Theoretical aspects of resonant X-ray emission spectroscopy

Abstract

We report recent theoretical topics for resonant X-ray emission spectroscopy (RXES) in d and f electron systems: (1) RXES in high T c cuprates, (2) effects of electric quadrupole excitation, and (3) magnetic circular dichroism (MCD) in RXES for ferromagnetic systems. In high T c cuprates, RXES with Cu 1s excitation detects a 6 eV charge transfer excitation whose intensity is affected by the Zhang–Rice (Z–R) singlet formation in the intermediate state. Very recently the Zhang–Rice singlet excitation is studied both theoretically and experimentally in RXES with O 1s excitation. On the electric quadrupole excitation at the L edge of rare earth systems, RXES is a powerful tool to detect it, whereas the quadrupole excitation is almost invisible in the conventional X-ray absorption spectroscopy (XAS). Theoretical calculations have recently been made for a spectral splitting of the quadrupole excitation in light rare earth elements from La to Eu, and compared with experimental results so far made. Finally, recent theoretical calculations of MCD in RXES for ferromagnetic systems show that when the incident X-ray direction is perpendicular to the magnetization of the system the MCD is caused by the interference effect characteristic of the coherent second order optical process. This effect has been confirmed by experimental observations of MCD-RXES with Gd L edge of a Gd–Co amorphous alloy.