Osnabrück 2002 – wissenschaftliches Programm

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MO: Molekülphysik

MO 2: Femtosekundenspektroskopie I: Ladungstransfer

MO 2.7: Vortrag

Montag, 4. März 2002, 15:30–15:45, HS 15/E07

Ultrafast site-specific infrared spectroscopy of excited state proton transfer — •Matteo Rini, Andreas Kummrow, Jens Dreyer, Erik T. J. Nibbering, and Thomas Elsaesser — Max Born Institut fuer Nichtlineare Optik und Kurzzeitspektroskopie, Berlin-Adlershof

We demonstrate that an elementary photochemical process, excited- state proton transfer (ESIPT), can be studied in a site-specific manner by use of femtosecond mid-infrared spectroscopy, giving direct access to transient molecular structure. We monitor changes in vibrational spectra of 2-(2’-hydroxylphenyl)benzothiazole (HBT), where the photoinduced ESIPT process transforms the enol into the keto species of HBT. We excite HBT with 40 fs near-UV pulses throughout the absorption band of HBT, and probe the C=O stretching mode of the product keto state with mid-infrared pulses. The C=O band builds up on a time scale of 50 fs, after which the strength, and line width remain unchanged indicating the formation of keto-HBT in the v=1 state of the C=O vibration. Blue-shifting of the band occurs on picosecond time scales indicating vibrational cooling. For the first time we provide, with femtosecond infrared spectroscopy, new insights on anharmonic coupling between vibrational modes during a chemical reaction. The modulation of the C=O band frequency indicates the coupling to impulsively excited low frequency modes.