From time-resolved studies on molecular dissociation, the time scales for a roaming H 2 molecule are determined and have been found to be of the order of few hundred femtoseconds. Such experiments usually employ pump-probe techniques to determine the yield of a fragment as a function of time delay, with the chosen fragment being the one resulting from a hydrogen migration process. Since the scrambling and migration of H-atoms is dependent on the internal energy and lifetime of the molecular precursor, these experiments sample a small section of the potential energy landscape of the molecular precursor. On the other hand excitation by ion-impact is known to be much more violent leading to a much wider distribution of internal energy in the target molecule.