Search

The dependence of emission and excitation spectra of luminescent centres on the nature of the host lattice is discussed and exemplified by the following cases: Eu3+, Eu2+, NbO 6 7- and Pr3+. The quantum efficiency of isolated luminescent centres is reviewed from the point of view of a general approximation and from the point of view of some specific examples (trivalent rare-earth ions; including the effects of charge-transfer and 4fn-15d states). Concentration quenching of luminescence is dealt with starting from a discussion of energy transfer. The concept of energy migration through a host lattice is outlined. Concentration quenching by cross-relaxation in rare-earth ions closes this section. Sensitized luminescence is discussed as a consequence of energy transfer between unlike, luminescent centres. From the foregoing, rules are derived to predict the composition of efficient phosphors. This is illustrated for two special cases in the application field, viz. the Eu3+ ion (red colour television phosphor) and the rare-earth activated phosphors applied in a new generation of fluorescent lamps.

This contract had as target the development of a silicate laser glass with the following properties: (1) fluorescence lifetime greater than LG-660, (2) induced emission cross-section greater than 1.5 x 10/sup -20/ cm/sup 2/, and (3) tank meltability. In order to obtain such a glass, the compositional space between LG-660 and LG-650 was to be investigated. The Nd/sup 3 +/ concentration for a megajoule laser glass will be between 5 and 8 x 10/sup 20/ ions/cm/sup 3/. Since concentration quenching is most severe at higher concentrations, we elected to test all glasses with a concentration of approximately 8 x 10/sup 20/ Nd/sup 3 +/ ions/cm/sup 3/, with the exception of glass which is to be used to test tank meltability (refractory wear). In these glasses all Nd/sub 2/O/sub 3/ was replaced by La/sub 2/O/sub 3/. Besides investigating the compositional space between LG-660 and LG-650, a few excursions were made testing some existing optical glasses for their suitability as a laser glass (melts LLS-4, 5, and 6).

Fundamental fluorescence equations were examined. Deviations from the theoretical were observed by varying the light path length to determine effects on "apparent" fluorescence. It was found that the decrease in emission of morphine solutions with the increase in concentration was caused by absorption effects that prevented excitation of the whole system.

— Quenching of the degradative photobleaching of FMN in anaerobic aqueous solution, and of the flavin-sensitized photo-oxidation of EDTA, alcohols, glycols, and glycerol, has been examined using flash techniques. The quenching can be produced by addition of KI, by successive flashes, and by increasing the flavin concentration (self-quenching). It is concluded that the lowest triplet is the photoreactive species in all systems, that the successive flash effect is due to triplet quenching by a reaction product, and that the concentration quenching may be due to either triplet-triplet annihilation or to reaction between a ground-state flavin and a flavin triplet.

THE luminescence associated with the wetting of finely divided solids has already been reported1–3. With greatly improved arrangements for heating the solid, redistilling the wetting liquid and wetting the solid in the same vacuum, it has now been found that silica from various sources heated in vacuo to 700° C. for a week retains some of its blue luminescence. In dry air or oxygen, heating at 650° C. or more for not less than twenty-four hours completely destroys its luminescent efficiency. Subsequent heating in pure dry hydrogen to 300° C. or above for a few minutes gives again brightly luminescent products the efficiency of which increases to a saturation value with time and temperature of heating. At room temperature hydrogen does not restore the luminescent efficiency destroyed by previous heating in air, so that presumably a chemical reaction is responsible for the effect at higher temperatures. None of the samples heated in hydrogen showed an afterglow of visibly appreciable time of decay except those heated for not more than ten minutes to 300° C. This indicates a concentration quenching of afterglow.

Quantitation of fluorescence intensity at the microscope level can now be accomplished by using one of several highly sensitive and commercially available micro-fluorimeters on the market. However, exact quantitative measurement of fluorescence intensity does not necessarily prove to be a correct quantitation of the fluorescent substance. The results to be discussed below on the microfluorimetric quantitation of biogenic monoamines by the Falck-Hillarp formaldehyde method (Falck et al. 1962; Corrodi and Hillarp 1963? 1964) indicate that careful methodological studies are necessary before a new fluorescence method is used for quantitative purposes.

The preparation and properties of zirconium/barium/sodium fluoride glasses containing high Er(III) contents are discussed in detail. Glasses containing up to 33×1020 Er(III) cm−3 (18 mol%) have 1530 nm fluorescence lifetimes in the region of 10 ms and show no evidence of concentration quenching in their emission at various wavelengths in the infra-red. The lack of substantial concentration quenching was an unexpected result and possible reasons for this are discussed.

Studying ESR spectra, one can conclude that the formation of Mn 2+ ion pairs and clusters is the main factor of the luminescence concentration quenching of ZnS:Mn, Cu. The dependence of luminescence characteristic of the preparation technique of the samples is also discussed.

Concentration quenching of Gd3+ emission occurs by energy migration to quenching sites. The critical distance for Gd3+–Gd3+ transfer amounts to 5–6A, depending on the Gd3+ site symmetry. For larger distances the transfer vanishes rapidly.

The luminescence spectra, decay rates, concentration quenching and sublinearity under cathoderay excitation of La(Al0.5, Ga0.5)O3:Tm is investigated. Experimental values agree with the results of calculations based on the Judd-Ofelt intensity parameters.

Oscillator strengths of Nd3+in Nd x La 1-x P 5 O 14 for 0.01 \leq x \leq 1 are shown to be constant. Consequently, through the whole concentration range Nd3+behaves essentially as one center with respect to transition probabilities. This situation is discussed in relation to concentration quenching.

Electron transfer rates from excited F centers to nearby F centers via bound polarons are calculated using a semicontinuum model in a two-mode coupling. Averaging over the Ffx461-1-F pair separations for a random F center distribution, a concentration dependence is derived of the intrinsic F center photoluminescent efficiency in KC1. It agrees reasonably well with experimental data.

The results of the absorption, infrared, Raman and fluorescence spectra are reported. Owing the analysis of vibrational levels based upon the IR and Raman spectra the assignment of vibronic peaks of fluorescence spectra is proposed. The anomalous concentration dependence of fluorescence decay rates is found. A discussion of the possible mechanism responsible for concentration quenching of Nd 3+ fluorescence in the Cs 2 Na(La,Nd)Cl 6 crystal indicates that at low temperature the non-resonant cross relaxation occurs whereas at room and the anti-Stokes resonant cross relaxation contribute in the Process.

A modification of Forster's equation for the quantum yield of heterogeneous radiationless energy transfer improves its description of homogeneous concentration quenching, at the expense of a new parameter which can be related to the number of steps taken by the excitation in its random walk and which seems to be independent of concentration.

The triplet yield of pyrene in poly(methylmethacrylate) decreases with increasing concentration in the same way as does total fluorescence yield, showing that concentration quenching of total fluorescence is to be attributed to enchanced internal conversion rather than intersystem crossing.

No general theory exists that can predict the efficiency of phosphors with electronic transitions in localized centers. In this paper a survey and discussion are given of the correlation between fluorescent properties and chemical properties, like composition and structure. Points of discussion are (a) choice of the activator together with concentration quenching and critical concentration for this phenomenon, (b) influence of the host lattice on the photoluminescence efficiency for direct excitation of the activator, and (c) energy transfer between fluorescent centers. These considerations are illustrated by a large number of experimental examples.