Absolute diffraction probabilities for the scattering of D-2 from a clean Cu(001) surface along the [100] azimuth have been measured at incident kinetic,energies between 20 and 250 meV. The measured attenuation of the diffraction intensities with surface temperature corresponds to a surface Debye temperature of Theta(D) = 341. K. The high-resolution angular distributions show clear evidence of rotationally inelastic diffraction (RID) peaks. The RID probability increases with incident energy and represents as much as 30% of the elastic diffraction probability at energies above E-i = 200 meV. An Eikonal approximation analysis gives a value h = 0.075 Angstrom for the surface corrugation which is independent of incident energy. The rotational transition probabilities correspond to an effective value of delta = 0.3 for the molecular eccentricity. The experimental results indicate that diffraction of D-2 from Cu(001) can be accounted for by a hard-wall collision mechanism over the whole range of investigated energies.