Molecular beams of rare gas atoms and D_2 have been diffracted from
100 nm period SiN_x transmission gratings. The relative intensities of the
diffraction peaks out to the 8th order depend on the diffracting particle and
are interpreted in terms of effective slit widths. These differences have been
analyzed by a new theory which accounts for the long-range van der Waals
-C_3/l^3 interaction of the particles with the walls of the grating bars. The
values of the C_3 constant for two different gratings are in good agreement and
the results exhibit the expected linear dependence on the dipole
polarizability.Comment: 4 pages Revtex + 3 figures. Physical Review Letters (in press
