Reactions at suprathreshold energy: Evidence of a kinematic limit to the internal energy of the products

In numerous state-to-state dynamics studies of atom transfer reactions at energies substantially in excess of the barrier height, population is not observed in many of the energetically accessible product quantum states. Frequently, the experimental state distributions end for internal energies well below the total energy limit. The lack of observable population in these high energy states has led us to conclude that not all of the total available energy is accessible to the rovibrational states of the products. To explain this behavior we describe a model in which the internal energy is constrained by kinematic factors. We use this model to show that the energy constraint is quantitatively determined by the skew angle for the reaction. We apply this model to the results of state-to-state experiments on a large number of reactions and get quantitative agreement between our predictions and actual observations. (C) 2001 American Institute of Physics.