Solvent effects in the Raman spectra of the triiodide ion: Observation of dynamic symmetry breaking and solvent degrees of freedom

Resonance Raman spectra, including absolute cross sections, have been measured for the triiodide ion in ethanol, ethyl acetate, and acetonitrile solvents using eight excitation wavelengths that span the two lowest absorption bands. Preresonance Raman spectra have also been obtained with 488 nm excitation. The apparent vibrational line widths observed on resonance are approximately twice the preresonant line widths in all three solvents. By quantitatively modeling the triiodide/solvent system using two intramolecular modes (the symmetric and antisymmetric stretches) and one low-frequency intermolecular or solvent mode, we are able to reproduce the vibrational broadening on resonance, which, within the context of our model, is due to unresolved combination bands between the intramolecular degrees of freedom and the intermolecular mode. The Raman spectra in ethanol clearly show one-quantum transitions in the antisymmetric stretching mode (nu(3)) which should be symmetry forbidden in the linear D-infinity h geometry, indicating symmetry breaking by the local solvent environment. These ''forbidden'' transitions are weaker in ethyl acetate and are essentially undetectable in acetonitrile, indicating that the solvent environment, averaged over the Raman time scale, appears to be the least symmetric in ethanol and most symmetric in acetonitrile. Variation of the ionic strength in ethanol over 2 orders of magnitude resulted in much smaller changes in the intensity of the antisymmetric stretch feature than were obtained by varying the solvent, indicating that only a small fraction of the symmetry breaking is due to the local ionic atmosphere. The importance of symmetry breaking on the overall photodissociation dynamics is discussed.