ABSTRACT
Optical absorption or emission within a single ion or a group of ions in a solid, it is appropriate to treat an optical transition with a localized model rather than the band model. The huge number of actual vibrational modes of the lattice can be approximated by a small number or a combination of specific normal coordinates. These normal coordinates are called the configurational coordinates. It explains optical properties of a localized center on the basis of potential curves, each of which represents the total energy of the molecule in its ground or excited state as a function of the configurational coordinate. In summarizing the discussion of the spectral shape based on the configurational coordinate model, one can review the experimental results on luminescence bandwidths. The shape of an optical absorption or emission spectrum is decided by the Franck-Condon factor and also by the electronic population in the vibrational levels at thermal equilibrium.
