Spectroscopy of binary solutions. The benzene–iodine charge-transfer spectrum

The theoretical formalism introduced in an earlier publication to treat the spectroscopy of excess electrons in condensed media is generalized to the case of a mixed solvent. With a simple redefinition of the reference state, the theory is shown to be equally applicable to the spectroscopy of neutral species, and is employed to calculate the familiar benzene–iodine charge-transfer spectrum. The zeroth order contribution to the intensity spectrum is seen to involve a direct dipole transition from the donor (benzene) to the acceptor (iodine), whereas the first and second order contributions involve dipole transitions modified by the effects of intermolecular electron ’’hopping’’ and its fluctuations, respectively. Using the so-called hopping (or resonance) and fluctuation integrals as adjustable parameters, the theory is shown to be capable of reproducing the main features of the experimental spectrum.