RELATIVISTIC TIGHT-BINDING CALCULATIONS OF X-RAY ABSORPTION AND MAGNETIC CIRCULAR-DICHROISM AT THE L2 AND L3 EDGES OF NICKEL AND IRON

The soft-x-ray absorption and magnetic circular dichroism (MCD) cross sections at the L2 and L3 core-level edges of Ni and Fe have been calculated using a one-electron tight-binding band-structure approach. The tight-binding scheme is taken from the work of Papaconstantopoulos, supplemented with the inclusion of spin-orbit coupling in the d bands and fully relativistic dipole selection rules. In Ni, a fit to the various L2 and L3 intensity ratios can be achieved, but only with values of the d-band spin-orbit parameter-xi and exchange parameter DELTA(ex) at variance with the ground-state band-structure values. For Fe, there is no plausible value of xi-capable of explaining the intensity ratios; also, the predicted substructure within the L2 and L3 White lines is not seen in experiment. These failures of the one-electron approach are qualitatively consistent with expected many-body electronic rearrangements associated with core-hole creation. Some discussion is offered on sum rules and on orbital versus spin magnetic moments.