Towards molecular movies: exploring reaction dynamics using electron diffraction

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So much of our knowledge and understanding of the world around us comes from a consideration of the structures of molecules. But how do scientists know what is happening at a molecular or atomic level? Diffraction techniques can give us directly information such as the geometry that a molecule adopts, whether that geometry changes depending on the physical state of the substance, and what products are yielded when two or more molecules react. In the 20th century no fewer than 22 Nobel Prizes were awarded for work based around structural studies using X-ray and electron diffraction, leading to such important discoveries as the double-helix structure of DNA and the role of haemoglobin in the life cycle. In the 21st century the new goal is to understand the dynamics of chemical reactions. This requires us not just to observe structures before and after reactions have occurred, but also to gain a deeper knowledge of how and why reactions proceed in particular ways and, ultimately, to use this information to control reactions.The use of pump-probe experiments to study ultrafast events in chemistry, biology and materials science has already begun to revolutionise our understanding of chemical reactions. Such experiments use an intense laser beam to provide energy to molecules (the pumping), changing their fundamental structures, which are then observed (probed). Until now the emphasis has been on using lasers for both the pump and probe phases or, more recently, using X-ray diffraction to probe the structures. Diffraction methods yield transient structures of molecules directly, which is greatly preferable to inferring structural information from spectroscopy.My research takes this one step further and uses electron diffraction as a probe to study the structures of chemical species undergoing changes that occur on a variety of timescales. Electrons are particularly well suited to studying structures in the gas phase, where the lack of influence from neighbouring molecules (an issue with solid-state techniques) allows model systems to be studied. Electrons are efficient probes of molecular structure, with a high scattering cross section and a low proportion of inelastic scattering (which contains little or no structural information). Because electrons are charged they repel one another. This has consequences when very short pulses of electrons are required, and the theoretical limit of temporal resolution in a laboratory is 0.5 picoseconds. Experiments have been performed elsewhere and reported as femtosecond electron diffraction - this is misleading as the technology dictates that the picosecond limit remains. However, it is possible to break through this barrier using electrons with very high energies. Such electrons are routinely used in accelerator physics, where they are sped up until X-rays are emitted. I will ultimately harness these electrons to give pulses with a length of around 100 femtoseconds; when used in a diffraction experiment these electrons will allow the formation and breaking of chemical bonds to be observed.One area where I will use ultrafast electron-diffraction methods is in the study of hydrogen bonds, which are of utmost importance in chemistry and biology and are common in many molecular species such as water, DNA and proteins. Despite many years of work into the mechanisms of the formation and breaking of hydrogen bonds there are still many unanswered questions. A process related to hydrogen bonding, called fast proton transport, is believed to occur in many biological systems where energy is converted from one form to another. It has been proposed that, in systems with more than one hydrogen bond, fast proton transport follows set patterns. I will also work closely with synthetic chemists to ensure that I am studying the systems that really matter to chemists today, setting my work apart from others who are currently practicising ultrafast electron diffraction.