Absolute momentum transfer in gas-surface scattering

Two new parameters called the reduced force coefficients are introduced that eliminate the singularity problems associated with the momentum accommodation coefficients and completely describe the macroscopic average momentum transfer to a surface by an incident gas. In addition, the reduced force coefficients can be used to determine the flux-weighted average velocity and translational energy of molecules scattered from a surface, and can be used to approximate the energy accommodation coefficient for many gas-surface interactions. The reduced force coefficients for molecular beams of ${\mathrm{H}}_{2}$ and ${\mathrm{N}}_{2}$ incident upon ${\mathrm{SiO}}_{2}$-coated Kapton and Z-93--coated aluminum were obtained using a specialized torsion balance to measure the magnitudes and directions of the forces exerted on the surfaces by the molecular beams.