Real-Time Kinetic Measurements of the Condensation and Evaporation of D₂O Molecules on Ice at 140 K < T < 220 K

The kinetics of condensation of D 2 18 O water vapor on D 2 16 O ice was studied in the temperature range 140−220 K. The measurements have been performed in real time using the pulsed valve technique in a low-pressure flow reactor. Ice samples were prepared by several different methods. The uptake coefficient γ was observed to decrease with increasing surface temperature, and varied from γ = 0.06 to 0.8. A significant dependence of γ on the method of preparation of the ice has been found. At 180 K, γ = 0.13 for single-crystal ice, 0.18 for ice condensed from the vapor phase, and approximately 0.25 for bulk ice obtained from freezing liquid D 2 16 O. The uptake coefficient attained a value of γ = 0.8 at 140 K for cubic ice prepared by vapor condensation at 140 K. The activation energy for evaporation of D 2 O at low temperatures (140−190 K) has been measured as 12.2 kcal/mol, whereas it decreases to 8.3 kcal/mol at higher temperatures (190−220 K). The rate of evaporation at 200 K corresponds to the loss of approximately 70 ± 10 formal monolayers per second. The experimental results suggest the formation of loosely bound water adsorbed to the surface of ice whose bond energy is estimated to be 4.0 ± 0.4 kcal/mol, independent of the type of ice.