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The condensation of atmospheric gases on cold surfaces

Data for the deposition of argon, nitrogen and carbon dioxide on cold, metal surfaces are considered in detail. It is concluded that argon and nitrogen deposit when the incident gas pressure equals the sublimation pressure at the respective surface temperature, and growth therefore proceeds without any significant intermediate nucleation barrier. Carbon dioxide, however, requires considerable supersaturation of the gaseous phase and consequently bulk deposition is inhibited by a nucleation barrier. The results are analysed with a view to determining the critical nucleus size and the adsorption energy. The ‘classical’ method of analysis gives unsatisfactory and inconclusive results. In contrast, the ‘atomistic’ approach is found to give a good account of the critical deposition phenomenon. The onset of gross deposition is found to be due entirely to capture of single molecules by stable nuclei, rather than by the formation of critical nuclei as the ‘ classical ’ theory wrongly assumes. The number of molecules in the critical nucleus is found to be about nine and a value of 9.4 kJ mol-1 is obtained for the adsorption energy, suggesting that nucleation occurs on top of a strongly bound adsorbed layer of contaminants or carbon dioxide itself, rather than on bare metal.