As the amount of cerium ions increases more strict compliance
of the luminescence decay kinetics of the
5 D 4 -level relaxation with
the exponentially law indicates the direct (resonant) excitation of
this level by energy transfer from the cerium ion. According to our
results (Fig. 3, the excitation spectrum), there are two parallel
channels of Tb 3þ excitation: direct excitation of the ion and
excitation through the cerium ion. As the concentration of Ce 3þ
ions increases, the number of Tb 3þ ions in which the
5 D 4
level is
excited in the interaction of Ce 3þ -Tb 3þ grows, and the lumines-
cence kinetics tends to the exponential dependence. The expo-
nential character of the decay kinetics indicates that the level is
excited directly, and the other excitation channels of the
5 D 4
level
and/or its non-radiative relaxation are not very effective if the
ratio of the concentration of Tb 3þ /Ce 3þ ions is 5/1.