Hyperpolarizability dependent dispersion forces and absorption spectra in simple liquids

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Abstract
A general quantum statistical perturbation expansion for molecular fluids, with term ordering in increasing order of molecular susceptibilities, reproduces results of recent theories on absorption spectra in liquids when taken to lowest order, i.e. including only linear polarizability terms. Inclusion of higher order, cubic polarizability terms produces solvent effects on impurity electronic absorption spectra. The impurity line shift obtained is a functional of the dipole pair correlations of the pure solvent and of the two-frequency dependent cubic polarizability tensor γ(ω, ξ, -ξ) of the impurity molecule. A simple form of γ based on a 3-level isotropic molecular model, together with a linear polarizability approximation for the solvent dipole correlations leads to a blue shift of the impurity lowest excitation frequency. The magnitude of the shift depends of the position of the impurity line relative to the excitonic band spectrum of the pure solvent and on molecular parameters of the impurity obtainable from the Kerr effect and other third order, two field frequency optical processes.