The Dynamics of Polar Solvation

Fluorescence upconversion experiments with subpicosecond resolution are described. The time dependent fluorescence spectra of several probe molecules in polar solvents are measured and used to construct the Stokes shift correlation function C( t ). The correlation functions decay nonexponentially in contrast to the predictions of simple Debye/Onsager continuum theory. At high values of the static dielectric constants the solvation times are much slower than predicted from standard continuum theory. Generalizations of continuum theory to include non‐spherical shapes, non‐Debye dielectric response and saturation effects are discussed. The experimental data are qualitatively in accord with predictions based on a frequency and position dependent dielectric response.