Chemistry Reference
In-Depth Information
of terbium doped ZnMnS nanoparticles due to the 4f-4f transition of
the Tb
3+
2+
ions.
Another energy transfer process that enhances efficiency of rare
earth phosphors is the cascade transition process [48]. Currently,
the phosphors used in fluorescence lamps seem fairly efficient in
converting the UV photons emitted by mercury (Hg) vapor into visible
light. This conversion reported very high quantum efficiency as one
UV photon emitted by Hg vapor is converted to exactly one visible
photon, which relates to 100% conversion efficiency. However, if
the conversion is considered in energy terms, the efficiency is much
lower. This efficiency of fluorescence phosphors can be improved if
one UV photon can excite two or more visible photons in a cascade
transition process. This process has been found to exist in various
host systems doped with Praseodymium (Pr
ion, via Mn
3+
)
ions.
3+
3
3
1
1
3
1
The energy structure of Pr
ions consists of
H,
F,
D,
G,
P,
I,
1
and
S multiplets split by spin-orbit interaction, as shown in Fig. 6.5.
3+
Figure 6.5
The energy level scheme of Pr
lattice with the
indicated scheme of cascade processes following the initial 4f-
5d charge transfer or host excitation [48].
ion in the ZrO
2
The photon cascade process observed shows an efficient 4f
2
-4f
1
5d
1
excitation, followed by carrier trapping via the highest
