Ultrafast electronic charge dynamics in solids mapped by femtosecond X-ray diffraction

Summary form only given. Many functional processes in condensed matter involve atomic motions and charge relocations on ultrashort time scales. Ultrafast spectroscopy has given insight into the dynamics of such events but provides - if at all-very limited structural information. In contrast, x-ray diffraction methods with a femtosecond time resolution allow for spatially resolving transient structures and charge distributions in a most direct way. This knowledge is highly relevant for understanding the structure-function relationship of crystalline materials. Femtosecond x-ray diffraction is based on a pump-probe approach in which a femtosecond pump pulse initiates the structural change and a delayed hard x-ray pulse is diffracted from the excited sample to generate a diffraction pattern reflecting the momentary crystal structure [1]. In particular, transient x-ray powder diffraction patterns consist of many reflections measured simultaneously and, thus, allow for deriving time-dependent electron density maps [2].In this talk, recent results on ultrafast electron and lattice motions in ionic crystals are presented. Such measurements reveal the interplay of lattice and charge motions in the photoexcited prototype material KDP (KH2PO4) [3,4]. Upon photoexcitation, the low-frequency TO soft mode is elongated impulsively and modulates the electronic charge distribution on the length scale of interatomic distances, much larger than the vibrational amplitude. The results demonstrate a net transfer of electronic charge from the P-atoms to the O-H...O units in the crystal lattice. The different length scales of vibrational elongations and charge relocations originate from the long-range Coulomb forces the ionic lattice exerts on the highly polarizable valence electrons of the (PO4) units, and are rationalized with the help of Cochran's model of ferroelectricity in ionic materials [5]. As a second example, the field-driven transfer of valence electrons between ions in a superposition of quantum states will be addressed for the materials LiBH4 [6] and LiH. Such crystals consist of light elements only with a small number of inner electrons. As a result, the valence electrons make a major contribution to the x-ray diffraction signal. In LiBH4, the strong external field provided by a sub-40 fs laser pulse at photon energies far below the bandgap drives an interionic transfer of electronic charge from the (BH4)to the Li+ ion, i.e., over a distance of some 250 pm. The charge density map shows that this transfer occurs exclusively during the pump pulse and is fully reversible. The strong electric field of the pump pulse generates a coherent superposition of valence and conduction band quantum states in which electronic charge is shifted compared to the initial valence band states. This fully reversible transfer mechanism makes a major contribution to the overall optical polarizability of the material.