f-d Transitions of Divalent Ytterbium Ions in Halide Crystals

Introduction Lanthanide (rare-earth) ions, are characterized by the most complicated opti- cal spectra of the elements. This is because the 4f shell, which is incomplete, produces a very large number of low-lying levels and the transitions between them give a many-line spectrum without apparent regularities. One of the most impressive features about the optical spectra of the rare- earth ions in a large number of dierent types of crystals is the sharpness of many lines in both the absorption and emission spectra. This is explained by the weak coupling of transitions within the 4f n conguration with vibrations in the surrounding host lattice. The rare-earth ions can be present in solids either in their divalent or trivalent state. Their electronic congurations are given by 4 f n 5s 2 5p 6 or 4f n 1 5s 2 5p 6 , respectively. Thus, the 4f electrons are not the outermost ones, and are shielded from external elds by two other electronic shells, i.e. the 5 s and 5p with larger radial extension. The result of this shielding is that, when a rare-earth ion is introduced into a solid, the 4f electrons are only weakly perturbed by the charges of the surrounding ligands. Therefore, many of the spectroscopic properties of the rare-earth ions in solids are expected to be understood from considerations of the free ions. These features make the rare-earths ions excellent materials for some appli- cations. From uorescent lamps, solid-state lasers, to optical ampliers in ber optics, rare-earth elements, have been widely used to activate lumines- cent and photonic materials. A majority of applications involve electronic transitions between states within a 4f n conguration of trivalent or diva- 1