TOF spectra of direct recoils. I. Chemisorption of O₂, H₂O, and CH₂OH on magnesium

Time-of-flight (TOF) analysis of directly recoiled surface atoms (neutrals and ions) produced by pulsed Ar⁺ ion irradiation has been used to monitor chemisorption of O₂, H₂O, and CH₃OH on polycrystalline magnesium. The intensities of the H, C, O, and Mg direct recoils are obtained as a function of gas exposure dose. The reaction with O₂ is fast, with saturation occurring at ∼ 10 L (1 L = 10^-6 Torr s), and following either a one- or a two-site adsorption model while the H₂O and CH₃OH reaction are slower, with saturation occurring at ∼ 20 L, and following a nucleation and growth expression. The hydrogen direct recoil intensitiy has been calibrated to the amount of hydrogen on a methoxylated surface. This calibration allows determination of H coverages during the O₂ and H₂O reactions and shows a surface stoichiometry consistent with Mg(OH)₂ for the water reaction. The analysis indicates that the oxygen forms an oxide at sites that are within the outermost Mg layer while the H₂O and CH₃OH form hydroxide and methoxide layers which are above the outermost Mg layer. Scattering and direct recoil cross-sections and shadow cones are calculated by means of a program which uses the Molière approximation to the interaction potential for single binary encounters.