A classical study of rotational effects in dissociation of H₂ on Cu(100)

Abstract

We perform classical trajectory and quasiclassical trajectory calculations treating all six molecular degrees of freedom of H₂ dissociatively adsorbing at normal incidence on Cu(100). Comparison of reaction probabilities with earlier quantum calculations for the same potential energy surface (PES) reveals the quasiclassical approach to be far superior to the classical approach for this system. We get qualitative agreement between the quantum and quasiclassical quadrupole alignments for (ν=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 the (ν=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 desorption of D₂ from Cu(111). We use the quasiclassical trajectories to gain insight into the dynamics, and find that the reactive mechanism is very much dependent on the initial rotational state. For (ν=0, j=0) reaction occurs at the site that includes the minimum energy barrier (i.e., bridge), but for (ν=0, j=4) reaction is predominantly at the hollow site, and for (ν=0, j=11, m_j=11) at low-symmetry sites near the top site. Parallel translation is found to play an important role in reaction of the (ν=0, j=11, m_j=11) state, with molecules converging on the top site as they react. We explain these findings with respect to potential topography, and some dynamical effects. We also show that the PES used in earlier quantum calculations contains a small flaw (a bump of approximately 0.03 eV) in the entrance channel, and present new quantum and quasiclassical results for an improved PES in which this flaw has been corrected. Using this PES we are able to reproduce the effect seen experimentally that rotation hinders reaction for low j (4).

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