SUBLIMATION OF VAPOR-DEPOSITED WATER ICE BELOW 170-K, AND ITS DEPENDENCE ON GROWTH-CONDITIONS

The sublimation of vapor-deposited H2O ice films was investigated betw een 60 and 170 K using a piezoelectric microbalance and a mass spectro meter. The films were grown in vacuum at substrate temperatures betwee n 30 and 120 K and rates between 0.2 and 7 mum/h. Above 140 K, the sub limation rate did not depend on the growth conditions tested and yield ed a sublimation energy of 0.45+/-0.03 eV. Below 140 K, we found that fresh films have an enhanced sublimation rate, which decays in time an d which depends on film thickness and growth temperatures. This is att ributed to sublimation from regions containing amorphous ice, which tr ansforms with time into the less volatile crystalline ice, with an act ivation energy of 0.44+/-0.04 eV. The finding of a dependence of subli mation and crystallization rates on growth conditions can explain larg e discrepancies in the data published during the last four decades.