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

Experimentelle Elektronendichtestudien als Schlüssel zum Verständnis chemischer und physikalischer Eigenschaften in ausgewählten Modellsystemen

Im Rahmen dieser Dissertation wurden anhand der topologischen Analyse von experimentell bestimmten Elektronendichteverteilungen verschiedene chemisch oder physikalisch motivierte Fragestellungen in geeigneten Modellsystemen untersucht. Dazu wurde die von Richard F. W. Bader entwickelte Quantentheorie der Atome in Molekülen verwendet. So wurden Übergangsmetallkomplexe mit aktivierten C–H Bindungen oder Si–H Bindungen untersucht, was zum besseren Verständnis von agostischen Wechselwirkungen führte. Eine wichtige Rolle spielt dabei der sogenannte atomare Graph, der die charakteristische räumliche Anordnung der kritischen Punkte des Laplacefeldes der Elektronendichteverteilung in der Valenzschale der betrachten Atome beschreibt. Er zeigt Orte mit einer lokal konzentrierten oder verarmten Elektronendichteverteilung an. Daraus ergab sich die empirische Regel, dass eine starke Aktivierung von C–H Bindungen oder Si–H Bindungen nur beobachtet wird, wenn sich das Wasserstoffatom gegenüber einer ausgeprägten Ladungsverarmungszone am Übergangsmetallatom befindet. Zudem wurden die quasi-eindimensionalen Seltenerd-Übergangsmetallcarbide Sc3FeC4, Sc3CoC4 und Sc3NiC4 untersucht. Obwohl alle drei Verbindungen bei Raumtemperatur isotyp sind, zeigte sich, dass nur Sc3CoC4 bei einer Temperatur von ~ 70K einen strukturellen Phasenübergang durchläuft und unterhalb einer kritischen Temperatur 4.5K supraleitend wird. Die zentrale Ursache hierfür ist die Variation der Valenzelektronenzahl durch den Austausch von Fe durch Co oder Ni. Dadurch werden schrittweise höherenergetische elektronische Zustände besetzt und somit die Fermikante angehoben. Die Änderung des Charakters der Zustände an der Fermikante spiegelt sich wiederum in den unterschiedlichen atomaren Graphen der Übergangsmetallatome und ihren deutlich verschiedenen physikalischen Eigenschaften wieder.The topological analysis of experimentally determined electron density distributions, employing the quantum theory of atoms in molecules developed by Richard FW Bader, was used in this thesis to study chemically or physically motivated questions in appropriate model systems. First, transition metal complexes with activated C -H bonds or Si -H bonds were examined which led to a better understanding of agostic interactions. An important tool during these investigations is the so called atomic graph, which describes the characteristic spatial arrangement of the critical points of the Laplacefield of the electron density distribution in the valence shell of the relevant atoms. It reveals zones with a locally concentrated or depleted electron density distribution. This leads to the empirical rule, that a strong activation of C -H bonds or Si -H bonds is only observed when the hydrogen atom faces a pronounced charge depletion zone at the transition metal atom. In addition, the quasi one-dimensional rare-earth transition metal carbides Sc3FeC4, Sc3CoC4 and Sc3NiC4 were examined. Although all three compounds are isotypic at room temperature, it was revealed during this thesis, that only Sc3CoC4 undergoes a structural phase transition at a temperature of ~ 70K and becomes superconducting below a critical temperature of 4.5K. The main reason for this behaviour is the variation of the valence electrons through the exchange of Fe by Co or Ni. This results in the occupation of progressively higher energy electronic states and a raising of the Fermi level. The change in the nature of the electronic states at the Fermi level is in turn reflected by the different atomic graphs of the transition metal atoms and the distinct physical properties of these three compounds

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

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

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 Kα 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

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 Kα 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