Relaxation Mechanism of β‑Carotene from S₂ (1B_u⁺) State to S₁ (2A_g^–) State: Femtosecond Time-Resolved Near-IR Absorption and Stimulated Resonance Raman Studies in 900–1550 nm Region

Carotenoids have two major low-lying excited states, the second lowest (S₂ (1B_u⁺)) and the lowest (S₁ (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 S₂ 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 β-carotene from the S₂ state to the S₁ state is investigated by femtosecond time-resolved multiplex near-IR absorption and stimulated Raman spectroscopy. The energy gap between the S₂ and S₁ states is estimated to be 6780 cm^–1 from near-IR transient absorption spectra. The near-IR stimulated Raman spectrum of S₂ β-carotene show three bands at 1580, 1240, and 1050 cm^–1. When excess energy of 4000 cm^–1 is added, the S₁ 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 S₂ state to the S₁ state is selectively accepted by one of the vibronic levels of the S₁ state and is redistributed among all the vibrational modes