CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

luminescence from the 5 D J states of Eu 3+ . The intensity of these transitions depends strongly
on the site symmetry in a host crystal. Magnetic dipole allowed f-f transitions are not affected
much by the site symmetry. The J selection rule in this case is ΔJ = 0, ±1 (except for 0→0).
For electric dipole transition, the difference in the J values (ΔJ) is ±2. Oscillator strengths
are of the order of 10 –5 to 10 –8 for electric dipole transitions, and 10 –8 for magnetic dipole
transitions [31].
1.5.3 Luminescence quenching: Luminescence can be quenched due to the non-radiative
decay of the excited state. This non-radiative decay is brought about by different mechanisms
involving energy transfer from the excited state of the donor to different types of acceptors
like host lattice, organic molecules on the surface, defect levels or nearby ions which may or
may not act as activator. In the following section different types of non-radiative path ways of
the excited state is given.
Multiphonon emission: As discussed in the previous section (equation 5) it is clear that the
extent of quenching due to phonons depends on the energy gap between the emitting states
and value of phonon energy. General observation is that if the energy difference is more than
5 times the phonon energy, then luminescence is the main de-excitation process than the
multi-phonon quenching [42]. This also explains the quenching of lanthanide ions excited
state by organic molecules containing functional groups with high phonon energy. Hence,
efforts for the development of new efficient luminescent materials based on Ln 3+ ions are
directed towards the search for low effective phonon energy host materials. The general order
of phonon energy of hosts are fluorides < sulfides < oxides < phosphates.
Energy transfer: An excited state can also relax to the ground state by non-radiative energy
transfer to a second nearby state/ center. For energy transfer to take place the energy levels of
the donor and acceptor should match. However, when there is a mismatch between the energy
levels/ transitions of the donor and acceptor ions, the energy transfer process needs to be
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