Time resolved laser induced luminescence

Using time resolved laser induced luminescence spectroscopy the relative yields of T-T Intra-ET and S-S Intra-ET were determined. Typical examples are shown in Figs. 3-6 where comparison is made between the relative quantum yields of fluorescence and phosphorescence of the o-diketone chromophore upon direct excitation of the a-diketone chromophore at 430 nm to that obtained by exciting the aromatic moiety at 266nm followed by S-S and T-T Intra-ET. The results indicate competition between T-T Intra-ET and S-S Intra-ET leading to a more efficient T-T process whenever S-S Intra-ET becomes less efficient. 

Esperite is a calcium lead zinc-silicate mineral. The crystal structure is mono-clinic-prismatic (P21/m) with a B2 jm group. Steady-state laser-induced luminescence of esperite was ascribed to Mn " in Zn and Ca positions, accompanied by Dy " and Sm " lines. Besides that reabsorption lines of Nd " and have been found (Gorobets and Rogojine 2001). The laser-induced time-resolved... 

Fig.4.59. Laser-induced steady-state (a) and time-resolved (b-d) luminescence spectra of zoisite demonstrating Cr, Eu, Dy and possibly V centers...

We studied laser-induced time-resolved lirminescence of synthetic BaS04 artificially activated by Sn and foimd several intensive UV and blue bands evidently connected with Sn emission (Fig. 5.55). Similar bands have been also found in natural barite laser-induced luminescence spectra and we think that their connection with an Sn center is quite possible. 

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. 

The Applications of Laser-induced Time-resolved Spectroscopic Techniques chapter starts with a short description of laser-induced spectroscopies, which may be used in combination with laser-induced luminescence, namely Breakdown, Raman and Second Harmonic Generation. The chapter contains several examples of the application of laser-based spectroscopies in remote sensing and radiometric sorting of minerals. The proljlem of minerals as geomaterials for radioactive waste storage is also considered. 

Photoderacemization is the simplest case of direct asymmetric photoreactions induced by cpl. The enantiomers are interconverted, and the mixture becomes optically active. Reaction scheme 2 is a modification of Scheme 1 ground state racemization is excluded. The enantiomerization step R S was observed directly by Metcalf et al. [9] by means of the time-resolved circularly polarized luminescence of europium-tris(bipicolinate). By means of a cpl laser pulse, a difference in the excited state population is created, and the decay of circular... 

Fig. 5.80 (a-c) Laser Induced time-resolved excitation and luminescence spectra of Fe in sillimanite... 

The possibilities of time-resolved laser based spectroscopies have been demonstrated, combining such techniques as luminescence, Raman, breakdown and second-harmonic generation. New type of luminescence excitation mechanism. Plasma Induced Luminescence, was found. UV Gated Raman spectroscopy proved to be an effective tool for minerals Raman detection on the strong luminescence background. 

Diffuse reflectance laser flash photolysis and laser-induced luminescence, hoth in time-resolved mode or ground-state absorption spectroscopy in the diffuse reflectance mode, are important techniques that have been used by several research groups to study opaque and crystalline systems [1—8]. These solid-state photochemical methods have been applied by us to study several organic compounds adsorbed onto different hosts such as microcrystalline cellulose [7, 8], p-tertbutylcalix[n]arenes (n = 4, 6, and 8) and their derivatives [10—12], silicalite, cyclodextrins [7, 12, 13], and silica [l4j. 

However, luminescence lifetime, which is a measure of the transition prob-abihty from the emitting level, may be effectively used. It is a characteristic and unique property and it is highly improbable that two different luminescence emissions will have exactly the same decay time. The best way to determine a combination of the spectral and temporal nature of the emission is by using laser-induced time-resolved spectra. The time-resolved technique requires relatively complex and expensive instrumentation, but its scientific... 

Time-resolved luminescence spectroscopy may be extremely effective in minerals, many of which contain a large amount of emission centers simultaneously. With the steady state technique only the mostly intensive centers are detected, while the weaker ones remain unnoticed. Fluorescence in minerals is observed over time range of nanoseconds to milliseconds (Table 1.3) and this property was used in our research. Thus our main improvement is laser-induced time-resolved spectroscopy in the wide spectral range from 270 to 1,500 nm, which enables us to reveal new luminescence centers in minerals previously hidden by more intensive centers. 

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

Fig. 3.4. Laser-induced time-resolved micro-luminescence system...

Fig. 4.2. a-d Laser-induced time-resolved luminescence spectra of apatite demonstrating Pr, and two centers. Besides that Mn " and Dy are detected... 

Fig. 4.3. a-c Laser-induced time-resolved IR luminescence spectra of apatite demonstrating Nd ", Yb " andMn " centers... 

Fig. 4.5. a-c Laser-induced time-resolved luminescence spectra of sedimentary apatite (francolite) (a) and francolite activated by Pr (b,c) after heating demonstrating uranyl and Pr +, Eu + and Sm + centers... 

The fluorite in our study consisted of 40 samples from different environments. Concentrations of luminescence impurities in several samples are given in Table 4.6. By using laser-induced time-resolved spectroscopy we were able to detect and ascribe the following emission centers Eu +, Ce ", Gd +, Sm +, Dy3+, Eu +, Pr +, Er +, Tm +, Ho +, Nd +, Mn + and the M-center (Figs. 4.10-4.12). 

Fig. 4.14. a-f Laser-induced time-resolved luminescence spectra of calcite demonstrating Mn ", Pb " and radiation-induced centers... 

The luminescence centers Eu ", Yb, Ce, Dy, and Sm + characterize the steady-state spectra of danburite (Gaft et al. 1979 Gaft 1989). By using laser-induced time-resolved spectroscopy we were able to detect the following emission centers Ce, Eu, Eu, Sm ", Dy (Fig. 4.15)... 

Two different Mn " luminescence centers have been found in steady-state spectra of apophyllite in the Ca position with orange luminescence peaking at 620 nm and in the K position with green emission peaking at 500 nm (Tarashchan 1978). The apophyllite in our study consisted of three samples from different environments. The laser-induced time-resolved technique enables us to detect the following emission centers Ce ", Mn " " with orange emission and possibly (U02) (Fig. 4.19). 

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