X-ray "filming" of atomic motions in chemical reactions

A recent theory of time-resolved X-ray diffraction is used to show how atomic motions in chemical reactions can be monitored. Only quasi-static reactions which evolve slowly as compared to the time scale of laser excitation are considered. The general expression for the X-ray signal is expressed in terms of temporally varying pair distribution functions, and a procedure is described to extract them from time resolved X-ray experiments. The density changes induced by laser heating are also included into the theory, and their signature is discussed. New experimental results concerning the I-2/CCl4 solution are reported. The recombination of the laser-dissociated iodine is monitored at pico- and nanosecond time scales. The CCl4 molecules take part of the reaction; the solvent thus appears as a reaction partner rather then an inert medium hosting it. (C) 2004 Published by Elsevier B.V.