Luminescence parameters

Table II. Luminescence Parameters that can be Combined for Selectivity Enhancement...

Several fundamental luminescence parameters were calculated for the anion of p-aminobenzoic acid on sodium acetate (11). The triplet formation efficiency (( ), the rate constant for phosphorescence... 

Table I. Luminescent Parameters for Photoexcited [Ru(phen)2(dppz)]2+ Bound to Nucleic Acids of Varying Conformations...

A good correlation between luminescence parameters and inhibition of electron transport exists, which indicates that the DCMU-type inhibitors shift the redox potential value of B to a more electronegative value. This may be responsible for inhibition of electron transport. 

The main luminescence parameters traditionally measured are the frequency of maximal intensity Vmax, intensity I, the quantum yield < >, the hfetime of the exited state T, polarization, parameters of Raman spectroscopy, and excited-state energy migration. The usefulness of the fluorescence methods has been greatly enhanced with the development of new experimental techniques such as nano-, pico-, and femtosecond time-resolved spectroscopy, single-molecule detection, confocal microscopy, and two-photon correlation spectroscopy. 

The sensitivity of the luminescence IP s in the systems employed here decreases with increasing x-ray energy more strongly than in the case of x-ray film. Therefore, this phenomenon must be compensated by using thicker lead front and back screens. The specific contrast c,p [1,3] is an appropriate parameter for a comparison between IP s and film, since it may be measured independently of the spatial resolution. Since the absorption coefficient p remains roughly constant for constant tube voltage and the same material, it suffices to measure and compare the scatter ratio k. Fig. 2 shows k as a function of the front and back screen thickness for the IP s for 400 keV and different wall thicknesses. The corresponding measured scatter ratios for x-ray films with 0,1 mm front and back screens of lead are likewise shown. The equivalent value for the front and back screen thicknesses is found from the intersection of the curves for the IP s and the film value. 

The Configuration Coordinate Model. To illustrate how the luminescent center in a phosphor works, a configurational coordinate diagram is used (2) in which the potential energy of the luminescent or activator center is plotted on the vertical axis and the value of a single parameter describing an effective displacement of the ions surrounding the activator, is plotted on the horizontal axis (Fig. 2). At low temperatures, near room... 

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. 

A critical parameter in determining the operating efficiency of polymer LEDs is the luminescence quantum efficiency of singlet excilons in the polymer i.e. the probability that a singlet exciton will decay radiatively. The luminescence quantum elft-... 

Many current multidimensional methods are based on instruments that combine measurements of several luminescence variables and present a multiparameter data set. The challenge of analyzing such complex data has stimulated the application of special mathematical methods (80-85) that are made practical only with the aid of computers. It is to be expected that future analytical strategies will rely heavily on computerized pattern recognition methods (79, 86) applied to libraries of standardized multidimensional spectra, a development that will require that published luminescence spectra be routinely corrected for instrumental artifacts. Warner et al, (84) have discussed the multiparameter nature of luminescence measurements in detail and list fourteen different parameters that can be combined in various combinations for simultaneous measurement, thereby maximizing luminescence selectivity with multidimensional measurements. Table II is adapted from their paper with the inclusion of a few additional parameters. 

Requirements for standards used In macro- and microspectrofluorometry differ, depending on whether they are used for Instrument calibration, standardization, or assessment of method accuracy. Specific examples are given of standards for quantum yield, number of quanta, and decay time, and for calibration of Instrument parameters. Including wavelength, spectral responslvlty (determining correction factors for luminescence spectra), stability, and linearity. Differences In requirements for macro- and micro-standards are considered, and specific materials used for each are compared. Pure compounds and matrix-matched standards are listed for standardization and assessment of method accuracy, and existing Standard Reference Materials are discussed. 

In general, luminescence measurements are relative rather than absolute, since the Instrument characteristics and sample properties that determine the fluorescence Intensities are often not well defined. Absolute luminescence measurements are difficult to perform and require time and Instrumentation not available In most laboratories. Thus, luminescence measurements rely heavily on standards to determine Instrument responses and parameters, the chemical composition of samples, and the characteristics of chemical systems. To... 

Definition and Uses of Standards. In the context of this paper, the term "standard" denotes a well-characterized material for which a physical parameter or concentration of chemical constituent has been determined with a known precision and accuracy. These standards can be used to check or determine (a) instrumental parameters such as wavelength accuracy, detection-system spectral responsivity, and stability (b) the instrument response to specific fluorescent species and (c) the accuracy of measurements made by specific Instruments or measurement procedures (assess whether the analytical measurement process is in statistical control and whether it exhibits bias). Once the luminescence instrumentation has been calibrated, it can be used to measure the luminescence characteristics of chemical systems, including corrected excitation and emission spectra, quantum yields, decay times, emission anisotropies, energy transfer, and, with appropriate standards, the concentrations of chemical constituents in complex S2unples. 

Requirements of Standards. The general requirements for luminescence standards have been discussed extensively (3,7-9) and include stability, purity, no overlap between excitation and emission spectra, no oxygen quenching, and a high, constant qtiantum yield independent of excitation wavelength. Specific system parameters--such as the broad or narrow excitation and emission spectra, isotropic or anisotropic emission, solubility in a specific solvent, stability (standard relative to sample), and concentration--almost require the standard to be in the same chemical and physical environment as the sample. 

The determination of the electronic structure of lanthanide-doped materials and the prediction of the optical properties are not trivial tasks. The standard ligand field models lack predictive power and undergoes parametric uncertainty at low symmetry, while customary computation methods, such as DFT, cannot be used in a routine manner for ligand field on lanthanide accounts. The ligand field density functional theory (LFDFT) algorithm23-30 consists of a customized conduct of nonempirical DFT calculations, extracting reliable parameters that can be used in further numeric experiments, relevant for the prediction in luminescent materials science.31 These series of parameters, which have to be determined in order to analyze the problem of two-open-shell 4f and 5d electrons in lanthanide materials, are as follows. 

Calculation of the Slater-Condon, spin-orbit coupling and ligand field parameters. The luminescence of CsMgBr3 Eu2+ is crucially dependent on the local coordination geometry of the Eu2+ dopant. Besides, a geometry change occurs in the excited state 4f 5d1 (see Table 2), leading to shifts... 

Parameters Radiometric proximity assays (SPA, Flashplate) Fluorescence polarization (FP) Time- resolved fluorescence (HTRF) Amplified luminescence (ALPHAScreen) Enzyme (p-galactosidase) complementation Electrochemilumines cence... 

There is a risk in using circular reasoning when introducing a new overall quality concept (such as the IQC) that contains new aspects and new parameters. It is difficult to introduce an unknown aspect such as integration, to associate it with crop management measures and to measure it with experimental parameters such as luminescence and biocrystallisations. A methodological... 

In order to understand the significance of experimental parameters such as luminescence and biocrystallisation, we assessed them for products grown in controlled conditions. We also correlated the results with as many established parameters as possible (step 7). 

Individual parameters must be validated by the laboratories that produce them. The biocrystallisation method and the luminescence method have been validated for selected crops almost up to steps 5 and 6 (Busscher et al., 2004 Kahl et al, 2003). More correlations with products grown in controlled conditions and evaluations with regard to human health are necessary in order to understand the significance of these methods. 

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