3.2.2. Photoluminescence Excitation Spectra
In order to determine the excitation channels of Tb3+
ions, the PL excitation spectra of C12A7:Tb samples were
measured at room temperature, monitoring the 5D4 →7F5
transition of Tb3+ ions located at 551 nm. As shown
in Figure 5, these samples exhibited a broad absorption
peak centered at 206 nm, which corresponded to the band
absorption of the host lattice. A slight red shift of this
absorption peak was observed when the terbium concentration
was increased, in agreement with the result of the
absorption spectra. Besides, two other absorption bands
were observed: a broad band located at 247 nm and a
very weak shoulder around 280 nm. These bands could
be attributed to the transitions from the lower energy level
of the 4f8 configuration to the energy level of the 4f75d
configuration of the Tb3+, resulting in a direct excitation
of Tb3+.21 23–25 Indeed, Tb3+ ions exhibited a 4f8
electron configuration for which the ground state is 7F6.
When one electron was promoted to the 5d shell, it gave
rise to two 4f75d1 excitation states: the high spin state
with 9DJ configurations and the low spin state with 7DJ
configurations.24 25 According to Hund’s rule, the 7DJ levels
were higher than the 9DJ ones, and the transitions
between 7F6 and 7DJ were spin allowed, while the transitions
between 7F6 and 9DJ were spin forbidden. As a
consequence, excitation spectra of a Tb3+-doped host generally
exhibited two groups of absorption bands: the first,
corresponding to spin-allowed f–d transitions, was strong
and was located at high energy side, and the second,
related to spin-forbidden transitions, was weak and was
situated at low energy side.24 25 The band at 247 nm presented
a relatively strong intensity and thus seemed to be