Hot-Atom vs Eley-Rideal dynamics in hydrogen recombination on Ni(100). I. The single-adsorbate case

We compare the efficiency of the Eley–Rideal (ER) reaction with the formation of hot-atom (HA) species in the simplest case, i.e., the scattering of a projectile off a single adsorbate, considering the Hydrogen and Hydrogen-on-Ni(100) system. We use classical mechanics and the accurate embedded diatomics-in-molecules potential to study the collision system over a wide range of collision energies (0.10–1.50 eV), both with a rigid and a nonrigid Ni substrate and for impact on the occupied and neighboring empty cells. In the rigid model metastable and truly bound hot-atoms occur and we find that the cross section for the formation of bound hot-atoms is considerably higher than that for the ER reaction over the whole range of collision energies examined. Metastable hot-atoms form because of the inefficient energy transfer to the adsorbate and have lifetimes of the order 0.1–0.7 ps, depending on the collision energy. When considering the effects of lattice vibrations we find, on average, a consistent energy transfer to the substrate, say 0.1–0.2 eV, which forced us to devise a two-step dynamical model to get rid of the problems associated with the use of periodic boundary conditions. Results for long-lived HA formation due to scattering on the occupied cell at a surface temperature of 120 K agree well with those of the rigid model, suggesting that in the above process the substrate plays only a secondary role and further calculations at surface temperatures of 50 and 300 K are in line with these findings. However, considerably high cross sections for formation of long-lived hot-atoms result also from scattering off the neighboring cells where the energy transfer to the lattice cannot be neglected. Metastable hot-atoms are reduced in number and have usually lifetimes shorter than those of the rigid-model, say less than 0.3 ps. In addition, ER cross sections are only slightly affected by the lattice motion and show a little temperature dependence. Finally, we find also that absorption and reflection strongly depend on the correct consideration of lattice vibrations and the occurrence of trapping.