▪ Abstract Experimental and theoretical studies of polar solvation dynamics and spectral line broadening are described. Photon echo and fluorescence Stokes shift experiments are discussed, and the relationship between the information content of the two techniques clarified. The interaction of the chromophore with its solvent bath is described by a spectral density, and the connection of this spectral density with the liquid's instantaneous normal modes discussed. The ubiquity of the ultrafast phase of solvation dynamics is demonstrated, and a discussion of its physical origin given. The slower phases of solvation vary strongly from solvent to solvent. Molecular theories of liquid dynamics are able to rationalize such variations over at least a 2000-fold range of time scale. The review concludes with a brief discussion of the relevance of ultrafast solvation to chemical dynamics.

We present an instantaneous-normal-mode analysis of liquid water at room temperature based on a computer simulated set of liquid configurations and we compare the results to analogous inherent-structure calculations. The separate translational and rotational contributions to each instantaneous normal mode are first obtained by computing the appropriate projectors from the eigenvectors. The extent of localization of the different kinds of modes is then quantified with the aid of the inverse participation ratio—roughly the reciprocal of the number of degrees of freedom involved in each mode. The instantaneous normal modes also carry along with them an implicit picture of how the topography of the potential surface changes as one moves from point to point in the very-high dimensional configuration space of a liquid. To help us understand this topography, we use the instantaneous normal modes to compute the predicted heights and locations of the nearest extrema of the potential. The net result is that in liquid water, at least, it is the low frequency modes that seem to reflect the largest-scale structural transitions. The detailed dynamics of such transitions are probably outside of the instantaneous-normal-mode formalism, but we do find that short-time dynamical quantities, such as the angular velocity autocorrelation functions, are described extraordinarily well by the instantaneous modes.