Analyzing gas-surface interactions using the reduced force coefficients

Abstract

The reduced force coefficients are introduced as a new set of parameters that can be used to analyze the momentum transfered to surfaces by incident molecules in rarefied flow regimes. These coefficients are well-defined for all angles of incidence, thereby eliminating the singularity problems associated with the momentum accommodation coefficients. The reduced force coefficients can be obtained from measurements of the forces exerted on surfaces by molecular beams, and can be used to obtain the flux-weighted average velocity of the molecules scattered from surfaces. Also, a method is developed that can be used to approximate the flux-weighted average translational energy of the scattered molecules and the energy accommodation coefficient for many gas-surface interactions. Measurements of the forces exerted on SiO{sub 2}-coated Kapton and Z-93-coated Al by molecular beams of N{sub 2} and H{sub 2} are used to demonstrate the singularity problems associated with the momentum accommodation coefficients. The same force measurements are then used to calculate the reduced force coefficients and several macroscopic average quantities of the scattered molecules. {copyright} {ital 1998} {ital The American Physical Society}