ATOM CLUSTER INTERACTION POTENTIALS AND THERMAL COLLISION RATES

A point-continuum model for evaluating atom-cluster interaction potent ials is developed and applied to the calculation of capture cross sect ions and thermal association rate constants for atom-cluster associati on in the framework of the Langevin model. The approach is illustrated by treating the initial capture step in the association reaction of a n iodine atom and iodine-doped van der Waals cluster I + I (Ar)n --> I 2 + n Ar. Two cluster sizes n = 12 and 54 are considered. The cluster structures correspond to an initial icosahedral geometry of Ar atoms s urrounding a central I impurity, denoted Ar6(I)Ar6, and Ar27(I)Ar27 fo r n = 12 and 54, respectively. For I + Ar6(I)Ar6 at lower relative tra nslational energies, Langevin capture cross sections predicted by the point-continuum model agree with classical trajectory results within s tatistical uncertainty. However, as the relative translational energy increases, the predicted capture cross sections start to deviate sligh tly from the classical trajectory values. For the larger cluster size (n = 54), the predicted capture cross sections and collisional rate co efficient are in quantitative agreement with the predictions of classi cal trajectory simulation, illustrating the emergence of bulk-like con tinuum behavior as cluster size is increased.