title: Ultrafast large-amplitude relocation of electronic charge in ionic crystals.
abstract: The interplay of vibrational motion and electronic charge relocation in an ionic hydrogen-bonded crystal is mapped by X-ray powder diffraction with a 100 fs time resolution. Photoexcitation of the prototype material KH(2)PO(4) induces coherent low-frequency motions of the PO(4) tetrahedra in the electronically excited state of the crystal while the average atomic positions remain unchanged. Time-dependent maps of electron density derived from the diffraction data demonstrate an oscillatory relocation of electronic charge with a spatial amplitude two orders of magnitude larger than the underlying vibrational lattice motions. Coherent longitudinal optical and tranverse optical phonon motions that dephase on a time scale of several picoseconds, drive the charge relocation, similar to a soft (transverse optical) mode driven phase transition between the ferro- and paraelectric phase of KH(2)PO(4).
authors.affiliation: Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany.
authors.forename: Flavio
authors.initials: F
authors.lastname: Zamponi
authors.forename: Philip
authors.initials: P
authors.lastname: Rothhardt
authors.forename: Johannes
authors.initials: J
authors.lastname: Stingl
authors.forename: Michael
authors.initials: M
authors.lastname: Woerner
authors.forename: Thomas
authors.initials: T
authors.lastname: Elsaesser