Ersin Yurtsever

The vibrational quenching cross sections and corresponding low-temperature rate constants for the v = 1 and v = 2 states of CN- colliding with He and Ar atoms have been computed ab initio using new three dimensional potential energy surfaces. Little work has so far been carried out on low-energy vibrationally inelastic collisions for anions with neutral atoms. The cross sections and rates calculated at energies and temperatures relevant for both ion traps and astrochemical modelling, are found by the present calculations to be even smaller than those of the similar C2- /He and C2-/Ar systems which are in turn of the order of those existing for the collisions involving neutral diatom-atom systems. The implications of our finding in the present case rather small computed rate constants are discussed for their possible role in the dynamics of molecular cooling and in the evolution of astrochemical modelling networks.

The stable structures and melting properties of ion clouds in isotropic octupole traps are investigated using a combination of semi-analytical and numerical models, with a particular emphasis at finite size scaling effects. Small-size clouds are found to be hollow and arranged in shells corresponding approximately to the solutions of the Thomson problem. The shell structure is lost in clusters containing more than a few thousands of ions, the inner parts of the cloud becoming soft and amorphous. While melting is triggered in the core shells, the melting temperature unexpectedly follows the rule expected for three-dimensional dense particles, with a depression scaling linearly with the inverse radius.