Ultrafast X-Ray Diffraction: Theory

Abstract

Ever since their discovery by Röntgen (more than 100 years ago), x-rays have made the unseen visible. In particular, much of our experimental knowledge about the structure and electronic densities of atoms and molecules is due to x-ray and electron diffraction measurements. X-ray and electron diffraction have been used to measure the structures of almost all small molecules and x-ray diffraction has been the basis (along with nmr) of most of our structural knowledge about biomolecules. Recent advances in the production of ultrashort x-ray and electron pulses1-3 suggest that diffraction (and absorption) techniques may be used to observe evolving, non-equilibrium structures of systems that are undergoing chemical (or biochemical) reactions or physical changes such as a phase transition or annealing. In such an ultrafast diffraction (or absorption) experiment, an ultrashort optical pulse can be used to initiate a chemical reaction and a second delayed x-ray (or electron pulse) can interrogate the reacting system. By varying the time delay between the two pulses, the motions of atoms during a chemical reaction may be reconstructed.4-6 In addition to watching the nuclear motion, at least in principle, x-ray diffraction could be used to follow the dynamics of the electron density involved in chemical bonding and electron flow, and x-ray absorption in the form of chemical shifts of atomic absorption edges could be used to follow the charge or oxidation state of chosen types of atoms. Hence, time resolved x-ray absorption and diffraction may serve as direct ways to watch the evolution of chemical reactions en route from reactants to products, to observe the microscopic processes by which biomolecules perform their tasks and to observe ultrafast process in solid state materials.