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(c) 2002 BMO

"Exploring the multidimensional character of ultrafast molecular processes"
S. Lochbrunner and E. Riedle
Recent Res. Devel. Chem. Physics 4, 31 - 61 (2003)

The interplay of many degrees of freedom is characteristic for molecular processes and one dimensional models are inadequate. With a UV-visible pump-probe spectrometer based on noncollinearly phase matched optical parametric amplifiers (NOPAs) providing an unprecedented time resolution of 30 fs we investigate the dynamics of molecular processes with prototype character on the time scale of nuclear motions. The spectrally well separated UV excitation and visible probe pulses allow for the first time to observe coherent wavepacket dynamics unequivocally associated with a reactive process in a multidimensional system. We identify the role of the various nuclear and electronic degrees of freedom in real time and gain direct insight into the mechanisms of photoreactions and electronic relaxation processes.
In a number of molecules showing ultrafast excited state intramolecular proton transfer (ESIPT) we observe characteristic oscillatory signal contributions due to coherent wavepacket motion. They reveal that up to four skeletal normal modes contribute significantly to the reaction path whereas the proton itself plays a rather passive role. We are able to reconstruct the geometry changes from the amplitudes and phases of coherently excited vibrations observed in the product state. By applying this procedure to the ESIPT we resolve the course of the reaction. These results lead to a new multidimensional ESIPT model and explain the appearance of irreversibility without loss of coherence. Previously the existence of vibrational wavepackets has been demonstrated in several systems. However, this is the first time that the observation and analysis of its evolution reveals the reaction mechanism. In addition we show that wavepacket dynamics associated with different reaction channels can be distinguished by their dependence on the excess energy. In the case of intramolecular double proton transfer it enables us to identify and discern the mechanisms relevant for the stepwise and the concerted double proton transfer.
For internal conversion (IC) processes and photo-Fries rearrangements we find a common electronic relaxation mechanism which arises from the coupling between three electronic states. The process proceeds from the originally excited ππ* state via a barrier due to an avoided crossing onto a dark πσ* state. For the IC a subsequent conical intersection leads to a very efficient return to the electronic ground state. The first real time observation of a photo-Fries rearrangement and the associated radical generation and recombination shows that in this case a part of the population proceeds on the dissociative πσ* state resulting in a cleavage of the O-C-bond. In both cases the vibrational normal modes relevant for the electronic coupling are orthogonal to those primarily excited by the applied UV laser pulse.

BMO authors (in alphabetic order):
Stefan Lochbrunner
Eberhard Riedle

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Letzte Änderung: 2016-09-14 13:34