CHARGE-TRANSFER EFFECT ON THE LINEWIDTH OF FE-K-ALPHA X-RAY-FLUORESCENCE SPECTRA

High-resolution Kα x-ray fluorescence spectra of FeO, Fe2O3, Fe3O4, K3[Fe(CN)6]3H2O, K4[Fe(CN)6], and Fe (metal) are measured. It is found that the linewidth of Fe Kα1 (K-L3) does not follow Van Vleck's theorem for oxides, but it follows Van Vleck's theorem for cyanides; the line shapes of FeO, Fe2O3, and Fe3O4 are nearly identical. These results are rationalized by the spin-unrestricted DV-Xα molecular-orbital calculations of model clusters. The model clusters we tried are [FeO6]10-, [FeO6]9-, [Fe(CN)6]3-, and [Fe(CN)6]4- as model clusters of FeO, Fe2O3, K3[Fe(CN)6]3H2O, and K4[Fe(CN)6], respectively. The electronic structures of these clusters are calculated for their ground states and 1s-1 hole states. It is concluded from these calculations that the charge-transfer effect induced by the creation of the 1s-1 core hole, which is the initial state of the Kα x-ray emission, reduces the effective number of 3d unpaired electrons of iron oxides, resulting in the Kα1 width being reduced from that expected from Van Vleck's theorem. On the other hand, the effective number of 3d unpaired electrons of cyanides in the ground state is conserved in the 1s-1 core-hole state. Thus Van Vleck's theorem holds for cyanides. © 1994 The American Physical Society.