A tale of negative ions

Negative ions exist widely in gases, liquids, and solids, playing an important role in many physical and chemical processes. The binding between the extra electron and the neutral core is very weak, which gives negative ions unique properties that are significantly different from neutral atoms and positive ions. For example, negative ions usually have no excited state. This article first reviews the electron affinity of atoms in the periodic table and the energy levels of their negative ions, as well as the corresponding measurement methods. A detailed introduction to recent research on the transition element negative ions is then presented, followed by a description of laser cooling and the dipole bound states of negative ions. Finally, future prospective research on gas-phase negative ions is summarized.