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laser pulse is very short, on the order of femtoseconds, the   amongst the resulting fragments. Overall, it is hoped that this will
          resulting “Coulomb explosion” occurs before the structure of the   allow for a deeper understanding of the intricate details behind
          molecule changes in response to the lost electrons. The original   photocatalytic reactions, and perhaps allow the development of
          molecular structure can be revealed by using the PImMS sensor   new catalysts that selectively produce socially- and industrially-
          to image and compare the momenta of each ionic fragment using   relevant chemicals through the use of light.
          a technique known as ion imaging. In collaboration with Henrik   The shared effort across the collaboration has pioneered
          Stapelfeldt and his group at Aarhus University in Denmark, the   new experiments, both within the United Kingdom and with
          Oxford team have used this technique to establish the structures   researchers based in Denmark, France, Germany, Russia,
          and chirality of several gas-phase molecules. Images from the   Canada, Sweden, and the United States. The new methods are
          PImMS sensor showing the atoms scattering from the Coulomb   being applied to a variety of chemical reactions and to investigate
          explosion of a substituted biphenyl molecule are shown on the   fundamental physical phenomena such as reaction rate
          cover of this issue.
                                                               constants, charge transfer, and the energy flow into the quantum
          Changes in molecular structure with time can be observed   states of molecular fragments. Each step brings us closer to the
          by acquiring Coulomb explosion images at different stages   goal of achieving a complete understanding and potentially even
          throughout a chemical process. In the example illustrated here,   control of the fundamental chemical dynamics governing our
          two benzene rings oriented in a staggered conformation are   environment.
          linked by a single bond.  Torsional motion about the bond can   References
          be kick-started using a laser pulse and, after a user-defined
          delay, a second pulse can induce a Coulomb explosion, creating   • “Dynamic Stark Control of Torsional Motion by a Pair of
          a snapshot of the position of the rings relative to one another.   Laser Pulses. “ L. Christensen, J. H. Nielsen, C. B. Brandt, C. B.
          Combining images from multiple delays creates a “molecular   Madsen, L. B. Madsen, C. S. Slater, A. Lauer, M. Brouard, M. P.
          movie” of the torsional dynamics.                    Johansson, B. Shepperson, and H. Stapelfeldt. Phys. Rev. Lett.
                                                               113, 073005 (2014).
          The remarkable properties of the PImMS sensor also allow
          mode-selective chemistry to be explored. The research group   • ‘
          of Stuart Mackenzie is currently investigating how to influence   • Studies of Photoinduced Molecular Dynamics Using a Fast
          the ratio of products that form from particular reactants by   Imaging Sensor’.  C. S.  Slater.  Springer monograph: http://
          using infrared light to vibrationally excite the starting materials.
          This opens up new reaction pathways that allow the reaction
          to be directed towards a desired product. For instance, one
          intuitive way to direct a reaction is to vibrationally excite a
          bond to encourage it to break to form a particular product.
          One of the key aims of the collaboration is to use the PImMS
          sensor to scrutinise this phenomenon in detail. This is achieved
          by photodissociating a species of interest (typically a metal-
          bound molecular complex) and utilising the multi-mass
          imaging functionality of the PImMS sensor to view the resulting
          fragments. By comparing the images recorded in the presence
          and absence of infrared light, insight is gained into how the
          energy available in these dissociation processes is partitioned

                                                            The Magazine of the Department of Chemistry
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