  We have studied the wetting properties of He4 adsorbed on the surface of
heavy alkali metals by using a non-local free-energy density-functional which
describes accurately the surface properties of liquid He4 in the temperature
range 0<T<3 K.Our results for He4 on the Cs surface give both the temperature
dependence of the contact angle and the wetting temperature in good agreement
with the experimental findings. For the He/Rb system we find that a wetting
transition on the Rb surface occurs at T=1.4 K, whereas the experiments show
either wetting down to T=0 or a wetting transition at T=0.3 K. We suggest that
this disagreement is due either to an inaccuracy of the fluid-substrate
potential used in our calculations or the consequence of substrate roughness,
which is known to affect the Rb surface, and whose effect would be to lower the
wetting temperature. The sensitivity of the wetting transition to the
He-surface potential are stressed for the He/Rb surface, which may justify the
controversial experimental findings for this system.
