Exciton and impurity states in alkali halides

The formalism for studying central-cell corrections to the effective-mass equation (which has been successfully used to calculate deep exciton states in rare-gas solids) is extended to the case of alkali halides. Beginning with a generalized impurity equation for both exciton and impurity states, a transformation is made to a representation in which the core parts of the Bloch functions are removed. The resulting pseudowave equation is then solved, making use of an improved r-dependent dielectric function to screen the electron-hole interaction, relativistic wave functions for calculating pseudopotentials, and recent band-structure data. Calculations are then made of (i) $1s$ exciton states in KI, RbI, KCl, and RbCl; and (ii) $1s$ impurity states of dilute iodine in K and Rb chlorides. Hydrogenic defects calculated in this scheme are in good agreement with the experimental values for both the small-defect ({~}0.1-eV) cases of KI and RbI and the large-defect ({~}1-eV) impurity cases.