Interpretation of the postpeak in iron fluorides and oxides

A broad post-edge peak (postpeak) in the core-loss spectrum of a relatively thick TEM specimen is readily accounted for by plural scattering of the transmitted electrons, involving a core-loss and a bulk plasmon. However, we observe a prominent postpeak, about 40 eV above iron L3 edge, even in very thin films of iron fluoride. The peak gradually disappears as fluorine is removed by electron irradiation, as indicated by decay of the fluorine K-edge and change in white-line ratio, but appears when an iron film is oxidized, therefore it appears to be characteristic of iron in an oxidized state. To study its origin, we performed real-space multiple-scattering (RSMS) calculations of the near-edge fine structure of the Fe L-edge, allowing us to discuss the origin of the postpeak within the framework of electron-scattering theory. In the belief that oxygen and fluorine anions in transition-metal compounds are strong backscatters, we propose that the postpeak is a common feature of transition metals in an oxidized state and can be used as an additional verification of such oxidization in an unknown sample.