Molecular dynamics simulations are used to examine the dissociation of halogen molecules that impact a surface at collision velocities up to 10km/s. For iodine molecule, the yield of dissociation of vibrationally cold molecules saturates below 50%, although the center of mass energy is up to 80 times the bond energy. The origin of this phenomenon is the dissociation dependence on the collision angle. Two segments of collision angles are non-reactive due to deactivation of the internal energy, which happens before the atoms separated. The importance of the vibration phase to the dissociation process is also discussed. (C) 2003 Elsevier B.V. All rights reserved.