Femtosecond stimulated Raman spectroscopy of the dark S1 excited state of carotenoid in photosynthetic light harvesting complex

Vibrational dynamics of the excited state in the light-harvesting complex (LH1) have been investigated by femtosecond stimulated Raman spectroscopy (FSRS). The native and reconstituted LH1 complexes have same dynamics. The ν1 (C=C stretching) vibrational mode of spirilloxanthin in LH1 shows ultrafast high-frequency shift in the S1 excited state with a time constant of 0.3 ps. It is assigned to the vibrational relaxation of the S1 state following the internal conversion from the photoexcited S2 state.