Referierte Publikationen

We present a method to determine the bulk temperature of a single crystal diamond sample at an X-Ray free electron laser using inelastic X-ray scattering. The experiment was performed at the high energy density instrument at the European XFEL GmbH, Germany. The technique, based on inelastic X-ray scattering and the principle of detailed balance, was demonstrated to give accurate temperature measurements, within 8 % for both room temperature diamond and heated diamond to 500 K. Here, the temperature was increased in a controlled way using a resistive heater to test theoretical predictions of the scaling of the signal with temperature. The method was tested by validating the energy of the phonon modes with previous measurements made at room temperature using inelastic X-ray scattering and neutron scattering techniques. This technique could be used to determine the bulk temperature in transient systems with a temporal resolution of 50 fs and for which accurate measurements of thermodynamic properties are vital to build accurate equation of state and transport models.

We propose a setup, based on a periodically driven spin chain, that can realize a high-quality quantum router. We present two protocols, which utilize this setup, that can either generate highly entangled two-qubit states over an arbitrary distance or transfer single-qubit states with high fidelity to any desired location on the chain. In addition, we can execute several protocols at the same time and also store quantum states on the spin chain. Our protocols exploit the effect of coherent destruction of tunneling to control, which spins on the chain couple to each other. This control is acquired by suitably shaping the external driving field. The success of our protocols does not depend on the values of the couplings between the spins as long as they are finite and much smaller than the driving frequency. Our setup is scalable, robust against errors, and may be of practical use for future quantum information technologies.

Quantum electrodynamics (QED) is currently considered to be one of the most accurate theories of fundamental interactions. As its extraordinary precision offers unique scientific opportunities, e.g., search for new physics, stringent experimental tests of QED continue to be of high importance. To this end, highly charged ions represent an exceptional test-bed due to enhanced QED effects. Recently, forbidden transitions in F-like ions have been analyzed to few ppm precision, resolving previous discrepancies between theory and experiment. Here we further test the accuracy of QED calculations with three new (Re, Os, Ir), and two improved (Kr, W) measurements of the P1/22-P3/22 transition energy in F-like ions using the NIST electron-beam ion trap and extreme-ultraviolet and x-ray spectrometers. Good agreement between theoretical and experimental energies is found for all considered elements.

Photoelectron angular distributions of the two-photon ionization of neutral atoms are theoretically investigated. Numerical calculations of two-photon ionization cross sections and asymmetry parameters are carried out within the independent-particle approximation and relativistic second-order perturbation theory. The dependence of the asymmetry parameters on the polarization and energy of the incident light as well as on the angular momentum properties of the ionized electron are investigated. While dynamic variations of the angular distributions at photon energies near intermediate level resonances are expected, we demonstrate that equally strong variations occur near the nonlinear Cooper minimum. The described phenomena is demonstrated on the example of two-photon ionization of magnesium atom.

Multipass high harmonic generation from plasma surfaces is a promising technique to enhance the efficiency of the generation process. In this paper it is shown that there is an optimal distance between two targets where the efficiency is maximized, depending on the laser and plasma parameters. This can be explained by the Gouy phase shift, which leads to the relative phase between the colours being changed with propagation in free space. A simple model is used to mimic the propagation of light from one target to another and to observe this effect in 1D particle-in-cell (PIC) simulations. The results are also verified using 2D PIC simulations.

We present Emission and absorption cross sections of thulium doped calcium fluoride (Tm:CaF2) in the visible to short wave infrared (SWIR) wavelength range for temperatures between 80 K and 300 K. For spectral regions of high and low absorption the McCumber relation and the Fuchtbauer–Ladenburg equation have been used to give reliable results. Furthermore, an estimation for the cross relaxation efficiency is derived from the emission spectra as a function of doping concentration and temperature. In addition, nearly re-absorption-free fluorescence lifetimes for various doping concentrations were studied. It was found that a double exponential fit model is better suited than a migration model to represent the fluorescence decay curves. The measurement results are interpreted in the light of the application of Tm:CaF2 as an efficient active medium in high-energy class diode-pumped solid state lasers.

An improved design of a longitudinally sensitive resonant Schottky cavity pickup for the heavy ion storage rings of the Facility for Antiproton and Ion Research in Europe (FAIR) project is reported. The new detector has a higher measured Q value of ∼3000 and a higher simulated shunt impedance of 473.3 kω. It is possible to vary the sensitivity of the cavity with a motorized mechanism by inserting a dissipative blade during the operation based on experimental needs. Apart from a lower price tag, the new design features a more robust and production-friendly mechanical structure suitable for a series production in the future FAIR project. The manufactured cavity was built temporarily into the experimental storage ring and had delivered its first results using stored heavy ion beams. The structure, simulation results, and performance of this cavity are presented in this work.

Asymptotic safety is a theoretical proposal for the ultraviolet completion of quantum field theories, in particular for quantum gravity. Significant progress on this program has led to a first characterization of the Reuter fixed point. Further advancement in our understanding of the nature of quantumspacetime requires addressing a number of open questions and challenges. Here, we aim at providing a critical reflection on the state of the art in the asymptotic safety program, specifying and elaborating on open questions of both technical and conceptual nature. We also point out systematic pathways, in various stages of practical implementation, toward answering them. Finally, we also take the opportunity to clarify some common misunderstandings regarding the program.

Current laser technology enables table-top high flux XUV sources with photon energies from several tens to several hundreds of eV via high harmonic generation in noble gases. Here we present a compact versatile coupling unit to establish windowless, and thus lossless coupling of such light sources to ultra high vacuum (UHV). The particular coupling unit has been developed to couple a XUV laser source to a heavy ion storage ring. Three-stage differential pumping allows to reduce the input pressure of ~10−6 mbar down to the 10−12 mbar range at the output. The unit particularly reduces the partial pressure of argon, which is used to generate the XUV radiation, by 6 orders of magnitude. Measurements of the pressure distribution inside the different chambers agree well with theoretical simulations. In principle, this unit can also serve for other purposes, where a windowless vacuum coupling is needed, with a transition from High Vacuum (HV) levels to deep UHV, such as coupling to cryogenically cooled detectors, ion traps or to photoelectron emission spectroscopy experiments.

We review the recent experimental and theoretical progress in K-shell detachment studies of atomic anions. On the experimental side, this field has largely benefitted from technical advances at 3rd generation synchrotron radiation sources. For multiple detachment of C-, O-, and F- ions, recent results were obtained at the photon-ion merged-beams setup PIPE which is a permanent end station at beamline P04 of the PETRA III synchrotron light source in Hamburg, Germany. In addition to a much increased photon flux as compared to what was available previously, the PIPE setup has an extraordinary detection sensitivity for heavy charged reaction products that allows one to study detachment processes with extremely low cross sections in the kilobarn range, e.g., for processes involving the simultaneous creation of two core-holes by a single photon as observed in the net triple detachment of F- and the net five-fold detachment of C-. Moreover, hitherto disregarded photodetachment resonances have been discovered, which exhibit a variety of line shapes. For O- the core-hole lifetime could be determined precisely from a high-resolution measurement of a photodetachment resonance. These experimental findings pose new challenges for state-of-the-art atomic theory and require calculations combining photoexcitation (ionization) with decay cascade processes that follow after initial core-hole production.

In this Letter, we present a novel, to the best of our knowledge, single-shot method for characterizing focused coherent beams. We utilize a dedicated amplitude-only mask, in combination with an iterative phase retrieval algorithm, to reconstruct the amplitude and phase of a focused beam from a single measured far-field diffraction pattern alone. In a proof-of-principle experiment at a wavelength of 13.5 nm, we demonstrate our new method and obtain an RMS phase error of better than $łambda /70$. This method will find applications in the alignment of complex optical systems, real-time feedback to adaptive optics, and single-shot beam characterization, e.g., at free-electron lasers or high-order harmonic beamlines.

Using a three-dimensional semiclassical method, we theoretically investigate frustrated double ionization (FDI) of Ar atoms subjected to strong laser fields. The double-hump photoelectron momentum distribution generated from FDI observed in a recent experiment [S. Larimian, Phys. Rev. Research 2, 013021 (2020)2643-156410.1103/PhysRevResearch.2.013021] is reproduced by our simulation. We confirm that the observed spectrum is due to recollision. The laser intensity dependence of FDI is investigated. We reveal that the doubly excited states of Ar atoms and excited states of Ar+ are the dominant pathways for producing FDI at relatively low and high intensities, respectively. The information of which pathway leads to FDI is encoded in the electron momentum distributions. Our work demonstrates that FDI is a general strong-field physical process accompanied with nonsequential double ionization and it can be well understood within the context of recollision scenario.

To celebrate the leading role Computer Physics Communications (CPC) has played in publishing open-source software in computational physics for over 50 years the editors are delighted to announce this Virtual Special Issue. Since 2018, coinciding with the 50th anniversary of the start of the CPC venture, thirty-two invited articles have been published. Each has been peer reviewed and each bears the header ‘CPC 50th anniversary article’. The special issue is in keeping with CPC's ethos: it is focused on computational physics software and is accompanied by twenty-five software systems. The introduction to the collection also includes a personal reflection on Phil Burke, CPC's founder, by Alan Hibbert, a lifelong colleague, who joined Queen's University with Phil in the autumn of 1967. The distinctive feature of CPC is its Program Library which houses and distributes over 3500 open-source programs in computational physics. The introduction concludes with a description of key events in the history of the Program Library, its association with Queen's University Belfast and its transfer to Elsevier's Mendeley Data repository.

Using quantum-mechanical, one-dimensional, non-Born-Oppenheimer simulations we study the control over the strong-field dynamics of the helium hydride molecular ion HeH+ due to interaction driven by short and strong two-color laser pulses. We calculate yields of two competing fragmentation channels: electron removal and dissociation. We find that by changing the relative phase of the two colors, we can select the dominating channel. Nuclear motion is decisive for explaining ionization in this target. Ionization yields are vastly underestimated when nuclear motion is excluded and they are substantially reduced in the heavier isotopologue HeD+. Coupling of the two lowest electronic states is crucial even for the ground-state dissociation process.

The propagation of ultra-intense circularly polarized laser pulses in a relativistically transparent plasma is investigated with the help of particle-in-cell (PIC) simulations. When the incident laser pulse is strong enough to expel almost all electrons from the focal volume, the propagation of the laser front edge is found to be dominated by the balance between the laser radiation pressure and the laser-driven electrostatic pressure. Based on a one-dimensional (1D) model, the laser front edge velocity is predicted to depend on , where n0 is the initial plasma density, nc the critical density and a0 the laser amplitude. PIC simulations show that the theoretical prediction works well for not only 1D but also 2D and 3D geometries.