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.

We report on the dynamics of ultrafast heating in cryogenic hydrogen initiated by a ≲300  fs, 92 eV free electron laser x-ray burst. The rise of the x-ray scattering amplitude from a second x-ray pulse probes the transition from dense cryogenic molecular hydrogen to a nearly uncorrelated plasmalike structure, indicating an electron-ion equilibration time of ∼0.9  ps. The rise time agrees with radiation hydrodynamics simulations based on a conductivity model for partially ionized plasma that is validated by two-temperature density-functional theory.

Abstract:The experimental characterization of the spatial and temporal coherence properties of the free-electron laser in Hamburg (FLASH) at a wavelength of 8.0 nm is presented. Double pinhole diffraction patterns of single femtosecond pulses focused to a size of about 10×10 µm 2 were measured. A transverse coherence length of 6.2 ± 0.9 µm in the horizontal and 8.7 ± 1.0 µm in the vertical direction was determined from the most coherent pulses. Using a split and delay unit the coherence time of the pulses produced in the same operation conditions of FLASH was measured to be 1.75 ± 0.01 fs. From our experiment we estimated the degeneracy parameter of the FLASH beam to be on the order of 10 10 to 10 11 , which exceeds the values of this parameter at any other source in the same energy range by many orders of magnitude. (4) were well satisfied in our experimental geometry. The maximum time delay introduced through the path length difference was τ max ≈ 0.6 fs and was smaller than the temporal coherence length τ c = (1.75 ± 0.0.01) fs measured by the split and delay unit (see below). Therefore, we could safely assume that in transverse coherence measurements |γ eff 12 (τ)| ≈ |γ eff 12 (0)| and α 12 (τ) ≈ α 12 (0). 36. An unconstrained fit yields a value of |γ eff 11 | ≈ 0.8 in both directions and provides slightly larger values for the transverse coherence length. We attribute this to inhomogenities in the transmission through the pinholes. 37. J. Chalupsky, J. Krzywinski, L. Juha, V. Hajkova, J. Cihelka, T. Burian, L. Vyain, J. Gaudin, A. Gleeson, M.Jurek, A. R. Khorsand, D. Klinger, H. Wabnitz, R. Sobierajski, M. Störmer, K. Tiedtke, and S. Toleikis, "Spot size characterization of focused non-Gaussian X-ray laser beams", Opt. Express 18, 27836 (2010). 38. We attribute this positional uncertainty to both, instabilities of the sample stages and beam positional jitter. 39. In our experiment the maximum of |γ 12 (τ)| did not reach unity but rather a value of 0.14. The reason for this is that the full beam was split in the middle and overlapped again meaning that parts of the center of the beam were overlapped with parts of the edge of the beam (see [27]

The monochromator beamline at the FLASH facility at DESY is the worldwide first XUV monochromator beamline operational on a free electron laser (FEL) source. Being a single-user machine, FLASH demands a high flexibility of the instrumentation to fulfil the needs of diverse experiments performed by a multidisciplinary user community. Thus, the beamline has not only been used for high-resolution spectroscopy that it was originally designed for, but also for pump-probe experiments controlling the temporal-spectral properties at moderate resolution, and as a filter for high harmonics of the FEL at very low resolution. The present performance and capabilities of the beamline are discussed with emphasis on particularities arising from the nature of the FEL source, and current developments are presented aiming to enhance its capabilities for accommodating a wide variety of experiments.

Beamline BL1 at the FLASH free-electron laser facility at DESY was upgraded with new transport and focusing optics for the installation of the new permanent CAMP end-station, a multi-purpose instrument optimized for electron- and ion-spectroscopy, imaging and pump–probe experiments. An overview of the layout, beam transport, focusing capabilities, and experimental possibilities of this new end-station, as well as results from its commissioning and first experiments, are presented.