Molecular dynamics study of Ar scattering from Pt(100): Reflected velocity distributions, accommodation coefficients, and a Gaussian tangential kernel

Gas–surface scattering kernels used in rarefied flow and vacuum modeling commonly assume Maxwellian reflection with constant accommodation coefficients. Molecular dynamics simulations were performed to quantify Ar scattering from a thermostated Pt(100) surface using a molecular-beam configuration. Incident speeds spanning 0.5–8 times the most probable speed and incident angles of 5°, 45°, and 75° were considered. Three trajectory classes were observed: single-bounce reflection, multibounce reflection, and adsorption following multiple bounces. Escape from the surface potential well occurred when the peak normal momentum during the interaction exceeded the incident normal momentum. Reflected velocity distributions were strongly non-Maxwellian. At low incident speeds, the tangential component was well described by a Gaussian and the normal component lay between Maxwellian and Gaussian forms. With increasing incident speed, the tangential distribution developed a shoulder and ultimately became bimodal, accompanied by tangential–normal coupling. Increasing incident angle reduced the spread of reflected velocities and shifted scattering from diffuse toward specularlike reflection. Accommodation coefficients were markedly condition dependent: tangential momentum accommodation decreased monotonically with incident speed at large angles; normal momentum and energy accommodation exhibited opposite trends between low- and high-speed regimes; and tangential momentum accommodation exceeded unity at small angles, indicating tangential velocity reversal. Motivated by the low-speed behavior, a tangential scattering-kernel parameterization is proposed in which a Gaussian distribution is used with incident-speed- and angle-dependent mean and variance, providing a compact model form for gas–surface boundary conditions.