The Electronic Structure of NaCl:Cu

The one‐electron energy levels of NaCl:Cu have been calculated by a charge‐consistent extended Hückel approach. The lowest transition is found to be electric‐dipole forbidden and its energy agrees with experiment when a Hückel constant of 2.0 is used. The ground state of the center is found to be partially covalent, with a net charge of +0.75 e on the Cu. Similar calculations on KCl:Cu tend to yield transitions too high in energy, although an estimate of the KCl valence band structure yields reasonable results. These results support the idea that the primary transition for on‐site Cu is the crystalline analogue of S → D, and that the cases of apparent allowed transitions arise from admixture of P into D through the electric field associated with a stable off‐center position of the Cu. It is suggested that the crystalline increase of the d 10 → d 9 s transition energy arises in part from the penetration of d‐orbitals into the ligand cores, with consequent lowering of their energies.