Recoil effects in surface dissociation

Using real-time wave packet propagation we consider the effects of lattice recoil, inelasticity and surface temperature in strongly activated dissociation reactions of diatomic molecules at surfaces. The energy diagram governing the dissociation, modeled as suggested by electronic structure calculations for H2 dissociation at Cu surfaces, consisted of an entrance channel barrier separated from the chemisorption region by a ridge, where dissociation takes place. Lattice recoil is simulated by coupling this ‘‘stiff-barrier’’ PES to a harmonic oscillator. Calculations were carried out for masses and potential parameters appropriate to H2/D2 dissociation on Cu and N2 dissociation on Fe. Barrier recoil was found to suppress the dissociation probability as compared with its stiff-barrier value. The effect, marginal for H2 and D2 but pronounced in the case of N2, can be understood in terms of dynamical increases in the barrier width and height. Simulations where the N2–Fe barrier was excited in the initial state showed that the influence of surface temperature on the dissociation can be quite dramatic and led to a strong enhancement in the tunneling region.