Specular Reflection ofHe4Atoms from the Surface of LiquidHe4

Abstract

Measurements with a pulsed ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atomic beam incident on the surface of liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at $T=0.03\mathrm{}$ K show that the probability of specular reflection depends only on the perpendicular momentum of the atom $\hslash kcos\theta$ and that it varies smoothly from 4×${10}^{-2\mathrm{}}$ to less than ${10}^{-4\mathrm{}}$ for $kcos\theta$ in the range from 0.05 to 0.6 ${\mathrm{\AA }}^{-1\mathrm{}}$. The probability of diffuse inelastic scattering is less than 2×${10}^{-3\mathrm{}}$. Contrary to theoretical expectations, there is no significant change in the reflection probability at the roton threshold at $k=0.5\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$.