Water gets in shape for VUV absorption

The absorption of vacuum‑ultraviolet (VUV) light by liquids reveals the dynamics of valence electrons. Yet, conventional flow cells force experiments to stop at 10.5 eV because even the best LiF or MgF₂ windows turn opaque at this energy. A team at PSI has now removed that barrier, quite literally.

Using the gas‑squeezed micro‑fluidic nozzle built at PSI's Photon Science Department in cooperation with researchers from SLAC National Accelerator Laboratory, USA, a team from SwissFEL and SLS produced free‑standing water sheets just 20–50 nm thick.​ This corresponds to how much a human fingernail grows each minute and is required as valence electrons absorb VUV light very efficiently. Any thicker water sheet would block the radiation entirely. Another challenge is to maintain the vacuum in the experiment, as both the squeezing gas and the water vapor must be pumped away quickly to allow for the synchrotron beam to pass through the sheet.

With their setup, the researchers recorded the first liquid‑phase absorption spectrum of water from 7 to 13 eV. The data reveal an n → σ* peak at 8.52 eV, at a 0.06 eV higher energy than in room‑temperature bulk water, allowing the measurement of the water jet temperature of 0 ± 3 °C, right at the edge of supercooling. At these dimensions, the sample is thinner than the VUV wavelength (λ ≈ 145 nm at 8.5 eV). Interference, the same physics that gives soap bubbles and thin oil films their iridescent colors, will also affect 20–50 nm liquid sheets in the VUV. Multiple internal reflections alternately reinforce and cancel, brightening or dimming specific wavelengths. Simulations based on Fresnel propagation model these effects and show that neglecting them would overestimate the cross section by up to 20% below 8 eV.

The windowless jet concept can now be extended to organic solvents and time‑resolved pump–probe experiments at SwissFEL. The approach paves the way for valence‑band spectroscopy of solvated molecules, heterogeneous interfaces and ultrafast reaction intermediates at energies long thought inaccessible to liquid studies.