Dynamics of Molecular Adsorption on Metal Surfaces

Doctoral thesis, 1993

In this Thesis different aspects of dissociative chemisorption and of scattering and trapping in a physisorption potential are investigated, mainly by means of classical trajectory calculations. In papers I-III the sticking coefficients of hydrogen on copper and nickel are calculated using a multidimensional effective-medium potential which takes the molecular orientation as well as the surface structure into account. In paper I it is shown that in order to describe and understand the trends found experimentally for the variation of the sticking probability with crystal face on copper and nickel the dynamics of all six molecular degrees of freedom must be included. In paper II it is found that six-dimensional effects lead to substantial shifts of the sticking onset energies. The change of onset energy due to vibrational excitation or isotope exchange is also sensitive to six-dimensional effects. In paper III an attempt is made to explain the non-monotonic dependence of the sticking coefficient on the initial rotational state and theoretical support is found for hypotheses expressed in the literature. It is concluded that a two-dimensional analysis of the relation between observed onset energies and potential barriers sometimes may be misleading. The surface atom motion, on the other hand, is found to have a negligible effect at surface temperatures used in recent experiments. In paper IV the non-equilibrium diffusion of highly hyperthermal oxygen atoms resulting from dissociative adsorption of O2 on Al(111) is investigated using molecular dynamics and the effective-medium theory. The results are discussed in relation to recent STM experiments. We find that the transient displacement along the surface is less than 30 ] and that it exclusively takes place in a ballistic phase which only lasts for less than 0.5 ps. The key factors for the transient motion are the chemisorption energy and the potential corrugation, determining the duration of the ballistic phase, whereas the energy transfer to the substrate phonons is less important. In paper V the role of the anisotropy of the molecule-surface potential in trapping and scattering of molecules weakly interacting with a surface is investigated using classical trajectories. A model potential is used which is based on a potential describing N2/Bg(111) and where a parameter controlling the anisotropy is introduced. The effects of this parameter on the rotational properties of scattered molecules and on the trapping behavior are investigated.