The soft X-ray monochromator at the SASE3 beamline of the European XFEL: from design to operation.

The SASE3 soft X-ray beamline at the European XFEL has been designed and built to provide experiments with a pink or monochromatic beam in the photon energy range 250-3000 eV. Here, the focus is monochromatic operation of the SASE3 beamline, and the design and performance of the SASE3 grating monochromator are reported. The unique capability of a free-electron laser source to produce short femtosecond pulses of a high degree of coherence challenges the monochromator design by demanding control of both photon energy and temporal resolution. The aim to transport close to transform-limited pulses poses very high demands on the optics quality, in particular on the grating. The current realization of the SASE3 monochromator is discussed in comparison with optimal design performance. At present, the monochromator operates with two gratings: the low-resolution grating is optimized for time-resolved experiments and allows for moderate resolving power of about 2000-5000 along with pulse stretching of a few to a few tens of femtoseconds RMS, and the high-resolution grating reaches a resolving power of 10 000 at the cost of larger pulse stretching.

Compact setup for spin-, time-, and angle-resolved photoemission spectroscopy.

We present a compact setup for spin-, time-, and angle-resolved photoemission spectroscopy. A 10 kHz titanium sapphire laser system delivers pulses of 20 fs duration, which drive a high harmonic generation-based source for ultraviolet photons at 21 eV for photoemission. The same laser also excites the sample for pump-probe experiments. Emitted electrons pass through a hemispherical energy analyzer and a spin-filtering element. The latter is based on spin-polarized low-energy electron diffraction on an Au-passivated iridium crystal. The performance of the measurement system is discussed in terms of the resolution and efficiency of the spin filter, which are higher than those for Mott-based techniques.

Time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser.

Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of nonequilibrium electronic processes, transient states in chemical reactions, or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical, and structural analyses requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines free-electron laser capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in (kx, ky, E) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å-1 diameter in a binding-energy range of several eV, resolving about 2.5 × 105 data voxels simultaneously. Using the ultrafast excited state dynamics in the van der Waals semiconductor WSe2 measured at photon energies of 36.5 eV and 109.5 eV, we demonstrate an experimental energy resolution of 130 meV, a momentum resolution of 0.06 Å-1, and a system response function of 150 fs.

Early Stages of Ultrafast Spin Dynamics in a 3d Ferromagnet.

Prior to the development of pulsed lasers, one assigned a single local temperature to the lattice, the electron gas, and the spins. With the availability of ultrafast laser sources, one can now drive the temperature of these reservoirs out of equilibrium. Thus, the solid shows new internal degrees of freedom characterized by individual temperatures of the electron gas T_{e}, the lattice T_{l} and the spins T_{s}. We demonstrate an analogous behavior in the spin polarization of a ferromagnet in an ultrafast demagnetization experiment: At the Fermi energy, the polarization is reduced faster than at deeper in the valence band. Therefore, on the femtosecond time scale, the magnetization as a macroscopic quantity does not provide the full picture of the spin dynamics: The spin polarization separates into different parts similar to how the single temperature paradigm changed with the development of ultrafast lasers.

Erratum: "Ultrafast demagnetization by hot electrons: Diffusion or super-diffusion?" [Struct. Dyn. 3, 055101 (2016)].

[This corrects the article DOI: 10.1063/1.4964892.].

Ultrafast demagnetization by hot electrons: Diffusion or super-diffusion?

Ultrafast demagnetization of ferromagnetic metals can be achieved by a heat pulse propagating in the electron gas of a non-magnetic metal layer, which absorbs a pump laser pulse. Demagnetization by electronic heating is investigated on samples with different thicknesses of the absorber layer on nickel. This allows us to separate the contribution of thermalized hot electrons compared to non-thermal electrons. An analytical model describes the demagnetization amplitude as a function of the absorber thickness. The observed change of demagnetization time can be reproduced by diffusive heat transport through the absorber layer.

The influence of the excitation pulse length on ultrafast magnetization dynamics in nickel.

The laser-induced demagnetization of a ferromagnet is caused by the temperature of the electron gas as well as the lattice temperature. For long excitation pulses, the two reservoirs are in thermal equilibrium. In contrast to a picosecond laser pulse, a femtosecond pulse causes a non-equilibrium between the electron gas and the lattice. By pump pulse length dependent optical measurements, we find that the magnetodynamics in Ni caused by a picosecond laser pulse can be reconstructed from the response to a femtosecond pulse. The mechanism responsible for demagnetization on the picosecond time scale is therefore contained in the femtosecond demagnetization experiment.

Imaging spin filter for electrons based on specular reflection from iridium (001).

As Stern-Gerlach type spin filters do not work with electrons, spin analysis of electron beams is accomplished by spin-dependent scattering processes based on spin-orbit or exchange interaction. Existing polarimeters are single-channel devices characterized by an inherently low figure of merit (FoM) of typically 10⁻4-10⁻³. This single-channel approach is not compatible with parallel imaging microscopes and also not with modern electron spectrometers that acquire a certain energy and angular interval simultaneously. We present a novel type of polarimeter that can transport a full image by making use of k-parallel conservation in low-energy electron diffraction. We studied specular reflection from Ir (001) because this spin-filter crystal provides a high analyzing power combined with a "lifetime" in UHV of a full day. One good working point is centered at 39 eV scattering energy with a broad maximum of 5 eV usable width. A second one at about 10 eV shows a narrower profile but much higher FoM. A relativistic layer-KKR SPLEED calculation shows good agreement with measurements.

Serological responses in Lyme disease: the influence of sex, age, and environment.

In the laboratory, the serodiagnosis of Lyme disease is a difficult decision, especially in early disease. The variability in the immune response to the Borrelia burgdorferi spirochete and the lack of specificity and sensitivity of commercial assays for the detection of antibodies in early disease have contributed to the difficulty of serodiagnosis. This study examines the serological data of over 20,000 serum specimens submitted for enzyme linked immunosorbent assay (ELISA) and/or Western Blot analysis to detect IgM and IgG antibodies to the Lyme spirochete. These samples were submitted to SmithKline Beecham Clinical Laboratories in Philadelphia from the five state region of New York, Pennsylvania, New Jersey, Delaware, and Massachusetts. These areas of the northeastern United States are considered endemic for infestation with the Borrelia burgdorferi spirochete and its vector, the deer tick. Samples were examined by positivity rate for ELISA and/or Western Blot Analysis (WBA). Specimens were broken down by age (greater than or equal to 13 years and less than 12 years) sex (male versus female), State (NY, NJ, PA, DE, MA), and by month of submission. Using the established criteria of the manufacturers for a positive response, these studies demonstrate an overall positivity rate for ELISA testing at 5.2 percent, while WBA alone had a positivity rate of 1.6 percent. Specimens examined by both ELISA and WBA had a positivity rate of 1.0 percent. Females > or = 13 years of age had the highest positivity rate of 8.5 percent on ELISA testing, while females < or = 12 years gave a positive reaction in 2.6 percent of the samples.(ABSTRACT TRUNCATED AT 250 WORDS)

Al Eskan disease: Desert Storm pneumonitis.

The authors observed an acute desert-related disease when the mixture of the fine Saudi sand dust and pigeon droppings triggered a hyperergic lung condition. It was further aggravated by various kinds of organic pathogenic components contributing to an opportunistic infection of the lung. These all lead to the recognition of a new clinicopathological entity, Desert Storm pneumonitis or Al Eskan disease. For the first time, the Saudi sand dust's elemental composition was studied by ultrastructural and microanalytical means. The authors concluded that, contrary to previous beliefs, sand particles less than 1 microns (0.1 microns to 0.25 microns) in diameter are present in substantial quantities in the Saudi sand and are pathogenic, causing hyperergia. Pathogenesis of the sand dust, induced hyperergia, and its immunopathologic background are highlighted.