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Figure 6
Average electron time-of-flight spectrum of 400 single shots measured with xenon at a general FLASH wavelength of 11.2 nm. A retardation voltage of 25 V was applied to the electron spectrometer. The prompt signal (orange) from fluorescence and scattered photons defines the time zero of the time-of-flight period. The photoelectron signals of different xenon orbitals (green) shift in arrival time with changing photon energy enabling FEL wavelength determination. Auger electron signals are fixed in arrival time due to their constant kinetic energy. Hence, they can serve as intrinsic calibration and for energy scale corrections when necessary. In this example, the wavelength derived by OPIS was 11.27 nm and only a minor correction of −0.01 nm has to be applied. This is in good agreement with the wavelength of 11.24 nm measured with the PG spectrometer.

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