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AG Zinth - Projects
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Ultrafast Fluorescence Spectroscopy - The Kerr-Gate Setup (Development of most modern methods in quantum optics for studies of ultra fast processes in chemistry and biology) Our aim is to resolve the fastest steps in photochemistry and photobiology using femtosecond laser pulses. As fluorescence spectroscopy selectively monitors the dynamics of molecules in the primarily excited state it perfectly complements the often used transient absorption technique. Employing femtosecond time resolution we are capable of tracking the first steps after photo-excitation and getting detailed information about the excited state topology. Some processes we have investigated so far:
We time resolve the fluorescence using a optical Kerr-gate setup. Here, the fluorescence light emitted by a molecule which has been excited with a femtosecond laser pulse is imaged onto this gate. This consists of two crossed polarizers and a Kerr-medium (a fused silica glass plate). A second laser pulse - the gate pulse - is overlapped with the fluorescence in the Kerr-medium. The birefringence of the Kerr-medium induced by this gate pulse opens the Kerr-cell for the duration of this pulse. Using a variable delay line this allows to record the fluorescence intensity at defined times after excitation. Since the fluorescence light is spectrally dispersed after the Kerr cell a complete spectrum can be recorded at a time. Some technical details: We use a Clark CPA2001 system to pump a collinear optical parametrical amplifier (OPA) and a non-collinear OPA (NOPA). The OPA delivers the gate pulses at 1100 nm and the NOPA is used as excitation source for our samples. Employing second harmonic generation and large aperture UV-vis optical elements we are able to excite samples from 200 nm to 680 nm and detect fluorescence from 300 nm to 1000 nm. The time resolution of our set-up is ~ 70 fs.
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