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Hannover 2013 – wissenschaftliches Programm

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MO: Fachverband Molekülphysik

MO 10: Femtosecond Spectroscopy II

MO 10.4: Vortrag

Dienstag, 19. März 2013, 12:00–12:15, F 102

Time-Resolved Vibrational Spectroscopy of a Short-lived Dark Electronic State — •Marie S. Marek, Tiago Buckup, and Marcus Motzkus — Physikalisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, D-69120 Heidelberg, Germany

Dark electronic states play an important role in photobiology. Many photophysical and photochemical processes taking place in biological systems after light absorption such as light-harvesting, vision or photo-damage are strongly influenced by the presence of dark states. On the other hand, such dark states are extremely challenging for optical spectroscopy, all the more when they have short lifetimes in the sub-picosecond timescale. Here, however, we present direct observation of vibrational coherences in such a short-lived intermediate dark state in a prominent representative of the carotenoid family, spheroidene. Using the method of pump-degenerate four-wave-mixing (pump-DFWM), we are able to follow the wavepacket dynamics from the initially excited S2 state to the S1 state via an additional excited electronic state. In spheroidene, the S2 state is energetically degenerate with the dark 3Ag state. A mixing of these two states leads to symmetry breaking and allows for an additional excited state absorption, which is forbidden in the pure states. This ESA appears in the pump-DFWM signal at the same delay as a well-known stimulated emission to a hot ground state and interference between the signals leads to a characteristic splitting of the modes in spheroidene’s vibrational spectra for early delays.

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