Study of dipole-bound negative ions: Formational dynamics and collisional properties

Electron transfer in collisions between atoms in high Rydberg states and polar targets can lead to the formation of dipole-bound negative ions in which the excess electron is weakly bound by the dipole potential of the neutral molecule. Their lifetimes and collisional destruction rates are studied using a Penning ion trap. Ion decay in the trap is characterized by a single exponential lifetime, ∼60 to 100 mus governed by BBR-induced photodetachment. The rate constants for destruction of these ions by rotational energy transfer in collisions with residual target gas present in the trap are large, ∼ 10-7cm3s-1. The dynamics of CH3CN- ion production through electron transfer are examined using velocity selected Rydberg atoms. The CH3CN- ion formation rate shows a strong velocity dependence and is relatively small, the associated rate constants being ∼ 0.5--1.0 x 10-8cm 3s-1. A curve-crossing model, which considers the effect of crossings between the diabatic potential curves for the covalent K(np)/CH3CN system and the ionic K+/CH 3CN- system is discussed and provides a clear explanation for the observations. Electron transfer in collisions between dipole-bound negative ions (CH 3CN- and C2H3CN-) and target gas, including attaching molecules like SF6 and polar molecules like CH3NO2, is examined using the Penning trap. A free-electron capture model is used to describe electron transfer to attaching molecules. A near-resonant charge transfer model is employed to describe collisions between dipole-bound anions and polar molecules. The rate constants predicted by these models are consistent with the experimental observations