Molecular Dynamics Calculations of Surface Processes involving O Atoms on Silica Surfaces

The interaction of atomic and molecular oxygen with silica and silica-based materials under high temperature conditions can lead to various chemico-physical processes of great importance in many research areas of fundamental and technological interest, in aerothermodynamics as well as in the chemistry of oxygen-based laboratory plasmas. Among the many possible chemical processes going on at the oxygen-silica interlayer, a primary role is played by the atom recombination process because of the large exothermic energy released in the reaction that can be transferred to the surface causing damage both thermal and chemical. Oxygen atom recombination may also be a very effective heterogeneous channel for the formation of roto-vibrationally excited O2 molecules. Besides atom recombination, other collisional surface processes include inelastic scattering, adsorption and adsorption-desorption processes. In this study the aforementioned processes are simulated using Molecular Dynamics (MD) technique. We follow a fully ab intio approach carried out through two main steps: firstly DFT electronic structure calculations at the GGA level of accuracy are performed to calculate the interaction forces exerted between the gas-phase O,O2 species and the model silica surfaces, then the semiclassical approach1 is applied to describe the dynamics of the nuclear motions of the gas-phase atoms over the calculated potential energy surfaces. In the MD simulations the surface temperature is kept constant to 1000K, while the impact energy of the O atoms colliding with the silica surface is varied in a wide range. The results of the simulations will be presented and discussed in relation to the computed probabilities for the various surface processes, global and state-to-state recombination probabilities and coefficients, reaction energetics, roto-vibrational distributions for the formed O2 molecule and the energy exchanged between the gas-phase oxygen atoms and the silica phonons due to the multi-phonon excitation-deexcitation processes. Correlations between the dynamics of the surface processes and the structural behaviours of the silica surface will be also highlighted. [1] G. D. Billing, Dynamics of Molecule Surface Interactions, John Wiley&Sons, NY, 2000