  A recently proposed model for steady-state spectra of polar-polarizable
chromophores is extended to describe time-resolved spectra. The model, based on
a two-state picture for the solute and on a continuum overdamped description
for the (polar) solvent, grasps the essential physics of solvation dynamics, as
demonstrated by the comparison with experimental spectra. The solute
(hyper)polarizability is responsible for spectroscopic features that cannot be
rationalized within the standard picture based on a linear perturbative
treatment of the solute-solvent interaction. In particular, the temporal
evolution of band-shapes and the appearance of temporary isosbestic points, two
common puzzling features of observed spectra, are natural consequences of the
molecular hyperpolarizability and of the consequent coupling between solvation
and vibrational degrees of freedom.
