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

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 100 nm thick Si3N4 or SiC membranes of 2 × 2 mm2 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 0.12 MPa. 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 50 μm and the helium pressure of 0.1090 MPa (0.1038 MPa), it is confirmed that the liquid water layer has flat thickness around 200 nm (320 nm) at the center position of 2 × 2 mm2 sample area with allowance of ±350 μm (±150 μm), where the thickness is ca 900 nm 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.