Adsorption isotherms of He-4 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, whi
ch may become superfluid at lower temperatures. The results on Rb indicate
clear prewetting jumps near coexistence from a thin film (less than 0.1 mon
olayers) 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).
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