Lehrstuhl für Physikalische Chemie II
Am Hubland, 97074/Würzbug
Other participating persons and organisations:
Research foci (and basic equipment-based research projects):
Due to the devolopment of complex laser systems during the last 15 years physiscists suceeded in generating light pulses with a duration of a few femtoseconds. We have two state-of-the-art femtosecond laser systems capable of producing 100 femtoseond pulses. The interaction of femtosecond pulses with molecules leads to a coherent superposition of several molecular eigenstates. Such a coherent superposition is called a wavepacket in quantum mechanics. Wavepackets move along the potential surfaces and reflect the energy- or the reaction-dynamics taking place after excitation. In several projects we are investigating the development of rovibrational coherences within bound potential energy surfaces by means of several femtosecond time-resolved four-wave mixing techniques ( DFWM = degenerate four-wave mixing, CARS = coherent anti Stokes Raman scattering, CSRS = coherent Stokes Raman scattering) or other coherent Raman techniques (e.g. stimulated Raman scattering). For the first time, the observation of totally separated wave packet motion in the electronically excited and ground states of molecules could be attained by means of femtosecond time-resolved nonlinear four-wave mixing spectroscopy. By changing the laser parameters (wavelength, polarisation), the relative timing between the pulses and the detection wavelength state-selectivity can be achieved. Spectral selectivity for the investigation of the dynamics is possible by the application of broadband detection of the nonlinear signals. Especially for more complex molecular or solid-state systems a much more detailed investigation of the dynamics of the different modes can be achieved. The monitoring of the transport of the excitation energy deposited in the vibronic states of the delocalized p-electron system of polydiacelylene chains by a laser pulse between the internal molecular modes was made possible by femtosecond time-resolved CARS spectroscopy.
In this connection, a great success was the direct control of the excitation of normal vibrations by the change of the timing and/or the phase structure (``chirp'') of the exciting laser pulses. This is the first realization of selective mode excitation in the ground state of complex molecular systems. Furthermore the electronic ground state vibrational dynamics of large, biologically relevant molecules namely porphyrins have been investigated by means of femtosecond time resolved CARS spectrtoscopy. Porphyrins have attracted considerable attention in the last couple of years, because they form the active sites of many important biological systems. The capability of the CARS process to interrogate individual normal modes has been utilised for exploring the influence of nuclear dynamics on the process of internal conversion (IC), by placing the CARS process in a pump-probe scheme, where it plays the role of a mode selective probe. This selectivity is utilised for interrogating the population flow in normal modes of beta-carotene, a molecule which plays a primary role in photosynthesis, subsequent to the S1/S0 IC. Ultrafast intramolecular non-radiative processes such as internal conversion (IC) play a crucial role in the mechanism of photochemical and photobiological processes such as vision and photosynthesis and underline the general concepts associated with molecular electronics. Of special interest is the influence of different normal modes on the rate of internal conversion. It could be shown that the C=C-stretching motion acts as a primary acceptor mode in the S1/S0 IC, i.e. the population flow during the transition from the S1 to the S0 potential energy surface (PES) is funneled predominately into this normal mode. This methods opens a lot of new posibilities since it is possible to monitor the population flow taking place after an electronic excitation mode selectively.
For the results in this research area, please consult the above-mentioned publications.
Rewards and prizes:
Prof. Dr. W. Kiefer: Honorary Fellow of the Laser and Spectroscopy Society of India (F.L.S.S.), 1999. - Distinguished Service Award of the Society of Applied Spectroscopy, 2000. - Foreign Councillor of the Institute for Molecular Science, Okazaki National Research Institutes, Okazaki, Japan, 1999-2001. - Technical Expert Monitor of the International Science and Technology Center, Moscow, Russia, 1999-2002. - Chief Editor of the: Journal of Raman Spectroscopy, 2000. -- PD Dr. J. Popp: Zehetmaier Award (postdoctoral lecture qualification scholarship) awarded by the Freistaat Bayern, 1999. - Cooperation Prize of the University of Würzburg, 2001, - Bunsen-Kirchhoff Prize awarded by the German Bunsen-Society, 2002. -- PD Dr. A. Materny: Heisenberg-Scholarship, 1999. -- Dr. M. Schmitt: DFG Fellowship for a Research Stay in Ottawa, Canada 1999-2000. -- Dr. Sebastian Schlücker: VCI Graduate Research Fellowship, 1999. - Mathias-Manger Award (Diploma) of the Faculty of Chemistry and Pharmacy, 1999. - Faculty Award (Dissertation) of the Faculty of Chemistry and Pharmacy, 2002. -- Dr. T. Chen: Volkswagen-Foundation, 1999. - ICORS 2000 Young Investigator Award, 2000. -- Dr. M. Heid: ICORS 2000 Young Investigator Award, 2000. -- Dr. D. Akimov: DAAD scholarsip, 2002.
Multi colour femtosecond laser system (repetition rate 1kHz, CLARK): Acton SpectraPro mono chromator / CCD camera / accessories; Multi colour femtosecond laser system (repetition rate 100 kHz, Coherent): Acton SpectraPro monochromator / CCD camera / accessories.