**Date/Time**

Date(s) - Aug 15 2019

*1:00 PM - 2:30 PM*

**Location**

Sycamore Room, Building 40, room 195

**Category(ies)**

**John J. Rehr**

**Adjunct Professor of Photon Science, SLAC**

and

**Dept. of Physics, University of Washington**

**Seattle, WA 98195-1560**

** III Many-body effects and Inelastic losses in XAS**

** IV Real-time approaches**** in XAS**

The first two Lectures of this series covered: I) Introduction to X-ray Spectroscopy Theory, and II) Real-space Green’s function Theory and FEFF. Now we aim to cover some more advanced aspects of the theory including more recent developments.

Lecture III is devoted to the theory of many-body effects in XAS, which are essential for quantitative calculations and interpretation. Recent advances now permit parameter-free calculations of the key many-body effects [1]. Physically, these effects arise from electronic correlations and atomic vibrations that lead to inelastic losses and damping. Quasi-particle (QP) approaches with a GW self-energy and vibrational damping with Debye-Waller factors [2] yield significant improvements. More advanced methods based on the GW/Bethe-Salpeter equation are also discussed [3]. Inelastic losses such as multi-electron excitations can then be treated using cumulant Green’s function techniques [4], in terms of a convolution of a quasi-particle theory with a spectral function that builds in inelastic losses.

Lecture IV) describes real-time approaches, which are becoming increasingly important in photon spectroscopies. These range from linear and non-linear optical response to XAS with pulsed sources. One approach is based on real-time, time-dependent density functional theory (RT-TDDFT) and time-correlation functions. Another is the investigation of dynamic structure in nano-scale materials using ab initio molecular dynamics and the real-space Green’s function approach in FEFF9 [5]. The extension to high intensity pulsed x-ray sources is also discussed [6].

[1] John J. Rehr et al., Comptes Rendus Physique **10**, 548 (2009).

[2] F. Vila et al., Phys. Rev. B **78**, 121404(R), (2008).

[3] K. Gilmore et al., Comput. Phys. Comm. **197**, 109 (2015).

[4] J. Sky Zhou et al., J. Chem. Phys. **143**, 194109 (2015).

[5] A.I. Frenkel et al., J. Vac. Sci. Technol. A **32**, 020801 (2014).

[6] J.J. Kas and J. J. Rehr, Phys. Rev. Lett. **119**, 176403 (2017).