Scattering of Atomic Hydrogen Off a H-Covered W(110) Surface: Hot-Atom versus Eley–Rideal Abstraction Dynamics

Abstract

Normal incidence scattering of hydrogen atoms off a H-covered tungsten W(110) surface is simulated via quasiclassical trajectories. A density functional theory (DFT) based multiadsorbate potential is developed to model a wide range of surface coverages, θ = 0.25−1 monolayer (ML), reproducing the surface arrangements observed at low temperature. The competition between hot-atom (HA) and Eley−Rideal (ER) abstraction mechanisms is studied for collision energies of the projectile atom in the range E p = 0.1− 5.0 eV (E p = 0.1−2.0 eV) for θ = 0.25 ML (θ = 0.5, 0.75, and 1 ML) coverage. Cross sections, final energies of the recombination products, and reaction times are analyzed. At low coverage and low collision energy, HA dominates the abstraction, whereas HA and ER cross-sections become similar when collision energy increases. The vibrational distribution of recombined H 2 molecules at finite coverage is found to be in better agreement with experiments than the one computed within the single adsorbate limit. At high surface coverage, ER dominates abstraction but the dynamical observables highlight the similarity between both reaction mechanisms, thus suggesting that abstraction may be considered as a unique process.

Communication: Hot-atom abstraction dynamics of hydrogen from tungsten surfaces: The role of surface structure

“…Recent theoretical works have thus focused on this issue, in particular for H 2 recombining from W(110). 43,44 Interestingly, it was predicted that, if hot-atom abstraction dominates recombination at low coverage when neglecting possible energy dissipation to the metal (adiabatic approximation), this process is importantly reduced in favor of adsorption as soon as dissipation to the surface 44 or to preadsorbed species 43 is significant. For hydrogen atoms and molecules interacting with metallic surfaces at low coverage, for which collisions with pre-adsorbed species are unlikely, the main energy dissipation channel in the picosecond time scale is the excitation of low lying e-h pairs.…”

“…Note that in selecting these coverages, the surface area per adsorbate is similar, ≈20.1 Å 2 and ≈28.4 Å 2 , respectively. Related multiadsorbate potential energy surfaces (PESs), which rely on density functional theory (DFT) calculations, were developed 43,53 as an expansion up to two H-terms 39,40 of a diatom-surface interpolated PES, derived from an adaptation of the corrugation reducing procedure (CRP). [54][55][56] QCT calculations use a 6 × 6 supercell with periodic boundary conditions in order to model an infinite covered surface.…”

“…To ensure convergence, respectively, 30 000 and 120 000 trajectories have been computed for W(100) and W(110). Since the multiadsorbate PESs ignore possible interaction between three hydrogen atoms, 43 trajectories are stopped whenever one H atom has two neighbouring H atoms closer than 1.5 Å. Since the actual fate of such trajectories is unknown, the corresponding contribution is taken as an uncertainty to any possible outcome of scattering.…”

“…Since the actual fate of such trajectories is unknown, the corresponding contribution is taken as an uncertainty to any possible outcome of scattering. 35,36,43 The vibration of the formed H 2 molecules has been semiclassically quantized via the Gaussian weighting procedure. [57][58][59] In order to rationalize nonadiabatic effects upon scattering, molecular dynamics simulations are performed within the Born-Oppenheimer static surface (BOSS) approximation and within the local density friction approximation (LDFA).…”

“…This result originates from the lower probability of adsorption mediated by energy dissipation to H-preadsorbed atoms, which has been previously shown to be a very efficient process. 43,44 Secondary HA abstraction is accordingly also more efficient on W(110). As observed in the single adsorbate limit, 35,37 ER abstraction proceeds, on the W(100) surface, via collision of the projectiles not only with neighbouring W surface atoms on the top most layer but also with second layer W atoms.…”