Calculation of charge-transfer satellites in x-ray absorption spectroscopy of transition metal oxides

Charge-transfer (CT) satellites in x-ray absorption spectroscopy (XAS) require treatments of correlation effects beyond the quasi-particle approximation. Here we present an approach for including CT effects in XAS that follows the model of Lee \textit{et al.}\footnote{J.D. Lee, O. Gunnarsson and L. Hedin, Phys. Rev. B \textbf{60}, 8034 (1999)} The approach is based on a three level system coupled to an itinerant state, with parameters obtained from either ab inito calculations or x-ray photoemission spectroscopy. The model yields an approximation to CT satellites in XAS in terms of a convolution of the quasi-particle spectrum with an energy-dependent spectral function that accounts for both localized CT excitations and solid state effects. The approach illustrates the crossover from the sudden to adiabatic approximations. Calculations for transition metal oxides, e.g. NiO and CoO, give reasonable agreement with XAS experiment. Finally, an extension of the present approach to CT satellites in resonant inelastic x-ray spectroscopy is also discussed.