Time Resolved X-ray Pump/X-ray Photoelectron Spectroscopy Probe

Recently, X-ray Free Electron Lasers proved the ability to produce two intense femtosecond x-ray pulses with controlled time delay and color. Combining these unique capabilities with X-ray photoelectron spectroscopy (XPS) probing, we can probe electronic and nuclear dynamics at a specific atomic site with femtosecond resolution. In our initial study we used carbon monoxide, i.e., a small heteronuclear molecule that can be well described with theoretical models. The pump pulse excites a core-electron resonantly at the oxygen K-edge and then we probe the molecular response with photoemission from the carbon core levels. The data gives a glimpse of the electronic and nuclear relaxation pathways proceeding on time scales \textless 40 fs upon x-ray excitation on the oxygen site.~Such experiments lay the ground for future time resolved core level spectroscopy at FELs that can be combined with more complex pulse modes and non-linear x-ray excitation modes.