PICOSECOND TRANSIENT NEAR-INFRARED EXCITED RAMAN SPECTRA OF β– CAROTENE: DYNAMICS AND SOLVENT EFFECTS ON THE IN-PHASE C=C STRETCHING MODE IN THE S2 (1Bu+) EXCITED STATE

Abstract: By using a near-infrared excited Raman spectrometer with about 3.3 ps time resolution, we succeeded in obtaining picosecond transient Raman spectra of β–carotene in the S2 (1Bu+) excited state. The in-phase C=C stretching mode of β–carotene in the S2 state was observed at about 1550 cm–1. The solvent effects on this band are discussed in comparison with the previous results on the S0 (1Ag–) ground state and the S1 (2Ag–) excited state. Carotenoids play an important role in the light–harvesting function in photosynthesis. All– trans–β–carotene (β–carotene) is a typical carotenoid with a linear polyene structure that has pseudo C2h symmetry. The S0 ground state of β–carotene has 1Ag – symmetry. The lowest singlet excited state, S1 (2Ag–), is therefore optically forbidden. The lowest optically active state is the S2 (1Bu+) state. The relaxation kinetics in carotenoids have been extensively investigated by using time-resolved spectroscopy. The photogenerated S2 (1Bu+) state converts to the S1 (2Ag–) state within 100 – 350 fs, and the lifetime of the S1 (2Ag–) state is several picoseconds. Recently, Yoshizawa and co-workers reported that the S2 (1Bu+) state of β–carotene has a transient absorption in the near-infrared region over 1000 nm [1]. In the present study, we measured the transient Raman spectra of β–carotene in the S2 (1Bu+) state in resonance with this near-infrared transien