Ultrafast internal conversion and vibrational relaxation in singlet excited-state all-trans-β-carotene as revealed by femtosecond time-resolved stimulated Raman spectroscopy

Abstract

A femtosecond time-resolved stimulated Raman scattering (FSRS) apparatus was built, and was used to study the ultrafast internal conversion and vibrational relaxation processes between the singlet excited states of all-trans-β-carotene in solution. The FSRS apparatus, constructed on the basis of a three-pulse pump-probe scheme, achieved a time resolution of 150fs, a spectral resolution of 23.7cm-1, and a detection spectral window of 300—4000cm-1. The FSRS results are interpreted in terms of a serial internal-conversion scheme between the low-lying singlet excited states, i.e. S2→SX→S1→S0. Spectral dynamics analysis and kinetics analysis proved that the intra-molecular vibrational relaxation in the S1 state proceeds on a time scale of 0.3—0.6ps, while the ‘hot’ S1 state decays with a time constant similar to the lifetime of this state (～11ps). Furthermore, the time scale of intra-molecular vibrational relaxation and vibrational cooling processes in the S0 state are shown to be ～0.3 and ～15ps, respectively. These results are useful for a deeper insight into the light-harvesting function of carotenoids in photosynthetic organisms.