A new route to obtain fluorescence X-ray absorption spectra of compounds and to remove the self-absorption induced nonlinearity in the spectra

A new route to obtain fluorescence X-ray absorption spectra of compounds and to remove the self-absorption induced nonlinearity in the spectra is described. The fluorescent intensity I-f is linearly proportional to the absorption coefficient mu. For studies of surface structures around an element (kappa) the fluorescence detection is often the mode of choice. However, the measurement may suffer from a self-absorption (SA) effect which nonlinearly distorts the spectra. The effect is severe when kappa is concentrated or the measurements are carried out in certain geometries. Here, the correlations among emission events in compounds are examined following resonance X-ray core-electron excitation within kappa. Under conditions leading to SA, I-f emitted from kappa apparently has a conjugated relationship with the fluorescent intensities simultaneously emitted from other elements (xi). Normalizing the former (kappa) by the latter (xi) will largely remove SA effects and reduce this nonlinear problem to a tractable linear problem. This does result in a moderate reduction of the spectral amplitude due to the so-called secondary emission from xi excited by the emission from kappa. Nonetheless, the resulting spectra will allow one to accurately determine bond distances and disorder and, in some respects, can be superior to spectra obtained via the absorption channel. For mu(xi) < mu(kappa) and grazing incidence geometry, the amplitude reduction can be small and simple normalization is sufficient to restore the spectral integrity with remarkable accuracy. This has been instrumental in unravelling the surface and subsurface structures around cations in amorphous Ga-In-O and Zn-Sn-O films which are otherwise inaccessible due to severe SA effects. This method has also been applied to several samples with mu(xi) similar or equal to mu(kappa) to examine its applicability. For these samples, the amplitude reduction is 12 +/- 4% versus their standards for the data measured with the classical 45 degrees/45 degrees geometry. This experimental method is easy to implement. Since I-f from kappa and xi are measured by the same detector system, it is also superior to other methods in removing systematic errors such as detector system nonlinearity, electronic noise, and some beam instabilities, and in removing spectral imperfections due to, for example, SA effects, diffraction effects and sample inhomogeneity. The distortions resulting from the latter can be severe in the spectra measured in transmission mode.