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July 29, 2010

Breaking Up Is Hard To Do

Femtosecond Science: Technique tracks structure, electron dynamics during reactions

Jyllian N. Kemsley

National Research Council of Canada
Villeneuve inspects a laser setup at the Joint Attosecond Science Laboratory, a collaboration between the National Research Council of Canada and the University of Ottawa.
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A spectroscopic method that involves temporarily removing an electron from a molecule enables femtosecond monitoring of molecular structure and electron dynamics during chemical reactions, report Paul B. Corkum, David M. Villeneuve, and colleagues at the National Research Council of Canada and the University of Ottawa (Nature 2010, 466, 604).

The new technique, based on high harmonic generation, allows researchers "to quite literally observe the breaking of the bromine-bromine bond, through the changes in the valence orbital that accompany the breaking," says Marc Vrakking of the Max Born Institute for Nonlinear Optics & Short Pulse Spectroscopy, in Berlin.

In its experiment, the Canadian group used lasers to excite some molecules in a sample of gaseous Br2 while leaving others in the ground state. Another laser pulse drove an electron from each of the molecules. As the laser field oscillated, the electrons were forced to recollide with either dissociating or unexcited Br2, depending on the electrons' origins.

With this method, the recollisions release a burst of extreme-ultraviolet radiation. The emissions from the unexcited molecules act like a fixed background, allowing the researchers to extract the amplitude and phase of the light emitted from the dissociating atoms. In this case, the amplitude yielded information on the separation of the Br nuclei, and the phase yielded the evolution of the electronic structure of the molecule as it broke apart.

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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