Key insights in materials at extreme temperatures and pressures can be gained by accurate measurements that determine the electrical conductivity. Free-electron laser pulses can ionize and excite matter out of equilibrium on femtosecond time scales, modifying the electronic and ionic structures and enhancing electronic scattering properties. The transient evolution of the conductivity manifests the energy coupling from high temperature electrons to low temperature ions. Here we combine accelerator-based, high-brightness multi-cycle terahertz radiation with a single-shot electro-optic sampling technique to probe the evolution of DC electrical conductivity using terahertz transmission measurements on sub-picosecond time scales with a multi-undulator free electron laser. Our results allow the direct determination of the electron-electron and electron-ion scattering frequencies that are the major contributors of the electrical resistivity.

We present the design of an extreme ultraviolet (XUV) pulse shaper relying on reflective optics. The instrument will allow tailoring of the time-frequency spectrum of femtosecond pulses generated by seeded free-electron lasers (FEL) and high-harmonic generation (HHG) sources down to a central wavelength of ~15 nm. The device is based on the geometry of a 4f grating compressor that is a standard concept in ultrafast laser science and technology. We apply it to shorter wavelengths using grazing-incidence optics operated under ultra-high vacuum conditions. The design blaze angle and the line density of the gratings allow the manipulation of all different harmonics typical for seeded FEL and HHG photon sources without the need of realignment of the instrument and even simultaneously in multi-color experiments. A proof-of-principle pulse shaping experiment using 266 nm laser light has been performed, demonstrating relative phase-control of femtosecond UV pulses.

Intense pulses from a short wavelength free-electron laser turn xenon nanoparticles into a high energy density nanoplasma within femtoseconds. Recently, the generation of multiply charged xenon ions during the initial phase of plasma evolution has been studied by energy-resolved XUV fluorescence detection as a function of cluster size and cluster composition [1]. In the present contribution we give a detailed analysis of the corresponding radiative transitions after resonant excitation of the 4d electron shell at intensities of 2 x 10(12) - 2.45 x 10(15) W cm(-2). The evaluation of charge-state specific fluorescence yields as a function of FEL power density demonstrates that plasma effects such as ionization potential lowering, electron impact excitation, ionization, and energy redistribution govern the laser-induced non-equilibrium dynamics in xenon clusters.

Our experiment aims to investigate the dynamics of electrons and nuclei in molecular systems using UV-XUV light pulses in a one colour pump-probe scheme with a delay controlled with subfemtosecond precision. The traditional split mirror design consisting of two halfs will not allow to resolve the relative phases of the light pulses after focusing due to the diffraction effects [1]. Thus, the phase sensitive experiment requires a special split and delay unit which becomes the key element of the setup. Simulations of different split unit designs were performed and showed that utilization of a multipixel reflective split unit allows to avoid the phase uncertainty in the area of a tight focus and perform the phase resolved experiments. The comparative performance of the two types of a split mirror design are shown in the Figure 1. One can observe a destructive interference corresponding to the π phase shift occuring in the center of the focus. The dark central spot is accompanied by two bright spots of constructive interference on the sides arising due to diffraction. The interaction area of the target with the excitation pulse must be restricted to the size of region with a well defined phase of the light field (the ”dark” spot), otherwise the phase uncertainty will not allow to record an interferometric autocorrelation trace. As seen from the picture such a restriction can hardly be achieved with a traditional split mirror design since the target size is usually larger than the separation between the areas of constructive and destructive interference. The multipixel split mirror design can solve this problem.