Luminescence interpretation

Determination of tint is the essential and the most intricate aspect in diamonds evaluation and may be useful for luminescence interpretation. The coloration of diamond is a reflection of its complex structural pecuharity. To describe a slight shade of color, one has to use a lot of physics, crystallography and analytical tools. The following types of diamond coloration are generally distinguished. 

As mentioned above, the interpretation of CL cannot be unified under a simple law, and one of the fundamental difficulties involved in luminescence analysis is the lack of information on the competing nonradiative processes present in the material. In addition, the influence of defects, the surface, and various external perturbations (such as temperature, electric field, and stress) have to be taken into account in quantitative CL analysis. All these make the quantification of CL intensities difficult. Correlations between dopant concentrations and such band-shape parameters as the peak energy and the half-width of the CL emission currently are more reliable as means for the quantitative analysis of the carrier concentration. 

Emission spectra have been recorded for four aryl-substituted isoindoles rmder conditions of electrochemical stimulation. Electrochemiluminescence, which was easily visible in daylight, was measured at a concentration of 2-10 mM of emitter in V jV-dimethylformamide with platinum electrodes. Emission spectra due to electrochemi-luminescence and to fluorescence were found to be identical, and quantum yields for fluorescence were obtained by irradiation with a calibrated Hght source. Values are given in Table X. As with peak potentials determined by cyclic voltammetry, the results of luminescence studies are interpreted in terms of radical ion intermediates. ... 

The role of disorder in the photophysics of conjugated polymers has been extensively described by the work carried out in Marburg by H. Bassler and coworkers. Based on ultrafast photoluminescence (PL) (15], field-induced luminescence quenching [16J and site-selective PL excitation [17], a model for excited state thermalizalion was proposed, which considers interchain exciton migration within the inhomogenously broadened density of states. We will base part of the interpretation of our results in m-LPPP on this model, which will be discussed in some detail in Sections 8.4 and 8.6. 

In our report on the bioluminescence of Meganyctiphanes (Shimomura and Johnson, 1967), the extremely unstable nature of the substance P caused us to interpret the functions of P and F incorrectly, the former as a photoprotein and the latter as a catalyst, as pointed out by Hastings (1968). The error was corrected 28 years later (Shimomura, 1995a), F being unambiguously shown to be a luciferin and P, a luciferase, on the basis that the quantum yield of F is about 0.6 at 0°C, while P can be recycled many times in the luminescence reaction. 

According to Reichl et al. (2000), the oxidation of pholasin by compound I or II of horseradish peroxidase induces an intense light emission, whereas native horseradish peroxidase shows only a small effect. The luminescence of pholasin by native myeloperoxidase (verdoperoxidase) is diminished by preincubation with catalase, which is interpreted as the result of the removal of a trace amount of naturally occurring H2O2 in the buffer (about 10-8 M) that forms compound I... 

Frequently, electrochemical information can be interpreted better in the presence of additional nonelectrochemical information. Typically, however, there is one significant restriction electrochemical and spectroscopic techniques often do not detect exactly the same mechanisms. With spectroscopic measurements (e.g., infrared spectroscopy), products that are formed by electrochemical processes may be detected. In other cases (luminescence techniques) mechanisms may be found by which charge carriers are trapped and recombine. Other techniques (electroreflection studies) allow the nature of electronic transitions to be determined and provide information on the presence or absence of an electric field in the surface of an electrode. With no traditional technique, however, is it... 

The immense growth in the luminescence literature during the period between these two reviews had little to do with developments in fundamental theory. It was mainly due to the availability of new instrumentation, such as the photomultiplier (around 1950), the laser (around 1960), transistor and microcircuit electronics (around 1970), and ready access to laboratory computers (around 1975). All aspects of luminescence theory now being used to interpret luminescence measurements have been known since the early 1900 s and nearly all of the types of measurements now being made had been initiated with cruder techniques by 1930. We discuss here many of the latest techniques in luminescence analysis with selected highlights from the historical development of luminescence and a look at several recent developments in luminescence applications that appear likely to be important to future research. 

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. 

In 1935, after studying the luminescence of various colorants, Jablonski suggested the electronic energy diagram of the singlet and triplet states to explain the luminescence processes of excitation and emission. The proposed diagram of molecular electronic energy levels formed the basis of the theoretical interpretation of all luminescent phenomena [21],... 

An alternative path includes oxidation, in the absence of light, of the diazaquinone with weak chemiluminescence (154). The effect of iron(II) on the luminescent intensity was interpreted by considering that it can efficiently generate the O - radical in a reaction with 02 and, as a consequence, increase the importance of reaction (108) in the overall process (155). 

Recently, the time-dependence kinetics of such phenomena as luminescence has been interpreted in terms of detrapping of species (43). An alternative treatment Involves the migration of an excited particle In a lattice (44). 

A first generation poly(amido amine) dendrimer has been functionalized with three calyx[4]arenes, each carrying a pyrene fluorophore (4) [30]. In acetonitrile solution the emission spectrum shows both the monomer and the excimer emission band, typical of the pyrene chromophore. Upon addition of Al3+ as perchlorate salt, a decrease in the excimer emission and a consequent revival of the monomer emission is observed. This can be interpreted as a change in the dendrimer structure and flexibility upon metal ion complexation that inhibits close proximity of pyrenyl units, thus decreasing the excimer formation probability. 1H NMR studies of dendrimer 4 revealed marked differences upon Al3+ addition only in the chemical shifts of the CH2 protons linked to the central amine group, demonstrating that the metal ion is coordinated by the dendrimer core. MALDI-TOF experiments gave evidence of a 1 1 complex. Similar results have been obtained for In3+, while other cations such as Ag+, Cd2+, and Zn2+ do not affect the luminescence properties of... 

Anodization of Si in HF under an applied magnetic field produces an enhancement of the PL efficiency at RT, accompanied by an enhanced porosity compared to PS samples prepared without an applied field. The degree of polarization of the emitted PL is reduced for field-assisted preparation [Na3]. At low temperatures (4.2 K), the Stokes shift and the decay time of the PL are found to be increased, if compared to PS formed under zero magnetic field. This has been interpreted as Zeeman splitting of the spin-triplet exciton states. It indicates that the ground state of the luminescing silicon crystallite is a triplet state [Kol3]. 

An IR study of electro-oxidized PS showed a decrease in the OH signal and an increase in the SiO signal with anodization time. This can be interpreted as oxide formation on the PS surface and a removal of electrolyte from the pores. Furthermore a correlation in intensity of localized carrier IR absorption and luminescence indicates that localized states are involved in the red EL [Du4]. 

The surface-state model, in which the luminescent recombination occurs via surface states, was proposed to explain certain properties of the PL from PS, for example long decay times or sensitivity of the PL on chemical environment. In the frame of this model the long decay times are a consequence of trapping of free carriers in localized states a few hundred meV below the bandgap of the confined crystallite. The sensitivity of the PL to the chemical environment is interpreted as formation of a trap or change of a trap level by a molecule bonding to the surface of a PS crystallite. The surface-state model suffers from the fact that most known traps, e.g. the Pb center, quench the PL [Me9], while the kinds of surface state proposed to cause the PL could not be identified. 

The visible luminescence from PS has been ascribed to QC effects [Cal]. This interpretation is supported by the observation of a similar luminescence from silicon clusters in the nanometer regime or large silicon molecules that are not produced by anodization but by alternative methods. 

Fitting luminescence decay data to sums of exponentials, even with rather good statistics, can present very serious problems in data fitting and in uniqueness of the solutions. This difficulty can severely cloud interpretation of data from... 

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