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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-^4He 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 ^3He does not significantly affect the results.
Berkhout et al. (Mon,) studied this question.