Probing gas-surface potential energy surfaces with diffraction of hydrogen molecules

We review recent experiments where diffraction of deuterium molecules from single-crystal surfaces was employed to characterize the potential energy surface for hydrogen dissociative chemisorption. Scattering experiments were carried out on Cu(001), which is the prototype of a system with a high barrier for dissociative chemisorption, on Ni(110), which is the prototype of a system with a low barrier, and on NiAl(110), which is the prototype of an alloy surface with A high barrier. The experiments were carried out on two different set-ups at incident energies between about 20 and 250 meV. Elastic and rotationally inelastic diffraction (RID) peaks were observed in all experiments. Diffraction probabilities are presented and compared for each surface. Elastic diffraction intensities were interpreted with a simple corrugated hard-wall scattering potential. The values of the corrugation are h = 0.075 Angstrom for D-2/Cu(001), and h = 0.091 Angstrom for D-2/Ni(110). A satisfactory fit of the diffraction intensities could be obtained for D-2/NiAl(110) only over a very limited range of incident beam energies. A major finding of our experiments is that dissociative chemisorption strongly affects RID. probabilities. These are higher by about a factor of 3-5 for scattering from Ni(110) than for the high-barrier systems Cu(001) and NiAl(110). The high rotational transition, probability for systems with low barrier is interpreted as a consequence of the angular anisotropy of the chemisorption potential. Perspectives for future experiments are discussed.