Thermoluminescence
is a record of light produced while a material is heated. In the case
of a polymer like Ultra-High Molecular Weight Polyethylene (UHMWPE),
the heating process causes recombination of Free Radicals that are
produced during sterilization or other material process. Trapped
electrons and holes that have been “frozen” in the material can be
released by an increase in temperature (i.e. thermal stimulation).
When the temperature is increased at a steady rate, a plot of luminescence
versus temperature, called the glow curve (shown above), can be recorded.
Important information about the radiation-induced traps (such as thermal
activation energy, frequency, and initial number of traps) can be
obtained from analysis of the position and shape of the glow peaks.
This type of emission is known as thermally stimulated luminescence
(TSL). It is common to refer to TSL as thermoluminescence (TL).
The
absorption and emission processes can easily be described using the
configurational coordinate diagram where energy, E, is plotted versus
displacement, R. The configurational coordinate represents the arrangement
of nuclei in the vacinity of an absorption center.
The
energy levels for the harmonic oscillator are quantized. Within the
ground state, represented by the parabola U, the wavefunctions of
the vibrational levels are labeled v0, v1, v2, . . . vn. For the lowest
vibration level, v0, the system has the highest probability of being
located in the center of the parabola at R0. For higher vibration
levels, the probability is greater for finding the system at the turning
points than in the center of the parabola.
Absorption
of energy causes transition from the lowest vibrational level, v0,
of the ground state, U, to some high vibrational level, v'n,
in the excited energy state represented by the parabola U?. Since
the probability is greatest for transitions initiating from R0, the
absorption band will have a maximum energy corresponding to that transition
(given by the bold arrow labeled Eabs). However,
it is possible to initiate transitions from R < R0 or R > R0.
This distribution of probabilities leads to the width of the absorption
band.
The
excited ion releases some of its energy through non-radiative relaxation
to lower vibrational levels within the excited state. Decay to the
ground state can occur radiatively, non-radiatively, or excitation
energy can be transferred to other ions in the lattice. The process
of relaxation and radiative return to the ground state is of particular
interest, and is shown in the figure below. The shift in the configurational
coordinate from R0 to R'0 can be explained by the fact that nuclei
must compensate for the excited state so that their interatomic distances
correspond to an equilibrium position.
Emission
from the excited state to the ground state is spontaneous (i.e. without
stimulation). Again, there is a distribution of probabilities for
the starting points of the emission, with the greatest probability
being at R?0. Because the relaxation process dissipates some of the
energy of the excited ion, radiative decay to the ground state occurs
at a lower energy than the absorption process (i.e. Eabs > Eem).
This energy difference is known as Stokes shift. It can be seen from
Figure 8 that a larger value for ?R will result in a larger Stokes
shift. The remaining energy is dissipated through relaxation to low
vibrational levels in the ground state.
Excitation
and emission can also be described using the band theory of solids
shown schematically below. The valence band (VB) and conduction band
(CB) are separated by the band gap, Eg. Impinging
radiation causes excitation (a) and the formation of traps (horizontal
lines) within the band gap. The traps are partially filled (b) by
electrons (solid circles) and holes (open circles). Luminescence can
result from recombination of free electrons with trapped holes (c),
trapped electrons with free holes (d), or by donor-acceptor pair emission
which involves recombination of a trapped electron and trapped hole
(e).
Ref:
G. Blasse and B. C. Grabmaier, Luminescent Materials, (Springer-Verlag,
New York, 1994)
Radioluminescence