We present a theory of time- and frequency-domain spectroscopy of a dilute nonpolar solute in a nonpolar liquid or supercritical fluid solvent. The solute and solvent molecules are assumed to interact with isotropic pair potentials. These potentials, together with the solute and solvent masses, are the only input in the theory. We arrive at expressions for the absorption and emission line shapes, which include the possibility of motional narrowing, and for the time-resolved fluorescence 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 normalized frequency fluctuation time-correlation functions. Within our formalism the former are obtained from the solute-solvent and solvent-solvent radial distribution functions, which are calculated using integral equations. The time-correlation functions involve the time-dependent solute-solvent Green’s function. Its solution depends upon the solute and solvent diffusion constants, which in turn are determined from the radial distribution functions. The theory compares favorably with computer simulation results of the same model. We then investigate the dependence of the various spectroscopic observables on the solvent density, the temperature, and the difference between the ground- and excited-state solute’s pair interaction with the solvent molecules. For example, 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 results are similar to a variety of experiments on solutes in both nonpolar and polar solvents.

Energy- and angle-resolved intensities are reported for the scattering of argon atoms off the surface of liquid indium just above the melting temperature, for two different argon incident energies. The higher incident energy results show significant energy transfer from argon to liquid atoms, and the angular distribution of scattered argon atoms is relatively sharply peaked near the specular angle. The lower incident energy results show a small amount of energy transfer from liquid atoms to argon atoms, the energy distribution of the scattered argon atoms is nearly thermal, and the angular distribution is much less sharply peaked, although still not completely thermal. Molecular dynamics simulations of these experiments are performed, and most of the results are in reasonable agreement with experiment. Analysis of the simulation trajectories helps to provide a microscopic understanding of the experimental results.

We consider the electronic spectroscopy of dilute CH3I in supercritical Ar fluid. Absorption line shapes for the B←X transition of CH3I have been measured previously in low-density argon, which yielded results for the CH3I/Ar pair potentials. Using these potentials, Kalbfleisch et al. [J. Chem. Phys. 105, 7034 (1996)] have performed molecular dynamics simulations to calculate the absorption line shapes at higher densities, and also the solvation correlation function. We compare the results of several analytic theories to the simulated line shapes and solvation correlation functions.

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