A classical study of rotational effects in dissociation of H-2 on Cu(100)

We perform classical trajectory and quasiclassical trajectory calculations treating all six molecular degrees of freedom of H-2 dissociatively adsorbi ng at normal incidence on Cu(100). Comparison of reaction probabilities wit h earlier quantum calculations for the same potential energy surface (PES) reveals the quasiclassical approach to be far superior to the classical app roach for this system. We get qualitative agreement between the quantum and quasiclassical quadrupole alignments for (nu = 0,j = 4), and find that, as in recent experiments for Cu(111), the quasiclassical alignment is for the most part a positive decreasing function of incidence energy for each of t he (nu = 0,j) states studied; however, we do find negative values for some states at low energies. The alignment at a fixed collision energy tends to increase with increasing j, an effect also measured in associative desorpti on of D-2 from Cu(111). We use the quasiclassical trajectories to gain insi ght into the dynamics, and find that the reactive mechanism is very much de pendent on the initial rotational state. For (nu = 0,j = 0) reaction occurs at the site that includes the minimum energy barrier (i.e., bridge), but f or (nu = 0,j = 4) reaction is predominantly at the hollow site, and for (nu = 0,j = 11, m(j) = 11) at low-symmetry sites near the top site. Parallel t ranslation is found to play an important role in reaction of the (nu = 0,j = 11, m(j) = 11) state, with molecules converging on the top site as they r eact. We explain these findings with respect to potential topography, and s ome dynamical effects. We also show that the PES used in earlier quantum ca lculations contains a small flaw (a bump of approximately 0.03 eV) in the e ntrance channel, and present new quantum and quasiclassical results for an improved PES in which this flaw has been corrected. Using this PES we are a ble to reproduce the effect seen experimentally that rotation hinders react ion for low j (less than or similar to 4).