Raman excitation profiles and second-derivative absorption spectra of β-carotene

Abstract

The absorption and second-derivative spectra of β-carotene in n.hexane, isopentane, and CS2, as well as the Raman excitation profiles for six modes in n.hexane and isopentane have been measured. The combined analysis of the absorption and second-derivative spectra in terms of model summing over the vibronic excited states furnished reliable values of the excited state displacement parameters. The input parameters for the calculation of the full spectra were taken from the Raman band relative intensities, while the line shape functions and their widths were determined by the second-derivative absorption spectra. The Raman excitation profiles were then calculated in terms of the transform theory, both the observed and calculated absorption spectra being transformed. Very good fits with the experimental profiles were found by including a small contribution of inhomogeneous broadening to the total width. It was concluded that the homogeneous broadening mainly derives from dephasing of nondegenerate low-frequency vibrations in the resonant electronic excited state. The low temperature profiles were easily calculated by decreasing this homogeneous contribution. The agreement between calculated and experimental results was improved by assuming that the vibrational frequencies of the carbon–carbon stretching modes increase at the excited state. The resulting nuclear diplacements and frequency shifts at the excited state as well as the Gaussian widths correlate very well with the absorption spectral properties of linear polyenes.