Computational studies of halogen chemistry on rare-gas surfaces. III. Photodissociation in submonolayer chlorine films on Ar(111) and Xe(111)

Abstract

The photodissociation reaction of submonolayer molecular chlorine films on rare-gas surfaces has been studied using molecular dynamics computer simulation. In particular, we have considered the coverage and temperature dependence of escaping photofragment yield, angular distribution, and translational energy distribution of the reaction h(ν=29 661 cm−1) +Cl2(1Σ)−Cl2[Θ]ads−Rg(111)[T]→2Cl⋅(1Π)[E=9699 cm−1]+Cl2[Θ]ads−Rg(111)[T′], where E is the initial potential energy of the dissociating fragments, Θ indicates the film coverage, T is the substrate temperature, and Rg indicates the rare-gas substrate which is either argon or xenon. Even at the submonolayer coverages studied here (X2[adsorbed]/Rg[surface]<1), fewer than half of the photodissociation fragments escape the surface. The mean translational energy of the escaping fragments as a function of coverage indicates that, on average, escaping fragments suffer collisions and lose energy before leaving the surface. At all coverages, the translational energy distribution is roughly Gaussian and peaked at approximately one-third of the total photodissociation energy. However, at higher coverages, the escaping fragment may actually carry more than half of the photodissociation translational energy. The average direction of escaping photofragment velocities as a function of coverage generally reflects the original diatom orientation in the adlayer but is shifted toward the surface normal. With increasing coverage, the angular distribution of fragment velocities goes from roughly Gaussian in the range between 0° and 90° to highly peaked about the surface normal.