The European XFEL delivers up to 27000 intense (>10¹² photons) pulses per second, of ultrashort (≤50 fs) and transversely coherent X-ray radiation, at a maximum repetition rate of 4.5 MHz. Its unique X-ray beam parameters enable groundbreaking experiments in matter at extreme conditions at the High Energy Density (HED) scientific instrument. The performance of the HED instrument during its first two years of operation, its scientific remit, as well as ongoing installations towards full operation are presented. Scientific goals of HED include the investigation of extreme states of matter created by intense laser pulses, diamond anvil cells, or pulsed magnets, and ultrafast X-ray methods that allow their diagnosis using self-amplified spontaneous emission between 5 and 25 keV, coupled with X-ray monochromators and optional seeded beam operation. The HED instrument provides two target chambers, X-ray spectrometers for emission and scattering, X-ray detectors, and a timing tool to correct for residual timing jitter between laser and X-ray pulses.

Nonlinear optical frequency conversion has been challenged to move down to the extreme ultraviolet and x-ray region. However, the extremely low signals have allowed researchers to only perform transmission experiments of the gas phase or ultrathin films. Here, we report second harmonic generation (SHG) of the reflected beam of a soft x-ray free-electron laser from a solid, which is enhanced by the resonant effect. The observation revealed that the double resonance condition can be met by absorption edges for transition metal oxides in the soft x-ray range, and this suggests that the resonant SHG technique can be applicable to a wide range of materials. We discuss the possibility of element-selective SHG spectroscopy measurements in the soft x-ray range.

Time-resolved soft X-ray photoelectron spectroscopy (PES) experiments were performed with time scales from picoseconds to nanoseconds to trace relaxation of surface photovoltage on the ZnO(0001) single crystal surface in real time. The band diagram of the surface has been obtained numerically using PES data, showing a depletion layer which extends to 1 μm. Temporal evolution of the photovoltage effect is well explained by a recombination process of a thermionic model, giving the photoexcited carrier lifetime of about 1 ps at the surface under the flat band condition. This lifetime agrees with a temporal range reported by the previous time-resolved optical experiments.

We present a new experimental magneto-optical system that uses soft X-rays and describe its extension to time-resolved measurements using a free electron laser (FEL). In measurements of the magneto-optical Kerr effect (MOKE), we tune the photon energy to the material absorption edge and thus induce the resonance effect required for the resonant MOKE (RMOKE). The method has the characteristics of element specificity, large Kerr rotation angle values when compared with the conventional MOKE using visible light, feasibility for M-edge, as well as L-edge measurements for 3d transition metals, the use of the linearly-polarized light and the capability for tracing magnetization dynamics in the subpicosecond timescale by the use of the FEL. The time-resolved (TR)-RMOKE with polarization analysis using FEL is compared with various experimental techniques for tracing magnetization dynamics. The method described here is promising for use in femtomagnetism research and for the development of ultrafast spintronics.

We have systematically investigated relaxation of the surface photovoltage effect on the atomically controlled In/Si(111) surfaces with distinctive surface states and different amounts of the surface band bending. The temporal variations were traced in $real$ $time$ by time-resolved photoemission spectroscopy using soft x-ray synchrotron radiation. The relaxation is found to be temporally limited by two steps of the carrier transfer from the bulk to the surface: the tunneling process at a delay time $\le 100$ ns and the thermionic process on the following time scale ($\ge 100$ ns). Crossover of the two mechanisms can be understood by breakdown of the quantum tunneling regime by the increase in width of the space-charge layer during the relaxation.