Fluorescence process, characteristics

Triplet-triplet annihilation In concentrated solutions, a collision between two molecules in the Ti state can provide enough energy to allow one of them to return to the Si state. Such a triplet-triplet annihilation thus leads to a delayed fluorescence emission (also called delayed fluorescence of P-type because it was observed for the first time with pyrene). The decay time constant of the delayed fluorescence process is half the lifetime of the triplet state in dilute solution, and the intensity has a characteristic quadratic dependence with excitation light intensity. 

Due to the short lifetime of the fluorescence decay process (10 — 10 s) time-resolved fluorescence studies provide a number of experimental difficulties and require the use of more sophisticated apparatus. Several tedini(pes have been applied to measure fluorescence decay characteristics but all require the use of a pulsed or modulated excitation source (see reviews by Birks, Ware, Knight and Selinger ). 

In this review we ha outlined experimental methods available for the study of the decay characteristic of fluorescence on a nanosecond and sub-nanosecond time-scale. These time-resoli d methods have been available for a comparative ort period of time, and are improving with the provision of superior light sources. The time-scale referred to above is one which coincides with that for various molecular motions and other processes in macromolecular systems, of both synthetic and biological origin. This permits the use of fluorescence decay characteristics as a probe of such events. It is hoped that in reviewing the applications of these methods to macromolecular systems we will have stimulated the interest of polymer chemists in their use, notwithstanding the difficulties in interpretation of results which has been emphasised. 

The marked excess-energy dependence of the fluorescence-decay characteristics of tetracene vapour has again been interpreted in terms of the Sx - S0 internal-conversion process being enhanced by population of higher vibrational levels of the Sx state.67 The process has been observed directly in the case of pentacene by the recognition of transient absorption bands ascribed to hot SQ levels produced in the internal conversion.68 Non-linear absorption in the laser photolysis of anthracene,69 the intramolecular photocycloaddition reaction of l-(9-anthryl)-3-(l-naphthyl)propane,80 and the quenching of fluorescence of aromatic hydrocarbons by caesium chloride81 have been reported. 

When a sample is placed in abeam of X-rays, some of the X-rays are absorbed. The absorbing atoms become excited and emit X-rays of characteristic wavelength. This process is called X-ray fluorescence. Since the wavelength (energy) of the fluorescence is characteristic of the element being excited, measurement of this wavelength enables us to identify the fluorescing... 

The x-ray fluorescence spectrometer consists of three main parts the excitation source, the specimen presentation apparatus, and the x-ray spectrometer. The function of the excitation source is to excite the characteristic x-rays in the specimen via the x-ray fluorescence process. The specimen presentation apparatus holds the specimen in a precisely defined position during analysis and provides for introduction and removal of the specimen from the excitation position. The x-ray spectrometer is responsible for separating and counting the x-rays of various wavelengths or energies emitted by the specimen. In this book the term x-ray spectrometer denotes the collection of components used to disperse, detect, count, and display the spectrum of x-ray photons emitted by the specimen. When referring to the entire instrument, including excitation source, sample presentation apparatus, and x-ray spectrometer, the term x-ray fluorescence spectrometer will be used. In this latter sense the term x-ray fluorescence analyzer is sometimes encountered in the literature. 

Fluorescence The characteristic of a material to produce Hght when excited by an external energy source. Minimal or no heat results from the process. 

Contaminated groundwaters might be expected to comprise the same fluorescence EEM characteristics of the pollutant source that has undergone dilution and chemical and microbial processing in the aquifer. Almost certainly, a wider range of contaminant fluorescent signatures exist than those reported. The two reported contaminant fluorescence organic matter characteristics are ... 

Subsequent studies (63,64) suggested that the nature of the chemical activation process was a one-electron oxidation of the fluorescer by (27) followed by decomposition of the dioxetanedione radical anion to a carbon dioxide radical anion. Back electron transfer to the radical cation of the fluorescer produced the excited state which emitted the luminescence characteristic of the fluorescent state of the emitter. The chemical activation mechanism was patterned after the CIEEL mechanism proposed for dioxetanones and dioxetanes discussed earher (65). Additional support for the CIEEL mechanism, was furnished by demonstration (66) that a linear correlation existed between the singlet excitation energy of the fluorescer and the chemiluminescence intensity which had been shown earher with dimethyl dioxetanone (67). 

The requited characteristics of dyes used as passive mode-locking agents and as active laser media differ in essential ways. For passive mode-locking dyes, short excited-state relaxation times ate needed dyes of this kind ate characterized by low fluorescence quantum efficiencies caused by the highly probable nonradiant processes. On the other hand, the polymethines to be appHed as active laser media ate supposed to have much higher quantum efficiencies, approximating a value of one (91). 

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. 

Functions of Standards. Fluorescent standards can be used for three basic functions calibration, standardization, and measurement method assessment. In calibration, the standard is used to check or calibrate Instrument characteristics and perturbations on true spectra. For standardization, standards are used to determine the function that relates chemical concentration to Instrument response. This latter use has been expanded from pure materials to quite complex standards that are carried through the total chemical measurement process (10). These more complex standards are now used to assess the precision and accuracy of measurement procedures. 

The derivatization process (5) is accomplished in aqueous media at basic pH (pH 7-10) in a matter of approximately 15 min to yield a 2-cyanobenz[f]isoindole (CBI), which is stable for 10 to 12 hr in solution. As shown in Figure 1, the absorption characteristics of the CBI adducts are also readily accessible for assay by standard fluorescence or ultraviolet detection. In addition to the absorption between 200 and 300 nm, there are two maxima in the visible spectrum at approximately 420 and 440 nm accessible for fluorescence or ultraviolet detection. A probable mechanism (5,11) for the CBI formation is illustrated in Scheme 1. 

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