Femtosecond X-ray Powder Diffraction Research Articles

Phonon-Induced Relocation of Valence Charge in Boron Nitride Observed by Ultrafast X-Ray Diffraction.

The impact of coherent phonon excitations on the valence charge distribution in cubic boron nitride is mapped by femtosecond x-ray powder diffraction. Zone-edge transverse acoustic (TA) two-phonon excitations generated by an impulsive Raman process induce a steplike increase of diffracted x-ray intensity. Charge density maps derived from transient diffraction patterns reveal a spatial transfer of valence charge from the interstitial region onto boron and nitrogen atoms. This transfer is modulated with a frequency of 250GHz due to a coherent superposition of TA phonons related to the ^{10}B and ^{11}B isotopes. Nuclear and electronic degrees of freedom couple through many-body Coulomb interactions.

Phonon driven charge dynamics in polycrystalline acetylsalicylic acid mapped by ultrafast x-ray diffraction.

The coupled lattice and charge dynamics induced by phonon excitation in polycrystalline acetylsalicylic acid (aspirin) are mapped by femtosecond x-ray powder diffraction. The hybrid-mode character of the 0.9 ± 0.1 THz methyl rotation in the aspirin molecules is evident from collective charge relocations over distances of some 100 pm, much larger than the sub-picometer nuclear displacements. Oscillatory charge relocations around the methyl group generate a torque on the latter, thus coupling electronic and nuclear motions.

Femtosecond X-ray powder diffraction

Femtosecond X-ray powder diffraction

Transient electron density maps from femtosecond x-ray powder diffraction

Ultrashort hard x-ray pulses are sensitive probes of structural dynamics on the picometer length and femtosecond time scales of electronic and atomic motions. Using short hard x-ray pulses as probe in a pump-probe scheme allow to do femtosecond x-ray diffraction experiments [1], which provide transient electron density maps at a femtosecond timescale with a sub-angstrom spatial resolution. In a typical femtosecond x-ray powder diffraction experiment many Debye-Scherrer rings, up to a maximum diffraction angle 2θmax, are recorded for each time delay between the optical pump and the hard x-ray probe. From the diffraction pattern, the change of the diffracted intensity of each rings are monitored. The interference of diffracted x-rays from the many unexcited cells, with known structure factors coming from steady-state measurement, and diffracted x-rays from the few excited cells allows for the detection of the transients structure factors. Problems could arise if the 3D-Fourier transform is directly used because of the abrupt end of the collected information in the reciprocal space (maximum diffraction angle 2θmax). In order to overcome this problem, the Maximum Entropy Method is apply to the data and the transient electron density maps are derived. We apply the femtosecond x-ray powder diffraction technique and the Maximum Entropy Method to study the induced transient polarization by high optical fields on ionic crystals. Such polarizations are connected to a spatial redistribution of electronic charge, which corresponds to a charge transfer between the two ionic compounds [2]. While the charge transfer originates from the anion to the cation in the LiBH and the NaBH4, the LiH exhibits a peculiar behavior: the charge transfer occurs from the cation to the anion. As result from comparison with calculations in the COHSEX framework, this behavior is due to the strong electronic correlations in the LiH [3].

Femtosecond X-ray diffraction maps field-driven charge dynamics in ionic crystals.

X-Ray diffraction provides insight into the distribution of electronic charge in crystals. Equilibrium electron distributions have been determined with high spatial resolution by recording and analysing a large number of diffraction peaks under stationary conditions. In contrast, transient electron densities during and after structure-changing processes are mainly unknown. Recently, we have introduced femtosecond X-ray powder diffraction from polycrystalline samples to determine transient electron density maps with a spatial resolution of 0.03 nm and a temporal resolution of 100 fs. In a pump-probe approach with a laser-driven tabletop hard X-ray source, optically induced structure changes are resolved in time by diffracting the hard X-ray probe pulses at different time delays from the excited powder sample and recording up to several tens of reflections simultaneously. Time-dependent changes of the atomic arrangement in the crystal lattice as well as modified electron densities are derived from the diffraction data. As a prototypical field-driven process, we address here quasi-instantaneous changes of electron density in LiBH(4), LiH and NaBH4 in response to a non-resonant strong optical field. The light-induced charge relocation in LiBH(4) and NaBH(4) exhibits an electron transfer from the anion (BH) to the respective cation. The distorted geometry of the BH4 tetrahedron in LiBH(4) leads to different contributions of the H atoms to electron transfer. LiH displays a charge transfer from Li to H, i.e., an increase of the ionicity of LiH in the presence of the strong electric field. This unexpected behavior originates from strong electron correlations in LiH as is evident from a comparison with quasi-particle bandstructures calculated within the Coulomb-hole-plus-screened-exchange (COHSEX) formalism.

Field-Driven Dynamics of Correlated Electrons in LiH andNaBH4Revealed by Femtosecond X-Ray Diffraction

We study the quasi-instantaneous change of electron density in the unit cells of LiH and NaBH4 in response to a nonresonant strong optical field. We determine for the first time the related transient electron density maps, applying femtosecond x-ray powder diffraction as a structure probe. The light-induced charge relocation in NaBH4 exhibits an electron transfer from the anion (BH(4)(-)) to the Na(+) cation. In contrast, LiH displays the opposite behavior, i.e., an increase of the ionicity of LiH in the presence of the strong electric field. This behavior originates from strong electron correlations in LiH, as is evident from a comparison with quasiparticle band structures calculated within the Coulomb-hole-plus-screened-exchange formalism.

Ultrafast inter-ionic charge transfer of transition-metal complexes mapped by femtosecond X-ray powder diffraction.

The transient electronic and molecular structure arising from photoinduced charge transfer in transition metal complexes is studied by X-ray powder diffraction with a 100 fs temporal and atomic spatial resolution. Crystals containing a dense array of Fe(II)-tris(bipyridine) ([Fe(bpy)3](2 +)) complexes and their [Formula: see text] counterions display pronounced changes of electron density that occur within the first 100 fs after two-photon excitation of a small fraction of the [Fe(bpy)3](2 +) complexes. Transient electron density maps derived from the diffraction data reveal a transfer of electronic charge from the Fe atoms and-so far unknown-from the [Formula: see text] counterions to the bipyridine units. Such charge transfer (CT) is connected with changes of the inter-ionic and the Fe-bipyridine distances. An analysis of the electron density maps demonstrates the many-body character of charge transfer which affects approximately 30 complexes around a directly photoexcited one. The many-body behavior is governed by the long-range Coulomb forces in the ionic crystals and described by the concept of electronic polarons.

Ultrafast inter-ionic charge transfer of transition-metal complexes mapped by femtosecond x-ray powder diffraction

Transient electron density maps are derived from x-ray diffraction patterns of photoexcited [Fe(bpy)3 ]2+ (PF6 − )2 powder. Upon photoexcitation, the 5 T2 quintet state reveals a charge transfer from the PF6 − ions and from the Fe atoms to neighboring bpy units. The charge transfer from the Fe points to a partial and weak charge-transfer character of this state.

Ultrafast charge relocation in an ionic crystal probed by femtosecond x-ray powder diffraction

Transient electron density maps of potassium dihydrogen phosphate(KH2PO4, KDP) are derived from femtosecond x-ray powder diffraction patterns. 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 of the nuclear motion. The results demonstrate a charge transfer from the volumes around the P-atoms to those containing the O—H···O units and a quadrupolar distortion of the K+ charge distribution. This behavior reflects the interplay of nuclear motions and electric polarizations in the ionic crystal lattice.

Electron Transfer in a Virtual Quantum State ofLiBH4Induced by Strong Optical Fields and Mapped by Femtosecond X-Ray Diffraction

Transient polarizations connected with a spatial redistribution of electronic charge in a mixed quantum state are induced by optical fields of high amplitude. We determine for the first time the related transient electron density maps, applying femtosecond x-ray powder diffraction as a structure probe. The prototype ionic material LiBH4 driven nonresonantly by an intense sub-40 fs optical pulse displays a large-amplitude fully reversible electron transfer from the BH4(-) anion to the Li+ cation during excitation. Our results establish this mechanism as the source of the strong optical polarization which agrees quantitatively with theoretical estimates.

Ultrafast soft-mode driven charge relocation in an ionic crystal

Transient electron density maps of potassium dihydrogen phosphate (KH(2)PO(4), KDP) are derived from femtosecond X-ray powder diffraction patterns. 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 charge transfer from the volumes around the P-atoms and K(+)-ions to those containing the O-HO units and a quadrupolar distortion of the K(+) charge distribution. This behavior reflects the interplay of nuclear motions and electric polarizations in the ionic crystal lattice.

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 [(NH(4))(2)SO(4)]. 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.