Effects of Elastic and Electric Fields on the Spectra of Transition-Metal Ion Centers

The change of broad absorption bands of electronic centers in a crystal subjected to elastic and electric fields is analyzed. The centers are assumed to have cubic symmetry ($O_h$) and an orbital singlet ground state, but excited states of arbitrary multiplicity. This is a reasonable model for $d$-shell transition-metal ions. The change of the zeroth and first moment of an absorption band is given as a function of the direction cosines of the field and the polarization of the incident light. Spin-orbit splitting and indirect transitions between states of equal parity are discussed.