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 beta-carotene in n-hexane. The short-lived excited electronic state S-2 was accessed in two ways: (i) by transient FSR after an actinic pulse to populate the S-2 state, exploiting resonance from an S-x <- S-2 transition, and (ii) by FSR without actinic excitation, using S-2 <-> S-2 resonance exclusively and narrow band Raman/broad-band feintosecond probe pulses only The two approaches have nonlinear optical susceptibilities chi((5)) and chi((3)), respectively. Both methods show low-frequency bands of the S-2 state at 200, 400, and,similar to 600 cm(-1), which are reported for the first time. With (ii) the intensities of lowfrequency vibrational resonances in S-2 are larger compared to those in S-0, implying strong anharmonicities/mode mixing in the excited state. In principle, for short-lived electronic states, the chi((3)) method should allow the best characterization of low-frequency modes.