Can a molecule on a solid surface undergo a true (RRKM) unimolecular reaction?

A model is constructed to simulate the behaviour of a vibrationally excited radical (or molecule) that is weakly coupled through a damped harmonic oscillator to an energy sink. The sink is designed to mimic the response of an insulating surface to the presence of a physisorbed species which, at the start of each calculation, is given an internal energy comparable to that received in the course of a laser photolysis experiment. The model examines the competition that exits between the rate of energy loss to the surface and the probability that the radical might undergo unimolecular decay. It is concluded that, for an underdamped (weakly coupled) system, there is a high probability of observing a unimolecular reaction on a surface and that the most interesting events might occur on a sub-picosecond time scale. Calculated kinetic energy and velocity distributions for the photofragment are comparable to those measured in recent experiments.