Concerted electron and proton transfer in ionic crystals mapped by femtosecond x-ray powder diffraction

X-ray powder diffraction, a fundamental technique of structure research in physics, chemistry, and biology, is extended into the femtosecond time domain of atomic motions. This allows for mapping (macro)molecular structure generated by basic chemical and biological processes and for deriving transient electronic charge density maps. In the experiments, the transient intensity and angular positions of up to 20 Debye Scherrer reflections from a polycrystalline powder are measured and atomic positions and charge density maps are determined with a combined spatial and temporal resolutions of 30 pm and 100 fs. We present evidence for the so far unknown concerted transfer of electrons and protons in a prototype material, the hydrogen-bonded ionic ammonium sulfate [(NH4)2SO4]. Photoexcitation of ammonium sulfate induces a sub-100 fs electron transfer from the sulfate groups into a highly confined electron channel along the c-axis of the unit cell. The latter geometry is stabilized by transferring protons from the adjacent ammonium groups into the channel. Time-dependent charge density maps derived from the diffraction data display a periodic modulation of the channel's charge density by low-frequency lattice motions with a concerted electron and proton motion between the channel and the initial proton binding site. Our results set the stage for femtosecond structure studies in a wide class of (bio)molecular materials.