## 2 Materials and Methods  
### Coated XUV Mirrors for 45\({}^{\circ}\) Incidence Angle  
Since the reflectivity of mirrors in the XUV spectral range is very low for normal incidence, it is necessary to use glancing incidence optics in experiments. However, here we investigate the reflectivity of metal mirrors for 45\({}^{\circ}\), which allows for an easier implementation in the beam path, even at the expense of a low reflectivity. In the energy range above 30 eV, tabulated values by the Center for X-ray Optics (CXRO) can be used to calculate the reflectivity, since, for photon energies from 30 eV to 30 keV, atomic scattering factors can be reliably used [18]. Below 30 eV, the assumption of independent scatterers is no longer strictly valid, since these photon energies approach the valence ionization threshold. Reflectivity values for four different materials are shown in Figure 2a. Figure 1: Principle scheme of pump-probe experiments with XUV (extreme ultraviolet) and IR (infrared) pulses. One part of a fs laser beam is focused into a noble gas for high-harmonic generation. Filtering of the fundamental laser beam is achieved by a metal foil filter that transmits a large amount of XUV radiation. Using, e.g., a toroidal mirror, the XUV beam can be focused to interact with atomic or molecular media. Electrons and ions created in the interaction region can be detected. When an IR beam is used for time-resolved measurements, a second metal foil filter must be used to protect the CCD (charge-coupled device) of the XUV spectrometer. In case filters are used before and behind the interaction region, there is an ambiguity about IR leak light due to imperfections of the first metal filter. Furthermore, characterization of the XUV focal spot size by fluorescence is also limited by the quality of IR filtering [17]. For our experiments, we have used mirrors made of uncoated stainless steel (Fe.72 Cr.18 Ni.10) and a cesium iodide-coated stainless steel (300-nm-thick coating). For reflectivity calculations, stainless steel is very well approximated by iron, and the cesium iodide-coated mirror can be well described by its bulk properties. In the spectral range of interest, we have several narrow features, which will be not seen in our experiments, because we use a broadband XUV source. To allow for a meaningful comparison, we have calculated the spectrally integrated reflected signal after convoluting the reflectivity data given in Figure 2a with a realistic spectral distribution. Here, we choose a Gaussian distribution with a standard deviation of 3 eV and vary the center photon energy between 40 eV and 110 eV with a step size of 2 eV (Figure 2b). The spectrally integrated XUV yield on a detector is shown for the center tablevalues of the spectral distributions in Figure 2c. Finally, we have calculated the ratio of the reflected XUV yield compared to CsI, as depicted in Figure 2d. It can be seen that, for central photon energies between 40 eV and 50 eV, the ratio between uncoated and CsI-coated stainless steel is approximately 10, resulting in an order of magnitude difference in the detected signal levels. For other possible substrate materials, this ratio quickly increases towards 50, which will cause experimental difficulties due to very different and very low signal levels. It can be seen that CsI coatings are not practicable for all elements in the range between 55 eV and 80 eV. However, measurement of different reflections from uncoated metal surfaces would be an alternative. For the range from 80 eV to 110 eV mean photon energy, the comparison with CsI-coated surfaces becomes feasible again. Above 120 eV, all surfaces have a very low reflectivity, which limits this measurement procedure. This can be overcome by choosing a smaller angle of incidence. Figure 3 illustrates the measurement procedure by taking two different reflective materials into account. Figure 2: **(a)** Calculated reflectivity values for iron, gold, silver, and cesium iodide for a 45\({}^{\circ}\) incidence angle and s-polarized light from [18].