Abstract
Following our recent system-bath modeling of the interaction between a hydrogen atom and a graphene surface [Bonfanti et al., J. Chem. Phys. 143, 124703 (2015)], we present the results of converged quantum scattering calculations on the activated sticking dynamics. The focus of this study is the collinear scattering on a surface at zero temperature, which is treated with high-dimensional wavepacket propagations with the multi-configuration time-dependent Hartree method. At low collision energies, barrier-crossing dominates the sticking and any projectile that overcomes the barrier gets trapped in the chemisorption well. However, at high collision energies, energy transfer to the surface is a limiting factor, and fast H atoms hardly dissipate their excess energy and stick on the surface. As a consequence, the sticking coefficient is maximum (∼0.65) at an energy which is about one and half larger than the barrier height. Comparison of the results with classical and quasi-classical calculations shows that quantum fluctuations of the lattice play a primary role in the dynamics. A simple impulsive model describing the collision of a classical projectile with a quantum surface is developed which reproduces the quantum results remarkably well for all but the lowest energies, thereby capturing the essential physics of the activated sticking dynamics investigated.
Highlights
IntroductionFor all but the smallest coverage, hydrogenation is driven by electronic and substrate-softening effects which lead to dimer formation and clustering; current experimental results leave the question open of how large is the initial hydrogen sticking coefficient on the carbon sheet
Carefully conducted scattering experiments which used low energy hydrogen atom beams — as opposed to high energy beams obtained by thermal cracking of H2 — found that the competing, non-activated hydrogen abstraction process dominates under these conditions,19 a result which was later confirmed by ab initio molecular dynamics simulations
In this transition range, sticking is mainly determined by the probability that the incoming hydrogen overcomes the barrier since any H atom that reaches the interaction region is able to dissipate the small amount of energy in excess to the barrier and gets trapped in the chemisorbed well
Summary
For all but the smallest coverage, hydrogenation is driven by electronic and substrate-softening effects which lead to dimer formation and clustering; current experimental results leave the question open of how large is the initial hydrogen sticking coefficient on the carbon sheet. A marked isotope effect has been found when hydrogenating epitaxial graphene grown on Au/Ni and it has been argued that it directly relates to the sticking cross sections.. The role of the specific graphenic substrate employed has been addressed, and substantial differences in hydrogen saturated structures have been reported between quasi-free-standing graphene and metal-bound graphene.. A complete and thorough description of this vast phenomenology is lacking, and experimental results, pointing towards strong dynamical effects, are affected by such a large variety of almost uncontrollable factors.
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