f-d Transitions of Divalent Ytterbium Ions in Halide Crystals

An exciton is a quasiparticle consisting of a bound state of an electron and an imaginary particle called an electron hole in insulators and semiconductors, and in some liquids. As such, it is a Coulomb-correlated electron-hole pair with no net charge. It is an elementary excitation of condensed matter characterized by a periodic lattice structure. The bound electron and hole pairs provide a means to transport energy without transporting net charge.

The concept of exciton was first proposed by Frenkel in 1931, when he described the excitation of atoms in a lattice of insulators. He proposed that this excited state would be able to travel in a particle-like fashion through the lattice without the net transfer of charge. Exciton formation may occur where a possible configuration of the lanthanide-trapped exciton in an MX2 alkaline-earth halide compound is depicted.
After fd excitation, the occupied 5d level is located just below the conduction band CB. It corresponds to point B that is described below.
Autoionization of the 5d electron to the conduction band leaves the lanthanide ion in the trivalent state which has an about 19 pm smaller ionic radius. The neighboring anions will relax towards Ln³⁺.

The Madelung potential at the lanthanide site becomes more negative and the (unoccupied) 5d levels of the lanthanide will move upward. The conduction band levels of the neighboring M²⁺ cation probably move downward. The Coulomb attraction between the electron and the Ln³⁺ ion, together with the reduced Madelung potential at the nearby M²⁺ sites, localizes the electron in the lanthanide-trapped exciton configuration.

The recombination of the electron and hole, that could give a photon emission, is limited by the overlap of the electron and hole wave functions, which results in extended lifetimes of the exciton. The emission from an impuritytrapped exciton is a competitive process with the emission from the excited fd state.
This emission was observed by Pedrini et al. in various fluoride host materials and was called “anomalous” emission. The emission mechanism. Initially the system is in the 4fⁿ ground state, indicated by point A on parabola a. After excitation to the 5d level, point B on parabola b is reached. Subsequent lattice relaxation brings the system to point C and several routes can be followed from here. In the case of SrF2 : Yb²⁺, the system relaxes to the impurity-trapped exciton state indicated by point E on parabola d.