CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

(IR), visible and ultra-violet (UV) region of the spectrum was presented by Dieke and
Crosswhite in 1963 (Fig.7) [37, 38]. Most of the emitting levels are separated from the next
lower level by an energy value of ~ 2 ×10 3 cm –1 or more.
Fig.6. Splitting of 4f n electronic energy levels due to coulomb repulsions, spin-orbit coupling
and crystal field effect.
The excited states of lanthanide ions relax via two competitive pathways namely by
light emission and phonon emission processes. The rate of phonon emission, ω, depends on
the number of phonons emitted simultaneously to bridge the energy gap and is expressed by
equation 5.
⎛ kΔE
⎞
ω ∝exp⎜−
⎟
hν
⎝ max ⎠
…………………… (5)
Where, ΔE is the energy gap to the nearest lower level and hν max is the maximum energy of
phonons coupled to the emitting states. The phonon emission rate, ω, decreases rapidly with
an increase in ΔE, so that the competitive light emission or radiative process becomes
dominant [39]. Thus, the well-known high luminescence efficiencies for 5 D 0 of Eu 3+ and 5 D 4
of Tb 3+ are based on the large energy gap of more than 10 4 cm –1 that needs to be bridged to
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