Optical Stimulated Luminescence

[ctydtgat]

In a measurement of an OSL spectrum the wavelength of the stimulation source is always higher than the wavelength of the emission of the material.
Is this due to the fact that at lower wavelengths one can no longer distinguish between stimulation and emission light?
Can electron traps be released by its own luminescence?

Thanks!

 

[Dr. Courtney]

Are you sure you don't have that backwards?

 

[ctydtgat]

In a thermo-luminescent material there are electron traps just below the conduction band and there are impurity luminescent centers in the crystal, e.g. Eu, Ce, ...
After UV illumination of the material, luminescent centers (Eu, ..) can be ionized (Eu2+ -> Eu3+) and the released electron will be delocalized through the conduction band where it can be trapped into a trap just below the conduction band. In an OSL experiment these electrons can be released by illuminating the material using infrared, just enough energy to release it from the trap back to the conduction band.
Assume now that at a certain position an electron is indeed released from its trap. Then the electron can go to a luminescent center (Eu3+ + e -> Eu2+,*) which we assume for now to decay radiatively (there is also the possibility to decay non-radiatively). Then this particular center will generate a photon. This photon travels through the material where it could be absorbed by another trapped electron which will thus escape from its trap. However, the energy from the released photon due to Eu2+,* -> Eu2+ + photon is higher. So if the photon would be absorbed by another trapped electron this means that it would get energy rather high into the conduction band. If there are levels available and the transition is allowed, this could in principle happen I guess.
Does this makes sense?