Electrons weakly bound to molecules by dipolar, quadrupolar or polarization forces

Within the framework of a: simple electrostatic model we here discuss the stability of very weakly bound molecular negative ions. In contrast with the case of conventional valence anions, the excess electron is then located in a very diffuse orbital and is mainly bound by electrostatic dipolar, quadrupolar and polarization forces at large distances from the neutral molecular core. By fitting a single repulsion parameter of the model to the available experimental data, it is possible to make quantitative predictions of the excess electron binding energies in these species. Critical values of dipole moment, quadrupole moment or polarizability required for the observation of stable dipole-bound, quadrupole-bound or polarization-bound negative ions are predicted.