We observe high-resolution diffraction patterns of a thermal-energy
helium-atom beam reflected from a microstructured surface grating at grazing
incidence. The grating consists of 10-μm-wide Cr strips patterned on a
quartz substrate and has a periodicity of 20 μm. Fully-resolved diffraction
peaks up to the 7th order are observed at grazing angles up to 20
mrad. With changes in de Broglie wavelength or grazing angle the relative
diffraction intensities show significant variations which shed light on the
nature of the atom-surface interaction potential. The observations are
explained in terms of quantum reflection at the long-range attractive
Casimir-van der Waals potential.Comment: 4 pages, 4 figure