Resonance Raman excitation profiles of methyl iodide in hexane

Abstract

Resonance Raman spectra, including absolute scattering cross sections, have been measured for methyl iodide in hexane at nine excitation wavelengths from 204.2 to 274.0 nm. Spectra excited in the 246–274 nm region, on resonance with the directly dissociative A state, exhibit overtone progressions in the C–I stretch and its combination bands with the methyl umbrella mode. The fundamental of the C–I stretch is weak relative to the overtones when excited near the peak of the A band, an effect shown to arise from interferences between the resonant amplitude and preresonant contributions from higher electronic states. Both the absorption spectrum and the Raman excitation profiles are quite similar in solution and vapor phases, suggesting that the dissociative A state is not strongly perturbed by solvation. This conclusion is consistent with a simple theoretical estimate of the solvation effect on the A state potential.