CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

the next lower level of these ions. The above formula implies that large values of hν max also
quench light emission. This is demonstrated by the fact that luminescence of Eu 3+ in aqueous
solution is almost quenched, but begins to appear if H 2 O is replaced by D 2 O [40].
Fig.7. Energy levels diagram for the lanthanide aquo ions. The main luminescent levels are
drawn in red, while the ground state level is indicated in blue [38].
In the energy region spanned by 4f levels, two additional electronic states (transitions)
with different characteristics from those of intra 4f levels are also observed for lanthanide
ions. They are the transition between 4f n–1 → 5d 1 states and the charge transfer transitions
(CT). In the former, one of the 4f electrons is transferred to a 5d orbital while in the latter
case, electrons in the neighboring anions are transferred to a 4f orbital. Both these processes
are allowed and result in strong optical absorptions. Unlike intra 4f transitions, transition
energies involved in the 4f n–1 → 5d 1 and CT transitions are strongly dependent on their
environments. The transition energies corresponding to 4f → 5d and CT transitions for
different lanthanide ions are shown in Fig.8 [31]. These energies are obtained based on the
absorption spectra of trivalent lanthanide oxides. As shown in the figure, 4f → 5d transitions
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