Grazing-Incidence X-Ray Resonant Raman Scattering (GI-RRS)

JUAN JOSÉ LEANI; H.J. SÁNCHEZ; C.A. PEREZ

Viena

Conferencia; EXRS 2012; 2012

IAEA

As the refractive index for x-rays is less than unity, external total reflection is possible if the glancing angle is less than a critical angle derived from the Snell´s law [1]. This fact gives origin to GIXRF, GEXRF and TXRF, largely proved spectroscopic techniques that allow the study of material surfaces and spectrochemical analysis. Using the GIXRF technique, different depths of a sample can be analyzed by means of the correct election of the incident radiation angle. In this way, analysis of the reflected intensity provide a method for studying surface properties, as variations of electron density with depth (e.g., corrosion, porosity, aging, etc.) with a resolution from Armstrong to hundred nanometers deep [2]. On the other hand, this technique presents a high signal/background ratio, allowing excellent detection limits. X-ray Resonant Raman Scattering (RRS) is an inelastic scattering process which presents fundamental differences compared to other scattering interactions between X-rays and atoms; when the energy of the incident photon approaches from below to an absorption edge of the target element, a strong resonant behavior takes place. Recently, a spectroscopic technique in formation based in this process showed to be useful to distinguish surrounded chemical environments [3,4]. In this work both total reflection and resonant Raman scattering techniques are used in combination in order to discriminate oxidation states in three cases: a) oxidized nano-layers of stratified transition metal samples; b) pure Cu and Fe foils oxidized in water and salty water respectively and c) arsenic speciation. All of the samples were studied at the XRF Beamline [5] of the Brazilian synchrotron facility. The measurements were carried out in total reflection geometry with incident energy lower and close to the K absorption edge of the element of interest in order to study the RRS emissions. In cases a) and b) the incident radiation angle was scanning around the critical angle. The RRS spectra were analyzed with specific programs for fitting the experimental data to theoretical expressions. Then, residuals were determined in the low energy side of the RRS peaks. These residuals were treated with FFT smoothing methods taking into account the instrument functions of the detecting system. The residuals show an oscillation pattern that depends of the observed nano-layer, i.e., the local oxidation state of the absorbing atom. The results show the possibility to obtain detail structural information by means of X-ray Raman scattering in total reflection geometry using a low resolution system, allowing a depth study of the oxidation state with nano-metric resolution. [1] Handbook on Synchrotron Radiation 1A, Edited by E.-E. Koch, North-Holland Publishing Company, Netherlands (1983). [2] Parrat L.G., Physical Review 95, 359 (1954). [3] J.J. Leani et al,. J. Anal. At. Spectrom. 26, 378 (2011). [4] J.J. Leani et al., X-Ray Spectrometry 40, 254 (2011).