Understanding gas-surface interactions from direct force measurements using a specialized torsion balance

Measurements of the magnitude and direction of the forces exerted on surfaces by molecular beams are used to obtain information about the microscopic properties of gas-surface interactions. The reduced force coefficients are introduced as a new set of parameters that completely describe the macroscopic average momentum transferred to a surface by incident molecules and can be used to obtain the flux-weighted average velocity and translational energy of the scattered molecules. This approach avoids the complications associated with obtaining macroscopic average quantities from averages of microscopic measurements and can be used to determine microscopic details concerning gas-surface interactions. Using a specialized torsion balance, the reduced force coefficients were determined from measurements of the forces exerted on surfaces of a solar panel array material, Kapton, SiO2-coated Kapton, and Z-93-coated Al by molecular beams of H-2, N-2, CO, and CO2, as a function of the angle of incidence. (C) 1997 American Vacuum Society.