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MO: Fachverband Molekülphysik
MO 24: Femtosecond Spectroscopy III
MO 24.7: Vortrag
Dienstag, 20. März 2007, 16:00–16:15, 6C
Femtosecond Optical and Vibrational Spectroscopy: Evidence for Vibronic Coupling — •Alexander Weigel, Alexander Dobryakov, and Luis Perez Lustres — Humboldt Universität zu Berlin, Fachbereich Chemie, Brook-Taylor-Str. 2, 12489 Berlin
Flavin derivatives serve as antenna pigments in blue light photoreceptors. To gain insight into primary activation steps the dynamics of riboflavin in solution was examined by ultrafast transient absorption spectroscopy. After vibrationless S1 excitation at 490 nm the signal rises on the timescale of solvation dynamics and decays in agreement with the fluorescence lifetime. The transient spectra shape exhibits a significant solvent dependence. In aprotic DMSO environment stimulated emission and ground state bleach are of similar intensity, whereas in protic aqueous solution the stimulated emission band is diminished already at time zero indicating strong coupling to nearby dark states.
Excitation with 5000 cm−1 excess of energy evidences vibronic coupling in DMSO as well. Most strikingly, the transient spectrum evolves non-exponentially and shows strong oscillation with a frequency of 95 cm−1. The quantum dynamics could be modelled by assuming a degenerate two state system with complete population of the S1 state at time zero while damping occurs through intramolecular vibrational redistribution of S1.
The results were further supported by femtosecond stimulated Raman experiments.