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

• Published in: Journal of synchrotron radiation Vol. 29; no. 2; pp. 470 - 479
• Main Authors: Ma, Qing, Moffitt, Stephanie L., Keane, Denis T.
• Format: Journal Article
• Language: English
• Published: 5 Abbey Square, Chester, Cheshire CH1 2HU, England International Union of Crystallography 01.03.2022; John Wiley & Sons, Inc
• Subjects: Absorption spectra; Absorptivity; amorphous transparent conducting oxides; Amplitudes; Emission spectra; Fluorescence; Inhomogeneity; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Nonlinearity; Normalizing; Research Papers; Secondary emission; self-absorption effect; Surface chemistry; Systematic errors; X-ray absorption spectroscopy; amorphous transparent conducting oxides; self-absorption effect; X-ray absorption spectroscopy
• ISSN: 1600-5775; 0909-0495; 1600-5775
• DOI: 10.1107/S1600577522000029

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 If is linearly proportional to the absorption coefficient μ. For studies of surface structures around an element (κ) 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 κ 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 κ. Under conditions leading to SA, If emitted from κ apparently has a conjugated relationship with the fluorescent intensities simultaneously emitted from other elements (ξ). Normalizing the former (κ) by the latter (ξ) 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 ξ excited by the emission from κ. 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 μξ < μκ 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 μξ ≃ μκ to examine its applicability. For these samples, the amplitude reduction is 12 ± 4% versus their standards for the data measured with the classical 45°/45° geometry. This experimental method is easy to implement. Since If from κ and ξ 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. This work describes an experimental method in tackling the self‐absorption effect encountered in fluorescence X‐ray absorption spectroscopy and its application to surface and subsurface.