Studies of the Vaporization Mechanism of Ice Single Crystals

The kinetics of the vacuum sublimation of ice single crystals has been investigated by a vacuum microbalance technique in the temperature range −90 to −40°C. The vaporization coefficient αv ≡ (observed vaporization rate) ÷ (theoretical maximum rate) and the activation enthalpy of sublimation, Δ Hs*, vary markedly with temperature in this range. At temperatures below about −85°C, α v=1 and Δ Hs* equals the thermodynamic enthalpy of sublimation Δ Hs°. Between about −85 and −60°C, αv decreases slowly with increasing temperature, Δ Hs*≲ΔHs°. Between about −60 and −40°C, αv decreases progressively more rapidly with increasing temperature and Δ Hs* decreases to a high-temperature limiting value of ≃ 12 Δ Hs°. The effects of various experimental parameters such as crystal orientation, doping with impurities and adsorbed gases on the ice vaporization kinetics have also been investigated. Neither grain boundaries nor crystal orientation has a measurable effect on the rate. Ice doped with monovalent impurities was found to vaporize at steady-state rates that were uniformly lower over the entire temperature range of the study. Also, NH3 (gas) and HF (gas), present in the ambient at pressures ∼10−3–10−2 torr, reduce and increase, respectively, the ice vaporization rate. The experimental results, along with previously reported physical—chemical properties of ice are used to arrive at a vaporization mechanism: Ice at equilibrium with the vapor has a surface population of a highly mobile species assumed to be water molecules hydrogen bonded to only one nearest neighbor. These energetic molecules are the source of the vapor flux leaving the surface. At sufficiently low temperatures, vacuum vaporization does not occur rapidly enough to alter this equilibrium surface population. Sublimation at higher temperatures, however, depletes the population to a progressively greater extent with increasing temperature. Thus the rate-limiting step in vaporization, which is the desorption of the mobile water molecules at low temperatures, changes to their formation at high temperatures.