Molecular-Orbital Model for KCl: Tl

A semiempirical molecular-orbital calculation is developed for describing the energy levels of KCl: Tl. This approach is conceptually more satisfactory than the ionic one, as the nature of a Tl-Cl bond is largely covalent. The computations are first carried out in the one-electron approximation, and then the Coulomb and spin-orbit interactions are taken into account. No attempt is made at an exact calculation, but it is demonstrated that the covalent calculation gives a consistent interpretation of the absorption spectrum.