Spectroscopy of lanthanide (rare-earth) and actinide ions in solids

Abstract IN the fifth period of the periodic table of elements after the element lanthanum (atomic number 57) the filling of the 4f shell takes place, from cesium with the outer configuration 5s2 5p64f15d16s2 to ytterbium with configuration 5s2564f13Sd16s2. These are the lanthanide or rare-earth elements, which occur as doubly or triply charged ions in ionic solids. In the triply charged rare-earth ions all 5d and 6s electrons are removed and the 4f shell is only partially occupied; Table 8.1 gives the number (n) of the electrons in the unfilled 4f shell. The many energy levels of the unfilled 4fn shell between which radiative transitions may occur cover a spread of energies of ⋍ 40000 cno-1. The next highest configurations are 4f(n-1), 5d, 4f(n-1)l6s and 4f(n-1)6p, which for the triply charged ions arc usually well above the lowlying 4fn energy levels. The divalent rare-earth ions contain one more electron (e.g. Sm2+ has the same configuration as Eu3+) and in these divalent ions the higher-lying energy configurations overlap some of the low-lying 4fn energy levels (Dicke 1968). In a similar way the filling of the Sf shell occurs after the element actinium (atomic number 89). These actinide ions, which have an incomplete Sf shell, have many low-lying energy levels in solids. In consequence they also have radiative transitions in the optical region.