Ultrafast Chemical Reaction Dynamics Probed by Time-resolved Dissociative Ionization

(at Max-Planck-Institut für Quantenoptik, Garching b. München, Germany)

We have been interested in dynamics of nonradiative processes such as internal conversion, solvent relaxation, and chemical reactions. Our aim is to study not only basic phenomena of nonradiative processes (the dynamics of intramolecular charge-transfer excited state: enhanced internal conversion induced by hydrogen bonding interaction) but also the reactions of highly vibrationally excited state formed after an internal conversion. It is concluded that an internal conversion is not a useless photophysical process (many people have thought it as an only energy dissipation process) but a very interesting process that forms an important intermediate in multiphoton hot molecule chemistry. Transient emission and absorption spectroscopy was used in the course of these studies to investigate the photophysical processes and photochemical reactions; however, the application of these techniques are limited to some extent. New transient spectroscopy that can measure the dark (non-emissive) state is strongly needed for more understanding about the nonradiative processes and photochemical reactions.
The member of Max-Planck-Institut fur Qunatenoptik (Germany) has developed a technique to monitor the path of the molecule all the way along the potential energy surfaces from the primary excited state (Franck-Condon region) down to the ground state of the products or reactant. There are no dark states in this method because nonresonant multiphoton ionization with mass selective detection of the ion yields was used. A crucial feature is that many (ion) signals are observed, and the success is based on the fact that the fragmentation pattern is different for different locations on the potential energy surfaces. We are also studying the nonresonant ionization process with a high intense femtosecond laser (operated with the laser power between 1013 ~ 1016 Wcm-2) in collaboration with physicists of Osaka university. Our aims are: (1) detection of a small amount of molecule, which is difficult to be detected by other techniques such as laser induced fluorescence (e.g. chlorinated compound such as dioxin); (2) investigation and application of highly energized and charged atoms formed by Coulomb explosion (X-ray generation etc.). On the other hand, the scientists of Max-Planck-Institut aimed to investigate the reaction dynamics. Although we and the members of Max-Planck-Institut made different approaches, both found similar and important features in such an ionization process. Phenomenologically it was clear that femtosecond intense laser pulses lead to enhanced formation of parent ions compared with nanosecond laser excitation. However, heavy fragmentation was observed in some molecules. We generalize that the non-resonance between the laser wavelength and the electronic energy levels of parent cation is a key factor in the formation of the parent ion during femtosecond laser excitation.
We started the joint-project on both, methodological development and its application to several interesting problems of ultrafast chemical reaction dynamics through conical intersection (intramolecular charge transfer excited state, highly vibrationally excited state etc.) based on the findings described above. (at the laboratory of Dr.Werner Fuß, Dr. Wolfram E. Schmid, and Dr. Sergei A. Trushin in Max-Planck-Institute for Quantum Optics, Garching, Germany).

Dr. Tomoyuki Yatsuhashi thanks to Max-Planck Society and Alexander von Humboldt foundation for research fellowship.

1) Coherent oscillations in the charge-transfer system 4-dimethylamino-benzonitrile
Trushin, S.A. ; Yatsuhashi, T.; Fuß, W.; Schmid, W. E.
Chem.Phys.Lett. 2003, 376, 282-291.
2) Ultrafast Charge Transfer and Coherent Oscillations in 4-Piperidino-benzonitrile
Yatsuhashi, T.; Trushin, S. A.; Fuß, W.; Rettig, W.; Schmid, W. E.; Zilberge, S.
Chem. Phys. 2004, 296, 1-12.
3)Ultrafast Temporary Charge Transfer in Pyrrolidinyl-benzonitrile and Pyrrolyl-benzonitrile in the Gas Phase
Fuß, W.; Rettig, W.; Schmid, W. E.; Trushin, S. A.; Yatsuhashi, T.
Faraday Discuss. (Hot article) 2004, 127, 23-33.

4) Ultrafast Relaxation and Coherent Oscillations in Aminobenzonitriles in the Gas Phase Probed by intense-field Ionization (Invited Review)

Fuss, W.; Schmid, W. E.; Pushpa, K. K.; Trushin, S. A.;
Yatsuhashi, T.
Phys. Chem. Chem. Phys
. 2007, 9(10), 1151-1169. (2007.03.14)

5) Probing of coherent oscillations by multiphoton ionization
Trushin, S. A.; Fuss, W.; Schmid, W. E.;
Yatsuhashi, T.
in "Femtochemistry and Femtobiology: Ultrafast Events in Molecular Science"
Edited by Monique, M. M.; Hynes, J. T.
p.299-302, Elsevier Science, 2004.(Proceedings of International Conference on Femtochemistry, 6th, Paris, France, July 6-10, 2003.)

ISBN 0-444-51656-5