Using molecular beams of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ with incident energies and angles in the ranges 5–50 meV and 40°–80°, we have measured diffractive selective adsorption scattering resonances from a Cu(100) surface kept at 80 K. From the dispersion relations—resonance energy versus incident angle—we have unambiguously identified the resonances and determined the appropriate bound-state level energies. We have analyzed these data and arrived at a level assignment that is compatible with a single gas-surface potential curve for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$. We find a well depth of 30.9 meV, which is substantially larger than the commonly accepted value around 22 meV for the ${\mathrm{H}}_2$-Cu system. We note that our data strikingly resembled those reported for the ${\mathrm{H}}_2$-Ag system and discuss these observations in relation to theoretical predictions.

• Received 4 December 1987

DOI:https://doi.org/10.1103/PhysRevB.38.2967