Time-Resolved Two-Color X-ray Pump/ X-ray Probe Photoelectron Spectroscopy at LCLS.

Recently, X-ray Free Electron Lasers (XFELs) 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), a powerful tool for extracting chemical information of a specific site by measuring the binding energy of core electrons, enables femtosecond time-resolved XPS experiments with chemical and site specificity. We will present our X-ray pump/X-ray probe XPS experiment aimed at studying energy flow and relaxation dynamics in CO, i.e. small hetero-nuclear molecules, in the gas phase. The data gives a glimpse of electronic and nuclear relaxation pathways upon resonant oxygen K-edge excitation proceeding on time scales \textless 40 fs. The experimental efforts are accompanied by theoretical work describing the time-resolved core-level photoemission with a time-dependent Schrodinger equation. This work lays the groundwork for further TR-XPS experiments following energy and charge transfer processes upon photo excitation in more complex molecules.