Femtosecond Stimulated Raman Spectroscopy of Linear Carotenoids

Target Analysis Resolves the Ground and Excited State Properties from Femtosecond Stimulated Raman Spectra

Target analysis is employed to resolve the ground and excited state properties from simultaneously measured Femtosecond Stimulated Raman Spectra (FSRS) and Transient Absorption Spectra (TAS). FSRS is a three-pulse technique, involving picosecond Raman pump pulses and femtosecond visible pump and probe pulses. The TAS are needed to precisely estimate the properties of the Instrument Response Function. The prezero "coherent artifact" present during the overlap of the three pulses is described by a damped oscillation with a frequency (ω - ωn) where ωn is a ground state resonance Raman frequency. Simultaneous target analysis of the FSRS and TAS allows the complete excited state dynamics to be resolved with a time resolution better than 100 fs. The model system studied is the carotenoid lycopene in tetrahydrofuran. The lycopene dynamics show a spectral evolution with seven states, including a biphasic cooling process during the S2-S1 internal conversion, multiple S1 lifetimes, and an S* state decaying with a lifetime of 7 ps.