A calculation of the electronic states of the trihalide anions

A valence-bond, pseudopotential calculation is presented of the electronic states of Cl3 -, Br3 - and I3 -. These anions are known in solution and in crystals and have been identified with defects in the alkali halides. The calculated bond lengths, dissociation energies, vibrational frequencies and optical excitation energies are compared with experiment and the optical spectrum is re-assigned. Calculations are also presented for the interaction of an X- anion with the X2 - molecule ion. These potentials are of interest in the study of the motion Vk centres in alkali and alkaline earth halides.