Resonance Femtosecond-Stimulated Raman Spectroscopy without Actinic Excitation Showing Low-Frequency Vibrational Activity in the S2 State of All-Trans β‑Carotene

Raman scattering with stimulating femtosecond probe pulses (FSR) was used to observe vibrational activity of all-trans β-carotene in n-hexane. The short-lived excited electronic state S2 was accessed in two ways: (i) by transient FSR after an actinic pulse to populate the S2 state, exploiting resonance from an S x ← S2 transition, and (ii) by FSR without actinic excitation, using S2 ↔ S0 resonance exclusively and narrow-band Raman/broad-band femtosecond probe pulses only. The two approaches have nonlinear optical susceptibilities χ(5) and χ(3), respectively. Both methods show low-frequency bands of the S2 state at 200, 400, and ∼600 cm–1, which are reported for the first time. With (ii) the intensities of low-frequency vibrational resonances in S2 are larger compared to those in S0, implying strong anharmonicities/mode mixing in the excited state. In principle, for short-lived electronic states, the χ(3) method should allow the best characterization of low-frequency modes.