High-Precision Measurements of Condensation Coefficients. Results for Carbon Dioxide and Water Molecules

Abstract

A molecular beam at temperature T G was impinged onto the surface of a copper block held at temperature T S . The fraction of molecules reflected was 1−γ, where γ is the condensation coefficient. Of the reflected molecules, a fixed fraction was condensed on the surface of a quartz crystal microbalance. Precision measurements of the mass flux onto the microbalance gave a direct determination of the rate of reflection of molecules from the surface of the copper block. This information, combined with the known flux of molecules onto the copper block, gave high precision (∼0.01%) values of γ. For CO 2 condensing on a film of solid CO 2 , γ>0.96 when T S ≤83 K and T S =274 K . Water molecules condensing on water icefilms give γ≥0.99 when T S ≤150 K and T G =273 K . Values of γ for CO 2 were found to depend on T S , T G and the molecular beam intensity ṅ CB . The range through which ṅ CB varied was 1.5×10 14 to 1.5×10 16 molecules cm −2 ⋅ sec −1 .