Post barrier channel effects during unimolecular decomposition: A trajectory study of energy term variations

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Abstract
Post barrier channel effects during the decomposition of a three-atomic molecular complex to an atom and a two-atom molecule have been investigated in a classical trajectory study. This was done by following the extent of energy exchange between the degrees of freedom outside the centrifugal barrier. An ideal dipole potential was used to model the potential energy between the atom and the molecule. In all runs the total energy used was approximately 4.66×10−20 J, corresponding to a temperature of close to 1000 K in a crossed molecular beam experiment, while total angular momentum Ltot has been varied in the runs. For large Ltot values the centrifugal barrier height can be a considerable fraction of the total energy. A recently developed statistical method (Holmlid, Rynefors 1981) has been used to generate the initial conditions at the top of the centrifugal energy barrier. The half-reaction KNaCl→K+NaCl was chosen as a model decomposition process but the effects of light and heavy product atom masses have also been investigated. When the magnitude of Ltot was close to the upper bound compatible with this total energy, considerable exchange occurred between the degrees of freedom. The molecular rotation energy shifted 11%, in the meanwhile changes for individual molecules of 40% were not uncommon. A redistribution between the rotational degrees of freedom is the primary effect when the atomic masses are approximately equal. For systems with other mass ratios large shifts in the translational energy can also occur.