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

Lead Research Organisation: Diamond Light Source
Department Name: Science Division

Abstract

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.

Planned Impact

Probably the most important contributor over the last 100 years to the way we can visualize chemical, materials and biochemical structures on an atomic scale is X-ray diffraction; in essence this forms the gold standard in international databases. In addition over the last 40 years X-ray spectroscopy has provided local, atomic scale pictures of sites in disordered materials. Being able to identify structures of transients in chemical reactions or in fluorescent states will extend this molecular viewpoint to the primary processes in chemical and biochemical reactivity and in photophysics and photochemistry; this is achievable with storage ring X-ray sources, and dynamic views of structural change is feasible with X-ray free electron lasers. Results from such experiments will form calibration points for theoretical calculations and the interplay between theory and experiment is an enticing possibility. The impact will be on the way we can envisage chemical change in solids and liquids.

In particular, the combination of fast laser spectroscopy with dynamic crystallographic experiments where a fast detector, or the time structure of the synchrotron, are synchronized with the laser pulses to determine the structures of molecules with nanosecond to millisecond lifetimes in the solid-state, will revolutionise the way that chemical and biological processes are viewed. We will be able to "watch chemistry happen" and this will provide exciting new insights into a whole range of molecular and macromolecular processes.

This radical change in our understanding of key processes can have enormous economic and societal benefit. For example it is estimated that 90% of all chemical processes use catalysis, and 35% of the world's GDP is reliant on catalysis. Sectors include energy, materials, food and agriculture. If the factors governing catalyst selectivity were better understood, then the basis of an industry and society with less environmental impact is evident. The strong links to the Catalysis Hub and to a major catalyst supplier (Johnson Matthey) provides a pathway for realising this opportunity.

Similarly, a better understanding of structural change during biochemical processes can also provide a similar impact, adding drug design and the mechanisms of drug-resistance. Diamond's Industrial Liaison Office operates directly with commercial concerns to provide techniques relevant to industrial users. There is thus a direct opportunity for impact in industrial developments, as well as the indirect one from the commercial community learning of developments at Diamond. The largest sector of industrial use is the pharmaceutical industry, who, to a substantial degree, rely on available macromolecular crystallography beamlines.

Publications

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