This project is the continuation of a previous research program also financed by the EU. Our goal is the experimental characterization of the potential energy surface (PES) governing the interaction between simple molecules and surfaces.
Two different approaches are being pursued. In one set of experiments we want to investigate the diffraction of D2 molecules from surfaces presenting barriers of different heights to dissociative chemisorption. Very recent theories by Hollloway (1992) predict the onset of dramatic changes in the diffraction intensities when the incident energies are comparable with the dissociation barrier.
In a second set of experiments the hydrogen exchange reaction H2 + D2 = 2IID is being studied with of high resolution time-of-flight technique on highly reactive surfaces like Ni(110). The residence time of the reactants on the surface and other dynamical parameters can be directly measured by means of a newly conceived molecular beam techniz ue. We plan the study of this exchange reaction on the Ni(100) and the stepped Ni(115 surfaces as a function of the incident energy, incident angle and the surface azimut . Other surfaces with structural defects and clusters will be investigated. We are going to extend the range of incident beam energies up to 360 meV em- ploying recently developed high temperature nozzle source. The combined use of high crystal temperatures and highly energetic monochromatic molecular beams should allow the observation of the transition between slow, trapping-mediated (Langmuir-IIinshel- wood) and fastj direct (Eley-Rideal) chemical reaction mechanisms as recently proposed by Weinberg (1990)