Interaction of Condensable Gases with Cold Surfaces

Deposition from a molecular beam onto a cold target surface only occurs when the beam pressure is many orders of magnitude larger than the vapor pressure of the depositing phase at the substrate temperature. The critical beam pressure for deposition has been interpreted as a critical supersaturation for heterogeneous nucleation of a condensed phase. It is shown that the critical beam pressure is much too large to be accounted for as a critical value for nucleation if the adsorbate temperature and the substrate temperature are alike. The interpretation of critical deposition phenomena as a nucleation event must include the additional assumption that the adsorbate temperature is somewhat higher than the substrate temperature.