Inelastic scattering dynamics of hyperthermal O atoms on engineering surfaces relevant to satellites in low Earth orbit

Abstract Understanding the gas-surface momentum transfer of oxygen atoms, a major component of the residual atmosphere at very low Earth orbit (VLEO) altitudes of 100–450 km, on materials on the exterior of satellites is important for estimating drag. We have thus investigated the scattering dynamics of orbital-velocity O atoms on four representative materials fluorinated ethylene propylene (FEP) polymer, aluminum with a chromate conversion Alodine coating (Al), solar cell cover glass with a MgF 2 coating (CG), and a glass-reinforced epoxy laminate circuit board material (FR4). A pulsed hyperthermal atomic-oxygen beam with a nominal translational energy of ~ 4.7 eV was directed at the target surface, and the scattered products were detected with a rotatable mass spectrometer. Time-of-flight (TOF) distributions were measured with various incident beam angles ( θ i = 60°, 45°, 30°, 15°, 0°) for O atoms scattered in and out of the plane defined by the incident beam and surface normal. For both in-plane and out-of-plane scattering experiments, TOF distributions of O atoms exhibited mostly impulsive scattering, with atom-surface scattering events occurring on a time scale too short for thermal equilibrium to be achieved. A small fraction of thermally desorbed O atoms was also observed. In addition, both the flux and energy of the scattered O atoms were found to be higher when exiting the CG and FEP surfaces than when exiting the FR4 and Al surfaces. The lower flux of O atoms scattering from FR4 and Al at a given final angle ( θ f ), is the result of the combined effects of reactive collisions leading to OH and H 2 O products and the multiple-bounce trajectories of the incident O atoms on the rough surfaces, which leads to scattering angle randomization. Characterization of the material surfaces was consistent with the observed scattering dynamics: CG and FEP surfaces are relatively smooth at the nanometer scale, while FR4 and Al surfaces are comparatively rough. Regardless of surface roughness, the average fractional energy transfer to the surface is well parameterized by the angle through which the incident O atoms were deflected as they scattered from the surface. The fraction of thermally desorbed O atoms tends to be higher for the FR4 and Al surfaces than for the CG and FEP surfaces, which is consistent with greater energy accommodation on the FR4 and Al surfaces. The results suggest that FR4 and Al surfaces will lead to increased drag compared to CG and FEP surfaces, as a result of the greater overall energy accommodation of incident O atoms on the rougher surfaces.