Characteristics luminescence

Another consequence of acceptor neutralization is the disappearance of excitons bound to acceptors. Their characteristic luminescence can be restored by the thermal release of the hydrogen. 

At low temperatures, donors and acceptors remain neutral when they trap an electron hole pair, forming a bound exciton. Bound exciton recombination emits a characteristic luminescence peak, the energy of which is so specific that it can be used to identify the impurities present. Thewalt et al. (1985) measured the luminescence spectrum of Si samples doped by implantation with B, P, In, and T1 before and after hydrogenation. Ion implantation places the acceptors in a well-controlled thin layer that can be rapidly permeated by atomic hydrogen. In contrast, to observe acceptor neutralization by luminescence in bulk-doped Si would require long Hj treatment, since photoluminescence probes deeply below the surface due to the long diffusion length of electrons, holes, and free excitons. 

Catalytic effects of certain substances and characteristic luminescences at various surfaces have been observed. 

The steady-state luminescence of water-organic complexes is strong and conceals the weaker characteristic luminescence of uranium containing centers, which can be detected by the difference in decay times only. The reason is that the decay time of water-organic complexes is characterized by two time intervals less then 30 ns and more then 10 ms. Since the uranium centers have decay times in the microseconds range, it is possible to detect them by time-resolved spectroscopy. In the time-delayed laser-induced spectroscopy, the luminescence spectra are recorded at a fixed moment after a laser pulse. These spectra maybe different from the integrated steady-state ones since after a certain time short luminescence will be practically absent. 

Our study of sedimentary apatite from Israel proved that laser-induced time-resolved luminescence is a perspective tool for evaluation of sedimentary phosphate ores with high dolomite content (Gaft et al. 1993b). The idea was based on the fact that natural apatite contains several characteristic luminescence centers, which enables us to differentiate it from dolomite. The most widespread characteristic luminescence center in sedimentary apatite is uranyl (U02) with a typical vibrational green band luminescence under nitrogen laser excitation (Fig. 8.13a,b). Nevertheless, it appears that such luminescence is absent in phosphate rock samples from Florida, evidently because of extremely low uranium concentration (Fig. 8.13c,d). hi order to find potential liuninescence centers, ICP-MS analyses of Florida phosphates was accompHshed. From discovered REE, theoretically Dy + is the best candidate... 

The high sensitivity and specificity of photoluminescence analysis should make it possible to individualize clue materials, e.g., hair and glass, by the characteristic luminescence properties of trace constituents or impurities. Of particular significance are the newer techniques of analyzing the luminescence decay curves. For example, even when the absorption and luminescence spectra of the impurities are similar, it is possible to determine their concentrations if their luminescence lifetimes differ. The usefulness of this technique is illustrated in Figs. 1 and 2, where it is shown that the fluorescence spectra of naphthalene (N) and 1,6-dimethyl napthalene (DMN) are too similar for fluorescence spectral analysis of their mixtures (Fig. l) yet their relative concentrations can be readily determined from the fluorescence decay curve (Fig. 2). As indicated by the dashed curve in Fig. 2, the observed decay is the sum of exponential decays from a shorter lived component, i.e., DMN (lifetime 50 nsec) and a longer lived component, i.e., N (lifetime 100 nsec). St. John and Winefordner (j) have discussed this technique in general and Hoerman and co-workers (8,9) have been... 

Meulenkamo, E.A., Nelly, J.J., and Blasse, G. 1993. Electrochemically induced characteristic luminescence of metal ions at anodic valve metal oxides. Journal of the Electrochemical Society 140, 84-91. 

The species [Re(2,2 -bipyridine)(3-ethynylpyridine)(CO)3](BF4) exhibits the characteristic luminescent properties and moderate cytotoxicity of this general class of compound. By attaching this species to a lipidated peptide known to increase cell permeability, enhanced cellular uptake of Re-myr-Tat (myr-Tat stands for myristoylated HIV-1 Tat peptide) compared with the model compound has been found. Moreover, cytotoxicity studies showed an increase in potency to a level comparable with cisplatin." ... 

As a result of the host-guest complexes formed by CDs with a variety of lumophores, characteristic luminescence properties of the included lumophore, sensitive to viscosity/polarity or dielectric, may change. In fact, the inclusion of a lumophore in a CD cavity has proved to provide a means of triplet-state stabilization, luminescence intensity enhancement, optical activity induction, improved solubility of poorly soluble analytes in water, etc. 

The reaction forms a characteristic luminescence due to an electronically excited NO2 molecule decaying to a lower energy state. The intensity of the luminescence is proportional to the concentration of NO. The light emission is detected by a photomultiplier tube, which generates a proportional electronic signal. 

It is also important to recognize the characteristic luminescence of each lanthanide ion so that the presence of unintentional impurities in a crystal can be recognized. This is generally easy because the transitions to different lower multi-plet levels provide a distinctive fingerprint. However, some reports have attributed unusual emission bands from trace lanthanide ion impurities to exotic phenomena. 

LIBS additional advantage is that it is a laser-based technique, therefore easily combined with other laser spectroscopy techniques, such as time-resolved luminescence and gated Raman. Specifically for the mining industry, such spectroscopic combination would enable analyses of both elements and minerals with characteristic luminescence or Raman signals, while PGNAA and XRF can only analyze elements. 

Our study of sedimentary apatite from Israel proved that laser-induced time-resolved luminescence is a perspective tool for evaluation of sedimentary phosphate ores with high dolomite content (Gaft et al. 1993b). The idea was based on the fact that natural apatite contains several characteristic luminescence centers, which... 

The physical processes involved in the phenomenon of characteristic luminescence are presented schematically in fig. 34.1. The figure shows part of a crystal M in which two kinds of foreign ions or ionic groups (centres) are incorporated. One centre of each type is shown, marked A and S. We assume that the host lattice absorbs no radiation. The centre in the right half is raised to an excited state as a result of radiation absorbed by that centre. The centre returns to the ground state by giving up the excitation energy as radiation or as heat. The former case is referred to as luminescence, and the centre involved is called an activator. 

In what follows, the role of activator is played by one of the ions of the rare earth metals (R ions). Research on these phosphors in particular has considerably advanced the understanding of characteristic luminescence, since the properties of these phosphors can be studied on simple model compounds. This is possible because of the similarity between these ions. The host lattice may be, for instance, a compound of the ions La , or Lu . The latter ions do not absorb ultraviolet radiation. Rare earth ions, for example Eu or Tb , are now substituted for a small proportion of the host lattice ions. These R ions occupy in the host lattice the crystallographic sites of La, Y or Lu in a virtually random distribution. It is possible in this way to make phosphors whose chemical constitution is well defined. 

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