Electronic and/or optical excitation at surfaces is assumed to modify the cohesive properties of localised sites. This leads to change of the activation energy and entropy for surface-assisted mechanisms (adsorption, desorption and reactions). These are introduced into the models leading to the Langmuir and BET isotherms. It is also shown how the Eley-Rideal and Langrnuir-Hinshelwood mechanisms are modified upon electronic excitation. In order to understand how electronic excitation may modify reaction rates, a reaction is considered in which a normally non-volatile intermediate product becomes volatile upon electronic excitation. This is discussed in the case of doping-assisted reaction and laserassisted reaction. In this last case, by taking into account the effects of laser irradiation on localised states and on the temperature of the irradiated zone, one may describe situations where either rapid saturation, or slow increase, or a sharp threshold for reaction takes place
From available relations between thermodynamical and microscopic parameters, the role of electronically excited sites on bond breaking and atomic migration is studied. It is demonstrated that, in covalent materials, electronic excitation leads to a decrease of the activation energy for bond breaking and migration, and of the entropy for migration, and to an increase of the entropy for bond breaking. It is also predicted that, under some circumstances, laser excitation might give rise t o a decrease of reaction rate above a critical temperature, independent on the photon flux. The shift of the equilibrium position between competing processes is discussed, in the presence of selective excitation. The previous arguments are compared with available experimental data. Hints for future experiments are given.Ausgehend von Beziehungen zwischen thermodynamischen und mikroskopischen Parametern wird die Rolle elektronisch angeregter Bereiche auf die Bindungsbrechung und Atomwanderung untersucht. Es zeigt sich, daS in kovalenten Materialien elektronische Anregung zu einer Abnahme der Aktivierungsenergien fur Bindungsbrechung und Wanderung sowie der Rntropie fur Wanderung, und zu einer Zunahme der Entropie fur Bindungsbrechung fiihrt. Ferner ergibt sich die Voraussage, daS unter gewissen Umstanden Laseranregung jenseits einer kritischen Temperatur zu einer Abnahme der Reaktionsrate fuhren kann, die vom PhotonenfluS unabhangig ist. Die Verschiebung des Gleichgewichts zwischen konkurrierenden Prozessen wird fur den Fall selektiver Anregung diskutiert. Die Ergebnisse der uberlegungen werden mit experimentellen Daten verglichen, und Anregungen fur weitere Experimente werden gegeben.
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