ABSOLUTE MOMENTUM-TRANSFER IN GAS-SURFACE SCATTERING

Two new parameters called the reduced force coefficients are introduce d that eliminate the singularity problems associated with the momentum accommodation coefficients and completely describe the macroscopic av erage momentum transfer to a surface by an incident gas. In addition, the reduced force coefficients can be used to determine the flux-weigh ted average velocity and translational energy of molecules scattered f rom a surface, and can be used to approximate the energy accommodation coefficient for many gas-surface interactions. The reduced force coef ficients for molecular beams of H-2 and N-2 incident upon SiO2-coated Kapton and Z-93-coated aluminum were obtained using a specialized tors ion balance to measure the magnitudes and directions of the forces exe rted on the surfaces by the molecular beams.