Fig. 9 is the schematic energy-level diagram showing the excitation,
emission and energy transfer mechanism of Ca0.96xEu0.04
BixSiO3 phosphors. Upon excitation at 280 nm, the photon energy
is absorbed by the sensitizer leading to the 1S0?1P1 transition. Further,
this energy is transferred to 3P2, 3P1 of Bi3+ and get transfer to
4f shell of Eu3+. Thus the electrons of Eu3+ at 5D0 excited state can
populate both from non-radiative charge transfer feeding and
non-radiative transitions from higher excited states. Then the
characteristic emissions at 580, 614 and 648 nm correspond to
the 5D0?7Fj (j = 1, 2 and 3) transitions, respectively [24]. The
transitions from 5D0 to 7Fj (j = 1–3) are radiative and produce the
emission lines.