Apparatus for dosing liquid water in ultrahigh vacuum

The structure of the solid-liquid interface often defines function and performance of materials in applications. To study the interface at the atomic scale, we extended an ultrahigh vacuum (UHV) surface-science chamber with an apparatus that allows to bring a sample in contact with ultrapure liquid water without exposure to air. In this process, a sample, typically a single crystal prepared and characterized in UHV, is transferred into a separate, small chamber. This chamber already contains a volume of ultrapure water ice, whose vapor pressure is reduced to UHV range by cooling it to cryogenic temperatures. Upon warming, the ice melts and forms a liquid droplet, which is deposited on the sample. First experiments carried out on rutile TiO2(110) single crystals using this apparatus exhibit unprecedented purity, as tested by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). These results enabled to separate the effect of pure water from the effect of low-level impurities present in the air. Other possible uses of the setup are discussed.