Polarization spectroscopy of Cr3+ ions in laser host crystals. II. The broadband transitions

This paper discusses the polarization of vibronically-broadened absorption and emission spectra of Cr³⁺ ions in ionic crystals such as MgO, CaO, YAlO₃ and Gd₃Sc₂Ga₃O₁₂. A molecular orbital theory is presented of the selection rules of the vibronic ⁴T₂→⁴A₂ emission, a transition of importance in tunable laser action of the Cr³⁺ ion in ionic host lattices. The selection rules of optical spectroscopy are normally determined using group theoretical methods. However, the molecular orbital approach adds to the group theoretical results an identification of the contributions to the strength of vibronic sideband transitions by covalent admixtures of ligand orbitals into ground and excited states wave functions of the optical centre. In particular molecular orbital theory is used to calculate the strengths of linearly and circularly polarized components of absorption and emission spectra in the absence or presence of a static magnetic field. The theoretical results are compared with experiments and used to discuss the relative contributions to the sideband by odd-parity distortions of T_1u and T_2u symmetry.