Non-linear effects in core photoemission from Real-time TDDFT.

In core photoemission spectroscopy a photon is absorbed and a core electron is emitted into the continuum, leaving behind a core hole. The remainder of the system responds to screen the core hole, significantly affecting the spectrum. State-of-the-art many-body perturbation theory (MBPT) methods such as the GW and the cumulant expansion are usually evaluated in the linear-response approximation. This requires a weak core hole; if this is not the case, one needs to go beyond linear response. This can be carried out with a non-linear cumulant derived in a MBPT framework, and evaluated from a real-time time-dependent density functional theory (TDDFT) approach. We discuss the application of this method with calculations carried out with an extension of our RT-Siesta code [1] for systems with d- and f valence electrons.

[1] J. J. Kas, F. D. Vila, J. J. Rehr, and S. A. Chambers, Phys. Rev. B. 91, 121112(R) (2015).