Portable femtosecond pump-probe facility (PORTO) for dynamic structural science

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Establishing the atomic arrangements in a molecule or a solid has been feasible for about 100 years by X-ray diffraction; most "pictures (stills)" of the structure of, for example, salt, insulin, haemoglobin and foot and mouse disease virus are based on this technique of scattering X-ray from crystals. For less ordered materials, like glasses and liquid solutions, partial, local structures can be derived from X-ray absorption spectroscopy. Both techniques require scattering off electrons and thus tell us about the atomic arrangements and some insight into electronic distributions.Chemical and light-induced changes are movements of electrons and atoms to new sites and so visualizing these evolutions by X-ray methods can provide chemical videos of reactions which have greater richness than before and after stills; this is the molecular parallel of picturing a galloping horse.Generally changes on the timescales of atomic motion occur between a 1/100 and 1 picosecond (1 ps = 1 millionth of a microsecond), and this has been monitored by changes in the uv and visible spectrum (colour). This provides little information about structure. Infra-red spectroscopy can be used for timescales greater than 1 ps, and is characteristic of functional groups within molecules. This proposal provides a means of approaching the detail of a molecular "still" through chemical changes. The Diamond Light Source is the brightest X-ray source in the UK, and provides the opportunity of studying structures on a timescale of 10s of picoseconds. This is fast enough to catch many excited states of fluorescent materials, and to observe the reactions of the most reactive of transient molecules. UV-visible and infrared spectroscopies will be monitored after changes induced by a laser pulse of about 1/5 of a picosecond. The fast laser spectroscopy will be combined with the rapidly developing technique of photocrystallography, where it is possible to obtain full 3-D solid-state structures of photoactivated species that have lifetimes in the nanosecond to millisecond range, so that it will be possible to make "molecular movies" showing how key chemical and biological processes occur. Thus, it will be possible to study important catalytic, sensor and non-linear materials across the time scales from picoseconds to milliseconds, to see how properties and functions develop over time. Sampling procedures for crystals, solutions and films will be developed and made available to other research groups. The whole approach should transform the way we think about chemical reactions.From such an approach there will be a fraction of problems for which even faster measurements would be fascinating. In recent years laser light in the X-ray region has become available in the USA and Japan (by X-ray free electron lasers, XFELs), and sources are being built in Europe (Germany and Switzerland). They provide an X-ray pulse of about 1/50 of a picosecond, faster than most molecular vibrations, and thus the X-ray movie of a chemical reaction is feasible.This proposal will provide a test-bed for researchers in the chemical sciences to develop their technique for visualizing their reactions. The facility will be based on the Harwell site adjacent to the equipment and expertise of the Diamond Light Source and Central Laser Facility, both of which are user facilities of the highest rank.