Kinetics of dissociative congruent evaporation based on the transition state theory.

Non-transition metal oxides, including major minerals of the early Solar System, are known to evaporate decomposing into multiple gas molecules, while maintaining their stoichiometric compositions (dissociative congruent evaporation). Here, we derived the absolute rate of this type of evaporation using the transition state theory. In our modified transition state theory, the activation energy closely corresponds to the average energy of the molecules at the transition state, reflecting the degree of decomposition at the potential energy barrier along the reaction coordinate of evaporation. By comparing the theoretical and experimental evaporation rates for the reaction MgO (s) → Mg (g) + O (g), we found that there is an activation barrier close to the product side (i.e., "late" barrier) where the decomposition is almost achieved. The present theory is advantageous to the Hertz-Knudsen equation, which is essentially formulated as the evaporation rate in equilibrium based on the detailed balance, in that it describes dissociative congruent evaporation as a non-equilibrium process and thus provides the link between the experiments and the reaction dynamics.