Berlin 2014 – wissenschaftliches Programm

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MO: Fachverband Molekülphysik

MO 5: Posters 1: Electronic Spectroscopy, Photochemistry, Femtosecond Spectroscopy

MO 5.20: Poster

Montag, 17. März 2014, 16:30–18:30, Spree-Palais

Ultrafast non-radiative relaxation dynamics of electronically excited pentafluorobenzene by femtosecond time-resolved mass spectrometry — •Ole Hüter, Hendrikje Neumann, and Friedrich Temps — Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel

The perfluoro effect in unsaturated aliphatic and aromatic hydrocarbons leads to a pronounced lowering of the πσ* electronic state located on the C–F bond. As a result, the πσ* state in pentafluorobenzene (PFB) becomes almost isoenergetic with the ππ* excited state. We have studied the ultrafast non-radiative excited-state dynamics of jet-cooled PFB using femtosecond transient time-of-flight mass spectrometry. The molecules were excited by femtosecond pump pulses of 30 fs duration at selected wavelenghts in the range 240–273 nm and probed by multi-photon ionization with 100 fs pulses at 800 nm. The observed temporal profiles exhibit a complex multi-exponential decay behaviour. Depending on the excitation energy, at least two and up to five decay time constants were necessary to fit the experimental data. These may be assigned to the transfer through the conical intersection between the S1 and S2 states, as well as the relaxation to the electronic ground state. The most prominent feature of all transients is a superimposed coherent oscillation with 75–78 cm−1 frequency and damping times between 0.8 and 6 ps, accounting for up to 40 percent of the total signal. In accordance with our previous work on HFB (Studzinski and Temps, JCP 128, 2008), this might hint at the vibrational coupling mode between the two electronic states.

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