Ruby (Al2O3 doped with Cr3+ ions), when exposed to ionizing radiation (for e.g. X-rays), emits luminescence. What is the mechanism that causes this 'Radio-luminescence? Does Cr3+ accept electrons or give up?
the following article may answer your question:
Lapraz, D., Iacconi, P., Daviller, D. and Guilhot, B., 1991. Thermostimulated Luminescence and Fluorescence of α‐Al2O3: Cr3+ Samples (Ruby). Influence of the Cr3+ Concentration. physica status solidi (a), 126(2), pp.521-531.
As the name implies injected radiation excites or pumps up the valence electron to the upper level which induces or simulates some of the electrons laying at the upper levels to drop down the vacant level while creating or emitting a photon having a proper energy equivalent to the difference between those levels taking place in the phenomenon. Actually there is a thermodynamic equilibrium established during this phenomenon which tells us that the rate of number of electrons excited by the radiation is exactly equal to the rate of those electrons drops down to the vacant level.
Chances of Reduction of Cr3+ to Cr2+ are less, it stays as Cr3+ even during irradiation/readout. May be Cr3+ gets excited resonantly from nearby centre emission (F centre) and emits in red. Redox model is used for RE doped phosphors, CaF2 or CaSO4 etc to a limited extent although without any experimental proof.
Check size/ionic radii of Cr2+ and Cr3+, is there reasonable difference?
Dear Munish,
Thank you for your comment. Then why do some papers say Cr3+ changes to Cr2+ or Cr4+ (for e.g. Lapraz et al.; above paper)?
Your comment indicates that the mechanism of RL is not known fully, is it?
@Raju: It may happen that we use such conversion (Not Ruled Out) for understanding but possibly no such experimental evidences are available in many cases. Even if conversion is there, it may be well below detection limit and negligible. See sizes of different states, they may find difficult to adjust in lattice etc.In CaSO4:Dy we assume Reduction as per Nambi Model, but experimental evidence is not there. There is no change in colour evan after huge irradiation for CaSO4:Dy, if reduction holds, colour should change. You may contact Dr. B. C. Bhatt, who is also on ResearchGate and has done pioneering work in these fields. I am also sharing the question with him.
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