Dissociative chemisorption of H2 on Cu(100): A four-dimensional study of the effect of parallel translational motion on the reaction dynamics

Abstract

We investigate the usefulness of a hybrid method for scattering with resonances. Wave packet propagation is used to obtain the time-dependent wave function ⌿(t) up to some time T at which direct scattering is over. Next, ⌿(t) is extrapolated beyond T employing resonance eigenvalues and eigenfunctions obtained in a Lanczos procedure, using ⌿(T) as starting vector to achieve faster convergence. The method is tested on one two-dimensional ͑2D͒ and one four-dimensional ͑4D͒ reactive scattering problem, affected by resonances of widths 0.1-5 meV. Compared to long time wave packet propagation, the hybrid method allows large reductions in the number of Hamiltonian operations N H required for obtaining converged reaction probabilities: A reduction factor of 24 was achieved for the 2D problem, and a factor of 6 for the 4D problem.

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.…”

“…This independence from collision energy was seen for the average parallel translational energy for molecules incident in the ͑v 0 ϭ0, j 0 ϭ0) state 14 and in 4D calculations in which the molecular orientation was fixed parallel to the surface. 10 This result suggests that energy transfer into parallel translation is not governed by competition with rotational excitation or normal translation but rather by a dynamical restriction on the number of quanta of momentum transferred during a collision. The energy spacing between diffraction states is small so energy transfer into parallel translational motion saturates at low collision energies.…”

“…The top site curve exhibits a great deal of structure, including sharp peaks previously identified as resonances associated with trapping at the surface as the intramolecular bond weakens and excitations to higher vibrational states occur. 9,10 At collision energies above 0.5 eV, the vϭ2 vibrational state ͑not shown͒ is energetically accessible and vibrational excitation competes with vibrational de-excitation, but this channel plays a minor role compared to reaction and rotational excitation. Figure 2 shows the energy dependence for vibrational de-excitation obtained by averaging over the values obtained for the fixed-site models.…”

“…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.…”

A note on the vibrational efficacy in molecule-surface reactions

“…The first theoretical [3][4][5][6][7] and experimental [8][9][10][11][12][13][14] studies related to the vibrational enhancement for specific molecule-surface systems were carried out on H 2 dissociation on Cu.…”