High resolution X-ray Spectrometer

G. TIRAO

Campinas

Congreso; XXII Reunión Anual de Usuarios del LNLS; 2012

X-ray emission spectroscopy (XES) has been widely used for elemental analysis and for studying the electronic structure of atoms. Technical developments in high-power X-ray sources, such as synchrotron radiation facilities, and high-resolution X-ray spectrometers, have allowed XES to become a powerful tool to study the chemical environment and the electronic structure of materials. The chemical environment mainly produces energy shifts and relative intensity variations of emission lines, which are commonly used for electronic and molecular structure determination. The electronic transitions from the molecular orbitals to a core-level are suitable candidates for chemically sensitive fluorescence lines, since the character of the molecular orbitals changes markedly between different chemical species, and its influence can be clearly observed in the structure of the emission spectrum [1]. Particularly, in transition metal compounds, Kb X-ray emission spectra, resulting from transitions 3p-->
1s, consist of the main Kb1,3 line and satellite structures, which can provide information about oxidation state, ligand type, coordination number, and bonding species [2].These measurements require high-precision intensity scans using high-resolution wavelengthspectrometers in order to detect the spectral parameters changes due to the chemical  environment. High-resolution spectra were obtained with a non-conventional spectrometer, based on quasibackdiffraction geometry [3], installed at the D12A-XRD1 beamline of the LNLS. It uses a focusing crystal analyzer in Johann-type configuration, in 1:1 Rowland geometry. The analyzer consists of a spherical surface of a certain crystal (Si or Ge) with a ~ 420 mm curvature radius, which corresponds to the Rowland circle diameter. The large analyzer effective area allows collecting fluorescent radiation in a solid angle up to 32 msr. The incident beam flux is monitored by registering scattered radiation from a Mylar film, placed in the path of the incident beam. The whole spectrometer is enclosed in a 1m diameter cylindrical vacuum chamber (~0.05 mbar) in order to avoid X-ray attenuation and scattering from air. Each spectrum was normalized to the incident intensity in order to take into account beams fluctuations. The energy analysis of fluorescence photons was performed by scanning the analyzer around the main Kb1,3 characteristic line and close to the backdiffractioncondition in order to achieve high energy resolution. From the analysis of high resolution spectra Kb, several spectral parameters reveal their dependence on chemical environment, making this technique a useful tool to characterize the chemical environment.