Experimental Studies of Exotic Negative Ions

Although negative ions have been studied extensively for quite some time, their study continues to offer insight into developing and refining quantum chemical modeling techniques. Negative ion states remain difficult to treat ab initio due to the significant electron-electron correlation, thus putting a premium on experimental results to guide the way. Experimental techniques such as Electrospray Ionization (ESI) and Rydberg Electron Transfer (RET) have made study of some exotic negative ions in the gas phase possible for the first time. Multiply Charged Anions (MCAs) of several families of molecules were studied using Collision Induced Dissociation (CID) and Infrared Multi Photon Dissociation (IRMPD) to consider the relative stabilities toward electron detachment and ionic fragmentation, as well as the influence of the Coulomb Barrier in these systems. The Coulomb Barrier is a potential barrier adding stability to MCAs due to the Coulombic repulsion between the excess charges. This study has shown for the first time that the magnitude of this barrier towards ionic fragmentation fits the simple electrostatic model of e2/r, as was found by Wang et al. for electron detachment.

Dipole-bound anions are another rare species of negative ions in which the excess electron is loosely bound to a polar molecule through its dipole moment. The electron affinity of these anions (as determined from RET formation and Field detachment measurements) has been shown to be primarily a function of dipole moment, yet this work has highlighted the significant influence of rotations on these anionic states. Observations of neutrals formed from detachment of these anions (black body v photodetachment and rotational autodetachment), has shown significant coupling to the moment of inertia of the molecule. In the first study of dipole bound anions of HCN, it has been shown that these rotational effects essentially truncate this anionic system such that only two rotational levels remain bound. Formation of the ground state and first rotationally excited state of the dipole bound anion has been observed in the RET spectra and field detachment measurements for the first time.