Michael Wörner, Flavio Zamponi, Zunaira Ansari, Jens Dreyer, Benjamin Freyer, Mirabelle Premont-Schwarz and Thomas Elsässer report in the latest issue of the Journal of Chemical Physics on the directly time-resolved observation of a chemical reaction in ammonium sulfate crystals [(NH₄) ₂SO₄] , Starting from a latest-generation short-pulse laser system, they generated a 50-femtosecond (1 fs = 10^-15 seconds) blue flash of light that triggered the chemical reaction. Only minimal time-delayed, they sent a synchronized 100 fs long X-ray flash behind, with which they could depict the event with high spatial resolution. The X-ray pulse is diffracted by a powder of small crystals (so-called Debye-Scherrer method). The physicists were able to reconstruct the instantaneous atomic distances in the crystal and the three-dimensional distribution of the electrons within the crystal from the multitude of simultaneously measured diffraction signals. By taking X-ray snapshots at different times after triggering the reaction, a moving film was created with the help of the stroboscopic effect.

Completely surprisingly, the Berlin physicists observed a reversible chemical reaction, which in principle differs from the known slow, i. thermal phase transitions of ammonium sulfate different. The blue flash of light causes the ammonium ion (NH₄) + to give off a proton, ie a positive charge, and the sulfate ion (SO₄) - an electron, a negative charge. The liberated elementary particles then combine to form a hydrogen atom, which eventually bounces back and forth between two distinct positions within the crystal. This movement is shown in the attached film. The circles indicate the original position of the protons. The red spots indicate the movement of the hydrogen atoms following the chemical reaction.

The X-ray powder diffraction in the femtosecond time range demonstrated here for the first time can be applied to many other systems, for example, to elucidate the properties of molecular magnets or to track the electron movements in (bio) molecular light receivers used in solar cells.