Page 17 - PERIODIC Magazine Issue 5

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 • pimms.chem.ox.ac.uk ‘
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. www.springer.com/us/book/9783319245157.
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

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Periodic
The Magazine of the Department of Chemistry