Molecular dynamics simulations of the bombardment of a organic film on a gold substrate with Au and Au-2 projectiles have been performed in order to understand the role of mass matching in the mechanisms for energy transfer. There will be an effective energy transfer from the projectile to the substrate atoms if the atom impacts the substrate unimpeded by molecules in the organic film. When a projectile atom hits an organic molecule, the process of fragmentation absorbs its incident kinetic energy and it enters the substrate region with little energy left. With Au,, there is a higher probability that one or more of the projectile atoms will hit a bare portion of the surface and initiate collision cascades with sufficient energy to result in ejection. Atoms in the top layer of the substrate lift off stable, intact molecules as they move out of the crystal. (C) 2004 Elsevier B.V. All rights reserved.