Photon Upconversion Spectroscopy
395
of chromophores. Moreover, organic molecules based system faces the problem of
photodegradation over the period of time. Nevertheless, this chapter shall remain
focused on lanthanide based UC process.
1.4 Energy Transfer Mechanism in Lanthanide Ions
Lanthanides are one of the most extensively studied group of elements in the periodic
table which start from lanthanum (La, Z = 57) to lutetium (Lu, Z = 71). Their
spectra consist sharp lines due to f-f electronic transitions. The sharp spectral bands
arise due to smaller radial extension of f-orbital than the outer 5s and 5p orbital.
Therefore, lanthanide shows very weak electron–phonon coupling which leads to a
lower susceptibility to crystal-field and exchange perturbations. All the lanthanide
ions show very stable oxidation state of +3. But in some cases, +2 (e.g. Sm
2+ , Yb
2+
and Eu
2+ ) or +4 (e.g. Ce
4+ and Tb
4+ ) oxidation states are also found. One of the
prominent processes that happen during luminescence in lanthanide doped material
is energy transfer. Increase in concentration of dopant ions in the host material triggers
the energy transfer from one excited ion to another ion in excited/ground state, as
it leads to contraction of spacing between ions allowing them to interact with each
other.
Four basic energy transfer mechanisms that have been observed are: (a) resonant
radiative energy transfer, (b) non-radiative energy transfer, (c) multiphonon assisted
energy transfer, and (d) cross-relaxation energy transfer. In radiative energy transfer
process, sensitizer (S) ion absorbs incident photons and is lifted to excited state.
Excited state energy of sensitizer is then transferred to the activator ion. This pumps
the activator in its excited state and the sensitizer returns to the ground state. For this
energy transfer, the emission spectra of sensitizer should overlap with absorption
spectra of activator. In the radiative ET, the decay time of the sensitizer does not show
any variation with a variation in the activator concentration. Pictorial representation
of radiative energy transfer is shown in Fig. 4a.
In non-radiative energy transfer process also, likewise the radiative energy transfer
process, initially sensitizer (S) ion absorbs incident photons and is promoted to
excited state, see Fig. 4b. But, sensitizer transfers its energy to activator ion nonradiatively (without emitting photons) either through exchange interaction or via
multipolar (dipole–dipole, dipole-quadruple, quadruple-quadruple, etc.) interaction.
This energy transfer happens if the energy difference between the ground sate and
excited states of the sensitizer equals to that of the energy of the activator ion.
Exchange interaction (proposed by Dexter) occurs for very short distances (<0.5 nm)
between sensitizer and activator as it dependents on the wave function overlap. On
the contrary, multipolar interaction (proposed by Forster) can occur even for larger
separations between sensitizer and activator (upto 10 nm) and depends on the strength
of the optical transitions. Different from radiative ET, the non-radiative ET often
leads to a decrease in the decay time of sensitizer luminescence transition with an
increase in activator concentration. Two other conditions required for the incidence
395
of chromophores. Moreover, organic molecules based system faces the problem of
photodegradation over the period of time. Nevertheless, this chapter shall remain
focused on lanthanide based UC process.
1.4 Energy Transfer Mechanism in Lanthanide Ions
Lanthanides are one of the most extensively studied group of elements in the periodic
table which start from lanthanum (La, Z = 57) to lutetium (Lu, Z = 71). Their
spectra consist sharp lines due to f-f electronic transitions. The sharp spectral bands
arise due to smaller radial extension of f-orbital than the outer 5s and 5p orbital.
Therefore, lanthanide shows very weak electron–phonon coupling which leads to a
lower susceptibility to crystal-field and exchange perturbations. All the lanthanide
ions show very stable oxidation state of +3. But in some cases, +2 (e.g. Sm
2+ , Yb
2+
and Eu
2+ ) or +4 (e.g. Ce
4+ and Tb
4+ ) oxidation states are also found. One of the
prominent processes that happen during luminescence in lanthanide doped material
is energy transfer. Increase in concentration of dopant ions in the host material triggers
the energy transfer from one excited ion to another ion in excited/ground state, as
it leads to contraction of spacing between ions allowing them to interact with each
other.
Four basic energy transfer mechanisms that have been observed are: (a) resonant
radiative energy transfer, (b) non-radiative energy transfer, (c) multiphonon assisted
energy transfer, and (d) cross-relaxation energy transfer. In radiative energy transfer
process, sensitizer (S) ion absorbs incident photons and is lifted to excited state.
Excited state energy of sensitizer is then transferred to the activator ion. This pumps
the activator in its excited state and the sensitizer returns to the ground state. For this
energy transfer, the emission spectra of sensitizer should overlap with absorption
spectra of activator. In the radiative ET, the decay time of the sensitizer does not show
any variation with a variation in the activator concentration. Pictorial representation
of radiative energy transfer is shown in Fig. 4a.
In non-radiative energy transfer process also, likewise the radiative energy transfer
process, initially sensitizer (S) ion absorbs incident photons and is promoted to
excited state, see Fig. 4b. But, sensitizer transfers its energy to activator ion nonradiatively (without emitting photons) either through exchange interaction or via
multipolar (dipole–dipole, dipole-quadruple, quadruple-quadruple, etc.) interaction.
This energy transfer happens if the energy difference between the ground sate and
excited states of the sensitizer equals to that of the energy of the activator ion.
Exchange interaction (proposed by Dexter) occurs for very short distances (<0.5 nm)
between sensitizer and activator as it dependents on the wave function overlap. On
the contrary, multipolar interaction (proposed by Forster) can occur even for larger
separations between sensitizer and activator (upto 10 nm) and depends on the strength
of the optical transitions. Different from radiative ET, the non-radiative ET often
leads to a decrease in the decay time of sensitizer luminescence transition with an
increase in activator concentration. Two other conditions required for the incidence