  Knudsen theory of scattering is established from quantum mechanical (QM)
principles. Using close-coupled QM simulations of elastic helium scattering
from a surface, we demonstrate that as disorder is introduced on the
atomic-scale, the scattering shifts from diffraction to diffuse, characterised
by cosine-distribution. This apparent thermalisation of the helium eigenstates
due to an elastic process is attributed to the bound-state resonances in the
helium-surface potential, which are shown to also exhibit chemical-like shifts.
Our work establish fundamental Knudsen scattering which precedes any known
macroscopic effects, as well as lay the basis for helium based metrology of
disorder in 2D materials and material surfaces.
