Specular Reflection of He4 Atoms from the Surface of Liquid He4

Measurements with a pulsed He4 atomic beam incident on the surface of liquid He4 at T=0.03 K show that the probability of specular reflection depends only on the perpendicular momentum of the atom kcosθ and that it varies smoothly from 4×10-2 to less than 10-4 for kcosθ in the range from 0.05 to 0.6 -1. The probability of diffuse inelastic scattering is less than 2×10-3. Contrary to theoretical expectations, there is no significant change in the reflection probability at the roton threshold at k=0.5 -1.