Surface X-ray dichroism for crystal field studies

The splitting of the free ion ground state of rare-earths can be investigated using X-ray absorption spectroscopy with polarized light at the M4.5-edges. The linear polarization of the light, combined with the dipole selection rules, allows one to study the ordering and separation of the crystal field levels over an energy range of a few meV, even using 1 keV photons with 1 eV energy resolution. Thanks to the high absorption cross section and to the elemental selectivity, this technique is particularly suited for surface studies.