Polarized X-ray Fluorescence as a Probe of Ground State Properties

Applicability of x-ray absorption sum rules to spectra detected with total fluorescence yield is studied by calculations in the ionic limit of the rare earth $3d\to 4f$ and transition metal $2p\to 3d$ spectra. We show that if no intermediate states with a high $\mathrm{LS}$ purity are reached, the dependence of the integrated intensity of fluorescence yield on the incoming polarization is mainly determined by the absorption step. Therefore, although in principle fluorescence yield is unequal to x-ray absorption, in the presence of a crystal field or of strong core-hole spin-orbit coupling fluorescence yield can be used to obtain ground state expectation values of $L_z$ and $S_z$.