Photodissociation of methyl nitrite on Ag(111): Simulation

Abstract

The photodissociation dynamics of methyl nitrite, CH3ONO, on Ag(111) have been simulated using a description that models 61 cis–methyl nitrite molecules adsorbed on a three-layer block of Ag(111). Based on classical intra- and intermolecular potentials and periodic boundary conditions, molecular dynamics (MD) simulation led to two domain structures at 100 K: those with CONO planes oriented nearly parallel and nearly perpendicular to the Ag(111) surface. To simulate photodissociation dynamics of NO, many NO trajectories were determined, each carried out as follows. At some instant of the MD simulation, a CH3ONO molecule was randomly selected from within the group of 61 and its internal CH3O–NO bond was stretched to a defined dissociation transition state. The nascent NO was given momentum along the direction of the bond broken and NO translational and internal energies were chosen to match those determined experimentally in collision-free gas phase photodissociation. The motion of the whole adsorbate–substrate system was then calculated while following the trajectory of NO. Analyzing the ensemble of NO trajectories, we conclude that, while the initial orientation of the dissociating CH3ONO influences the number of subsequent collisions, the exit direction, and the final translational and internal energy of NO, it does not fully account for the properties of ejected NO. Furthermore, for those molecules lying nearly parallel to the surface, a transition state prepared by simply stretching the O–N bond is often located away from the lowest potential energy exit path due to interactions with nearest neighbor species. As a result, coordinates, e.g., internal twisting, other than the internal CH3O–NO stretching mode are intimately involved in the dissociation channel.