Studies of elementary collision processes of H2 with metal surfaces can provide benchmark tests of theoretical methods that are increasingly used to aid the design of new heterogeneous catalysts [1]. Molecular beam and associative desorption experiments have been carried out to understand the main factors that govern H2 dissociation at the surface. In addition, vibrationally inelastic and rotationally inelastic scattering experiments, complemented by theoretical research, have provided useful information on how certain features of the potential energy surface (PES) control the experimental observations [2,3].
A different point of view is provided by diffraction experiments. H2 diffraction from metal surfaces is more complex than He diffraction, since the PES is 6-dimensional and the coupling with the dissociative adsorption channels comes into play. Thus, H2 diffraction has been proposed some time ago as a promising (and may be unique) technique to gauge the molecule-surface PES and dynamics [4]. In addition, it can provide useful information on the dynamics of the rotational inelastic transitions of the scattered molecules, as shown by our current work.
We have measured in-plane diffraction of D2 molecular beams scattered from a NiAl(110) surface at incident energies from 25 to 150 meV. The experiments were performed with a high-resolution time-of-flight (TOF) apparatus, which allows measurements of the rotational inelastic diffraction (RID) transitions with high accuracy. Elastic first and second-order diffraction as well as 0<->2 and 1<->3 RID peaks were observed over the whole incident energy range explored. The absolute RID transition probabilities were extracted from the data, and compared with classical trajectory calculations performed using an ab initio determined potential energy surface. The results will be discussed at the conference.
[1] G.J. Kroes and M. F. Somers, J. Theor. Comput. Chem. 4, 493 (2005).
[2] A. Gross, Surf. Sci. Rep. 32, 291 (1998).
[3] M. Bertino and D. Farías, J. Phys. C 14, 6037 (2002).
[4] D. Farías, C. Díaz, P. Rivière, H.F.Busnengo, P. Nieto, M.F. Somers, G.J.Kroes, A. Salin and F. Martín, Phys. Rev. Lett. 93, 246104 (2004).
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