  We present first experimental data on the high energy behavior of helium
atoms quantum reflecting from the nanoscopically disordered surface of a quartz
crystal. The use of the light, stable and inert He atom not only opens the
unique possibility of measuring quantum reflectivity in the thusfar
inaccessible limit of high energies, but also allows the determination of the
gas-solid interaction potential. The specularly reflected intensity from the
rough surface shows a change of 5 orders of magnitude within an incident
angular range of less than 6 degrees. By separating out the influence of
surface disorder the quantum reflection coefficient for the smooth surface is
deduced. Firstly, the data confirm the high energy asymptotic behavior of the
reflection, defined by the non-retarded attractive van der Waals potential. The
experiment shows very good agreement with our calculations covering the entire
energy region, in which also Casimir forces play a role. Parameters for the
gas-solid interaction perfectly match those reported in literature in the
vicinity of the potential minimum.
