RESIDUAL MASS-FLOW OF HE-4 ON A NONWETTED CS SURFACE

At temperatures well below the wetting temperature for He-4 on Cs it i s expected that there is an extremely low-density two-dimensional (2D) gas of He-4 on the Cs surface. We have detected this low-density gas by measuring the residual mass flow of He-4 atoms over the Cs surface. We find that this transport rate is 10(-9) of the flow when 4He wets the Cs. We estimate that there is < 4 x 10(-6) monolayers on the Cs at 0.12 K. The fact that Cs is extraordinarily free of He-4, even when a thick superfluid He-4 film abuts it, has useful practical consequence s.