Calculations of accommodation coefficients for diatomic molecular gases

A theoretical study of energy and momentum accommodation coefficients and reduced force coefficients for molecular gases exchanging energy with surfaces has been carried out. The theoretical model uses classical mechanics for describing translational and rotational motions while internal molecular vibrational modes are treated quantum mechanically. Calculations for diatomic molecular gases are compared with recent measurements using hypersonic beams of N2 incident on SiO2 layers deposited on Kapton substrates. The theory gives good qualitative predictions of the behavior of the various accommodation coefficients as functions of the available experimentally controllable parameters such as incident translational energy, incident beam angle, molecular and surface masses, and surface temperature. Quantitative comparisons with measurements for energy and normal momentum accommodation indicate that these experiments can be used to obtain basic physical information about the molecule-surface interaction such as the physisorption potential well depth and the extent of surface roughness.