  The scattering of gas from surfaces underpins technologies in fields such as
gas permeation, heterogeneous catalysis and chemical vapour deposition. The
effect of surface defects on the scattering is key in such technologies, but is
still poorly understood. It is known empirically that unordered surfaces
result-in Knudsen, random-angle, scattering, with the effect thought to be
classical. We here demonstrate the transition from quantum mechanical
diffraction to an apparent Knudsen scattering, and show that quantum
bound-state resonances can greatly affect this transition. Further, we find
that randomly distributed defects induce a blue-shift in the bound-state
energies. We explore this phenomena, which can lay the basis for helium based
quantum metrology of defects in 2D materials and material surfaces.
