The origin of fluorescence

Over the course of fluorescence that accompanies the return to the initial state, the molecule can retain a part of the energy absorbed in the form of vibrational 

Sensitivity in fluorimetry is often 1000 times greater to that of the UV/Vis absorption. However, the correct use of these techniques requires a sound knowledge of the phenomenon in order to avoid a number of errors. 

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The origin of fluorescence is at 18332 cm-1 and is b-polarized. Comparing the origin of fluorescence and absorption (see Fig. 6-8) we notice that, although both... 

Schematic representations of the origins of fluorescence (a, b), and UV-vis absorption (c, d). (Adapted from reference 34.)...

Though theories have been proposed (32-35) to explain this phenomenon, the mechanism of fluorescence is still not yet fully understood. Jankow and Willis (36) proposed a mechanism which involves a direct excitation of the molecule or an impurity to an excited state, followed by internal conversion and then reversion back to the original state with emission of light. This mechanism can be explained as follows A molecule in the lowest vibrational level of the ground state A is transferred to a certain vibrational level in the excited state D. The molecule tends to cascade into the lowest vibrational level of state D by collisions with other excited molecules. It passes from state D to state C and then to state B by radiationless transi-... 

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

An inscribed thick plate of brass attributed to the landing, in 1579, of Francis Drake on the coast of California, is retained in safekeeping at the University of California, Berkeley. Since its discovery, in the San Francisco Bay area in 1936, however, there have been doubts about the authenticity of the plate, although an early chemical study had apparently confirmed its authenticity. Regardless of this initial study, doubts about the origin of the plate persisted, and a new study, based on the composition of the brass as determined by neutron activation, X-rays fluorescence, and atomic absorption analysis was initiated to reevaluate the earlier authentication of the plate. The results of this study were then compared with the composition typical of brass from Drake s time as well as from modem brass, and it was then concluded that the plate was probably made during the latter part of the nineteenth century or the early years of the twentieth century (Hedges 1979). 

However, the case of luminescence of PET fibers and films is not so easily interpreted and has recently been the subject of several studies (2,7,9,21,22,23). There is general agreement that PET does have a luminescent state and that the observed emission is not merely an impurity. The origin of the fluorescence has remained the subject of debate for the past decade. 

Liu T, Callis PR, Hesp BH, de Groot M, Buma WJ, Broos J (2005) Ionization potentials of fluoroindoles and the origin of nonexponential tryptophan fluorescence decay in proteins. J Am Chem Soc 127(11) 4104-4113... 

FRET is a nonradiative process that is, the transfer takes place without the emission or absorption of a photon. And yet, the transition dipoles, which are central to the mechanism by which the ground and excited states are coupled, are conspicuously present in the expression for the rate of transfer. For instance, the fluorescence quantum yield and fluorescence spectrum of the donor and the absorption spectrum of the acceptor are part of the overlap integral in the Forster rate expression, Eq. (1.2). These spectroscopic transitions are usually associated with the emission and absorption of a photon. These dipole matrix elements in the quantum mechanical expression for the rate of FRET are the same matrix elements as found for the interaction of a propagating EM field with the chromophores. However, the origin of the EM perturbation driving the energy transfer and the spectroscopic transitions are quite different. The source of this interaction term... 

Over the course of the last three decades, a variety of techniques have been used to characterize the circuitry of serotonergic neurons in the central nervous system. The density of serotonergic innervation in the forebrain was initially underestimated because the original histo-fluorescence method was limited in sensitivity and did not permit the detection of many fine axons and terminals. 

L. Romaner, A. Pogantsch, P.S. de Freitas, U. Scherf, M. Gaal, E. Zojer, and E.J.W. List, The origin of green emission in polyfluorene-based conjugated polymers on-chain defect fluorescence,... 

Fluorescence intensity increases with the laser fluence, while its change was quite smooth even around the threshold. If a new process leading to ablation was involved in addition to the Si - Si annihilation, the relative fluorescence intensity of two excimers would not change furthermore above the threshold. Although the details are beyond our current knowledge, we conclude that the Si -Si annihilation is the origin of laser ablation in thiB fluence range. 

It is worth giving a brief account of the early stages in the history of fluorescence and phosphorescence (Table 1.2), paying special attention to the origin of these terms. 

We understand now why fluorescence contains the term fluor, but what is the origin of fluorspar or fluorspath and why are these materials fluorescent Spar (in English) and spath (in German) were the names given in the eighteenth century8 to stones that are more or less transparent and crystallized with a lamellar texture. Because these materials can be easily melted, and some of them can help to melt... 

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