Quenching center

Quadrupole splitting constants, 37 129 Quantum mechanical methods, 38 424 Quantum mechanical predication, lattice energies and, 1 181-186 2,2 6, 2 6",2"-Quaterpyridine coordination compounds, 30 104-106 Structure, 30 70 Quenching center, 35 321 organometallics, 19 93-98 chemical, 19 98... 

Luminescence intensity with very rare exceptions is much higher in artificial gemstones compared to natural counterparts. It may be explained by the fact that the activator contents are usually higher in laboratory made gems, while the quenching center concentrations are lower. 

Luminescence lifetime depends upon radiative and nomadiative decay rates. In nanoscale systems, there are many factors that may affect the luminescence lifetime. Usually the luminescence lifetime of lanthanide ions in nanociystals is shortened because of the increase in nomadiative relaxation rate due to surface defects or quenching centers. On the other hand, a longer radiative lifetime of lanthanide states (such as 5Do of Eu3+) in nanocrystals can be observed due to (1) the non-solid medium surrounding the nanoparticles that changes the effective index of refraction thus modifies the radiative lifetime (Meltzer et al., 1999 Schniepp and Sandoghdar, 2002) (2) size-dependent spontaneous emission rate increases up to 3 folds (Schniepp and Sandoghdar, 2002) (3) an increased lattice constant which reduces the odd crystal field component (Schmechel et al., 2001). 

The compounds SrTiOa and KT10P04 show S-state emission with a low thermal activation energy. In KTi0P04 the titanate groups form linear chains by comer sharing, so that the delocalization is onedimensional (186). Compounds in which this delocalization plays a role often do not luminesce at all, since the excitation energy can easily reach quenching centers. 

Interestingly enough there is another sharp drop in luminescence intensity at the transition to the liquid-crystalline phase 215). This is shown in Fig. 45. In the latter phase there is an orientation of the phthalocyanine molecules that is more favorable for migration, so that more quenching centers are reached. The transition shows hysteris (Fig. 45) and coincides with thermodynamic measurements. Therefore the luminescence is used to probe the crystalline to liquid-crystalline transition. A further analysis yielded an estimation of the number of phthalocyanine molecules in the stack. 

Maximum possible light yields can be predicted from the maximum possible efficiency (Sect. 4.4). Some examples are given in Table 9.4 [10], As a matter of fact the quantum efficiency of the luminescent center should be high, and competing (quenching) centers should be absent (Sect. 4.4, Refs [10,11]). 

Moses et al. [36] have determined the quantum efficiency (Sect. 4.3) of the CeF3 luminescence. For direct Ce excitation it is high. Lower quantum efficiencies are found if the excitation starts at the F ion (2p). For lOOeV the total quantum efficiency is about 0.7. The cncigy efficiency is then 3%. Also this is relatively low, and the authors suggest nonradiative recombination on quenching centers in order to explain this. 

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