On the Multipole Character of the X-Ray Transitions in the Pre-Edge Structure of Fe K Absorption Spectra. An Experimental Study

The K absorption edge of pure Fe and the pre-edge Fe K absorption of FeS$_2$ (pyrite), $Fe_2O_3$ (hematite), and $FeCO_3$ (siderite) are investigated with single crystal absorbers of these materials using linearly polarized synchrotron X-radiation. From the angular dependence of the pre-edge K absorption the existence of predominantly ligand-field-induced dipole transition in $Fe_2O_3$ and of quadrupole transitions in $FeCO_3$ is concluded. The multipole character and the mechanism of the pre-edge K absorption are discussed