Development of a Tunable Femtosecond Stimulated Raman Apparatus and Its Application to β-Carotene

We have developed a tunable femtosecond stimulated Raman spectroscopy (FSRS) apparatus and used it to perform time-resolved resonance Raman experiments with <100 fs temporal and <35 cm -1 spectral resolution. The key technical change that facilitates this advance is the use of a tunable narrow-bandwidth optical parametric amplifier (NB-OPA) presented recently by Shim et al. (Shim, S.; Mathies, R. A. Appl. Phys. Lett. 2006, 89, 121124). The practicality of tunable FSRS is demonstrated by examining the photophysical dynamics of β-carotene. Using 560 nm Raman excitation, the resonant S 1 state modes are enhanced by a factor of ∼200 compared with 800 nm FSRS experiments. The improved signal-to-noise ratios facilitate the measurement of definitive time constants for β-carotene dynamics including the 180 fs appearance of the S 1 vibrational features due to direct internal conversion from S 2 and their characteristic 9 ps decay to So. By tuning the FSRS system to 590 nm Raman excitation, we are able to selectively enhance vibrational features of the hot ground state (S hot 0 ) and monitor its ∼5 ps cooling dynamics. This tunable FSRS system is valuable because it facilitates the direct observation of structural changes of selected resonantly enhanced states and intermediates during photochemical and photobiological reactions.