Evaporation of small fragments during the scattering of argon clusters at thermal kinetic energies from a graphite surface

Abstract

We present a thermokinetic model together with new experimental results for the scattering of large argon clusters off a graphite surface. Both angular and time-of-flight distributions are shown for a large range of surface temperatures, incidence angles, and incident cluster sizes. A quantitative comparison between the proposed thermokinetic model and our measurements allows one to interpret most of the experimental results as due to thermal evaporation of very small fragments from their parent clusters gliding along the surface. The coefficient of tangential velocity conservation cF and the local temperature Tlocal of the evaporating fragments have been determined quantitatively. Although the investigated parameters were varied over a large range, Tlocal remains essentially constant around (140 ± 20) K. The coefficient cF turns out to be approximately (0.80 ± 0.05) independent of surface temperature and incident cluster size for all incidence angles larger than 40°. It increases, however, rapidly to 1.4 when the incidence angle is reduced to 20° suggesting a substantial transfer from normal to tangential kinetic energy. For high enough surface temperatures, incident cluster sizes, and incident angles, both experimental time-of-flight (TOF) and angular distributions show the appearance of a grazing exit angle component attributed to large cluster fragments leaving the surface before total evaporation.