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

摘要

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(lOO) 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.