Electronic structure calculation for nonlinear x-ray matter interactions

Intense and ultrashort pulses of X-ray Free Electron Lasers (XFELs) have revolutionized our ways to probe matter at atomic length and time scales. The intensities achievable at these light sources surpass those of storage-ring based x-ray sources (synchrotron x-ray sources) by up to 10-12 orders of magnitude, making it possible to study nonlinear x-ray matter interaction for the first time. Phenomena studied in the early phases of XFEL based research have been multiple sequential ionization of atomic and molecular gases, collective excitation and superfluoresecence of gas, liquid and solid samples and the creation and study of warm-dense matter. With recent capabilities to produce sub-fs pulses, stimulated electronic x-ray Raman scattering becomes experimentally tractable, opening the pathway to a plethora of nonlinear x-ray spectroscopies. Another intriguing class of nonlinear analysis techniques combining x-ray diffraction and spectroscopy are parametric wave-mixing processes. The underlying material properties addressable with these novel techniques are nonlinear, nonlocal electronic correlation functions, such as current-density current-density or density current-density correlators. In this presentation I will address the current experimental and theoretical status of nonlinear x-ray spectroscopy and will discuss the related theoretical challenges and future needs.