Stepwise Upconversion and Cooperative Phenomena in Fluorescent Systems
The general discussion of the preceding chapter of energy transfer in the regime of high sensitizer concentration is applied to several cases of stepwise, sequential transfer between rare earth ions, a process which leads to conversion of infrared sensitizer excitation to visible or ultraviolet activator luminescence. The process is compared with the generally less efficient cooperative excitation process in which two sensitizers transfer simultaneously their excitation to an activator. Both types of process are adequately described by rate equations for the populations of the relevant energy levels of the ions. Another class of photon-assisted two-ion processes is briefly discussed on the basis of Dexter’s theory.
KeywordsLuminescence Intensity Green Luminescence Cooperative Phenomenon Sequential Transfer Weak Excitation
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