We use a concave spherical mirror to focus at 18-mm-diam beam of H atoms down to 0.5 mm. The mirror consists of a fused-quartz substrate polished to optical precision and coated with a liquid${\mathrm{-}}^4$He film to obtain high reflectivity. The temperature dependence of the focused beam intensity enables us to study the influence of the dynamic surface roughness on the reflection of the H atoms. Both zero-point fluctuations and thermal excitations turn out to be of importance. A monolayer of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ does not significantly affect the results.

• Received 2 June 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.1689