Diffraction patterns, generated using X-ray pulses of only picosecond duration, reveal the motion of individual atoms as iodine molecules dissociate, then recombine, in solution. Location, location, location! That well-known saying about real-estate value is an apt paraphrase of what chemists want to know about chemical reactions, as atoms change position to transform one molecule into another. These changes govern the outcome of a reaction, and ultimately deter mine the biological function of the molecule. As they report in Physical Review Letters, Anton Plech et al. have extended the technique of X-ray diffraction to track such changes on a shorter timescale than ever before (Phys. Rev. Lett. 92, 125505; 2004). They have monitored the photo-dissociation of iodine dissolved in carbon tetrachloride at the level of picoseconds -- just 10~(-12) seconds.
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