Spectra and energy transfer phenomena in crystalline rare gas solvents

Abstract

The use of crystalline nonpolar gases at 4.2°K as solvents allows one to study spectra, solvent shifts, and energy transfer phenomena over a wide range of solvent properties. Contributions to the spectral shifts may arise from the usual London dispersion energy, exchange repulsions and attractions, and dipole-induced-dipole interactions. Except for the frequency shifts and certain intensity perturbations, the spectrum of a nonpolar solute in this sort of an environment is like that of a randomly oriented gas at 4.2°K. The advantages associated with the vibrational analysis of such a spectrum are obvious. A striking perturbation on the phosphorescence lifetime of benzene in heavy rare gases has been explained on the basis of an exchange interaction mixing states of the benzene-rare-gas complex, whose Russell Saunders components are further mixed by the large rare-gas spin-orbit perturbation. The oscillator strength for the purely radiative 3 B 1 u - 1 A 1 g transition in unperturbed benzene is found to be 7.7 × 10 −11, and it is estimated that a 150-cm path of highly purified liquid benzene should be sufficient to observe in absorption the first strong band of this system predicted to lie near 31,000 cm −1. One of the most important factors in a nonradiative process is the vibrational overlap integral associated with the two vibronic states between which energy transfer occurs. A type of tunnelling from the lowest triplet state back to the ground singlet state has been found. It involves usually high vibrational quantum numbers of the ground state. Because of the high sensitivity of the overlap integral to triplet states are therefore probably tunnelling lifetimes and have little to do with the purely radiative lifetimes. Deuterium substitution is expected to increase these lifetimes substantially and allow a number of new triplet states to be found. Energy transfer from the excited singlet to the excited triplet has been found to be highly sensitive to local solvent environment. This process apparently can be essentially stopped or made 100% efficient at will simply by changing the mass of the solvent molecules. This phenomenon is no doubt a function of the strength of the coupling between the internal modes and the lattice modes, the coupling being dependent, of course, on the polarizability of the solvent. Solid hydrogen or neon solvents are expected to provide the least opportunity for energy transfer during an electronic lifetime. Conversely, very polarizable solvents promote the population of the lowest triplet state in those cases where the nonradiative process is slow in hydrocarbon solvents. Applications of this effect to the study of molecular excited states are obvious.