Adsorption isotherms of helium on Na and on Rb

Adsorption isotherms of ⁴He on Na and on Rb measured with a quartz crystal microbalance are presented. The isotherms on Na indicate complete wetting at coexistence and the growth of a thin film at low relative pressures, which may become superfluid at lower temperatures. The results on Rb indicate clear prewetting jumps near coexistence from a thin film (less than 0.1 monolayers) to a thick superfluid film. Our results are in good agreement with the phase diagram found by Phillips et al. (Phys. Rev. B 58 (1998) 3361).