Blue shifts in helium-surface bound-state resonances and quantum effects in cosine-law scattering.

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 random-angle scattering, with the effect thought to be classical. We here demonstrate the transition from quantum mechanical diffraction to cosine-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.