The active lasing dopant shares some definite spectroscopic characteristics with the laser emission. These characteristics are a result of the spectral properties (wavelength, bandwidth, lifetime of the emitting level) of the active ion. The following section defines the relevant spectroscopic nomenclature. The most commonly used active ions belong to the ƒ element block (rare earth ions). The 4ƒ energy level of the rare earth elements after lanthanum (La) falls below that of the 5d level; therefore, subsequent electrons are added to the inner, shielded 4ƒ orbitals. One should note that although the 4ƒ orbitals are expected to be occupied regularly - namely, the electronic structure will be in the following arrangement: 4ƒn5d16s2 (n=1-14), starting from Ce to - the observed structure is associated with the electronic stability obtained by a complete configuration of 4ƒ7 (Eu) or 4ƒ14 (Yb) occupancy of any set of orbitals. The tendency to fill the ƒ orbitals relative to the 4ƒn5d1 configuration reflects the energetic stability of the complete set of ƒ orbitals, either in a complete single set ( ƒ7) or a complete double set ( ƒ14) occupancy. After 4ƒ7 occupation of Eu, the next atom, Gd, which is located at the center of the series just after Eu, has the following ground state configuration: 4ƒ75d16s2. This structure continues with the 4ƒn configuration until Yb (4ƒ146s2), and at the next atom, Lu, it has the configuration 4ƒ145d16s2. Table 2.1 presents a sample basic-ground-state electronic configuration in both the ideal (expected) and the observed (actual) cases of some commonly used ƒ elements.
The chemical and physical differences between the 4ƒn5d16s2 (idealized) configuration and the 4ƒn+16s2 (observed) configuration are small because the energy differences between these configurations are also very small. The 4ƒ electrons are well shielded by the outer 5d or 6s shells, and therefore these electrons are unaffected by the external crystal fields and are unavailable for chemical reaction. To summarize, the states resulting from the 4ƒn configurations are only slightly affected by the physical or chemical surroundings of these ions; thus, they have very similar properties under various conditions. Energy calculations show that the tripositive oxidation state of the ƒ elements is the most stable compared to other oxidation states, namely, tetrapositive or dipositive ions. Rare earth ions can be found in a triple- or double-charged state in ionic solids.
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