Novel wetting behavior of ^{4}He on cesium

We have measured ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ adsorption isotherms on a cesium substrate above and below ${\mathit{T}}_{\lambda }$ using a quartz-microbalance technique. The results show that the cesium surface remains dry until the pressure almost reaches the saturated vapor pressure. At liquid-vapor coexistence, however, the wetting film continuously thickens to more than 30 layers. Below ${\mathit{T}}_{\lambda }$, the film at coexistence is superfluid. Above ${\mathit{T}}_{\lambda }$, we resolve a feature in the isotherm just below saturated vapor pressure which is characteristic of a prewetting transition.