He atoms and ${\text{H}}_{2}$ molecules with energies of 2 keV are scattered under grazing angles of incidence ranging from $0.2\ifmmode^\circ\else\textdegree\fi{}$ to $1.8\ifmmode^\circ\else\textdegree\fi{}$ from the surface of a monolayer silica film grown on a Mo(112). We observe for scattering along low indexed atomic strings in the topmost surface layer pronounced diffraction pattern owing to diffraction effects for the elastically scattered projectiles. The diffraction patterns are analyzed in terms of semiclassical trajectory calculations making use of interaction potentials derived from density-functional theory. We find good agreement with the experiments, for a two-dimensional [Si-O-Si] network structural model for the ultrathin silica film.