Dynamics of substrate mediated photodesorption The role of the excited state potential

Abstract

The role of the excited state potential in photodesorption dynamics is examined theoretically within the framework of the generalized Menzel–Gomer–Redhead model, in which the adsorbed system undergoes electronic excitation and deexcitation mediated by energetic substrate charge carriers. The multi-dimensional desorption dynamics is represented by a time-dependent quantum wavepacket. We discuss two prototypical systems: the photon stimulated desorption of ammonia and nitric oxide from non-insulator surfaces. In the first system, we show that by simply shifting the excited state potential the desorption mechanism can be significantly altered. In the photodesorption of NO, both direct and indirect pathways are identified. The indirect desorption involves energy transfer from an internal mode to the translational mode. We also explore the correlation between the product rotational and translational energy distributions by systematically varying the excited state topology. Our results call for a critical reevaluation of the popular negative ion model for the NO photodesorption.