Ab Initio Molecular Dynamics Study of Alignment-Resolved O2 Scattering from Highly Oriented Pyrolytic Graphite

We present ab initio molecular dynamics simulations based on the density functional theory to study the alignment-dependent scattering of O-2 from highly oriented pyrolytic graphite (HOPG). Results are obtained for 200 meV O-2 molecules with different alignments impinging with an angle of incidence measured from a surface normal of 22.5 degrees on a thermalized (110 and 300 K) graphite surface. The choice of these initial conditions in our simulations is made to perform comparisons with recent experimental results on this system. Our results show that a lower number of molecules initially aligned normally to the surface is reflected in the scattering plane compared to the number of molecules initially aligned parallel to the surface. This is not due to an increase in the trapping of end-on-aligned molecules but due to the fact that a large amount of them are scattered out-of-plane. Additionally, we find that the final translational energy of end-on molecules is around 10% lower than that of side-on molecules. We show that this is due to a more efficient energy transfer from the translational degree of freedom to internal degrees of freedom for end-on molecules, and not due to a more efficient energy transfer to the surface. The results of our simulations are in overall agreement with the experimental observations regarding the alignment dependence of the in-plane scattering probabilities and the energy loss of the reflected molecules.