Journal Article free-electron laser FEL pump–probe Kirkpatrick–Baez mirrors VMI X-ray free-electron laser XFEL ab initio calculation beam profile calibration characterization charge state distribution electron spectroscopy electron-ion-spectrometers first-principles calculation imaging detector mass spectrometry micro-focus multilayer mirror pump–probe experiments soft X-rays spatial fluence distribution split-and-delay unit synchrotron temporal and spatial overlap time-of-flight spectrometer time-stamping camera ultrafast laser velocity map imaging