Clusters anharmonic decay

The classical anharmonic RRKM rate constant for a fluxional molecule may be calculated from classical trajectories by following the initial decay of a microcanonical ensemble of states for the unimolecular reactant, as given by equation 1A3.12.41. Such a calculation has been performed for dissociation of the Alg and A1j3 clusters using a model analytic potential energy function written as a sum of Lennard-Jones and Axelrod-Teller potentials [30]. Stmctures of some of the Alg minima, for the potential function, are shown in figure A3.12.6. The deepest potential minimum has... 

To examine the effects of anharmonicity on the decay of excited clusters, calculations were performed for hot clusters in which the alternate, truncated harmonic potential, equations (7) and (8), was employed as the atomic pair potential. While the interatomic pair potential was harmonic in form, the entire cluster was not a harmonic system since displacements along the normal mode coordinate were not subject to restoring forces which were linear functions solely of the displacements along those coordinates. The change in potential nonetheless causes a change in the density of states for the cluster. 

Fig. 15. Cluster decay rate coefficient as a function of fixed energy for "harmonic noncollisionally formed four-atom clusters. Decay rate as computed from trajectory calculations for harmonic clusters are represented by A and those for anharmonic hot clusters are represented by O. The solid lines are the best fits of the RRK equation to the results.

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