Isothermal Desorption Kinetics of Crystalline H₂O, H₂¹⁸O, and D₂O Ice Multilayers

Abstract

The isothermal desorption rates of crystalline H2O, H218O, and D2O ice multilayers were measured over a temperature range from 175 to 190 K. The desorption rates were measured with optical interferometry using ice multilayers grown epitaxially on a Ru(001) surface in an ultrahigh vacuum chamber. The Arrhenius parameters for the desorption of H2O and H218O were identical within experimental error. For H2O, the preexponential was ν = 1032.6±0.3 cm-2 s-1 and the activation energy was E = 13.9 ± 0.2 kcal mol-1. For H218O, the preexponential was ν18 = 1032.4±0.3 cm-2 s-1 and the activation energy was E18 = 13.8 ± 0.2 kcal mol-1. Despite the near equivalence in the Arrhenius parameters, H218O desorbed at a rate that was slower by ∼9% throughout the range of temperatures. In contrast, the desorption rate of D2O was slower by 49−62% compared with H2O over the measured temperature range. The Arrhenius parameters for the desorption of D2O were νD = 1033.4±0.5 cm-2 s-1 and ED= 14.8 ± 0.4 kcal mol-1. A transition state model was developed to explain the measured desorption kinetics. The transition state model predicts that total molecular mass has only a small effect on the desorption kinetics because the mass differences among the isotopomers are small. On the other hand, the principal moments of inertia of the desorbing molecules have a large effect on the desorption rate. About each of the three axes, D2O has roughly twice the moment of inertia of H2O or H218O. This larger moment of inertia affects the desorption rate in two ways. First, surface bound D2O molecules have lower frequencies for hindered rotations. These lower frequencies result in less zero-point energy for the surface bound molecule and a larger desorption activation energy. Second, a transition state D2O molecule has a larger rotational partition function. This larger rotational partition function yields a larger preexponential for desorption.