Isothermal desorption kinetics of crystalline H2O, H218O, and D2O ice multilayers

The isothermal desorption rates of crystalline H2O, H-2 O-18, 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 (H2O)-O-18 were identical within experimental error. For H2O, the preexponential was nu = 10(32.6+/-0.3) cm(-2) s(-1) and the activation energy was E = 13.9 +/- 0.2 kcal mol(-1). For (H2O)-O-18, the preexponential was nu(18) = 10(32.4+/-10.3) cm(-2) s(-1) and the activation energy was E-18 = 13.8 +/- 0.2 kcal mol(-1). Despite the near equivalence in the Arrhenius parameters, (H2O)-O-18 desorbed at a rate that was slower by similar to9% 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 nu(D) = 10(33.4+/-0.5) cm(-2) s(-1) and E-D = 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 (H2O)-O-18. 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.