Luminescence Cathodoluminescence

The book deals mainly with theoretical approach, experimental results and their interpretation of laser-induced time-resolved spectroscopy of minerals in the wide spectral range from 250 to 2000 nm, which enables to reveal new luminescence previously hidden by more intensive centers. Artificial activation by potential luminescence centers has been accomplished in many cases, which makes the sure identification possible. The mostly striking example is mineral apatite, which has been extremely well studied by many scientists using practically all known varieties of steady-state luminescence spectroscopy photoluminescence with lamp and laser excitations. X-ray excited luminescence, cathodoluminescence, ionolumi-nescence and thermoluminescence. Nevertheless, time-resolved spectroscopy revealed that approximately 50 % of luminescence information remained hidden. The mostly important new information is connected with luminescence of trivalent... 

Cathodoluminescence (CL), i.e., the emission of light as the result of electron-beam bombardment, was first reported in the middle of the nineteenth century in experiments in evacuated glass tubes. The tubes were found to emit light when an electron beam (cathode ray) struck the glass, and subsequendy this phenomenon led to the discovery of the electron. Currendy, cathodoluminescence is widely used in cathode-ray tube-based (CRT) instruments (e.g., oscilloscopes, television and computer terminals) and in electron microscope fluorescent screens. With the developments of electron microscopy techniques (see the articles on SEM, STEM and TEM) in the last several decades, CL microscopy and spectroscopy have emerged as powerfirl tools for the microcharacterization of the electronic propenies of luminescent materials, attaining spatial resolutions on the order of 1 pm and less. Major applications of CL analysis techniques include ... 

Spatial information about a system can be obtained by analyzing the spatial distribution of PL intensity. Fluorescent tracers may be used to image chemical uptake in biological systems. Luminescence profiles have proven useftil in the semiconductor industry for mapping impurity distributions, dislocadons, or structural homogeneity in substrate wafers or epilayers. Similar spatial infbrmadon over small regions is obtained by cathodoluminescence imaging. 

Cathodoluminescence is the luminescence observed upon excitation by high-energy electrons. 

A luminescent mineral is a sohd, which converts certain types of energy into electromagnetic radiation over and above thermal radiation. The electromagnetic radiation emitted by a luminescent mineral is usually in the visible range, but can also occur in the ultraviolet (UV) or infrared (IR) range. It is possible to excite the luminescence of minerals by UV and visible radiation (photoluminescence), by a beam of energetic electrons (cathodoluminescence), by X-rays (X-ray excited luminescence) and so on. A special case is so-called thermoluminescence, which is stimulation by the heating of luminescence, prehminary excited in a different way. 

Fig. 3.3. a-h Cathodoluminescence (a) and laser-induced time-resolved luminescence (b-h) spectra of the same apatite sample... 

A possible candidate may be Tm ". For example, the doublets at 803 and 817 nm and at 796 and 813 nm are the strongest ones in cathodoluminescence spectra of fluorite and scheelite activated by Tm " (Blank et al. 2000). It is possible to suppose that the strong fines at 805 and 820 nm with a relatively short decay time of 60 ps in the titanite luminescence spectrum belong to Tm " ". They appear under 532 nm excitation and are evidently connected with the electron transition. Similar emission of Tm " was also detected in... 

The cathodoluminescence of thin films containing rare-earth oxides was studied by Hansen and Myers (140). Films of yttrium oxide doped with rare earths were prepared in vacuum by electron-bombardment evaporation of the oxide powder mixtures. Luminescent rise and decay curves were obtained for activation with europium, gadolinium, terbium, and dysprosium. 

Helderberg carbonates are extensively cemented by clear equant calcite cement that postdates marine cement and fills most of the original pores. These cements are compositionally zoned as shown by cathodoluminescence and trace element analyses. These precipitates include nonluminescent, bright and dull cements. The luminescent cement is the earliest generation of cement followed by... 

By far the most important activators in mineral luminescence are the iron group ions which exhibit transitions between partly filled d-orbitals. These will dominate the discussion that follows. Luminescence arising from the trivalent rare earth ions occurs in some phosphate minerals but is dealt with elsewhere in this volume (Wright). The filled d-shell ions are activators for cathodoluminescence phosphors such as ZnS, however, most sulfide mineral phases contain too many luminescence poisons for the transitions from these ions to be observed. 

Energy Transfer Processes. Luminescence is a complex sequence of energy transfer processes. Figure 2 is a schematic of the most important of these for photoluminescence, cathodoluminescence, and candoluminescence. The ultimate source of energy is the excitation UV light, electron beam, ion beam, or radical recombination excitation. We are not concerned here about the triggered release of previously trapped energy such as occurs in thermoluminescence and triboluminescence. 

Excitation by impinging electrons (or ions) takes place by a somewhat different mechanism. Energetic electrons penetrate the phosphor grains and the lose energy by multiple collision processes. The energy is sufficient to pump the conduction band of the phosphor host and the excitation can move anywhere in the crystal to pump the localized luminescence centers. Cathodoluminescence thus appears at lower concentration thresholds than photoluminescence but is more susceptible to the influence of defects and other luminescence poisons. 

Figure 4. Photomosaics of the cathodoluminescence of three low petrologic type meteorites (a) Semarkona (type 3.0, top), (b) Bishunpur (type 3.1, middle), and (c) Krymka (type 3.1, bottom). The scale bar shown refers to Semarkona, the scale bar for the others is the same as in Figure 5. In general, red luminescence is produced by Fe-free olivine and pyroxene, and blue and yellow CL is produced by chondrule mesostases of plagioclase composition.

An efficiency of the cathodoluminescent light source depends directly from the efficiencies of its basic components an electron gun and luminescent covering. The diminishing of the power consumption and the increasing of the cathodoluminescent lamps efficiency are provided with application of field emission cathodes made of carbon fibers. 

The electron-optical system of the cathodoluminescent lamp consists of electron gun and luminescent screen, which is covered by phosphor. It represents the triode construction. The base of this triode is the cathode-modulator unit (CMU) that consists of field emission cathode and extraction electrode (modulator). 

FIGURE 6 Cathodoluminescence spectra obtained in the defective area of the crystal which appeared yellow in an optical microscope (a) and in the transparent area of the crystal (b). Note changes in the intensities of the yellow luminescence peak (2.4 eV) and band to band luminescence (3.5 eV). 

Other groups reported the solution synthesis of a cerium-doped inorganic phosphor library (73) to optimize their structure as luminescence down-converters in white Light Emitting Diodes (LEDs) and the use of cathodoluminescence (CL) (74) to identify... 

Houseknecht D. W. (1991) Use of cathodoluminescence petrography for understanding compaction, quartz cementation, and porosity in sandstones. In Luminescence Microscopy and Spectroscopy Quantitative and Qualitative Applications (eds. C. E. Barker and O. C. Kopp). Society for Sedimentary Geology (SEPM), Tulsa, OK, Short Course vol. 25, pp. 59-75. 

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