Pb2+ and Bi3+ impurity centres in alkali‐earth oxides vibronic spectra, lattice dynamics, and electron‐phonon interaction

The vibrational structure in the emission spectra of Pb2+‐ and Bi3+‐doped CaO and SrO single crystal is studied at 4.2 K. The emission spectra are caused by the forbidden 3A1u → 1A1g transition which is initiated by the mixing of 3A1u and 3T1u states by T1g lattice vibrations. A theory is given which takes into account, within the Cochran‐Cowley shell model, a linear vibronic coupling to A1g modes and a change of force constants in the final electronic state. In most cases the main source of the electron‐phonon interaction is found to be the coupling of the transition with the electronic polarization of the nearest anions. The T1g‐symmetry‐projected phonon density of states is shown to be concentrated within a narrow interval of the order of 10 cm−1. An estimate is presented showing that the hyperfine interaction can be responsible for the appearance of the weak zero‐phonon lines observed in the emission spectra.