ABSTRACT
Group theory helps us to understand optical spectra of crystals doped with metal ions. e description of optical spectra of metal ions introduced into a lattice is more qualitative but is quick to determine and good enough. e practical idea is that metal ion energy levels are influenced by the surrounding host ions. e central metal cation and the nearest surrounding anions form a molecular complex. ere are more ways to treat this molecular complex theoretically:
1. Crystal field theory. is theory was proposed by the physicist Hans Bethe in 1929 (Bethe 1929). e molecular complex is like an “ionic” molecule (Ballhausen 1962). Ions of the molecular complex are seen as point charges and the interaction between the central ion and surrounding ions is a pure electrostatic interaction between point charges. us, the valence orbitals of metal ions are influenced and just deformed in the direction of neighboring ions. e electrons of metal ions are not allowed to mix with electrons of the surrounding ions. As a consequence, their orbitals will have a lower symmetry inside the complex than the spherical symmetry as free ions.
