Quartz crystal microbalance isotherms of 4He on
rubidium are presented. The isotherms are used to locate the prewetting
transition and the onset of superfluidity in the adsorbed helium films.
The phase diagram of He on Rb is distinctly different from helium on either
conventional strong substrates or on cesium. In particular, the superfluid
transition is hysteretic and does not conform to the Kosterlitz-Thouless
paradigm.