Theory ofFA-center optical absorption in alkali halides: Symmetry-adapted wave functions

Flexible symmetry-adapted trial pseudowave functions are used to estimate variationally the optical-absorption energies of $F_A$ centers in KCl: Na, KCl: Li, RbCl: Na, and KF: Li and of $F$ centers in KCl, RbCl, and KF. The results are compared with previous work in which simple $1s$ and $2p$ functions were used. In the present work a form of the ion-size correction of Bartram, Stoneham, and Gash is used and lattice distortion is ignored except for a static displacement of the impurity cation. Contributions to the absorption splitting from kinetic, point-ion, and ion-size energies are obtained, and the effect of wave-function flexibility is analyzed.