Balanced Detection for XAS at the Photon Noise Limit
(Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California, USA)
XHQ / E1.173 (European XFEL)
XHQ / E1.173
Despite being a ubiquitous method at storage ring x-ray light sources the development of soft x-ray XAS at FELs has been sluggish. My talk will begin with a survey of why this method is challenging for such high intensity pulsed sources. Building on this experience we use diffractive optics to divide a monochromatic soft x-ray beam into two identical copies. Using this balanced approach where one beam copy illuminates the sample, the other copy serves as a reference and both are detected on a CCD—we demonstrated XAS limited only by the number of photons detected. We successfully applied this method to time resolved XAS on VO2 near the metal to insulator transition. This method will unlock new possibilities for XAS at FELs to study: dilute systems, subtle processes and nonlinear phenomena.