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 250 GHz 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.

Compact high-flux hard X-ray source driven by femtosecond mid-infrared pulses at a 1 kHz repetition rate.

A novel, to the best of our knowledge, table-top hard X-ray source driven by femtosecond mid-infrared pulses provides 8 keV pulses at a 1 kHz repetition rate with an unprecedented flux of up to 1.5×1012 X-ray photons/s. Sub-100 fs pulses at a center wavelength of 5 µm and multi-millijoule energy are generated in a four-stage optical parametric chirped-pulse amplifier and focused onto a thin Cu tape target. Electrons are extracted from the target and accelerated in a vacuum up to 100 keV kinetic energy during the optical cycle; the electrons generate a highly stable K α photon flux from the target in a transmission geometry.

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.

The Color of the Elements: A Combined Experimental and Theoretical Electron Density Study of ScB2 C2.

The chemical or physical control parameters for the onset of superconductivity in MB2 C2 hetero-graphene materials are unclear. This is mainly due to the almost ubiquitous positional B/C disorder, rendering the description of real structures of borocarbides into one of the most challenging problems in materials science. We will show that high-resolution X-ray diffraction data provides all the essential information to decode even complex coloring problems due to B/C disorder. Electron density studies and subsequent analyses of the fine structure of the Laplacian of the electron density resolves the local electronic structure of ScB2 C2 at sub-atomic resolution and allows for an unequivocal identification of all atoms involved in the coloring scenario. This information could finally be used to identify the electron deficient character of the B/C layers in ScB2 C2 and to synthesize the first bimetallic hetero-metallocene with lithium and scandium atoms embedded in the pentagonal and heptagonal voids, respectively.

Soft-mode driven polarity reversal in ferroelectrics mapped by ultrafast x-ray diffraction.

Quantum theory has linked microscopic currents and macroscopic polarizations of ferroelectrics, but the interplay of lattice excitations and charge dynamics on atomic length and time scales is an open problem. Upon phonon excitation in the prototypical ferroelectric ammonium sulfate [(NH4)2SO4], we determine transient charge density maps by femtosecond x-ray diffraction. A newly discovered low frequency-mode with a 3 ps period and sub-picometer amplitudes induces periodic charge relocations over some 100 pm, a hallmark of soft-mode behavior. The transient charge density allows for deriving the macroscopic polarization, showing a periodic reversal of polarity.

Towards shot-noise limited diffraction experiments with table-top femtosecond hard x-ray sources.

Table-top laser-driven hard x-ray sources with kilohertz repetition rates are an attractive alternative to large-scale accelerator-based systems and have found widespread applications in x-ray studies of ultrafast structural dynamics. Hard x-ray pulses of 100 fs duration have been generated at the Cu K α wavelength with a photon flux of up to 109 photons per pulse into the full solid angle, perfectly synchronized to the sub-100-fs optical pulses from the driving laser system. Based on spontaneous x-ray emission, such sources display a particular noise behavior which impacts the sensitivity of x-ray diffraction experiments. We present a detailed analysis of the photon statistics and temporal fluctuations of the x-ray flux, together with experimental strategies to optimize the sensitivity of optical pump/x-ray probe experiments. We demonstrate measurements close to the shot-noise limit of the x-ray source.

Charge-scaling effect in ionic liquids from the charge-density analysis of N,N'-dimethylimidazolium methylsulfate.

The charge scaling effect in ionic liquids was explored on the basis of experimental and theoretical chargedensity analyses of [C1MIM][C1SO4] employing the quantum theory of atoms in molecules (QTAIM) approach. Integrated QTAIM charges of the experimental (calculated) charge density of the cation and anion resulted in non-integer values of ±0.90 (±0.87) e. Efficient charge transfer along the bond paths of the hydrogen bonds between the imidazolium ring and the anion was considered as the origin of these reduced charges. In addition, a detailed QTAIM analysis of the bonding situation in the [C1SO4]- anion revealed the presence of negative πO→σ*S-O hyperconjugation.

Probing the Zintl-Klemm concept: a combined experimental and theoretical charge density study of the Zintl phase CaSi.

The nature of the chemical bonds in CaSi, a textbook example of a Zintl phase, was investigated for the first time by means of a combined experimental and theoretical charge density analysis to test the validity of the Zintl-Klemm concept. The presence of covalent Si-Si interactions, which were shown by QTAIM analysis, supports this fundamental bonding concept. However, the use of an experimental charge density study and theoretical band structure analyses give clear evidence that the cation-anion interaction cannot be described as purely ionic, but also has partially covalent character. Integrated QTAIM atomic charges of the atoms contradict the original Zintl-Klemm concept and deliver a possible explanation for the unexpected metallic behavior of CaSi.

On the chemical shifts of agostic protons.

Agostic hydrogen atoms in planar d(8) transition metal complexes display a remarkable wide range of chemical shifts from +5 to -10 ppm in the proton NMR spectra. It is therefore surprising that a simple recipe can be elaborated to predict the influence of the local electronic structure of the metal atom on the shielding of the coordinating protons: In cases where the agostic hydrogen atom is pointing to a local Lewis acidic center at the metal the (1)H NMR signal is shifted upfield relative to the scenario where the proton is opposing a local charge concentration at the metal. To trace the physical origin of this empirical relationship, a systematic study has been performed to understand how the (i) topology of the electron density and (ii) orientation of the magnetic field vector, B0, control the paratropic or diatropic characteristics of the induced current density at the metal atom and thus the shielding or deshielding of the agostic protons.

A unifying bonding concept for metal hydrosilane complexes.

Experimental and theoretical charge density studies and molecular orbital analyses suggest that the complexes [Cp2Ti(PMe3)SiH2Ph2] (1) and [Cp2Ti(PMe3)SiHCl3] (2) display virtually the same electronic structures. No evidence for a significant interligand hypervalent interaction could be identified for 2. A bonding concept for transition-metal hydrosilane complexes aims to identify the true key parameters for a selective activation of the individual M-Si and Si-H bonds.

On the electronic structure of Ni(II) complexes that feature chelating bisguanidine ligands.

In this work we report on the syntheses and properties of several new Ni complexes featuring the chelating bisguanidines bis(tetramethylguanidino)benzene (btmgb), bis(tetramethylguanidino)naphthalene (btmgn), and bis(tetramethylguanidino)biphenyl (btmgbp) as ligands. All complexes were structurally characterized by single-crystal X-ray diffraction and quantum chemical calculations. A detailed inspection of the magnetic susceptibility of [(btmgb)NiX(2)] and [(btmgbp)NiX(2)] (X=Cl, Br) revealed a linear temperature dependence of chi(-1)(T) above 50 K, which was in agreement with a Curie-Weiss-type behavior and a triplet ground state. Below approximately 25 K, however, magnetic susceptibility studies of the paramagnetic d(8) Ni complexes revealed the presence of a significant zero-field splitting (ZFS) that results from spin-orbit mixing of excited states into the triplet ground state. The electronic consequences that might arise from the mixing of states as well as from a possible non-innocent behavior of the ligand have been explored by an experimental charge density study of [(btmgb)NiCl(2)] at low temperatures (7 K). Here, the presence of ZFS was identified as one potential reason for the flat angle-spherical Cl-Ni-Cl deformation potential and the distinct differences between the angle-spherical X-Ni-X valence angles observed by experiment and predicted by DFT. An analysis of the topology of the experimentally and theoretically derived electron-density distributions of [(btmgb)NiCl(2)] confirmed the strong donor character of the bisguanidine ligand but clearly ruled out any significant non-innocent ligand (NIL) behavior. Hence, [(btmgb)NiCl(2)] provides an experimental reference system to study the mixing of certain excited states into the ground state unbiased from any competing NIL behavior.