title: Development of a tunable femtosecond stimulated raman apparatus and its application to beta-carotene.
abstract: 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 beta-carotene. Using 560 nm Raman excitation, the resonant S1 state modes are enhanced by a factor of approximately 200 compared with 800 nm FSRS experiments. The improved signal-to-noise ratios facilitate the measurement of definitive time constants for beta-carotene dynamics including the 180 fs appearance of the S1 vibrational features due to direct internal conversion from S2 and their characteristic 9 ps decay to S0. 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 approximately 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.
authors: Department of Chemistry, University of California, Berkeley, California 94720, USA.
authors: Sangdeok
authors: S
authors: Shim
authors: Richard A
authors: RA
authors: Mathies
mesh_terms: D012015:Reference Standards; D013059:Spectrum Analysis, Raman / Q000295:instrumentation* / Q000379:methods* / Q000592:standards; D013237:Stereoisomerism; D013997:Time Factors; D019207:beta Carotene / Q000737:chemistry*
chemical_list: D019207:beta Carotene