The effect of corrugation on the quantum dynamics of dissociative and diffractive scattering of H-2 from Pt(111)

We present results of two dimensional (2D) and three dimensional (3D) calcu lations for dissociative and diffractive scattering of H-2 from Pt(111), us ing a potential energy surface obtained from density functional theory (DFT ) employing the generalized gradient approximation (GGA) in conjunction wit h a slab representation of the metal surface. The present study is motivate d by the importance of Pt as a hydrogenation catalyst, and by a paradox reg arding the amount of corrugation of the H-2+Pt(111) potential energy surfac e (PES). Molecular beam experiments on dissociation of D-2 from a Pt(111) s urface suggest a rather corrugated PES, which is at odds with results from molecular beam experiments on rotationally inelastic diffraction of HD from Pt(111), where only very little diffraction is found, suggesting a weakly corrugated PES. Results of our 3D calculations for off-normal incidence sho w that the present 3D model does not obey normal energy scaling, and that p arallel motion inhibits dissociation at low collision energies, in agreemen t with the dissociation experiment. On the other hand, substantial diffract ion is found, where the diffraction experiment found almost none. For each impact site considered in the 2D calculations, the computed dynamical barri er height, E-0, is substantially lower than the barrier height in the PES, E-b, at that site. Both the 2D and the 3D calculations show a large vibrati onal enhancement of reaction. These effects are not due to a reduced mass e ffect, the barrier to dissociation being early, but to a decrease in the fo rce constant of the H-2 vibration upon approaching the barrier to dissociat ive adsorption from the gas phase. The vibrational enhancement computed for H-2+Pt(111) was not observed in seeded beam experiments on D-2+Pt(111) [A. C. Luntz, J. K. Brown, and M. D. Williams, J. Chem. Phys. 93, 5240 (1990)] . However, an analysis performed here strongly suggests that seeded beam ex periments will be unable to observe vibrational enhancement if the dissocia tion of the molecule in nu =0 proceeds without an energetic threshold, as i s the case for H-2+Pt(111). (C) 2000 American Institute of Physics. [S0021- 9606(00)70442-9].