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

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.