MOLECULAR THEORY OF ELECTRONIC SPECTROSCOPY IN NONPOLAR FLUIDS - ULTRAFAST SOLVATION DYNAMICS AND ABSORPTION AND EMISSION-LINE SHAPES

We present a theory of time- and frequency-domain spectroscopy of a di lute nonpolar solute in a nonpolar liquid or supercritical fluid solve nt. The solute and solvent molecules are assumed to interact with isot ropic pair potentials. These potentials, together with the solute and solvent masses, are the only input in the theory. We arrive at express ions for the absorption and emission line shapes, which include the po ssibility of motional narrowing, and for the time-resolved fluorescenc e and transient hole-burning observables, by assuming that the solute' s fluctuating transition frequency describes a Gaussian process. These expressions depend only on the average and variance of the transition frequency distributions in absorption and emission and on the normali zed frequency fluctuation time-correlation functions. Within our forma lism the former are obtained from the solute-solvent and solvent-solve nt radial distribution functions, which are calculated using integral equations. The time-correlation functions involve the time-dependent s olute-solvent Green's function. Its solution depends upon the solute a nd solvent diffusion constants, which in turn are determined from the radial distribution functions. The theory compares favorably with comp uter simulation results of the same model. We then investigate the dep endence of the various spectroscopic observables on the solvent densit y, the temperature, and the difference between the ground- and excited -state solute's pair interaction with the solvent molecules. For examp le, since our theory for the time-correlation functions captures both their short- and long-time behavior, we can see how the crossover from inertial to diffusive dynamics depends on these variables. Our result s are similar to a variety of experiments on solutes in both nonpolar and polar solvents. (C) 1997 American Institute of Physics.