Negative ion production by impinging I-2 at hyperthermal energies on a diamond surface was modeled. A quantum time dependent nonadiabatic approach in Lionville space was used. The primary coordinate of the I-2 distance from the surface was modeled explicitly while the surface degrees of freedom were included implicitly using Lindblads semigroup formalism. The calculations were directed at determining the mechanism of the ion production. Two nonadiabatic crossing points were considered. By fitting the nonadiabatic parameters both crossing points could yield the increase in the ion production as a function of energy observed in the experiment. The mechanisms can be discerned by analyzing the relation between the kinetic energy distribution of the ions to that of the incident molecules. Ions are produced predominantly at a nonadiabatic crossing point located high in energy and close to the surface fit better the experimental observations. (C) 2002 Published by Elsevier Science B.V.