Studies of gas-surface interaction dynamics with hyperthermal neutral beams

Hyperthermal neutral beams have been employed to study the gas-surface interaction dynamics for several systems, including initial reactions of hyperthermal atomic oxygen with a liquid hydrocarbon surface, collision-assisted erosion of polymers and graphite in an atomic oxygen environment, collision-assisted etching of Si in an atomic chlorine environment, and collision-induced desorption of Cl from a Si surface. Initial reactions between hyperthermal atomic oxygen bond breaking products were detected in the initial reactions. However, during the steady-state oxidation of polymers and graphite, volatile products of CO and CO₂ are produced, which are believed to account for a significant fraction of the mass loss of polymers or graphite that is observed under continuous atomic-oxygen attack. Exposure of these continuously-oxidized polymer or graphite surfaces to bombardment of hyperthermal inert species results in a dramatic increase in the production of CO and CO₂ and in the erosion rate of the surfaces. This synergism between energetic inert particles and chemically active species incident on surfaces is believed to be a generic effect. Both the production of volatile silicon chloride (SiCl_x, x = 1–4) compounds and the Si etching rate are dramatically enhanced with the bombardment of energetic inert species on a continuously chlorinated Si surface. Mechanisms of this collisional effect involve chemical sputtering and chemically-enhanced physical sputtering. In our experiment, the latter mechanism is essentially synonymous with collision-induced desorption. Studies of collision-induced desorption of Cl from a chlorinated Si surface reveal that impulsive bimolecular collision between the incident collider and adsorbate is the dominant process. Although the details of these collisional mechanisms are complicated, the dynamical behavior of the ejected products may be described in terms of a simple kinematic picture in which an incident energetic particle collides with a localized region of the surface that has an effective mass. The studies of initial reactions between atomic oxygen and a saturated hydrocarbon surface and collisional enhancement in material removal under steady-state oxidation conditions are directly relevant to material erosion and degradation on spacecraft in the low Earth orbit environment.