CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

orbitals with filled electronic configuration. As a result, the 4f electrons are only weakly
affected by the ions surrounding lanthanide ions (ligands) in the crystal. Because of this, all
trivalent lanthanide ions, except cerium, show atomic-like sharp absorption and emission
spectra.
1.5.1 Lanthanide Energy levels: The perturbation of the 4f levels of lanthanide ions caused
by the crystal field is rather weak as compared to the Coulomb interaction between the
electrons and spin-orbit coupling [34]. The Coulomb interaction between the electrons within
the 4f shell has the largest contribution to the energy level splitting and this leads to separated
energy levels with energy difference of the order of 10 4 cm −1 . Each of these levels split
further into several levels by the spin-orbit interaction with an energy difference of the order
of 10 3 cm −1 . All these levels form the 4f configuration of the free lanthanide ions and each
level can be characterized with the symbol 2S+1 L J , where S is the total spin, L the total orbital
angular momentum, and J the total angular momentum of the 4f electrons. When the Ln 3+
ions are put into a host material, the crystal field not only affects the strength of the intraconfigurational
f-f transitions but also brings about an additional splitting of the 4f states that
depends on the site symmetry of the lanthanide ions [34]. Although the 4f electrons are well
shielded, when lanthanide ion occupies a site with certain symmetry, the crystal filed effects
can lead to splitting of levels which may range up to several tenths of electron volts [35]. A
schematic diagram of 4f energy level splitting is given in the Fig.6. Unlike the 4f levels of
lanthanide ions, their 5d energy levels (like 5d levels of Ce, Tb, etc.) are largely perturbed by
the crystal field. The magnitude of the splitting of the 5d levels depends strongly on the shape
and size of the anion polyhedron coordinating the lanthanide ion [36]. The dependence of
shape of the coordinating polyhedron on the crystal field splitting can be explained
qualitatively within the point charge electrostatic model. The first energy level diagram
displaying the 4f levels of the trivalent free Ln 3+ ions in aqueous medium in the infra-red
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