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 <i>n</i>-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_<i>x</i> ← 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