Quantitation of homogeneous and inhomogeneous broadening mechanisms in t r a n s-stilbene using absolute resonance Raman intensities

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Abstract
The sensitivity of resonance Raman cross sections to the excited state homogeneous width is exploited to separate the homogeneous and inhomogeneous contributions to the optical absorption linewidth in trans-stilbene. Absolute Raman cross sections have been measured using excitation at 356, 299, 282, and 266 nm, and calculations have been performed to model the intensities of the eleven strongly Raman-active modes as well as the absorption spectrum. A simple analysis neglecting explicit temperature effects requires a homogeneous Lorentzian linewidth (Γ) of 310 cm−1 and an inhomogeneous Gaussian distribution of zero–zero energies with a standard deviation (θ) of 500 cm−1. Consideration of thermal excitation in the lowest-frequency single-bond torsional mode significantly improves the fit to the experimental absorption by broadening the spectrum asymmetrically to higher energies, while allowing Γ and θ to be reduced to 120 and 450 cm−1, respectively. The spectral broadening generated by thermal excitation of ground-state torsional modes thus contributes significantly to the homogeneous linewidth in trans-stilbene. The relatively large effective homogeneous width at room temperature is consistent with the low Raman intensity in the ∼200 cm−1 C=C–φ bend compared with the prominence of this vibration in resolved low-temperature electronic spectra.