Kinetic isotope effects in evaporation

<p>Kinetic isotope effects (KIEs) in evaporation result in isotope fractionation between the gas phase and the condensed phase, which provides important information about the thermal histories of extraterrestrial materials in the early Solar System. The KIEs can be predicted by the square root of the inverse mass ratio of the evaporating molecules, but the degrees of fractionation measured in experiments are smaller than this theoretical prediction and depend on the material and temperature. In the present study, we newly formulated the KIEs in evaporation based on transition state theory and found that (i) the KIEs at high temperature are smaller than the prediction when the activation barrier exists for the bond dissociation, and (ii) the KIEs decrease with the decrease of temperature and disappear when the temperature is comparable to the energy of the vibrational motion along the reaction coordinate. The different degrees of evaporative isotope fractionation of metallic iron and silicates measured in experiments suggest that there exists an activation barrier for the dissociation of an ionic bond which may originate from the avoided crossing of potential energy curves. The temperature dependence observed for evaporation of silicates is also consistent with (ii), which can be quantitatively evaluated by conducting experiments in a wider temperature range.</p>