  Typically one expects that when a heavy particle collides with a surface, the
scattered angular distribution will follow classical mechanics. The heavy mass
assures that the de Broglie wavelength of the incident particle in the
direction of the propagation of the particle (the parallel direction) will be
much shorter than the characteristic lattice length of the surface, thus
leading to a classical description. Recent work on molecular interferometry has
shown that by increasing the perpendicular coherence length, one may observe
interference of very heavy species passing through a grating. Here we show,
using quantum mechanical simulations, that the same effect will lead to quantum
diffraction of heavy particles colliding with a surface. We find that the
effect is robust with respect to the incident energy, the angle of incidence
and the mass of the particle. It may also be used to verify the quantum nature
of the surface and its fluctuations at very low temperatures.
