## 3 Results and Discussion
### IR Leak Light Detection  
For the characterization of the XUV signal by the spectrally integrated reflectivity measurement, we have assumed no residual IR light contribution due to efficient suppression with the metal filters. However, imperfections of the metal foil filters lead to a substantial portion of IR light in the XUV beam. With the attached XUV spectrometer, this IR light cannot be detected because of its limited displayed spectral range. Although the Channeltron is regarded as being solar blind, we will show that we were able to detect the residual IR light. Therefore, we varied the metal foil position horizontally and vertically to find imperfections in the foil that increase the amount of transmitted IR light. This has been monitored with an IR viewer observing the steel surface. After adjusting the filter, we performed the same data analysis as presented in Section 3.1, and the results are shown in Figure 7. We have made two very distinct observations: First, the measured data now reveal a substantial difference between the calculation and the measurement. The measured reflectivity ratio can be 10% to 40% lower. Second, as we have used a counting detector, we observed a substantial change in the count rate for both surfaces, uncoated and CsI-coated stainless steel. To further investigate the IR sensitivity of our measurement setup, the secondary electron signals of the Channeltron for high and low IR content are shown for both surfaces in Figure 8 for the XUV spectra shown in Figures 6c and 7a. Since we use the same detection electronics as for time-of-flight spectroscopy, the secondary electron signal of the Channeltron is sampled with a 0.5 ns temporal resolution. Figure 6: XUV spectra for three different pulse-shapes are shown in (**a**–**c**). A comparison between calculations and measurements is depicted in (**d**) for the associated colored spectrum. Using the spectral shape and tabulated reflectivity values calculation of the estimated reflectivity ratio of uncoated and cesium iodide (CsI)-coated stainless steel is possible (dashed columns) and shows good agreement with the measurement (filled columns). As it has been pointed out already, the peak of the XUV reflection extends from delays of 720 ns to 750 ns (i.e., the laser pulse is 720 ns delayed to the trigger signal obtained from the amplifier system). Figure 8: Time-resolved secondary electron signal of the Channeltron for XUV light with residual (black symbols) and suppressed IR light (red symbols), respectively. **(a)** Due to the low IR reflectivity of CsI only a small additional peak is visible between 780 ns and 800 ns. In the presence of increased IR light, the total counts in this temporal window increase from 9 to 36, which is only \(\sim\)\(10^{-4}\) of the total counts; **(b)** For the stainless steel surface, the IR reflectivity is higher. Therefore, the counts between 780 ns and 800 ns are more pronounced and increase from 1778 to 17,981, which is more than 1% of the total counts. Figure 7: XUV spectra for three different driving pulse-shapes are shown in **(a–c)** with increased IR content (not visible in the spectra). A comparison between our calculations and our measurements is depicted in **(d)** for the associated colored spectrum. Using the spectral shape and tabulated reflectivity values calculation of the estimated reflectivity ratio of uncoated and CsI-coated stainless steel is possible (dashed columns) and shows obvious discrepancy compared to the measurement (filled columns). This is caused by IR leak light through the aluminum foil filter. This is also the dominant part of the signal for both surfaces, with and without increased infrared leak light. However, for the increased infrared light, additional peaks can be observed in the time-resolved Channeltron signal. They appear mainly with an additional 60 ns delay in the range between 780 ns and 800 ns.