Ultrafast X-ray diffraction and absorption

Abstract

Abstract The conventional way to study ultrafast chemical dynamics is by ultrafast optical pump/probe experiments. Unfortunately, such optical measurements cannot be directly inverted to give the desired positions of the atoms as a function of time except in very favourable cases where the system is small and there is a prior knowledge of the potential energy surfaces on which the atoms move. Unlike optical spectroscopy, X-ray diffraction and X-ray absorption fine structure do in principle provide direct ways to reconstruct the motions of atoms during dynamic processes. In addition, at least in principle, X-ray diffraction can 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 threshold can be used to follow the charge or oxidation state of chosen types of atoms. Thus, time-resolved X-ray absorption and diffraction may serve as more 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 processes in solid-state materials.