Molecular beam-surface scattering techniques are used to understand in detail the interactions between fast atoms or molecules and surfaces. At collision energies of many electron volts, non-equilibrium interactions become important. Such interactions may control the outcome of processes such as etching, surface-chemistry modification, and materials degradation on spacecraft. Our research goals are to understand fundamental gas-surface interactions in the regime of hyperthermal collision energies and to apply the knowledge gained to the solution of real-world problems.
A molecular beam apparatus is used in conjunction with a unique hyperthermal molecular beam source (see photo below). The diagnostic power of the apparatus comes from a rotatable mass spectrometer, which enables the identification of species emerging from a surface and the determination of their velocities and directions. With the use of this information, the detailed gas-surface interactaction dynamics may be inferred. The molecular beam source is capable of producing neutral species with kinetic energies in the range 2-20 eV, thus permitting the study of gas-surface interactions in a relatively unknown regime of collision energies. The molecular beam techniques employed allow independent control of all parameters, including nature of reactant species, collision energy, incident angle, and surface temperature.
Recent experiments have shown that non-equilibrium processes dominate both the initial and steady-state interactions when a hydrocarbon surface is bombarded with oxygen atoms traveling at velocities on the order of 8 km s-1 (similar to the velocity of spacecraft to the velocity of spacecraft in low Earth orbit). Direct inelastic scattering, in which only a fraction of the initial kinetic energy is lost to the surface, is by far the most probable non-reactive interaction. The most likely reaction is gas-phase-like H-atom abstraction to form OH. Once formed, the OH may undergo further collisions and reactions with the surface. During steady-state oxidation, CO and CO2 produced. Formation of these species is believed to account for the erosion, or mass loss, of a polymer under O-atom attack. A new phenomenon that was recently discovered is that the rate of CO and CO2 production from the surface is significantly enhanced when high-energy (>10 eV) Ar atoms or N2 molecules collide with a surface undergoing continuous oxidation. Such a collisional process may be very important in some exposure environments for etching polymers.