We have observed the direct vibrational excitation (v = 0 --> v = 1) of H-2 and D2 molecules colliding with a Cu(111) surface. The dependence of this process on kinetic energy is found to be similar for both molecules, resembling the function determined for dissociative chemisorption of molecules in the v = 0 state. This suggests that translational-vibrational coupling is strong only when molecules closely approach the dissociation barrier.