Computational Pump-Probe Spectroscopy for XFEL Experiments

Pump Probe Spectroscopy is a widely used experimental technique that allows us to study dynamics on ultra-short timescales. An initial pump pulse is used to initiate the dynamics, and after a short time delay, a secondary probe laser pulse explores the dynamics present. X-Ray Free Electron Lasers (XFELs) facilities have recently become available, and have opened up a new area of pump probe spectroscopy, where the initial pump laser removes a tightly bound inner-shell electron from the system. Observation of the dynamics resulting from pump probe spectroscopy can be difficult to interpret. Therefore, accurate theoretical models are required to properly understand the dynamics involved. We make use of the R-Matrix with Time Dependence (RMT) approach to model XFEL pump probe spectroscopy on Ne+. We apply our newly developed density matrix approach to connect two single-ionization RMT calculations and demonstrate this approach through the sequential ionization of Ne+ to Ne3+. We analyse the dynamics of the angular distribution of the photoelectrons arising from the photoionization of Ne2+. We have found that the time delay between the pump and probe pulses manifests as a change in the angular distribution of the photoelectrons, through the time variation of the quantum superposition the intermediate states in the Ne2+ ion. Full study on pump-probe spectroscopy of inner-shell electrons requires a detailed description of the atomic structure, making these calculations computationally expensive. We therefore require the use of High-Performance Computing Facilities such as ARCHER2 for this work.