Sublimation of vapor-deposited water ice below 170 K, and its dependence on growth conditions

The sublimation of vapor-deposited ${\mathrm{H}}_2$O ice films was investigated between 60 and 170 K using a piezoelectric microbalance and a mass spectrometer. The films were grown in vacuum at substrate temperatures between 30 and 120 K and rates between 0.2 and 7 μm/h. Above 140 K, the sublimation rate did not depend on the growth conditions tested and yielded 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 and which depends on film thickness and growth temperatures. This is attributed to sublimation from regions containing amorphous ice, which transforms with time into the less volatile crystalline ice, with an activation energy of 0.44±0.04 eV. The finding of a dependence of sublimation and crystallization rates on growth conditions can explain large discrepancies in the data published during the last four decades.