Equilibrium-non-equilibrium "vibrational state" transition in exoergic reactions at solid surfaces

A kinetic model is presented to describe the vibrational populations of adatoms in exoergic reactions at solid surfaces. The exchange of vibrational quanta among adatoms (VV exchange) and between the adatoms and the substrate (VL exchange) has been modeled by rate equations. In the model case of four vibrational levels it is shown that non-equilibrium steady states can be attained, in which adatoms do not populate the vibrational ladder according to the Boltzmann distribution. By properly defining a parameter for the "measure" of the displacement of the distribution from the Boltzmann one, it is found that, depending on gas flux and rate constants for quantum exchange, the system undergoes a transition from equilibrium to non-equilibrium state. The transition is governed by the ratio between the probabilities for VV and VL transfers and by the gas flux. The larger the adsorption energy is, the sharper the transition. These results are also analysed and discussed in connection with the catalytic activity of the system. © 2004 Elsevier B.V. All rights reserved.