Broad Raman scattering and luminescence in β-carotene solution

Abstract

Resonant secondary radiation spectra of dilute β-carotene solution (10−4–10−5 M) are measured under stationary excitation. The excitation energy is varied within 0–0 and 0–1 transition energies in the S2–S0 transition of β-carotene. When the excitation energy is varied from the peak of the 0–0 absorption band to the low-energy side at 60 K and 175 K, (a) the line shape of 0–0 emission band changes from symmetric to asymmetric, and (b) the intensity of luminescence rapidly decreases as compared with the intensity of Raman scattering by ν1, ν2, and ν3 intramolecular vibronic modes of β-carotene. When the 0–1 absorption band is excited, we successfully separate luminescence and broad Raman component (BR), which is resonant Raman scattering of low-frequency phonon modes in solution. The line shape of the density of vibronic states weighed by the coupling strength between electronic states in a dye molecule and vibrations of the surrounding atoms (WDOS) is determined by BR. Taking into account inhomogeneous broadening and assuming linear electron–phonon coupling, the resonant secondary radiation spectra are calculated using obtained WDOS. The above characters of the experimental results (a) and (b) are well reproduced by the calculations.