Titanite luminescence

Another group of lines is detected in the titanite luminescence spectrum, which may be considered as connected with the Nd " " emission. Those lines at 589, 658, 743 and 846 nm are especially strong in the luminescence spectra with a narrow gate excited by Aex = 532 nm (Fig. 4.34b). Such a combination of emission lines with relatively short decay times is very unusual for minerals and may not be easily connected to any rare-earth element traditional for luminescence in the visible range. If we were to consider the possible connection with the visible emission of Nd " ", the detected lines correspond very well, for example, to electron transitions from 67/2 level to %/2> fii/2> fi3/2 and Ii5/2 levels. 

The line at approximately 600 nm has a long decay time of 1 ms. It is the strongest one in the titanite luminescence spectrum under 266, 355 and 532 nm (Fig. 4.33b,c), but its relative intensity is much lower under 514 nm excitation (Gaft et al. 2003b). It appears that from all lines found in titanite luminescence spectra only two weaker ones at 563 and 646 nm have similar kinetic and excitation characteristics with the line at 600 nm. Such a combination of luminescence lines is very typical for Sm ". Thus the emission spectrum... 

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 natural titanite in our study consisted of nine samples. Concentrations of potential luminescence impurities in one sample are presented in Table 4.12. The laser-induced time-resolved technique enables us to detect Sm +, Nd, Tm ", Pr ", Er ", Eu " and Cr " emission centers (Figs. 4.33-4.34). 

Fig. 4.33. a-d Laser-induced time-resolved luminescence spectra of titanite demonstrating Cr, Eu and Sm centers... 

Fig. 4.36. Luminescence (right) and excitation (left) spectra of titanite (1), vuon-nemite (2), epistolite (3), natisite (4), penkvUsite (5), Ti-silicate (6), vino-gradovite (7), leucosphenite (8), Ti-carbonatesUicate (9), fersmanite (10) (Gaftetal. 1981)...

Figures 4.34a,b demonstrate the emission lines of titanite, which according to their spectral positions may be confidently connected with Nd " ". The luminescence spectrum in the 860-940 nm spectral range, corresponding to the transition, contains six peaks at 860, 878, 888, 906, 930 and 942 nm, while around 1,089 nm corresponding to F3/2- fn/2 transition it contains five peaks at 1,047,1,071,1,089,1,115 and 1,131 nm. The decay time of IR luminescence of Nd " equal to approximately 30 ps in titanite is evidently the shortest one in the known systems activated by Nd ". The typical radiative lifetime of this level depends on the properties of the solid matrix and varies from approximately 100 ps to 600 ps (Kaminskii 1996). To explain the fast decay time of Nd " in titanite, the energy level quenching by the host matrix may be considered.

Titanite has rather unusual luminescence of Tm + (Gaft et al. 2003b). The lines near 800 nm are very characteristic for its luminescence spectra under different excitations (Fig. 4.34). It is interesting to note that the fines at 796,806 and 820 nm present also in optical absorption spectra of titanite (Fig. 5.21). Such absorption fines are usually ascribed to electron transitions to the fs/2 level of Nd " ". Thus in such cases it is logical to suppose that the emission fines at 800 nm have to be correlated with other Nd " luminescence, for example with IR emission from the " 3/2 level. Nevertheless, this is not the case. For example, with Aex = 488 nm excitation the fines at 800 nm are absent, while the IR emission of Nd " " is very strong. The same situation is present in the ionoluminescence spectrum, where the fines near 800 nm are absent and the luminescence of Nd " " presents (Yang 1995). Thus it is possible to suppose that the fines near 800 nm with a relatively short decay time of 60 ps belong to another luminescence center. 

The lines at 686 and 693 nm with a long decay time of approximately 1 ms in the titanite emission spectrum are not correlated with any other lines and bands (Fig. 4.34). Such lines are very typical for Cr in a high field coordination and may be connected with such a center. The broad luminescence band appears peaking at 765, which may be ascribed to Cr + in a weak field coordination. The band at 765 nm has distinct dips at 749, 762, 793, 798, 804 and 820 nm. Comparison with the titanite absorption spectrum (Fig. 5.19) demonstrates that those lines exactly coincide with the absorption spectrum of Nd (Bakhtin and Gorobets 1992). Cr is a good energy sensitizer, because it has broad, allowed absorption bands with a broad emission spectrum, which overlaps the absorption bands of the lasing ion (Nd " ", Ho " ). 

Working with titanite, one sample has been found with luminescent behavior strongly different from the others. Suspicion was raised that its identification is not correct. In order to check it, LIBS and Raman data have been received from the same area where liuninescence spectra were determined. Figure 9. la demonstrates that breakdown spectra of titanite are really characterized by the group of UV fines at 300 nm of Ti and by many fines of Ca, the strongest ones at 393 and 396 nm. Nevertheless, such fines are absent in the LIBS of the suspicious sample, where only a strong fine of Na presents at 589 nm and its Raman spectrum (Fig. 9.1c) is totally different from those of titanite (Fig. 9.1b). Subsequent EDX and XRD analyses enabled us to identify this mineral as catapleite. 

Detailed research of titanite REE luminescence under different cw laser excitations was done by Lenz et al. (2015) including the study of luminescence and its excitation of artificial titanite samples activated by different REE, such as Sm, Nd, Pr and Eu. Relative emission intensities of individual REEs depend strongly on the excitation wavelength. The Raman spectra of titanite obtained using a 473 nm laser excitation shows emissions of Pr, Sm " and Nd ", whereas green excitation (532 nm) excites preferentially the PL of Sm ", and Nd ", red excitation (633 nm) predominantly Cr " and Nd ", and NIR excitation (785 run) Nd " only. Those results have been confirmed by excitation spectroscopy of artificially activated titanite samples. Under 785 mn excitation, Nd " emissions are exceptionally strong (whereas Raman scattering is weaker under NIR excitation when compared to visible excitatirui). Therefore, Raman spectra of titanite samples obtained with IR excitation typically are obscured vastly by Nd " emissions. 

Fig. 4.81 Laser-induced CW 514 nm luminescence spectra of titanite demonstrating Nd centers (V vertical H horizontal polarization)...

Figures 4.80a, c demonstrate emission lines of titanite, which according to their spectral positions may be confidently connected with Nd. Luminescence spectrum in the 860-940 nm spectral range, corresponding to the " F3/2-" l9/2 transition, contains six peaks at 860, 878, 888, 906, 930 and 942 nm, while around 1089 nm corresponding to transition it contains five peaks at 1047, 1071, 1089,...

Three emission lines at 563, 853 and 978 nm (Fig. 4.80c) demonstrate correlated behavior and evidently connected with one emission center. Such emission is characteristic for Er +. The main feature of Et + luminescence in titanite is connected with " 83/2 excited state. It is responsible for green emission at approximately 550-560 nm corresponding to " S3/2-%5/2 transitimi and for near IR emission at 850 nm corresponding to " S3/2-" l9/2 transitiOTi. The other important state is the which is upper level for the " Iu/2-" Ii5/2 transitiOTi at about 980 nm. 

Drastic change takes place in luminescence spectrum of titanite at low temperatures (Fig. 4.80d). At 77 K, Nd luminescence intensity becomes lower and narrow line appears at 732 nm with long decay time of 2.5 ms accompanied by phonon repetitions. At even lower temperature of 20 K such emission totally dominates luminescence spectrum. Such behavior may be explained by Cr in intermediate crystal field sites for which the crystal field parameters lie in the crossing region of the T2 and states. Within the intermediate crystal field there is complicating mixing between doublet and quartet states with complicated spectra, non-radiative transfer and the temperature dependence of luminescence. In such case the emission from both T2 and E states may be expected. At 300 K the... 

Fig. 5.48 (a-b) Laser-induced time-resolved luminescence spectra of Cr in natural titanite activated by 0.2 % Cr... 

Luminescence of Ti was not confidently detected in steady-state luminescence spectra of minerals. In Ti minerals studied by laser-induced time resolved spectroscopy broad read band have been found with decay time of several ps at 660 nm in benitoite (Fig. 4.82) and 750 nm in titanite (Fig. 4.801). At room temperature the benitoite band with a maximum at 660 nm has half-width of 135 nm and may be approximated by one Gaussian (Gaft et al. 2004). One exponent with decay time of 1.1 ps approximates well its decay curve at room temperature in all spectral range of luminescence band. At lower temperatures up to 30 K this red luminescence intensity becomes approximately ten times higher and the spectrum undergoes certain changes, namely its maximum shifts in long wave direction to 668 mn and the band becomes a little narrower with half-width of 105 mn. Such red emission is not excited by laser sources in the visible part of the spectrum, such as 488, 514 and 532 mn. Excitation spectrum at lower temperatures, when... 

Sometimes luminescence lines appear in the midst of the regular Raman lines in the 200-900 cm spectral range. For example, titanite time resolved spectra under excitation by 532 nm reveal pure Raman spectrum with zero delay and 10 ns gate (Fig. 6.25a) while after delay of 500 ns when all Raman signals are definitely quenched, bivalent REE emission clearly dominates the spectrum (Fig. 6.25b). 

Fig. 6.25 (a-b) Raman (a) and luminescence (b) of titanite under excitation by 532 nm in different time windows... 

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