Energy transfer dynamics in the collision-induced dissociation of Al₆ and Al₁₃ 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 Al₆ and Al₁₃ with argon. For the octahedral Al₆ (O_h) cluster the CID threshold is ∼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 Al₆ internal energy. For the planar Al₆ (C_2h) cluster, the CID threshold is closer to the true threshold. For the spherically shaped Al₆ (O_h) and Al₁₃ (D_3d) clusters, T → V is the predominant energy transfer pathway. T → R energy transfer is important for the planar Al₆ (C_2h), Al₁₃ (D_2h), and Al₁₃ (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 ξ is decreased.