ENERGY-TRANSFER DYNAMICS IN THE COLLISION-INDUCED DISSOCIATION OF AL-6 AND AL-13 CLUSTERS

Using a model analytic potential energy function developed for Al-n clusters [J. Chem. Phys. 1987, 87, 2205] and a UMP2(fc)/6-31G* potential derived here for the Ar-Al interaction, classical trajectory simulations are performed to study collision-induced dissociation (CID) of Al-6 and Al-13 with argon. For the octahedral Al-6 (O-h) cluster the CID threshold is similar to 14 kcal/mol higher than the true threshold. This is because, near the threshold, there are no trajectories which transfer all the reactant relative translational energy to Al-6 internal energy. For the planar Al-6 (C-2h) cluster, the CID threshold is closer to the true threshold. For the spherically shaped Al-6 (O-h) and Al-13 (D-3d) clusters, T --> V is the predominant energy transfer pathway. T --> R energy transfer is important for the planar Al-6 (C(2)h), Al-13 (D-2h), and Al-13 (D-6h) clusters. T --> V energy transfer is enhanced as the cluster is softened (i.e., its vibrational frequencies lowered), the mass of the colliding atom is increased, and/or the relative velocity is increased. These effects are consistent with a previously derived impulsive model [J. Chem. Phys, 1970, 52, 5221], which says T --> V energy transfer increases as the collisional adiabaticity parameter xi is decreased.