Dissociative adsorption of H2 on Cu(100): Fixed-site calculations for impact at hollow and top sites

Abstract

We present results of six-dimensional ͑6D͒ quantum wave-packet calculations for the dissociative adsorption of (ϭ0,jϭ4,m j ) H 2 on Cu͑100͒. The potential-energy surface is a fit to points calculated using density-functional theory ͑DFT͒, with the generalized gradient approximation ͑GGA͒, and a slab representation for the surface. New aspects of the methodology we use to adapt the wave function to the symmetry of the surface, which relate to calculations for initial rotational states with odd m j ͑the magnetic quantum number͒, are explained. Invoking detailed balance, we calculate the quadrupole alignment for H 2 as it would be measured in an associative desorption experiment. The reaction of the helicopter (ϭ0,jϭ4,m j ϭ4) state is preferred over that of the (ϭ0,jϭ4,m j ϭ0) cartwheel state for all but the lowest collision energies considered here. The energy dependence of the quadrupole alignment that we predict for (ϭ0,jϭ4) H 2 desorbing from Cu͑100͒ is in good qualitative agreement with velocity-resolved associative desorption experiments for D 2 ϩCu͑111͒. The vibrational excitation probability P(ϭ0,j→ϭ1) is much larger for j ϭ4 than for jϭ0, and the m j -dependence of P(ϭ0,jϭ4,m j →ϭ1) is markedly different from that of the initial-state-resolved reaction probability. For all but the highest collision energies, vibrational excitation from the (ϭ0,jϭ4) state is accompanied by loss of rotational energy, in agreement with results of molecular beam experiments on scattering of H 2 and D 2 from Cu͑111͒.

Vibrational de-excitation of v=1 H2 during collisions with a Cu(100) surface

“…The domination of vibrational de-excitation by collisions at top sites is consistent with results obtained using a 2D model [9][10][11][12] to examine the scattering of ground state H 2 from Cu͑100͒ which showed that reaction is more likely in collisions with bridge or hollow sites and vibrational excitation occurs primarily at the top site. Specifically, twodimensional calculations 9,10 with the molecular axis held fixed parallel to the surface predict vibrational excitation probabilities of up to 40% for impact at a top site.…”

“…The results of 3D and 4D calculations, in which the position of the molecular center-of-mass is fixed above the high-symmetry bridge, 11 hollow, and top sites 12 are shown in Fig. 2.…”

“…11,12 Calculations using these models predict that vibrational excitation occurs readily for collisions at top sites but predict significant, yet smaller, probabilities for vibrational excitation at the bridge site. Examination of the results revealed that vibrationally inelastic scattering can occur at the bridge site when the molecular axis is tilted in the range 60°Ͻ Ͻ80°.…”

“…7,8 This conclusion was supported by dynamics calculations using calculated ͑DFT͒ PES's for H 2 on Cu͑100͒ using reduced-dimensional models to examine the dependence of reaction and vibrational excitation on the surface impact site. [9][10][11][12] Reactivity was highest at the bridge and hollow sites because they have the lowest energy barriers. Vibrational transitions occur readily during collisions at top sites because of the strong curvature of the reaction path in front of the barrier and its location far into the product channel.…”

Six-dimensional quantum dynamics of dissociative chemisorption of H2 on Cu(100)

“…5,6 Previous to these, theoretical studies of surface reactions were restricted to explaining experimental trends, because to obtain a quantitatively accurate picture of dissociative chemisorption of diatomic molecules on metal surfaces, it is necessary to consider all six molecular degrees of freedom. 2,[16][17][18] Nonetheless, reduced dimensionality ͑2D-4D͒ calculations 16,[19][20][21][22][23][24][25][26][27] have played, and will continue to play, a very important role in understanding the trends found experimentally.…”