We present a real-space Green's function theory and calculations of two-photon x-ray absorption (TPA). Our focus is on nonresonant $K$-shell TPA in metallic Cu, which has been observed experimentally at intense x-ray free electron laser (XFEL) sources. The theory is based on an independent particle Green's function treatment of the Kramers-Heisenberg equation and an approximation for the sum over nonresonant intermediate states in terms of a static quadrupole transition operator. XFEL effects are modeled by a partially depleted $d$ band. This approach is shown to give results for $K$-shell TPA in quantitative agreement with XFEL experiment and with a Bethe-Salpeter equation approach. We also briefly discuss many-body corrections and TPA sum rules.

Close

PrevNext

View figure in article

• Approximation methods for many-body systems
• Electronic structure
• First-principles calculations
• Green's function methods