Chemistry continues to flourish at the ESRF, with more than half of the beamlines receiving applications for experiments, illustrating the wide-ranging nature of chemistry itself and the many applications of synchrotron radiation in this discipline.

The first two examples show how the structural response of chemical systems to electronic excitation by laser light can be followed on the picosecond timescale, for either a crystalline organic solid, or for a simple diatomic molecule in solution. Direct knowledge of the resulting dynamic processes yields information about the potential-energy surface between the interacting species and complements theoretical and spectroscopic investigations. These are technically challenging experiments, for which the high brilliance and pulsed nature of the ESRF source are essential.

The covalent interactions between the atoms of a diatomic molecule can also be affected by the application of pressure. At high pressures halogen molecules dissociate. For iodine, metallisation precedes dissociation, but the onset of metallisation for the other halogens is uncertain. X-ray absorption spectroscopy measurements of the evolution of the pre-K-edge peak in bromine with pressure show convincingly that metallisation in bromine occurs before the subsequent dissociation, at pressures in accord with recent calculations. 

Synchrotron radiation is often brought to bear on systems exhibiting complex behaviour. Two examples of crystalline organic materials are illustrated, both were investigated by single-crystal X-ray diffraction. In the first, there is an intricate scheme of disorder, accompanied by extensive diffuse scattering and satellite reflections, which evolves from the tip to the centre of the crystal. Analysis of the data was performed using a distributed array of computers to obtain a detailed three-dimensional model. In the second, the material exhibits the unusual property of negative thermal expansion along one direction, contracting rather than expanding as the temperature is raised. The crystal structure was solved from a small single crystal, and the evolution of the unit-cell dimensions with temperature was followed using a powder.

EXAFS spectroscopy is used in conjunction with high-resolution powder X-ray diffraction to investigate copper ions in a copper-exchanged zeolite, a microporous material that has been found active for the catalytic conversion of nitrous oxide (NO) to inoffensive nitrogen and oxygen. The location of the ions within the structure and characterisation of their interactions with adsorbed molecules, by complimentary approaches, are important steps towards understanding the mechanism of the catalytic activity.

Investigation of radioactive materials is now possible at ESRF, at the BM20 CRG beamline, where all the necessary facilities for safe handling are installed. The EXAFS investigations included in these highlights are the study of the formation of a variety of monomeric and polynuclear complexes of the uranyl ion with hydroxide as a function of pH, a topic of interest for many years. Finally, the stability in solution of novel technetium-based complexes has been studied as part of an investigation into possible new radiopharmaceuticals, used in medical imaging.