A MOLECULAR THEORY OF THE LINE-SHAPE - INHOMOGENEOUS AND HOMOGENEOUS ELECTRONIC-SPECTRA OF DILUTE CHROMOPHORES IN NONPOLAR FLUIDS

Kubo's stochastic theory of the spectral line shape provides an elegan t phenomenological description of inhomogeneous and homogeneous broade ning and the transition between the two. This theory has been used pro fitably in the analysis of many experiments. In this paper we attempt to provide a microscopic foundation for the Kubo model by developing a completely molecular theory of the line shape. For definiteness we fo cus on the optical line shape of dilute chromophores in nonpolar fluid s. Many of the features of the Kubo theory are found in the molecular theory; indeed, the molecular theory produces microscopic expressions involving the solvent structure and dynamics for Kubo's phenomenologic al parameters, and provides some justification for the Gaussian assump tion in the stochastic theory. On the other hand, the molecular theory produces a transition frequency time-correlation function that is dis tinctly nonexponential, in contrast to the exponential assumption of t he Kubo theory, and it is found that this nonexponentiality is necessa ry for the accurate description of line shapes in the regime intermedi ate between inhomogeneous and homogeneous broadening. For a model of L ennard-Jones particles the molecular theory is compared with molecular dynamics computer simulations.