Luminescence excitation bands

Table 2.2. Main luminescence excitation bands and lines in minerals ...

Table 41.1 Luminescence excitation bands and emission lines in elements and minerals.

Table 11. Position of the first strong excitation bands of the luminescence of the WO and the UO octahedra in several host lattices. Values in kK...

The excitation spectrum demonstrates that for an effective luminescence not only the presence of an emitting level is important, but also the presence of the upper levels with a sufficiently intensive absorption. The excitation spectra enable us to choose the most effective wavelength for luminescence observation. The combination of excitation and optical spectroscopies enable us to determine the full pattern of the center s excited levels, which may be crucial for luminescence center interpretation, energy migration investigation and so on. The main excitation bands and fines of luminescence in minerals are presented in Table 2.2. 

Two types of Ce centers in calcite were detected by steady-state spectroscopy (Kasyanenko and Matveeva 1987). The first one has two bands at 340 and 370 nm and is connected with electron-hole pair Ce -COj". The second one has a maximum at 380 nm and was ascribed to a complex center with Ce and OH or H2O as charge compensators. Such a center becomes stronger after ionizing irradiation and disappears after thermal treatment. The typical example of Ce luminescence in the time-resolved liuninescence of calcite consists of a narrow band at 357 nm with very short decay time of 30 ns, which is very characteristic for Ce " (Fig. 4.13a). It was found that Ce " excitation bands occurs also in the Mn " " excitation spectrum, demonstrating that energy transfer from Ce to Mn " occurs (Blasse and Aguilar 1984). 

IR luminescence detected in ZrSi04-Cr has an excitation band peaking at 920 nm. Its luminescence spectrum at 300 K (Fig. 5.38) is characterized by a relatively unresolved broad band peaking at 1,200 nm. It is very similar to Cr luminescence in silicates, especially in forsterite except for a very short decay time shorter than the time resolution of our detection system about 200 ns. It is not suitable for Cr with a much longer decay in the ps range (Boulon 1997). Luminescence at lower temperatures is much more intensive and spectra are characterized by several strong narrow hnes with very short decay which appear already at 100 K. Once again, it is rather unusual for Cr +. 

Tarashchan (1978) already ascribed the UV liuninescence band at 325 nm with an excitation band at 237 nm in pink calcite to Pb +. A decay time of 120 ns measured in our sample is consistent with such interpretation. Such a UV band was unique in the calcite collection at our disposal and ICP-MS analyses of its impurities have been done (Table 4.7). It was found that Pb concentration in Franklin, NJ calcite of 450 ppm is approximately 50 times higher than in pink calcite, taken for comparison sake, while its Ce content is more than 10 times lower. Those data confirm the connection of the UV band at 325 nm with the Pb luminescence center. 

The emission and excitation peaks occur at 251 and 347 nm, respectively with a Stokes shift of 10,000 cm It is very close to luminescence and excitation bands detected in natural samples. In order to prove the possible relation of the UV luminescence band at 355 nm to Pb in natural hardystonite, its decay time as a function of temperature has been studied. These decay curves are very specific for mercury-like ions, where the emission at low temperatures is ascribed to the forbidden transition and has a long decay... 

Figure 4.39a represents the liuninescence band detected in all investigated natural zircons. It has an excitation band peaking at 310 nm. The broad yellow band at 575 nm with a decay time of 25-35 ps represents classical zircon luminescence. The picture is not principally changed with different delay times and gate width. After heating the sample up to 700 °C the luminescence intensity is nearly the same, but after that it is strongly reduced and at 800 °C the yellow luminescence disappears. Luminescence spectra at 77 and 4.2 K are not substantially different. 

Fig. 5.71. a-f Unidentified emission center in apatite laser-induced time-resolved luminescence spectra of apatite, a Steady-state luminescence spectrum b Time-resolved spectrmn with narrow gate where yellow band with short decay time dominates c-d Time-resolved spectra after heating at 800 °C e-f Excitation bands of Mn and short-lived yellow band, correspondingly... 

According to spectral-kinetic parameters, the optimal conditions of luminescence excitation and detection, so called selection window (SW) parameters, were calculated in the following way. At optimal for the useful component excitation, the liuninescence spectra, decay time and intensity were determined for this mineral and for the host rock. After that, on the personal computer was calculated the proportion between useful and background signals for the full spectral region for each 50 ns after laser impulse. For calculation the spectral band was simulated by the normal distribution and the decay curve by the mono-exponential function. The useful intensity was multiplied by the weight coefficient, which corresponds to the concentration at which this component must be detected. 

Fig. 14. Origin region of the low temperature excitation (luminescence monitored broad band below 20 200 cm 1) and luminescence (excitation at 457.9 nm with an Ar laser) spectra of [lr(ppy)2bpy]PF5 in [Rh(ppy)2bpy] PF6. M, C and D label electronic origins (from Ref. [45])...

The intensity of the UV-excited band simply falls off with increasing temperature. The hydrogen radical recombination luminescence spectrum peaks at 40 C and then falls off rapidly. Nitrogen and water vapor radical recombination luminescence peaks near 320 C while the candoluminescent spectrum peaks at 178 C. 

In addition, the adsorption of quencher molecules, such as O2 and H2, on AEOs has allowed the clarification of the nature of the luminescence sites present at the surface [81]. In particular, in the case of H2 adsorption on SrO, a change in the excitation band shape was observed. H2 may react at different rates with species absorbing in different parts of the excitation band, producing a change in the band shape. No corresponding change in band shape was observed in the emission spectrum. By contrast, O2 adsorption did not change the shape of either the excitation or the emission bands only a decrease in intensity was observed. This... 

The CT excited states usually are not emissive because the low energy gap and the strong distortion with respect to the ground state favor the occurrence of radiationless deactivation. Furthermore, the presence of the low-energy CT excited states causes rapid radiationless decay of the upper lying, potentially luminescent excited states localized on the molecular components. Therefore, rotaxanes and catenanes based on CT interactions usually do not exhibit any luminescence. For example, the intense emission band with maximum at 320 nm (t = 2.5 ns) exhibited by macrocycle 7 [1 la] is no longer present in catenane [23]. 

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