Structure and dynamics of cationic van-der-Waals clusters

Abstract.

A diatomics-in-molecules (DIM) model with ab-initio input data, which in part I successfully described the structure and bonding properties of protonated argon clusters Ar_nH⁺, is used here to investigate some aspects of the dynamics of such aggregates for n up to 30. The simple triatomic ionic fragment, Ar₂H⁺, is studied in some detail with respect to normal vibrations, characteristics of classical intramolecular dynamics as reflected in the Fourier spectra of dynamical variables, and accurate quantum states of the vibrational motion. For larger clusters Ar_nH⁺ (n ≤30), the normal vibrational frequencies (and displacement eigenvectors) are calculated and related to the cluster structure. In addition, the Fourier spectra are analyzed with respect to their variation with changing internal energy and cluster size. As expected, the clusters show some floppy character. Even a little vibrational excitation can lead to internal rearrangement and to Ar-atom evaporation from the clusters; this is studied in more detail for one small complex (n = 3). Electronic excitation to one of the low-lying excited states, which are all globally repulsive, leads to complete fragmentation (atomization) of the clusters. A variety of conceivable elementary collision processes involving protonated argon clusters are discussed. Some of these may play a role in the gas-phase formation of medium-sized Ar_nH⁺ aggregates.