Theory of Ca L_2,3-edge XAS using a novel multichannel multiple-scattering method

A new method for calculating X-ray absorption spectroscopy (XAS) at the L_2,3 edges of Ca and transition metals is presented. It is based on the multichannel multiple-scattering theory by Natoli et al. [Phys. Rev. B, (1990), 42, 1944–1968] combined with the eigen-channel R-matrix formalism. Atomic multiplet-like effects, owing to the Coulomb interaction of photoelectrons and the 2p hole, are taken into account through a configuration interaction ansatz for the final-state wavefunction. The various multiplet states lead to a set of channels for the photoelectron wavefunction, which is calculated in multiple-scattering theory. The method is applied to Ca, an important element for biological applications of XAS. An L:L branching ratio of 3:4 is found, in good agreement with experiment but in contrast to the statistical value 2:1 obtained in all one-electron approaches. By using a linear mixture between statically screened (∼90%) and unscreened (∼10%) core-hole potential, the line shape, too, agrees well with the experimental one.