A resonance Raman intensity study of electronic spectral broadening mechanisms in CS2/cyclohexane

Abstract

Complete resonance Raman spectra, including absolute cross sections, have been measured for CS₂ in cyclohexane using four excitation wavelengths from 223 to 204 nm, on resonance with the strongly allowed S₃←S₀ electronic transition. Absolute intensities have also been measured in CS₂ vapor using 200 nm excitation. These total Raman cross sections, together with the Rayleigh cross sections calculated from the Kramers–Kronig transform of the absorption spectrum, are used to determine the homogeneous linewidth Γ for the electronic transition under the assumption that the solvent induced broadening may be partitioned into a static, inhomogeneous part and a very rapidly modulated, homogeneous component. Γ is found to be ∼13 cm⁻¹ in the vapor and ∼200 cm⁻¹ in cyclohexane solution, indicating that homogeneous broadening is responsible for much of the increase in spectral breadth between vapor and solution phases. Direct modeling of the solution phase absorption spectrum and the intensities of the lower lying Raman transitions gives a best fit to the absolute cross sections with a somewhat smaller Γ of ∼115 cm⁻¹, probably due to the approximate nature of the model employed for the highly anharmonic excited state potential surface. The possible importance of solvent ‘‘memory’’ effects (finite solvent correlation time) on the electronic spectral broadening is discussed.