X-ray Emission Spectropolarimetry of Strongly Anisotropic Single Crystal Systems Using a Rowland Circle Geometry

Polarization dependence has historically seen extensive use in X-ray spectroscopy to determine magnetic and local geometric properties, but more broadly as a way to gain extra sensitivity to electronic structure at the level of individual magnetic orbitals. This is often done in the context of X-ray absorption through techniques like X-ray magnetic circular dichroism or X-ray linear dichroism, but it has seen little application to X-ray emission. Here, we explore the information contained in the polarized emission of two 3d transition metal systems across both core-to-core (CtC) and valence-to-core emission (VtC) lines. We demonstrate how the Rowland circle geometry can be used as a spectropolarimeter and apply it to the X-ray emission spectroscopy of spin-1/2 Cu(II) and spin-0 Ni(II) ions in LiVCuO4 and DyNiC2, respectively. From this, we explore how the polarized XES interrogates the occupied density of states at the valence level, either as a second-order effect through Coulomb exchange (CtC X-ray emission) or by direct transitions (VtC X-ray emission). We find that the polarized X-ray emission can provide insights into the valence electron orbital occupation, in much the same way that is achievable with polarized absorption or angle-resolved photoemission spectroscopy techniques. Finally, we highlight how the individually polarized dipole emission spectra can be extracted from a linearly independent suite of directed emission spectra, allowing for polarized measurements at high Bragg angle with lower experimental broadening.