Disentangling multipole resonances through a full x-ray polarization analysis

Abstract

Complete polarization analysis applied to resonant x-ray scattering at the Cr $K$ edge in ${\mathrm{K}}_{2}\mathrm{Cr}{\mathrm{O}}_{4}$ shows that incident linearly polarized x rays can be converted into circularly polarized x rays by diffraction at the Cr pre-edge $(E=5994\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. The physical mechanism behind this phenomenon is a subtle interference effect between purely dipole $(E1\text{\ensuremath{-}}E1)$ and purely quadrupole $(E2\text{\ensuremath{-}}E2)$ transitions, leading to a phase shift between the respective scattering amplitudes. This effect may be exploited to disentangle two close-lying resonances that appear as a single peak in a conventional energy scan, in this way allowing one to single out and identify the different multipole order parameters involved.