We report the relative efficacy of vibrational and translational energy in overcoming the barrier to dissociative adsorption of D2 on a Cu(111) surface. This is determined by measuring variation of adsorption probability with kinetic energies, Ei, and vibrational temperature. At the highest energy (Ei=0.83 eV) adsorption is predominantly due to ground-state D2; molecules in excited vibrational states play the dominant role for Ei below 0.5 eV. A detailed analysis provides estimated adsorption probability versus energy functions for D2 in vibrational states v=0 to v=3. © 1992 The American Physical Society.