[en] We have studied diffractive scattering of H_2 from LiF(100) under fast grazing incidence, as a function of the initial ro-vibrational state of the molecule. We show that diffraction patterns, i.e., the relative diffraction peaks intensity, vary significantly with the initial ro-vibrational state. This result indicates that in order to perform accurate comparisons between experimental and theoretical results, some knowledge about the initial ro-vibrational distribution of the molecular experimental beam is required. We hope that this result will encourage experimental groups working on the field to design and develop the techniques required to provide such information. (paper)