CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

in Ce 3+ , Pr 3+ , Tb 3+ and CT absorptions in Eu 3+ and Yb 3+ have energies less than ca. 40 × 10 3
cm –1 . Therefore, they can interact with 4f levels leading to emissions correspondinng to intra
4f transitions. In case, the energy levels of these states are lower than those of 4f levels, direct
luminescence transitions from these levels are found, such as 5d → 4f transitions in Ce 3+ , Pr 3+
and Eu 2+ . Luminescence due to charge transfer transitions has also been reported for Yb 3+
[41].
Fig.8. Energies for 4f →5d and CT transitions of trivalent lanthanide ions [31].
It can concluded from the above discussion that those ions that are easily oxidized to
the tetravalent state have lower 4f → 5d transition energies, while those that are easily
reduced to the divalent state have lower CT transition energies. So far the absorption or
excitation characteristic of Ln 3+ ions is discussed. In the following section, factors affecting
the emission/ luminescence properties of lanthanide ions are discussed.
1.5.2 Selection rules: Luminescence originating from electronic transitions between 4f levels
is predominantly due to electric dipole or magnetic dipole interactions. Electric dipole f-f
transitions in free 4f ions are parity-forbidden, but become partially allowed by mixing with
orbitals having different parity. Typical examples of this mechanism are demonstrated by the
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