Kohärenzeffekte in der Valenz-Photoionisation kleiner Moleküle

Kohärenzeffekte in der Valenz-Photoionisation kleiner Moleküle Die Untersuchung von Kohärenzeffekten in der Valenz-Photoionisation kleiner Moleküle ermöglicht Einblicke in molekulare Strukturen und Dynamiken und gibt dadurch Aufschluss über einige fundamentale quantenmechanische Prozesse. Der Schwerpunkt dieser Arbeit liegt auf derartigen Studien. Mittels Synchrotronstrahlung und der winkeldifferenzierenden Flugzeitanalyse von hierbei emittierten Photoelektronen werden molekulare Zwei-Zentren-Interferenzen für die Valenzelektronen der homonuklearen, diatomaren Moleküle N2 und O2 untersucht. Es wird gezeigt, dass durch die Inversionssymmetrie dieser Moleküle und die resultierende Unbestimmbarkeit des Emissionsortes der Photoelektronen Interferenzen auftreten. Die winkelaufgelöste Photoelektronenstudie in einem sehr großen Photonenenergiebereich von 20 bis 600 eV , die in dieser Arbeit vorgestellt wird, zeigt erstmalig sogenannte 'Cohen-Fano Oszillationen' in der Winkelverteilungsanisotropie. Es werden zudem weitere relevante Effekte der kohärenten Photoelektronenemission vorgestellt und im Lichte der gewonnenen experimentellen Daten analysiert. Eine energetisch hoch auflösende Studie von N2 und O2, besonders im niederenergetischen Photonenenergiebereich von 20 bis 50 eV, zeigt des Weiteren bisher unentdeckte Resonanzphänomene, die höchstwahrscheinlich verschiedenen Arten der Doppelanregung zuzuschreiben sind. Ausblickend wird eine Analyse für das polyatomare heteronukleare Molekül CH4 von 20 bis 300 eV vorgestellt, um eine Diskussion über molekulare Multi-Spalt-Systeme anzustoßen.Coherence Effects in the Valence Photoionization of Small Molecules Coherence effects in the valence photoionization of small molecules deliver insight into molecular structures and dynamics which in turn allow to unravel details of fundamental quantum mechanical processes. The work presented in this dissertation primarily deals with such investigations, using third generation synchrotron radiation. The applied technique is angle resolving photoelectron time-of-flight spectroscopy. It is shown that due to the inversion symmetry of homonuclear diatomic molecules like N2 and O2 an indistinguishability of the electrons and corresponding delocalization leads to interference pattern in the angular distribution anisotropy. Such two center interferences are determined in a wide photon energy range from 20 to 600 eV in the valence photoionization angular distribution anisotropy. The observed oscillations will be discussed in the light of the Cohen-Fano formalism. The relevant photoionization dynamics of the broad photon energy study will be discussed. In this context, a high energy resolution study of N2 and O2 is presented for the low photon energy region from 20 to 50 eV, showing hitherto unexplored resonance phenomena most likely originated by different types of doubly excited states. In order to initialize a discussion about more complex molecular multi-slit systems, an angle resolving study from 20 to 300 eV is presented for the heteronuclear polyatomic molecule CH4

Bayesian inferencing and deterministic anisotropy for the retrieval of the molecular geometry ∣Ψ(r)∣2|\Psi(\mathbf{r})|^2 in gas-phase diffraction experiments

Currently, our general approach to retrieve the molecular geometry from ultrafast gas-phase diffraction heavily relies on complex geometric simulations to make conclusive interpretations. In this manuscript, we develop a broadly applicable ultrafast gas-phase diffraction method that approximates the molecular frame geometry $|\Psi(\mathbf{r}, t)|^2$ distribution using Bayesian Inferencing. This method does not require complex molecular dynamics simulation and can identify the unique molecular structure. We demonstrate this method's viability by retrieving the ground state geometry distribution $|\Psi(\mathbf{r})|^2$ for both simulated stretched NO$_2$ and measured ground state N$_2$O. Due to our statistical interpretation, we retrieve a coordinate-space resolution on the order of 100~fm, depending on signal quality, an improvement of order 100 compared to commonly used Fourier transform based methods. By directly measuring the width of $|\Psi(\mathbf{r})|^2$, this is generally only accessible through simulation, we open ultrafast gas-phase diffraction capabilities to measurements beyond current analysis approaches. Our method also leverages deterministic ensemble anisotropy; this provides an explicit dependence on the molecular frame angles. This method's ability to retrieve the unique molecular structure with high resolution, and without complex simulations, provides the potential to effectively turn gas-phase ultrafast diffraction into a discovery oriented technique, one that probes systems that are prohibitively difficult to simulate.Comment: 16 pages, 8 figures, 2 tables. Please find the analysis code and templates for new molecules at https://github.com/khegazy/BIG

Multiple-core-hole resonance spectroscopy with ultraintense X-ray pulses

Understanding the interaction of intense, femtosecond X-ray pulses with heavy atoms is crucial for gaining insights into the structure and dynamics of matter. One key aspect of nonlinear light-matter interaction was, so far, not studied systematically at free-electron lasers -- its dependence on the photon energy. Using resonant ion spectroscopy, we map out the transient electronic structures occurring during the complex charge-up pathways. Massively hollow atoms featuring up to six simultaneous core holes determine the spectra at specific photon energies and charge states. We also illustrate how the influence of different X-ray pulse parameters that are usually intertwined can be partially disentangled. The extraction of resonance spectra is facilitated by the fact that the ion yields become independent of the peak fluence beyond a saturation point. Our study lays the groundwork for novel spectroscopies of transient atomic species in exotic, multiple-core-hole states that have not been explored previously.Comment: Supplementary information is include

Ultrashort Free-Electron Laser X-ray Pulses

For the investigation of processes happening on the time scale of the motion of bound electrons, well-controlled X-ray pulses with durations in the few-femtosecond and even sub-femtosecond range are a necessary prerequisite. Novel free-electron lasers sources provide these ultrashort, high-brightness X-ray pulses, but their unique aspects open up concomitant challenges for their characterization on a suitable time scale. In this review paper we describe progress and results of recent work on ultrafast pulse characterization at soft and hard X-ray free-electron lasers. We report on different approaches to laser-assisted time-domain measurements, with specific focus on single-shot characterization of ultrashort X-ray pulses from self-amplified spontaneous emission-based and seeded free-electron lasers. The method relying on the sideband measurement of X-ray electron ionization in the presence of a dressing optical laser field is described first. When the X-ray pulse duration is shorter than half the oscillation period of the streaking field, few-femtosecond characterization becomes feasible via linear streaking spectroscopy. Finally, using terahertz fields alleviates the issue of arrival time jitter between streaking laser and X-ray pulse, but compromises the achievable temporal resolution. Possible solutions to these remaining challenges for single-shot, full time-energy characterization of X-ray free-electron laser pulses are proposed in the outlook at the end of the review

Ultrafast Nuclear Dynamics in Double-Core Ionized Water Molecules

Double-core-hole (DCH) states in isolated water and heavy water molecules, resulting from the sequential absorption of two x-ray photons, have been investigated. A comparison of the subsequent Auger emission spectra from the two isotopes provides direct evidence of ultrafast nuclear motion during the 1.5 fs lifetime of these DCH states. Our numerical results align well with the experimental data, providing for various DCH states an in-depth study of the dynamics responsible of the observed isotope effect

Opportunities for Gas-Phase Science at Short-Wavelength Free-Electron Lasers with Undulator-Based Polarization Control

Free-electron lasers (FELs) are the world's most brilliant light sources with rapidly evolving technological capabilities in terms of ultrabright and ultrashort pulses over a large range of accessible photon energies. Their revolutionary and innovative developments have opened new fields of science regarding nonlinear light-matter interaction, the investigation of ultrafast processes from specific observer sites, and approaches to imaging matter with atomic resolution. A core aspect of FEL science is the study of isolated and prototypical systems in the gas phase with the possibility of addressing well-defined electronic transitions or particular atomic sites in molecules. Notably for polarization-controlled short-wavelength FELs, the gas phase offers new avenues for investigations of nonlinear and ultrafast phenomena in spin orientated systems, for decoding the function of the chiral building blocks of life as well as steering reactions and particle emission dynamics in otherwise inaccessible ways. This roadmap comprises descriptions of technological capabilities of facilities worldwide, innovative diagnostics and instrumentation, as well as recent scientific highlights, novel methodology and mathematical modeling. The experimental and theoretical landscape of using polarization controllable FELs for dichroic light-matter interaction in the gas phase will be discussed and comprehensively outlined to stimulate and strengthen global collaborative efforts of all disciplines

Resonance-Enhanced Multiphoton Ionization in the X-Ray Regime

Here, we report on the nonlinear ionization of argon atoms in the short wavelength regime using ultraintense x rays from the European XFEL. After sequential multiphoton ionization, high charge states are obtained. For photon energies that are insufficient to directly ionize a 1s electron, a different mechanism is required to obtain ionization to Ar17+. We propose this occurs through a two-color process where the second harmonic of the FEL pulse resonantly excites the system via a 1s -> 2p transition followed by ionization by the fundamental FEL pulse, which is a type of x-ray resonance-enhanced multiphoton ionization (REMPI). This resonant phenomenon occurs not only for Ar16+, but also through lower charge states, where multiple ionization competes with decay lifetimes, making x-ray REMPI distinctive from conventional REMPI. With the aid of state-of-the-art theoretical calculations, we explain the effects of x-ray REMPI on the relevant ion yields and spectral profile

Wie Freie-Elektronen-Laser Licht in den Auger-Prozess bringen

Der Auger-Effekt ist ein fundamentaler Prozess, bei dem Energie in Materie, etwa nach Aufnahme kurzwelliger Strahlung, durch Abgabe von Elektronen typischerweise innerhalb weniger Femtosekunden umverteilt wird. Experimente an Röntgenlasern machen es nun möglich, diesen bedeutsamen Effekt in bisher unerforschten Zuständen der Materie und in „Echtzeit“ zu untersuchen. Hierdurch lässt sich die interdisziplinäre Bedeutung des Auger-Prozesses aus ganz neuen Blickwinkeln studieren, und wir erlangen im Detail Zugang zur ultraschnellen Dynamik der komplexen elektronischen Struktur von Materie

In Situ X-ray Measurements to Follow the Crystallization of BaTiO3_3 Thin Films during RF-Magnetron Sputter Deposition

Here, we report on adding an important dimension to the fundamental understanding of the evolution of the thin film micro structure evolution. Thin films have gained broad attention in their applications for electro-optical devices, solar-cell technology, as well storage devices. Deep insights into fundamental functionalities can be realized via studying crystallization microstructure and formation processes of polycrystalline or epitaxial thin films. Besides the fundamental aspects, it is industrially important to minimize cost which intrinsically requires lower energy consumption at increasing performance which requires new approaches to thin film growth in general. Here, we present a state of the art sputtering technique that allows for time-resolved in situ studies of such thin film growth with a special focus on the crystallization via small angle scattering and X-ray diffraction. Focusing on the crystallization of the example material of BaTiO$_3$, we demonstrate how a prototypical thin film forms and how detailed all phases of the structural evolution can be identified. The technique is shaped to enable a versatile approach for understanding and ultimately controlling a broad variety of growth processes, and more over it demonstrate how to in situ investigate the influence of single high temperature sputtering parameters on the film quality. It is shown that the whole evolution from nucleation, diffusion adsorption and grain growth to the crystallization can be observed during all stages of thin film growth as well as quantitatively as qualitatively. This can be used to optimize thin-film quality, efficiency and performance

The Small Quantum Systems - SQS Instrument at the European X-Ray Free Electron Laser

This contribution will present the Small Quantum System (SQS) scientific instrument, which is one of six experimental end stations at the European XFEL planned to open for user operation in autumn 2017. This experimental platform is designed for investigations of atomic and molecular systems, as well as clusters, nano-particles and small bio-molecules. It is located behind the SASE 3 soft x-ray undulator, which will provide horizontally polarized FEL radiation in a photon energy range between 260 eV and 3000 eV (4.8 nm to 0.4 nm) with 0.1 to 2 × 10e14 photons per pulse and up to 27000 pulses per second. Two high-quality elliptical mirrors in Kirkpatrick-Baez configuration will focus the FEL beam to a FWHM spot size of approximately 1 µm diameter. This is going to result in an intensity of more than 10e18 W/cm2 within the interaction region, which will allow for studying non-linear multi-photon processes. Furthermore, the short FEL pulse duration between 2 fs and 100 fs in combination with a synchronized optical femtosecond laser will enable time-resolved studies of dynamic processes, thus capturing the motion of electrons and nuclei with unprecedented resolution in space on ultrafast time scales