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    A tachograph of photochemical reactions

    03/19/2014

    Many chemical reactions occur at such high speeds that it is virtually impossible to identify the single intermediate steps. Physical chemists from the University of Würzburg have now achieved astonishing successes with a new method.

    Dreidimensionales Spektrum, das ähnlich einem „Fahrtenschreiber“ den chemischen Reaktionsweg vom roten Startmolekül zum blauen Endmolekül aufzeichnet.(Grafik: Stefan Rützel, Universität Würzburg.)
    Dreidimensionales Spektrum, das ähnlich einem „Fahrtenschreiber“ den chemischen Reaktionsweg vom roten Startmolekül zum blauen Endmolekül aufzeichnet.(Grafik: Stefan Rützel, Universität Würzburg.)

    Breaking down chemical reactions into their single steps and exactly tracking the way from the original product over various intermediate stages up until the final product: that is one of the main tasks of physical chemistry. Not an easy one, since most of these reactions occur within incredibly short periods of time, usually in the magnitude of a few femtoseconds – that is a few millionths of one billionth of a second.

    Images have limited informative value

    Although so-called femtosecond lasers are capable of providing the necessary temporal resolution and shedding light on the reaction processes, the images they deliver have one flaw: Frequently, the lasers cannot isolate the overlapping signals of molecules, intermediate stages and end products. In certain cases, the images they provide therefore have limited informative value.

    Now, researchers of the University of Würzburg have succeeded in finding a solution for this problem. Contributors to the project included professor Tobias Brixner, who holds the chair of Physical Chemistry I, professor Bernd Engels from the Institute of Physical and Theoretical Chemistry as well as Stefan Rützel, Meike Diekmann, Patrick Nürnberger and Christof Walter. They presented the results of their work in the current issue of the journal PNAS - Proceedings of the National Academy of Sciences.

    A direct image of the transformation processes

    "We used the so-called three-dimensional optical spectroscopy that provides a direct image of which reactants can be transformed into which products", Tobias Brixner explains. And Patrick Nürnberger draws the analogy: "This allows us to track the reaction process from start to finish in much the same way as a tachograph does."

    Ultra-fast photochemical reactions are at the centre of Tobias Brixner and Bernd Engel's research activities. Such reactions occur when plants start the photosynthesis under light. They are essential for photovoltaic facilities to transform light into electricity, and they are also responsible for colourants to glow. Photochemical reactions also hold much promise when it comes to processing and storing data by means of light.

    A major step towards controlling reactions

    "In order to understand these chemical reactions on a molecular level and to actively regulate them if possible, we need to have a detailed knowledge of the various single steps they usually comprise", Brixner explains. The "multi-dimensional spectroscopy", applied by the Würzburg team for this purpose for the first time, now makes this possible.

    Multidimensional spectroscopy of photoreactivity. Stefan Ruetzel, Meike Diekmann, Patrick Nuernberger, Christof Walter, Bernd Engels, and Tobias Brixner, PNAS Early Edition, www.pnas.org/cgi/doi/10.1073/pnas.1323792111

    Contact

    Prof. Dr. Tobias Brixner, T: +49 (0) 931 31-86330, brixner@phys-chemie.uni-wuerzburg.de

     

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