PURELY MECHANICAL SOLVATION DYNAMICS IN SUPERCOOLED LIQUIDS - THE S-0[-T-1(0-0) TRANSITION OF NAPHTHALENE

We have measured the Stokes shift and its dynamics for the probe molec ules naphthalene (Delta mu approximate to 0) and quinoxaline (Delta mu approximate to 1.3 D) in n-propanol and other glass-forming solvents. The Stokes shift for naphthalene (approximate to 63 cm(-1)) turns out to be independent of the solvent polarity over a wide range of the st atic dielectric constants. Its time dependence is governed by the stru ctural (or alpha- or shear stress) relaxation time of n-propanol, with out any signature of the strong dielectric relaxation. For this solven t; structural and dipolar contributions can be distinguished because t he time scale for dipole reorientation is a factor of 25 slower than t he alpha-relaxation time. We conclude that the solvation of naphthalen e reflects excitation-induced changes in the van der Waals interaction s, which makes it an ideal probe for assessing shear stress or mechani cal relaxations on microscopic spatial scales near T-g.