Laser ions divalent rare earths

In the subsections to follow, trivalent and divalent rare earth ions and transitions used for crystal lasers are reviewed. The literature on rare earth crystal lasers is too voluminous to attempt a comprehensive listing of all ion-crystal combinations. Instead, the following references and tables therein should be consulted Kaminskii and Osiko (1970), Weber (1971b), and Kaminskii... 

Fig. 35.12. Energy levels and laser transitions of divalent rare earth ions. Wavelengths of transitions are in /xm.

The rare earth (RE) ions most commonly used for applications as phosphors, lasers, and amplifiers are the so-called lanthanide ions. Lanthanide ions are formed by ionization of a nnmber of atoms located in periodic table after lanthanum from the cerium atom (atomic number 58), which has an onter electronic configuration 5s 5p 5d 4f 6s, to the ytterbium atom (atomic number 70), with an outer electronic configuration 5s 5p 4f " 6s. These atoms are nsnally incorporated in crystals as divalent or trivalent cations. In trivalent ions 5d, 6s, and some 4f electrons are removed and so (RE) + ions deal with transitions between electronic energy sublevels of the 4f" electroiuc configuration. Divalent lanthanide ions contain one more f electron (for instance, the Eu + ion has the same electronic configuration as the Gd + ion, the next element in the periodic table) but, at variance with trivalent ions, they tand use to show f d interconfigurational optical transitions. This aspect leads to quite different spectroscopic properties between divalent and trivalent ions, and so we will discuss them separately. 

Alkaline-earth fluoride crystals have been the principal hosts for divalent lanthanide laser ions. These are relatively soft, optically isotropic materials. Rare earths occupy cubic Ca sites. The allowed radiative transitions between 4f states are magnetic-dipole or vibronic. Since these are relatively weak, the fluorescence lifetimes are long, —10 ms. In comparison, the fluorescence lifetime of the 5d 4f transition of SrF2 Sm, which is electric-dipole allowed, is short,... 

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