Phase-transition induced changes in the electron-phonon coupling of all-trans-β-carotene.

The resonance Raman spectra of the fundamental, combination, and second harmonic modes around the C-C and C=C stretches of all-trans-β-carotene in 1,2-dichloroethane solution are obtained in the 323-83 K temperature range. The Raman scattering cross-section of the fundamentals in the liquid and solid phases generally increases as the temperature decreases, except for the liquid-solid phase transition, which exhibits a decreasing trend. The relative Raman intensities of the combination and harmonic modes of the CC bonds increase as the temperature decreases. The Raman bandwidths of the C=C bonds gradually become narrow but then appear a turning-point in the phase transition. The temperature-induced fundamental modes are analyzed using a coherent weakly damped electron-lattice vibration model and resonant Raman effects. The changes in the combination and harmonic modes are interpreted using the aforementioned model as well as the theory of electron-phonon interaction.