Associative desorption of hydrogen isotopologues from copper surfaces: Characterization of two reaction mechanisms

Abstract

We report quantum-state resolved measurements of angular and velocity distributions of the associative desorption of H 2 , HD, and D 2 from Cu(111) and Cu(211) surfaces. The desorbing molecules have bimodal velocity distributions comprising a "fast" channel and a "slow" channel on both facets. The "fast channel" is promoted by both hydrogen incidence translational and vibrational energy, while the "slow channel" is promoted by vibrational energy but inhibited by translational energy. Using detailed balance, we determine state-specific reaction probabilities for dissociative adsorption and compare these to theoretical calculations. The results for the activation barrier for the "fast channel" on Cu(111) are in agreement with theory within "chemical accuracy" (1 kcal/mole). Results on the Cu(211) facet provide direct information on the effect of increasing step density, which is commonly believed to increase reactivity. Differences in reactivity on the (111) and (211) facets are subtle-quantum state specific reactivity on the (211) surface is characterized by a broader distribution of barrier heights whose average values are higher than for reaction on (111). We fully characterize the "slow channel," which has not been found in theoretical calculations although it makes up a large fraction of the reactivity in these experiments.

Quantum Dynamics of Dissociative Chemisorption of H2 on the Stepped Cu(211) Surface

“…In our calculations, we see no evidence of the “slow channel” reactivity reported by Kaufmann et al 45 in their very recent paper, that is, reaction at low translational energies. We can now rule out quantum effects during the dynamics as the source of this slow channel reactivity, in which the reaction supposedly is inhibited by translational and promoted by vibrational energy.…”

“…Recent associative desorption experiments on Cu(111) and Cu(211), 45 which were in good agreement with earlier theoretical and experimental works, 16,30,34,35,46,58 have shown a never before reported “slow” reaction channel to be active for both Cu(111) and Cu(211). In this channel, the reaction could be facilitated by trapping on the surface and distortion of the surface due to thermal motion forming a reactive site.…”

“…We opt for the simplest and most direct method to scale to the relative experimental associative desorption data. In order to compare to the experimental E 0 (ν, J ) parameters, where E 0 (ν, J ) is the translational energy for which the reaction probability of the (ν, J ) state is half of the maximum reaction probability measured for the (ν, J ) state, we use the maximum translational energy sensitivity reported in Tables S7 and S9 of ref (45). Theoretical E 0 (ν, J ) are taken to be the translational energy to which the reaction probability is half that of the reaction probability at the maximum translational energy for which the experiment is sensitive.…”

“…Blue dots represent QCT results, red dots represent QD results, green dots represent MDEF results, and black dots represent experimental results. 45…”

An improved static corrugation model

“…The consequences of fitting experimental results in such a way and comparing with theory have recently been discussed. 39 Figures 8(a) and 8(b) show the fitted E 0 parameters for v = 0 and v = 1 at increasing J. For all rovibrational states, the SRP-BOSS and SRP48-BOSS are in very close agreement, even though there is a small difference between the two PESs as discussed above.…”