Dipole-Bound Anions

Any molecule with a dipole moment above approximately 2.5 Debye can form a stable negative ion (dipole-bound anion). These anions are best produced by “resonance” charge exchange from atoms in high Rydberg states (Rydberg electron transfer, RET). RET to form dipole-bound anions occurs over a narrow range of effective principle quantum number, n*. Dipole-bound anions for 32 molecules with dipole moments between 2.5 and 6.0 Debye have been studied. The excess electron in such an anion is very diffuse and weakly bound. Binding energies (electron affinities, EAs) are estimated from the narrow range of n* at which charge exchange occurs and also from measurements of the electric field required to detach the electron. Electron affinities range from less than ~1 milli electron volt (meV) to 100 meV. Factors other than dipole moment affect these electron affinities. These include polarizability, molecular shape, and dispersion interactions of the excess electron with the molecule. One of the molecules studied has for one of its conformations a possible quadrupole-bound negative ion state.