[en] The influence of molecular vibrations on the reaction dynamics of H₂ on Si(001) as well as isotopic effects have been investigated by means of optical second-harmonic generation and molecular beam techniques. Enhanced dissociation of vibrationally excited H₂ on Si(001)2x1 has been found corresponding to a reduction of the mean adsorption barrier to 390 meV and 180 meV for ν=1 and ν=2, respectively. The adsorption dynamics of the isotopes H₂ and D₂ show only small differences in the accessible range of beam energies between 50 meV and 350 meV. They are traced back to different degrees of vibrational excitation and do not point to an important influence of quantum tunneling in crossing the adsorption barrier. The sticking probability of H₂ on the 7x7-reconstructed Si(111) surface was found to be activated both by H₂ kinetic energy and surface temperature in a qualitatively similar fashion as H₂/Si(001)2x1. Quantitatively, the overall sticking probabilities of H₂ on the Si(111) surface are about one order of magnitude lower than on Si(001), the influence of surface temperature is generally stronger