Gas–Surface Scattering Dynamics Applied to Concentration of Gases for Mass Spectrometry in Tenuous Atmospheres

A concept for a passive neutral gas concentrator that facilitates the analysis of rarefied atmospheres using mass spectrometry on spacecraft has been developed. The efficiency of the concentrator depends strongly on gas–surface scattering dynamics between the incoming gas molecules and the concentrator surface. We conducted beam–surface scattering experiments using hyperthermal beams containing atomic and molecular oxygen with speeds of approximately 5500 m s–1, with angular and velocity resolution of the inelastically scattered O and O2, on gold thin film, SiO2, and highly oriented pyrolytic graphite (HOPG) surfaces, which were chosen as candidate concentrator surfaces. The results show clearly that atoms and molecules scattering from HOPG have the narrowest and most superspecular angular distributions with the least energy transferred to the surface. A test particle model, referred to as the Statistical Program for Aerodynamic and Radiation Pressure Coefficient Simulation, utilized the experimental results to model gas concentration in three-dimensional cone and annular-ring geometries constructed of the representative materials chosen for study. The modeling results indicate that a cone concentrator with a 10° half angle opening yields the highest concentration factors. In addition, a cone constructed of HOPG yields a concentration factor that is an order of magnitude higher than what can be achieved using gold or SiO2 surfaces.