Thresholds for the collision-induced dissociation of clusters by rare gas impact

Classical trajectory simulations are used to study collision-induced dissociation (CID) of octahedral Al-6 and Mg clusters, by rare gas impact. M represents a first-row transition metal atom with a mass of 60 amu. Both an analytic Alb potential derived from ab initio calculations [J. Chem. Phys. 1987, 87, 2205] and a model analytic function with adjustable parameters were used for the Alb and Mg cluster potentials. Ab initio and model intermolecular potentials were also used for interactions between the Ne, Ar, and Xe rare gases and the Alb and Mb clusters. CID cross sections versus relative translational energy E(rel) are calculated for collisions between the rare gas atoms and clusters, from which CID thresholds E(0)(CID) are determined. For all cases E(0)(CID) is larger than the actual threshold, which results from inefficient transfer of E(rel) to cluster vibrational modes. The efficiency of energy transfer to the cluster depends on the cluster vibrational frequencies, masses of the rare gas and cluster atoms, repulsiveness of the intermolecular potential, and the collision's relative velocity. Increasing this energy transfer efficiency lowers E(0)(CID). An impulsive model [J. Chem. Phys. 1970, 52, 5221] qualitatively reproduces the trajectory results but misses important details. Using ab initio cluster and intermolecular potentials, the Ne, Ar, and Xe + Al-6 CID thresholds are similar and range from 8 to 11 kcal/mol higher than the actual thresholds.