BM23 XAS beamline

BM23 is one of the first projects in the Upgrade Programme. The beamline is a renewal of the previous XAS beamline BM29.

Synopsis

BM23 is the general purpose X-ray absorption spectroscopy beamline at the ESRF. It aims to meet the needs of the member countries in the area of conventional X-ray absorption spectroscopy. Principally, BM23 is designed to perform experiments which have requirements that fall beyond the capabilities of most second generation synchrotron sources, but which do not require the specialist characteristics of the other ESRF X-ray absorption instruments. The strengths to which BM23 operates arise from the intrinsic properties of the ESRF synchrotron, coupled with a bending magnet source and the high quality performance of the beamline's principle optical element, its monochromator. These strengths can be summarised as:

• a very large operational energy range with reasonable X-ray flux: 4 keV to 74 keV;
• high energy resolution: typically a factor 3 to 5 better than the intrinsic spectral broadening at any K or L absorption edge.;
• high spectral signal to noise ratio: above 7.0 * 10ˆ4 for well prepared samples;
• high beam stability: compatible with the demands of extreme sample environments such as pressure cells, where beam dimensions of 1.0 mm * 0.2 mm are required;
• a high level of automation which allows the user to perform non-conventional scanning measurements, like single energy temperature scan and energy scanning x-ray diffraction.

Scientific Applications

X-ray Absorption Spectroscopy (XAS) is a powerful structural technique to investigate the short-range environment around selected atomic species in condensed matter. While scanning the x-ray energy impinging onto the sample, a core level photoelectron is generated. This is scattered by the surroundings matter producing interference effects visible in the absorption cross-section and usually referred to as x-ray absorption fine structure (XAFS). The process itself is general and therefore fundamental to study structural properties in materials like:

• liquids, molecular solutions, liquid crystals;
• single- and poly-crystalline materials;
• amorphous and highly disordered solids;
• molecules and macromolecules containing metallic atoms or partially substituted with heavy atoms.

The energy range 4.5 keV < E < 74 keV is sufficient for K-edge studies of elements in the range 22 < Z < 76. For Z > 76 one must probe L-edges instead. The sample thicknesses for experiments are in the µm range for transmission experiments. However, thin film and/or dilute systems studies are still possible in fluorescence mode.