Luminescence excitation sources

Fig.8.1. Schematic representation of iuminescent sorter 1-funnei 2-vibrational feeder 3-frame 4-conveyer for concentrate 5-conveyer for waste 6-luminescence excitation source 7-collecting optics 8-optical filter 9-detector 10-air valve (Moskrousov and Lileev 1979)... 

Luminescent Pigments. Luminescence is the abihty of matter to emit light after it absorbs energy (see Luminescent materials). Materials that have luminescent properties are known as phosphors, or luminescent pigments. If the light emission ceases shortly after the excitation source is removed (<10 s), the process is fluorescence. The process with longer decay times is referred to as phosphorescence. 

Frequency domain measurements require the use of periodic excitation sources. The luminescent molecules respond to the periodic excitation exhibiting the same frequency of modulation. This luminescence exhibits a phase delay and a demodulation with respect to the excitation due to the inability of the sensor molecule to respond to the higher frequencies of the excitation. This inability of the sensor molecules roughly begins at modulation frequencies /modulation of the same order of magnitude or faster than the decay rate... 

EXAMPLE 1.5 The sensitivity of luminescence. Consider a photoluminescence experiment in which the excitation source provides a power of 100 ptW at a wavelength of400 nm. The phosphor sample can absorb light at this wavelength and emit light with a quantum efficiency of r] = O.I. Assuming that kg = 10 fii.e., only one-thousandth of the emitted light reaches the detector) and a minimum detectable intensity of l(f photons per second, determine the minimum optical density that can be detected by luminescence. 

At the present time luminescent sorters are mainly used for the processing of diamonds, but also for the scheehte, fluorite and others types of ores (Mokrousov and lileev 1979 Salter and Wyatt 1991 Gorobets et al. 1997a). The sources of luminescence excitation in these sorters are X-ray tubes and UV lamps, where the first is more powerful and the second is more selective. The UV impulse lasers employment as excitation sources enables us to combine the... 

Luminescence is the emission of light at RT under the influence of various physical agents as mechanical(tribo-l), electrical (electro-1), radiant (photo-1), thermal (thermo-1), or chemical (chemo-1) means. The exciting source also may consist of moving charged particles, such as alpha-, beta-, or gamma-. Certain substances luminesce on crystallization, as,... 

Figure 25. Excitation and photoluminescence (solid and dashed lines) of x% Mn2+ CdS nanocrystals, where. v = 0 (a), 0.8 (fe), 2.5 (c), and 4.8 (d). The solid luminescence spectra were collected in CW mode, and the dashed luminescence spectra were collected with a pulsed excitation source and a 2-ms delay between excitation and emission detection. Note that the intensities of (b)-(d) are referenced to that of (a). [Adapted from (82).]...

In luminescence studies it can often be observed that intensities decrease with increasing pressure. A decreasing luminescence intensity can be ascribed to two main effects. On one hand, the excitation efficiency can decrease due to a pressure-induced shift of absorption bands away from a fixed excitation energy. This effect can be minimized either by a tunable excitation source or by exciting into a band, whose shift is negligible compared to its width. 

Many minerals can be made to luminesce under various excitation sources, usually UV light, but in relatively few cases is the mechanism understood in detail. Best understood is luminescence due to transitions between localized states in the unfilled d-orbitals of transition metal ions and localized states in the unfilled f-orbitals of rare earth ions. Rare earth ions, important in the development of synthetic phosphor and laser materials, are uncommon among naturally occurring minerals. 

The emission spectra are similar but often not identical to those excited by UV (18). The energy source is the recombination of free radicals that occurs in the flame and thus flame-excited luminescence is the same as the radical recombination luminescence observed when free neutral radicals from plasmas are used as an excitation source. A simple hydrogen diffusion flame is the simplest source for demonstrating the phenomenon. 

The lunar transient events could be excited by protons in the solar wind but experiments with silicate minerals in proton beams show that the process is inefficient, quantum efficiencies from lxl0 4 to 1x10 , and given the concentration of protons in the solar wind the mechanism cannot account for the intensity of the observed luminescence (33). Another possibility is that neutral particles in the background solar wind or associated with disturbances on the sunfs surface provide the excitation source (34). This would be a process very similar if not identical to the candoluminescence and radical recombination luminescence observed in the laboratory. 

Photoluminescence is the radiation emitted by the recombination process and as such is a direct measure of the radiative transition. Information about non-radiative recombination can often be inferred from the luminescence intensity, which is reduced by the competing processes (Street 1981a). The most useful feature of the luminescence experiment is the ability to measure the emission spectrum to obtain information about the energy levels of the recombination centers. The transition rates are found by measuring the transient response of the luminescence intensity using a pulsed excitation source. Time resolution to about 10 s is relatively easy to obtain and is about the maximum radiative recombination rate. The actual recombination times of a-Si H extend over a wide range, from 10 s up to at least 10- s. 

The quantiun yield of luminol is relatively low and developments now in progress use the luminescent light source to excite a secondary fluorescent molecule. Luminescence, like fluorescence, is susceptible to quenching by a vM iety of drugs and endogenous compounds. 

Luminescent properties of samples were evaluated on a SPEX Fl-2 spectrofluorometer with a xenon arc lamp as excitation source at room temperature. 

H NMR spectra were recorded on a Bruker AC 400 spectrometer with tetramethylsilane (TMS) as internal reference. Infrared spectra were obtained as KBr pellets on a Perkin-Elmer 580 B FT-IR spectrometer. Electrospray (ES) mass spectra were recorded on an LCQ Finnigan Mat spectrometer. The excitation and emission spectra were obtained on a SPEX FL-2T2 spectrofluorimeter with slit at 0.8 mm and equipped with a 450 W lamp as the excitation source. Luminescence lifetimes were measured with a SPEX 1934D phosphorimeter using a 7 W xenon lamp as the excitation source with the pulse width at 3 ps. Powder X-ray diffraction patterns were recorded on Rigaku D/Max-IIB diffractometer using Cu-Ka radiation. 

If the researcher has commercial molecular luminescence instrumentation (e.g., a spectrofluorometer) available, then solid-state luminescence data should not be difficult to obtain. Many good references are available discussing the basic theory of luminescence, " so the focus herein will be on its use in solid-state applications. Instrumentation normally consists of an excitation source, excitation wavelength selector, sample compartment, emission wavelength selector, and detector. The largest issue for conducting measurements on... 

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