Oxidation of a graphite initiated from surface defects is investigated by using scanning tunneling microscopy (STM) and density functional (DF) calculation. The defects are created on a graphite surface with controlled depth and density by impact of Ar + ions at low energies (50-500 eV). Oxidation of the defects with O 2 at temperatures of 450-650°C produces shallow pits on the surface via layer-by-layer etching of the carbon layer. The etch pattern and speed vary depending on the depth of a pit. Round shaped pits are produced in most cases, but less frequently anisotropic patterns are also observed for monolayer etching. A multilayer pit grows a few times faster than a monolayer pit via simultaneous etching of the top and inner carbon layers. Kinetic parameters are extracted for the oxidation of depth-differentiated pits. The activation energies are quite comparable for the oxidation of monolayer and multilayer pits, but the pre-exponential factor is larger for multilayer oxidation, which gives rise to a faster etching speed. DF calculations are performed to identify the key intermediates involved in the oxidation reactions. The calculation shows that an O 2 molecule dissociates exothermally with no energy barrier and then forms oxide species at the top or bridge sites of a carbon vacancy. A metastable O 2 * precursor state, in which an O 2 molecule adsorbs at the bridge site without dissociation, is also found. These surface species can be desorbed under the etching conditions either thermally or via further chemical reactions. Reaction pathways involving these intermediates are proposed to explain the different etching behaviors observed for mono-and multilayer pits.

The interactions of hyperthermal chlorotitanium ions, TiCl x + (x ) 0-4) with HOPG surfaces were studied by low-energy ion scattering of TiCl x + from HOPG and by XPS depth profile analysis of the TiCl x + exposed HOPG surfaces. Positive ion scattering data for 5-500 eV collision energies showed exclusively dissociative scattering of the molecular TiCl x + into TiCl + and Ti + fragments. TiCl 3 + and TiCl 2 + daughter fragments were not detected in the surface-induced dissociation (SID) spectra of the corresponding larger incident TiCl x + ions. The observed survival of >100 eV TiCl 3 + and TiCl 2 + incident ions in the scattered ion spectra and the calculated trend in the electron affinities of TiCl x neutrals implicate the role of bond dissociation in the efficient electron transfer to the departing TiCl 3 and TiCl 2 fragments during SID. XPS depth profile analysis showed a good correlation between the collision energy and the type of incident TiCl x + with the quality of TiC thin films formed on the surface of HOPG. Higher collision energies resulted in higher carbidic C to Ti ratios and higher carbidization efficiencies whereas a larger number of Cl atoms in the incident TiCl x + ion gave lower stoichiometries and lower carbidization efficiencies in the resulting films.