NOVEL WETTING BEHAVIOR OF HE-4 ON CESIUM

We have measured He-4 adsorption isotherms on a cesium substrate above and below 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 T-lambda the film at coexistence is superfluid. Above T-lambda we resolve a feature in the isotherm just below saturated vapor pressure which is characteristic of a prewetting transition.