Sticking Coefficient of CO2 on Solid H2O Films

A molecular beam method has been used to measure the sticking coefficient γ of CO2 molecules on thin films of solid H2O. Impingement rates were below 1014 molecules cm−2 sec−1, and the CO2 gas temperature was 184 K. The substrate temperature TS was varied from 65.5–76.4 K. At TS=76.4 K, γ was initially near unity, but decreased with increasing CO2 coverage until γ≃0 at approximately monolayer coverage. When TS was below 76.4 K, the initial γ was also close to unity. As the CO2 coverage increased, γ decreased to a minimum value at about one monolayer coverage. However, γ again increased with time as CO2 continued to be deposited. These results are discussed in terms of a simple nucleation theory.