Formation of metastables and dissociative trapping in high-energy molecule-surface collisions

Dissociation of diatomic molecules in high-energy impact on a solid surface is studied by classical trajectory calculations for systems in the parameter regime E >> D ≳ B where E is the collision energy and D and B are respectively the molecular and the atom-surface binding energies. The calculations are for model of I₂ colliding with a smooth surface. The main results are as follows: (1) Dissociative trapping, corresponding to the reaction I₂ + surface → I⋯surface + I, remains appreciable even for E/B ≈ 10, decreasing only slowly with energy. (2) The velocity distribution of I atom fragments is doubly peaked, the structure, of the low-velocity peak reflecting the effect of dissociation trapping. (3) A substantial fraction (∼5%) of the scattered I₂ molecules emerge as metastables, in a rotational predissociation state, indicative of efficient translational-rotational energy transfer with large changes of angular momentum in the collision. (4) The average rotational excitation energy of the nondissociated molecules is higher than the vibrational one. Several experiments are suggested in light of these results.