PREWETTING OF HELIUM ON WEAK BINDING SUBSTRATES

Recent theoretical and experimental work has shown that alkali metal s urfaces have extremely weak potentials for physical adsorption, which leads to very unusual thermodynamic properties of adsorbed films. We h ave used a microbalance adsorption isotherm technique to investigate t he phase diagram of He films on cesium. This experimental system has p rovided the first example of a surface which is not wet by superfluid helium, and the first experimental observation of the prewetting line. Near the wetting temperature T(w) almost-equal-to 2 K, we have observ ed large hysteresis effects associated with the nucleation of the thin phase. At higher temperatures, the prewetting line terminates at a pr ewetting critical point T(pw)c almost-equal-to 2.5 K. This system is p articularly well suited to surface phase transition studies, since the interaction parameters can be tuned continuously by varying the thick ness of the cesium substrate, or by doping the film with He-3.