ROTATIONAL MOTION AND THE DISSOCIATION OF H-2 ON CU(111)

The influence of rotational state on the dissociation probability of H -2 on Cu(111) has been investigated with 3- and 4-dimensional close-co upling wave packet calculations. Recent experimental results have show n that the energetic threshold for dissociative adsorption increases a nd then decreases as the J state is continuously increased. This trend can be faithfully reproduced by modeling the H-2 as a planar (cartwhe el) rotor scattering from a flat surface. The agreement disappears whe n the model is extended to a 3-dimensional rotor. Further, the degener ate m(J) states have a spread of dissociation probabilities which resu lts in a broad smearing of the dissociation threshold. This effect, wh ich is absent from experiment, increases with J(i). These shortcomings can be partially corrected by corrugating the potential in the azimut hal coordinate in accord with recent ab initio results. The dynamical calculations also exhibit strong rotational inelasticity for the scatt ered fraction, during dissociation. Since this system has a late barri er for dissociation, we show that the rotational inelasticity should b e enhanced by initial vibrational state. Our 4-dimensional modeling is unable simultaneously to match the relative positions of dissociation and vibrational excitation thresholds. We speculate that these proces ses occur on different surface sites.