Reliable absorbance measurement of liquid samples in soft X-ray absorption spectroscopy in transmission mode

Abstract

In soft X-ray absorption spectroscopy in transmission mode (T-mode XAS), it is necessary to optimize the thickness of thin liquid layers for appropriate absorbance around 0.3–0.7 and keep the sample thickness flat within a photon beam. In the present work at the UVSOR Synchrotron Facility, a thin liquid layer is sandwiched between two 100nm thick Si3N4 or SiC membranes of 2×2mm2 in a helium chamber with enough working space, and the liquid thickness is optimized by using the elasticity of the membranes under controlling the helium pressure between 0.1013 and 0.12MPa. The flatness of the liquid sample within a photon beam is investigated with measuring position-dependent O K-edge T-mode XAS spectra of liquid water. In the case of the beam size of 50μm and the helium pressure of 0.1090MPa (0.1038MPa), it is confirmed that the liquid water layer has flat thickness around 200nm (320nm) at the center position of 2×2mm2 sample area with allowance of ±350μm (±150μm), where the thickness is ca 900nm at the brim of the liquid sample area. The pre-edge vs. main edge (Pre/Main) ratio is 0.38±0.01 for the center of the sample area and becomes considerably larger than 0.38 for the off-center position. The deviation from 0.38 is caused by the inhomogeneous thickness and is evaluated in comparison with a model simulation. The Pre/Main ratio can be used to evaluate the flatness in the liquid sample thickness within the beam area of soft X-rays. Positioning of as small a beam as possible on the center of the liquid sample area is essential to obtain reliable T-mode XAS spectra with the present liquid cells.