The translational temperature increases approximately linearly with fluence and is lower for D 2 than for H 2 . The energy partitioning into translational, vibrational, and rotational energies of the desorbing species is unequal, and there is a preference for one type of rotation over the other. The ultrashort response time of the reaction, as observed in two-pulse correlation experiments, along with these other characteristics, suggests an electronic, non-thermal reaction mechanism. In addition, experiments using mixtures of the two isotopes found that the desorption of D 2 is facilitated by the presence of H at the surface, i.e., a dynamical promotion effect was found, while the desorption of H 2 is hindered by the presence of D.