Soft x-ray spectroscopy of high pressure liquid.

We describe a new experimental technique that allows for soft x-ray spectroscopy studies (∼100-1000 eV) of high pressure liquid (∼100 bars). We achieve this through a liquid cell with a 100 nm-thick Si3 N4 membrane window, which is sandwiched by two identical O-rings for vacuum sealing. The thin Si3 N4 membrane allows soft x-rays to penetrate, while separating the high-pressure liquid under investigation from the vacuum required for soft x-ray transmission and detection. The burst pressure of the Si3 N4 membrane increases with decreasing size and more specifically is inversely proportional to the side length of the square window. It also increases proportionally with the membrane thickness. Pressures > 60 bars could be achieved for 100 nm-thick square Si3 N4 windows that are smaller than 65 μm. However, above a certain pressure, the failure of the Si wafer becomes the limiting factor. The failure pressure of the Si wafer is sensitive to the wafer thickness. Moreover, the deformation of the Si3 N4 membrane is quantified using vertical scanning interferometry. As an example of the performance of the high-pressure liquid cell optimized for total-fluorescence detected soft x-ray absorption spectroscopy (sXAS), the sXAS spectra at the Ca L edge (∼350 eV) of a CaCl2 aqueous solution are collected under different pressures up to 41 bars.