Journal
JOURNAL OF PHYSICAL CHEMISTRY A
Volume 118, Issue 23, Pages 4071-4078Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jp504272h
Keywords
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Funding
- Japan Society for Promotion of Science (JSPS KAKENHI) [24750023, 24350012]
- Ministry of Education, Culture, Sports, Science and Technology (MEXT KAKENHI) [26104534]
- Grants-in-Aid for Scientific Research [26104534, 24750023] Funding Source: KAKEN
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Carotenoids have two major low-lying excited states, the second lowest (S-2 (1B(u)(+))) and the lowest (S-1 (2A(g)(-))) excited singlet states, both of which are suggested to be involved in the energy transfer processes in light-harvesting complexes. Studying vibrational dynamics of S-2 carotenoids requires ultrafast time-resolved near-IR Raman spectroscopy, although it has much less sensitivity than visible Raman spectroscopy. In this study, the relaxation mechanism of beta-carotene from the S-2 state to the S-1 state is investigated by femtosecond time-resolved multiplex near-IR absorption and stimulated Raman spectroscopy. The energy gap between the S-2 and S-1 states is estimated to be 6780 cm(-1) from near-IR transient absorption spectra. The near-IR stimulated Raman spectrum of S-2 beta-carotene show three bands at 1580, 1240, and 1050 cm(-1). When excess energy of 4000 cm-' is added, the S-1 C=C stretch band shows a large upshift with a time constant of 0.2 ps. The fast upshift is explained by a model that excess energy generated by internal conversion from the S-2 state to the 5, state is selectively accepted by one of the vibronic levels of the S-1 state and is redistributed among all the vibrational modes.
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