A method of moments and projected green's function technique for electronic absorption and emission bands in crystals

Abstract

An analysis of the temperature variation of different moments of electronic absorption and emission bands of defects is given. The relaxation around a lattice defect and the changes of the force constants during an electronic transition are calculated. A Green's function formulation of the single moments allows a compact description of the coupling of all lattice oscillators to the electronic transition by perturbed projected phonon densities. The range of perturbed shells of ions around the defect can easily be adapted to the special model of the center.

A numerical discussion forF-centers in KBr gives simultaneously the experimentally observed Stake's shift and half width of the bands for fairly reasonable values of the relaxation parameters (lattice ion displacements and force constants). A plausible explanation is found for the wellknown discrepancy of the effective frequencies which define the temperature behaviour of the half widths in absorption and emission.