*2.4. X-Ray Microbeam Analysis: XRF and XANES*

XRF and imaging experiments were conducted using the Very Sensitive Elemental and Structural Probe Employing Radiation from a Synchrotron (VESPERS) [14,15] and the Soft X-ray Microanalysis Beamline (Soft X-ray Microanalysis Beamline (SXRMB)) [16,17] beamline of the Canadian Light Source. The former produces monochromatic hard X-rays from 5 to 30 keV, and the latter delivers tender X-rays from 1.7 to 10 keV. With VESPERS, an X-ray energy of 10 keV was selected for XRF excitation, which is above the Cu K-edge (edge: 8979 eV; Kα1: 8048 eV), an element of interest in our pursuit, providing a good cross-section and, hence, fluorescence yield from other first-row transition elements at the K-edge, such as Fe (edge: 7112 eV; Kα1: 6404 eV). The SXRMB beamline is equipped with a double-crystal monochromator with dual crystal sets of InSb(111) and Si(111) crystals. The former is used for Si K-edge studies with a slightly better photon throughput. It is also equipped with a tender X-ray microprobe endstation that is capable of tracking elements such as Si (K-edge: 1840 eV; Kα<sup>1</sup> at 1740 eV), K (K-edge: 3608 eV; Kα1: 3314 eV) and all the way up to Fe (K-edge: 7112eV; Kα1: 6404 eV). The micro-X-ray beam was scanned over the sample (fixed beam position with sample moving) with a pixel size of ~5 × 5 and ~10 × 10 microns for VESPERS and SXRMB, respectively, while the intensity of the relevant fluorescent X-ray was tracked by silicon drift detectors. Elemental maps were produced with the normalized fluorescence yield of the element of interest and analyzed with standard imaging software.

XANES tracks the modulation of the X-ray absorption coefficient of an element above an absorption edge. The modulation, often appearing as oscillations arising from the interference of the forwardand backscattered photoelectron by surrounding atoms, contains all the information about the local structure of the absorbing atom (the absorption coefficient of a free atom is monotonic), such as the identity of the neighboring atom, its interatomic distance from the absorbing atom and its local dynamics (root mean displacement from the equilibrium distance). The simplest but most powerful use of XANES is its fingerprinting features since different local environments will exhibit different yet specific features in XANES. Thus, microbeam XANES provides additional information beyond XRF in that it provides not only elemental information but also chemical information. The power of these capabilities in the X-ray microanalysis of the Dzi bead specimen will become apparent below.
