have applied the reference interaction-site model (RISM) theory to expla in our experiments on solvation dynamics of laser dyes in polar organic sol vents by means of the transient hole-burning and time-resolved fluorescence spectroscopy [K. Nishiyama and T. Okada, J. Phys. Chem. A, 101 (1997) 5729 ; 102 (1998) 9729]. The experiments show that the dynamic response function of the relaxation derived from the inhomogeneous spectral band-width of th e solute - solvent system relaxes about an order of magnitude slower than t hat of average energy of the system. On the basis of our present theoretica l prediction, we suggest that the relaxation dynamics of the spectral width might be mainly characterized by the solvent acoustic mode (translational motion) as well as the rearrangement of second plus further solvent shells.