Critical cluster size determination from sticking coefficient and flash desorption measurements

Abstract

Measurements were made of the sticking coefficient γ of CO 2 molecules on a H 2O ice substrate as a function of time. A molecular beam was used with a flux of 7.7 × 10 13 molecules cm −2 sec −1. The H 2O surface temperature T s was varied between 72.4 and 74.4 K. The behavior of γ as a function of time was interpreted in terms of a microscopic nucleation theory. γ was also measured for T s decreasing at a constant rate b from above 78 K to 72 K. These data, taken with different beam fluxes, showed that nucleation processes are continuous for this system rather than discontinuous at some critical temperature. Analysis of the data for T s = constant and b = constant gave a critical cluster size of four CO 2 molecules and an activation energy equal to 22 ± 2.4 kcal mole −1. Thermal desorption rate measurements yielded an activation energy of desorption for CO 2 on solid H 2O equal to 6007 cal mole −1.