Quantum dynamics studies of adsorption and desorption of hydrogen at a Cu(111) surface

The quantum dynamics of dissociative adsorption and associative desorption of hydrogen on Cu(111) surface over an atop site has been studied in detail using the S-matrix Kohn variation method for reactive scattering. We employed an empirical London-Erying-Polanyi-Sato (LEPS) type potential energy surface (PES) with parameters fitted to the available experimental adsorption data and to theoretical cluster calculations. The dissociation probability of hydrogen, as a function of normal kinetic energy, is calculated for individual rovibrational states with the v = 1 translational energy threshold being lower than that of v=0 by about 0.317 eV. Our calculation shows that dissociative adsorption of H₂ on Cu(111) at relatively low kinetic energies (<0.4 eV) is dominated by the component of vibrationally excited H₂(v=1), whereas ground H₂(v=0) plays the dominate role at higher kinetic energies. In addition to vibrational enhancement of hydrogen dissociation, the role of hydrogen rotation in dissociative adsorption has also been examined. In particular, in-plane rotation of H₂ (m=j) is found to be more favorable for dissociation than out-of-plane rotation (m=0), similar to the finding from a previous study on H₂/Ni(111) system. The present study also examined internal state distributions of H₂ desorbed from Cu(111). The vibrational population ratio P_v=1/P_v=0 in desorption is much larger than the thermal distribution at surface temperatures. The relation between the vibrational population ratio in desorption and the vibrational enhancement in adsorption is discussed and analyzed. Our theoretical results are compared to the recent experimental results for both adsorption and desorption of H₂ on Cu.