Calcite time-resolved luminescence spectra

Fig, 5.66. Laser-induced time-resolved luminescence spectrum (a) and excitation spectrum (b) of radiation induced center in calcite... 

The violet emission of the radiation-induced center (COs) " is well known in steady-state luminescence spectra of calcite (Tarashchan 1978 Kasyanenko, Matveeva 1987). The problem is that Ce also has emission in the UV part of the spectrum. In time-resolved luminescence spectroscopy it is possible to differentiate between these two centers because of the longer decay time of the radiation-induced center. Its luminescence peaking at 405 nm becomes dominant after a delay time of 100-200 ns while emission of Ce is already quenched (Fig. 4.14f). 

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 coimected with electron-hole pare Ce -C03. The second one has a maximum at 380 nm and was ascribed to complex center with Ce " " and OH or H2O as charge compensators. Such center becomes stronger after ionizing irradiation and disappears after thermal treatment. The typical example of Ce luminescence in time-resolved luminescence of calcite, aragonite and dolomite consists of narrow band at approximately 350-360 nm with very short decay time of 30 ns, which is very characteristic for Ce (Figs. 4.18a, 4.22a and 4.25b). It was found that Ce " " excitation bands occur also in the Mn excitation spectrum, demmi-... 

Figure 4.19c demonstrates time-resolved luminescence spectra of calcite, Franklin, under 266 nm excitation. A very intensive UV band at 312 nm with short decay time of 120 ns is detected. It may not be connected with Ce emission, because its spectrum situated at longer wavelength near 400 nm (Fig. 4.19e). The excitatimi spectrum of the band at 312 nm consists of one band at 240 nm (Gaft et al. 2002a).

Nevertheless, such interpretation cmitradicts the fact, that after heating at 800 °C the short-lived yeUow band disappeared and a usual long-lived Mn luminescence becomes visible (Fig. 5.111c, d). Time-resolved excitation spectrum of short lived yellow band consists of one main broad band with extremely low Stocks shift and is absolutely different from those for Mn + (Fig. 5.11 le-f). Certain similarity may be seen between this luminescence and short-lived orange emission in calcite, which has been ascribed to radiation-induced center. It is possible that natural irradiation may be a reason of orange luminescence in apatite also. 

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