The kinetics of condensation and evaporation of H2O from pure ice in the range 173-223 K: a quartz crystal microbalance study

The kinetics of evaporation and condensation on ice condensed from the vapor phase at 190 K has been studied in the range 173-223 K using a suitably calibrated quartz crystal microbalance that was put inside a vacuum chamber that could be used either as a molecular or a stirred flow reactor, depending on the measured pumping speed. The Arrhenius representation of the zero order evaporative flux J(ev) (molecule cm(-2) s(-1)) displays a discontinuity at 193 +/- 2 K that could be observed owing to the degree of precision of the present measurements. For 223 > T > 193 K, J(ev) = (1.6 +/- 0.7) x 10(28)exp((-10.2 +/- 0.5)/RT) molecule cm(-2) s(-1) and for 173 < T < 193 K, J(ev) (2.2 +/- 0.9) x 10(30)exp((-12.0 +/- 0.5)/RT) molecule cm(-2) s(-1) where R = 1.987 x 10(-3) kcal mol(-1) K-1. The corresponding measured rate constant k(con)d for H2O condensation on ice satisfies the known vapor pressure P-eq (H2O) together with Jev thereby affording thermochemical closure of the kinetics which results in the heat of sublimation DeltaH(subl)(0) = 11.7 +/- 0.6 and 12.3 +/- 0.5 kcal mol(-1) from the above high and low T-range, respectively. J(ev) is significantly lower than the maximum theoretically allowed value throughout the T-range in agreement with an uptake coefficient for H2O on ice that is significantly smaller than unity. In addition, the negative temperature dependence of kcond or g reveals a precursor-mediated adsorption/desorption process for the H2O/ice system. The increased evaporative lifetime of pure ice may be important under atmospheric conditions.