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

• Received 16 December 1993

DOI:https://doi.org/10.1103/PhysRevLett.73.692