Femtosecond snapshots of a powdered solid

X-ray diffraction has long been an important tool for finding crystal structures by mapping their electron densities. In recent decades, time-resolved x-ray diffraction has probed ever-faster structural changes in single crystals, including atomic motions on the femtosecond time scale. But many materials of interest, such as the transition metal complexes used in organic photovoltaic cells, can’t easily be made into crystals of sufficient size and quality. Now Michael Woerner, Thomas Elsaesser, and colleagues at the Max Born Institute in Berlin have demonstrated femtosecond x-ray powder diffraction, in which the sample is an ensemble of randomly oriented microcrystals of ammonium sulfate, (NH₄)₂SO₄, and the diffraction pattern is composed of concentric rings rather than discrete peaks. The innovation was in engineering the x-ray source—a laser-driven plasma source that produced an ultrafast x-ray pulse from an equally brief optical pulse—to operate stably and at high repetition rate for long enough to reveal small changes in the diffraction ring intensities. From those changes, the researchers calculated the change in the sample’s electron density. As shown in the figure, which depicts the equilibrium electron density and the resulting changes over one slice through the crystal, electrons briefly pool (red blobs) where no nucleus exists in the equilibrium structure—so a nucleus, specifically a proton, must have migrated there. Ultrafast IR spectroscopy confirmed that NH₄ ⁺ ions were reversibly breaking apart; surprisingly, the observed structural change bears no resemblance to either of ammonium sulfate’s known phase transitions. (M. Woerner et al., J. Chem. Phys., in press.)—Johanna Miller