Solvent Effects on Electronic Spectral Intensities

A theory is developed for determining the effect of the London–van der Waals interaction on the electronic spectral intensities of an absorbing substance dissolved in a transparent solvent. The formulation is based on quantum-statistical double perturbation theory: time varying in the applied field and time independent in the intermolecular force field. A virial expansion is developed for the extinction coefficient, integrated over a band, in terms of molecular parameters and molecular distribution functions, and the first and second virial coefficients are worked out in detail. It is shown that the leading term produces the oscillator strength of the free absorbing molecule in a rarefied gas. The results are analyzed in terms of local field contributions and distortions of electron distributions by the surroundings. The relevance of the theory to allowed transitions and forbidden transitions made allowed is discussed.