## 3 Results and Discussion  
### MBES Energy Resolution  
The spectrometer has been calibrated with the Kr M\({}_{4,5}\)NN Auger lines, appearing in the kinetic energy range shown in Figure 4. The background pressure in the interaction region was \(2\times 10^{-7}\) mbar before filling in Kr, although much lower pressures in the \(10^{-9}\) mbar range have been reached by pumping for longer times and baking. We backfilled the chamber with Kr gas at a pressure of \(5.5\times 10^{-7}\) mbar for calibration purposes. Figure 3: Cut-down scheme of the magnetic bottle time of flight spectrometer, with the magnified area around the interaction region. The interaction region is defined by the overlap of the optical and X-ray pulse with the region of highest sensitivity of the MBES. The magnetic field in the interaction region is dominated by the permanent magnet having a tapered soft-iron pole piece on top. The strong, inhomogeneous magnetic field drops quickly to a weak, homogeneous solenoid field surrounding the flight tube, thus guiding the electrons from the interaction region towards the detector. A \(\upmu\)-metal tube (orange) around the flight-tube and solenoid assembly (blue) shields the long flight region from external magnetic fields. At the entrance of the flight tube, an electrostatic lens stack can be used to retard the electrons to optimally utilize the energy resolution at a mean kinetic energy of choice. The micro-channel-plate (MCP) detector assembly contains a grid at the front to accelerate electrons towards the first plate. The grid and lens stack are equipotential so that the electrons travel through the flight tube with constant velocity. We chose an FEL photon energy of 270 eV; the spectral jitter was on the order of 2% as observed by an inline diagnostic instrument at the beamline [30]. The nonresonant Auger lines however are not affected by the spectral jitter of the light source. The permanent magnet of the spectrometer was moved close to the interaction region and scanned vertically to and horizontally along the FEL beam to optimize the position for highest electron signal. A halo of scattered light around the laser focus prohibited distances of the magnet tip to laser focus to be smaller than 4 mm. Even a small amount of scattered light produced many more photoelectrons from the solid magnet tip than from the gas in the real focus because of the orders of magnitude lower density of our gas compared to solid matter. The Auger spectra used for calibration are shown in Figure 4. The red line is recorded using a retardation potential of 0 V. The black reference spectrum is obtained from [PERSON] et al. [29]. One can clearly identify the two different groups of Kr M\({}_{4}\)5 NN Auger lines in our spectra. In the higher energy group, we identify the two lines at 37.7 and 38.7 eV as two separate lines. This results in an energy resolution without retardation of \(\Delta\)E/E \(\cong\) 1/40. The kinetic energy spectrum with 20 V retardation voltage (orange) shows deeper modulation indicating higher energy resolution, as the kinetic energy decreased. We now discuss the resolution-limiting factors in the calibration measurements. We note a large background in the kinetic energy range between 35 and 37.5 eV. This results from a saturation of the detector due to a large amount of electron counts per X-ray pulse. In the TOF-spectrum, the group of Auger electrons between 37 and 42.5 eV hit the detector at early times and thereby produces a long lasting background of several nanoseconds. By the time the next group between 30 and 32 eV hits the detector, this signal has decayed. This memory effect in the detector certainly degrades the resolution as background accumulates in a group of Auger lines. This issue can be easily solved by reducing  
Figure 4: Krypton M\({}_{4}\)5 NN Auger spectra recorded with the URSA-PQ instrument using 270 eV photons (red spectrum using a retardation of 0 V, orange spectrum using a retardation of 20 V).