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

A molecular beam at temperature TG was impinged onto the surface of a copper block held at temperature TS. 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 CO2 condensing on a film of solid CO2, γ>0.96 when TS≤83 K and TS=274 K. Water molecules condensing on water ice films give γ≥0.99 when TS≤150 K and TG=273 K. Values of γ for CO2 were found to depend on TS, TG and the molecular beam intensity ṅCB. The range through which ṅCB varied was 1.5×1014 to 1.5×1016 molecules cm−2⋅sec−1.