An Electronic Cotunneling Model of STM-Induced Unimolecular Surface Reactions
The STM-induced unimolecular decomposition of a molecule adsorbed at a surface is analyzed within the framework of a theory of electron transport involving three different electronic states. Transitions between the electronic states are considered to be provoked by an energy transfer associated with electron cotunneling. The subsequent chemical reaction is then produced by the nuclear dynamics in the excited electronic state, a similar phenomenon to what occurs in gas phase organic photochemistry. We apply our model to account for the main features of some recent experiments. The idea of cotunneling provides a simple physical picture of the mechanism for energy transfer involved in the initial stage of the chemical reaction. It also explains the most important features observed in the experiment: the existence of a voltage threshold; the weak current dependence; and the relatively long-time-scale of variation in the measured current.