Luminescence quenching Subject

The mineral matrix is formed chiefly by La " or Ce ". The last one is widely regarded as the luminescence impurity center, but in rare-earth bearing minerals it is subjected to concentration quenching because of strong exchange... 

The authors believe that the decreases in decay times are associated primarily with changes in quantum yield. This may be inferred from the fact that both the emission intensities and lifetimes are falling off at about the same rate with temperature. One thus concludes that the luminescence of sulfuric acid solutions of terbium sulfate is subjected to much greater temperature quenching than the luminescence in aqueous solution of the same salt. The increasing probability of radiationless transitions is undoubtedly connected in some manner with greater interaction of the radiating ion with the solvent molecules. 

Photo-electroswitching takes place in substances where an electrochemical change induces more or less pronounced variations in light absorption or emission properties. This occurs namely for metal complexes subjected to redox interconversion between their various oxidation states. A two-component device combining a luminescent centre and an electroactive unit may function as a photo-electroswitch in which the emission properties are modulated by redox interconversion via energy or electron transfer quenching (Figure 30). 

In the absence of chemical quenching, uranyl compounds have long luminescent lifetimes and high luminescent quantum efficiency [21]. Often, however, the excited state reacts chemically. The photochemistry of the ion, the most famous example of which is the uranyl oxalate actinometer, has generated an enormous body of work and been the subject of comprehensive reviews [22,23]. It can occur both in solution and in the solid state. The most common reaction is the oxidation of organic substrates. Both the photochemistry and the remarkable properties of the covalent bond, demand a satisfactory interpretation in terms of the electronic structure. 

The rapid development of lasers has led to the publication of increasing numbers of papers concerned this year with such subjects as superfluorescence and co-operative radiation processes,451 the thermodynamics of co-operative luminescence,452 saturation, collisional dephasing, and quenching of fluorescence of organic vapours in intense laser excitation studies,453 a theoretical model for fluorescence in gases subjected to continuous i.r. excitation,454 a quantum treatment of spontaneous emission from strongly driven two-level atoms,455 the development of site-selection spectroscopy,45 and measurements of relaxation times 457 using laser excitation. 

Where kp and km are the rate constant of fluorescence and non-radiative processes, respectively. The fluorescence quantum yield (Of) value in the range of 0.0 to 1.0. If the non-radiative relaxation is fast compared to fluorescence (km > k,), O will be small, that is the compound will fluoresce very little or not at all. Often different non-radiative events are limited in the solid phase, and long-lived luminescence (e.g. phosphorescence) is often studied in frozen solution or other solid phases. Quenchers make non-radiative relaxation routes more favorable and often there is a simple relation between 0 and the quencher concentration. The hest-known quencher is probably O2, which quench almost all fluorophores other quenchers only quench a limited range of fluorophores. If a molecule is subject to intramolecular quenching, O may yield information about the structure. 

The intense colors in 2,2 bipyridyl complexes of iron, ruthenium, and osmium are due to excitation of an electron from metal t2g orbitals to the empty jr -orbitals of the conjugated 2,2 bipyridyl. The photoexcitation of this MLCT excited state can lead to emission. However, not all complexes are luminescent because of the different competing deactivation pathways. This aspect is beyond the scope of this chapter the interested reader can refer to a number of publications on this subject [16-20]. The other potential deactivation pathways for the excited dye are donation of an electron (called oxidative quenching, Eq. 2) or the capture of an electron (reductive quenching, Eq. 3) or transfer of its energy to other molecules or... 

Porous silicon is a desirable surface material for sensors because of its photoluminescence properties. With excitation, the porous silicon surface can luminesce at a variety of easily detected wavelengths. Through various methods, such as quenching, the surface can show some evidence of interaction with a target molecule. However, plain porous silicon lacks the specificity needed to detect target molecules, and is subject to non-specific interferences. 

Mineral matrix is formed chiefly by La " or Ce ". The last one is widely met as luminescence impurity center, but in rare-earth bearing minerals it is subjected to concentration quenching because of strong exchange interaction of outer 5d orbitals of neighboring Ce " ions. Hereby its luminescence is not observed in REE minerals. From the other side, other trivalent REE can achieve some concentrations without quenching of luminescence, because outer electron shells, such as 5s and 5p , shield their inner 4f-4f transitions. 

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