It was theoretically predicted that the heavy alkali metals provide the only surfaces non-wetted by superfluid %e below a certain temperature Tw. This was experimentalIy proven both for Cs and for Rb. However, investigations have shown that the non-wetting thin-film state for the He-Cs system is extremely dilute for T Q T,, yet close to T, it can be much thicker and of the order of monolayers. Using the photoelectron tunneling method we have sensitively measured the growth of the non-wetting thin-film skate of 4He on a quench-condensed Cs surface. Tt turns out that far from coexistence there is littIe adsorption of helium. In contrast, close to GO-existence a rapid growth up to two to three monolayers of helium is observed, but the surface is still non-wet under the usual convention. These systems are excellent for studying the phenomena of non-wetting and pre-wetting, difficult to do with classical fluids. However, it turned out that the wetting behaviour, determined by measured contact angles [4,8,9] is strongly influenced 'by the method used to prepare the cesiated substrate: quench-condensed Cs, as it IS obtained by evaporation at low temperature, (expected to cause a rough surface) shows weaker nonwetting than Cs grown from the liquid state (which allows for a smooth surface to develop). This difference *Corresponding author.E-mail addresc: iov.valentin@hmi-konstanz.de (V. lov).in wetting behaviour is mainly attributd to the microscopic roughness of the Cs [lo].We have studied the non-wetting thin fiIm state on a quench-condensed (and hence rough) Cs surface using the photoelectron tunneling method [I 11. Close to liquid-vapour coexistence there is a sharp drop in photocurrent which indicates a transition from a monolayer to a multilayer film. Even in this regime the Cs surface shows non-wetting behaviour, according to the usual definition. From the temperature dependence of this transition a phase diagram for the monolayer and multilayer regime, respectively, can be constructed.
