Femtosecond time-resolved Raman signals on ultrafast dynamics in all-trans-β-carotene

Raman signals of excited states in all-trans-beta-carotene have been investigated using recently developed femtosecond time-resolved Raman spectroscopy. The Raman signals are distinguished from other absorbance changes due to highly excited states using temporal and spectral dependence on the Raman pump pulse. The negatively stimulated Raman signals due to the 2Ag(-) excited state are explained by the Raman transition from the vibrational excited levels. The vibrational relaxation of the 2Ag(-) state has fast and slow kinetics. The fast one has a time constant of 0.6 ps, but the other is slower than the internal conversion to the ground state. Since the molecule exists in the vibrationally excited levels through the relaxation kinetics, the vibrational features in beta-carotene are quite important with relation to energy transfer in photosynthesis.