Abstract
Computer simulations are performed to study the dynamics of the photodissociation of a single HBr adsorbed on a LiF (001) surface. A cluster model, including 50 moving atoms in two layers surrounded by five static walls consisting of 274 fixed atoms, is used to represent the surface. Since only one HBr molecule is allowed to sit on the surface, interactions between coadsorbates and possible collisions between adsorbed species are not considered in this model. The equilibrium properties of the HBr molecule on the surface prior to the photon absorption are obtained through Monte Carlo simulations, while the subsequent dissociation dynamics is described by molecular dynamics. Two dissociation channels corresponding to the ground and excited spin–orbit states of bromine are explicitly considered and transitions from one excited-state surface to another are treated with the Landau–Zener model. The kinetic energy and angular distributions of the dissociated H atoms as well as the Br/Br* ratio are determined from 600 trajectories. At 100 K and 193 nm, the calculated kinetic-energy distribution of the H fragment agrees with experimental data very well, as does the Br/Br* ratio. Neither the H kinetic energy nor the Br/Br* ratio is significantly affected by the presence of the surface. The energy exchange between the H atom and the surface is minimal, but increases when H is replaced by deuterium. The calculated H fragment angular distribution deviates from experimental data by approximately 15°. The deviation can be attributed to the scattering of the dissociating H fragment by a coadsorbate. We have also investigated the influence of the bulk temperature and the incident photon wavelength on the dynamics.
Published Version
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