Recent progress in ultrafast x-ray sources helped establish x-rays as an important tool for probing lattice and magnetic dynamics initiated by femtosecond optical pulses. Here, we explore the potential of ultrashort hard x-ray pulses for driving magnetic dynamics. We use a transient grating technique in which a spatially periodic x-ray excitation pattern gives rise to material excitations at a well-defined wave vector, whose dynamics are monitored via diffraction of an optical probe pulse. The excitation of a ferrimagnetic gadolinium bismuth iron garnet film placed in an external tilted magnetic field by x-rays at the Gd L3 edge results in both magnetic and nonmagnetic transient gratings whose contributions to the diffracted signal are separated by polarization analysis. We observe the magnetization precession at both longitudinal acoustic and spin wave frequencies. An analysis with the Landau-Lifshitz-Gilbert equation indicates that the magnetization precession is driven by strain resulting from thermal expansion induced by absorbed x-rays. The results establish x-ray transient gratings as a tool for driving coherent phonons and magnons, with the potential of accessing wave vectors across the entire Brillouin zone.

Optical-domain transient grating (TG) spectroscopy is the ideal tool to investigate transport phenomena in gases, liquids and solids, but it is limited to typically micron-size grating periods. Extreme-Ultraviolet TG has represented a major leap forward to access the mesoscopic scales. Hard X-ray TGs open access in principle to the nanoscale. Hard X-ray TGs were recently generated using the Talbot effect and probed by optical pulses, but these hinder exploiting the advantages of the nanoscale gratings. Here, we present an all-X-ray TG study, in which few-femtosecond hard X-ray pulses are used both for excitation and probing. Our experiment was performed on an amorphous film of an FeGd alloy and on a thin silicon single crystal. The results show a manifestation of the TG induced by the X-ray pump and probe pulses in the form of Talbot carpets, as well as temporal evolution of the grating in crystalline silicon showing coherent optical phonons. Ultrafast all-X-ray TG spectroscopy has the potential to study fundamental excitations with femtosecond time resolution and nanometer spatial sensitivity.

The use of advanced x-ray sources plays a key role in the study of dynamic processes in magnetically ordered materials. The progress in x-ray free-electron lasers enables the direct and simultaneous observation of the femtosecond evolution of electron and spin systems through transient x-ray absorption spectroscopy and x-ray magnetic circular dichroism, respectively. Such experiments allow us to resolve the response seen in the population of the spin-split valence states upon optical excitation. Here, we utilize circularly polarized ultrashort soft x-ray pulses from the new helical afterburner undulator at the free-electron laser FLASH in Hamburg to study the femtosecond dynamics of a laser-excited CoPt alloy at the Co L₃-edge absorption. Despite employing a weaker electronic excitation level, we find a comparable demagnetization for the Co 3d-states in CoPt compared to previous measurements on CoPd. This is attributed to the distinctly different spin–orbit coupling between 3d and 4d vs 3d and 5d elements in the corresponding alloys and multilayers.

Time-resolved absorption spectroscopy and magnetic circular dichroism with circularly polarized soft x-rays (XAS and XMCD) are powerful tools to probe electronic and magnetic dynamics in magnetic materials element- and site-selectively. By employing these methods, groundbreaking results have been obtained, for instance, for magnetic alloys, which helped to fundamentally advance the field of ultrafast magnetization dynamics. At the free-electron laser facility FLASH, key capabilities for ultrafast XAS and XMCD experiments have recently improved. In an upgrade, an APPLE-III helical afterburner undulator was installed at FLASH2 in September 2023. This installation allows for the generation of circularly polarized soft x-ray pulses with a duration of a few tens of femtoseconds covering the L 3 , 2 -edges of the important 3d transition metal elements with pulse energies of several μ J. Here, we present first experimental results with such ultrashort x-ray pulses from the FL23 beamline employing XMCD at the L 3 , 2 -edges of the 3d metals, Co, Fe, and Ni. We obtain significant dichroic difference signals indicating a degree of circular polarization close to 100%. With the pulse-length preserving monochromator at beamline FL23 and an improved pump-laser setup, FLASH can offer important and efficient experimental instrumentation for ultrafast demagnetization studies and other investigations of ultrafast spin dynamics in 3d transition metals, multilayers, and alloys.

Pump-probe methods are a ubiquitous tool in the field of ultrafast dynamic measurements. In recent years, x-ray free-electron laser experiments have gained importance due to their ability to probe with high chemical selectivity and at atomic length scales. Measurements are typically repeated many thousands of times to collect sufficient statistics and vary parameters like delay or fluence, necessitating that initial conditions are restored each time. An alternative is presented by experiments which measure the relevant parameters in a single shot. Here, we present a time-to-space mapping imaging scheme that enables us to record a range of delays and laser fluences in any single shot of the x-ray probe. We demonstrate the use of this scheme by mapping the ultrafast dynamics of the optically induced insulator-to-metal Verwey transition in a magnetite thin film, probed by soft x-ray resonant diffraction. By extrapolating our results toward the conditions found at x-ray free-electron lasers with higher photon energy, we demonstrate that the presented data could be recorded in a single shot.

The sustainability transition of the chemical industry hinges on the educated design of catalysts for reactions such as the CO hydrogenation, optimizing the materials for selectivity toward valuable products. So far, theoretical models have been used to predict reaction selectivity from the competition of elementary surface processes. Here, we provide an in situ experimental view of surface adsorbates during CO hydrogenation. We compare X-ray photoelectron spectra acquired at reaction conditions (200–325 °C, 150 mbar) over single crystals of Fe, Rh, Ni, Co, and Cu and infer which elementary steps decide the product distribution. We find that the chemisorption energies of C and O, as often used descriptors for catalytic activity, qualitatively predict the rate-limiting steps. They fail, although when reaction-induced carburization occurs on Ni and Fe, steering the selectivity toward methanation on Ni and hydrocarbon chain growth on Fe. For the noncarburized Co and Rh we show how the adsorbate distribution and the oxygen chemisorption energy allow for oxygenate production on Rh, but hydrocarbon chain growth on stepped Co. Ultimately, we show how in situ experiments provide a chemical and mechanistic understanding of CO hydrogenation selectivity, useful to tailor catalysts for a sustainable production of high-value chemicals.

In recent years, the multi-dimensional spectroscopy and inelastic X-ray scattering endstation MUSIX has become an extensively used instrument at the XUV/soft X-ray free-electron laser FLASH and the synchrotron source PETRA III at DESY in Hamburg. It was employed for static and time-resolved spectroscopy on quantum materials as well as for non-linear methods in the XUV/soft X-ray regime. Recently, the instrument was upgraded for more flexibility regarding the orientation of the sample and the spectrometer dispersion direction. These upgrades are described here, showcasing the scientific benefits. MUSIX is now even better suited for novel experimental schemes in the XUV and soft X-ray regime, especially at the upcoming facilities FLASH2020+ and PETRA IV.

The detection of inelastically scattered soft x-rays with high energy resolution usually requires large grating spectrometers. Recently, photoelectron spectrometry for analysis of x-rays (PAX) has been rediscovered for modern spectroscopy experiments at synchrotron light sources. By converting scattered photons to electrons and using an electron energy analyser, the energy resolution for resonant inelastic x-ray scattering (RIXS) becomes decoupled from the x-ray spot size and instrument length. In this work, we develop PAX towards high energy resolution using a modern photoemission spectroscopy setup studying Ba2Cu3O4Cl2 at the Cu L 3-edge. We measure a momentum transfer range of 24% of the first Brillouin zone simultaneously. Our results hint at the observation of a magnon excitation below 100 meV energy transfer and show intensity variations related to the dispersion of dd-excitations. With dedicated setups, PAX can complement the best and largest RIXS instruments, while at the same time opening new opportunities to acquire RIXS at a range of momentum transfers simultaneously and combine it with angle-resolved photoemission spectroscopy in a single instrument.

Photodoped states in strongly correlated charge-transfer insulators are characterized by 𝑑−𝑑 and 𝑑−𝑝 interactions and the resulting intertwined dynamics of charge excitations and local multiplets. Here, we use femtosecond x-ray absorption spectroscopy in combination with dynamical mean-field theory to disentangle these contributions in NiO. Upon resonant optical excitation across the charge-transfer gap, the Ni 𝐿₃ and O 𝐾 absorption edges red-shift for >10 ps, which we associate with photoinduced changes in the screening environment. An additional signature below the Ni 𝐿₃ edge is identified for <1 ps, reflecting a transient nonthermal population of local many-body multiplets. We employ a nonthermal generalization of the multiplet ligand field theory and argue that this feature originates from 𝑑−𝑑 transitions. Overall, the photodoped state differs significantly from a chemically doped state. Our results demonstrate that x-ray spectroscopies are helpful in revealing excitation pathways in correlated materials.