Time-resolved luminescence decay

Time Resolved Luminescence Decay. The luminescent decay profiles of porous silicon are long lived and inhomogeneous across the emission band. This is evident in Figure 3a where the emission at 750nm clearly exhibits a longer lifetime than emission... 

Silica makes up 12.6 mass-% of the Earth s crust as crystalline and amorphous forms. It was found that both modifications show similar main luminescence bands, namely a blue one centered at 450 nm ascribed to which substitutes for Si, red centered at 650 nm linked with non-bridge O, and dark-red at 700-730 nm linked with Fe. Time-resolved luminescence of hydrous volcanic glasses with different colors and different Fe, Mn, and H2O contents were measured and interpreted (Zotov et al. 2002). The blue band with a short decay time of 40 ns was connected with T2( D)- Ai ( S) and Ai C G)- Ai ( S) ligand field transitions of Fe " ", the green band with a decay time of approximately 250 ps with a Ti( G)- Ai( S) transition in tetrahedrally coordinated Mn ", while the red band with a much longer decay time of several ms with T1 (4G)- Ai( S) transitions in tetrahedrally coordinated Fe ". We detected Fe " " in the time-resolved luminescence spectrum of black obsidian glass (Fig. 4.43d). 

The steady-state luminescence of Pr + in minerals was found only in scheel-ite, where the hne near 480 nm has been ascribed to this center (Gorobets and Kudrina 1976) and possibly in fluorite (Krasilschikova et al. 1986). The luminescence of Pr in minerals is difficult to detect because its radiative transitions are hidden by the stronger lines of Sm in the orange range of 600-650 nm, Dy " " in the blue range of 470-490 nm and Nd in the near IR (870-900 nm). In order to extract the hidden Pr lines time-resolved luminescence was applied. The fact was used that Pr " usually has a relatively short decay time compared to its competitors Dy ", Sm " and Nd, especially from the Po level. In order to correct identification of Pr " lines in minerals several of them were synthesized and artificially activated by Pr (Fig. 5.5). Besides, comparison has been made with CL spectra of synthetic minerals artificially activated by Pr (Blank et al. 2000). 

The luminescence center of divalent europium in fluorite is well known (Haber-land et al. 1934 Tarashchan 1978 Krasilschikova et al. 1986 Barbin et al. 1996). It is clearly seen in laser-induced time-resolved luminescence spectra with a decay time of 600-800 ns (Fig. 4.10a). In several samples the band with a spectrum similar to those of Eu + has a very long decay time and remains even after a delay of several ms. Principally it may be connected with energy migration from a UV emitting center with a long decay time, for example, Gd ". ... 

Time-resolved luminescence spectroscopy of zircon revealed luminescence lines, which maybe confidentially ascribed to a Eu center (Fig. 4.38d). Usually they are hidden by a broad band yellow emission of zircon and may be detected only with a long delay time using its much longer decay time compared to yellow luminescence. 

The fines of Er " " are difficult to point out because they have short decay times, which are comparable with the decay of broad band luminescence of scheelite at the same spectral region. Using a dye laser with 357 nm emission solved this problem. It is effective for Er " " but not suitable for the excitation of the WO4-center (Fig. 4.8c). These fines of Er " " are most clearly seen with a narrower gate width when the luminescence of short-lived centers dominates. Investigation of the time-resolved luminescence of synthetic CaW04 Er confirmed our interpretation (Fig. 5.19). 

Laser-induced time-resolved luminescence spectra of natural emeralds also demonstrate l -lines of Cr at 680 and 684 nm accompanied by a narrow band peaking at 715 nm, which have similar decay times of approximately 55ps(Fig. 4.53). 

Laser-induced time-resolved luminescence spectra of natural alexandrite revealed two narrow doublets, the first at 679 and 680 and the second at 693 and 694 (Fig. 4.54). They have strongly different polarizations and decay times and evidently may be connected with 7-Unes of Cr in two different structural sites. Besides that a narrow Une at 690 nm with a decay time of 120 ps is also found, which also may be connected with Cr + emission. 

Fig. 5.28. a-d Laser-induced time-resolved luminescence spectra of topaz at different temperatures demonstrating different decays of Cr and possibly Mn centers... 

The broad band peaking at 730 nm accompanied by a narrow doublet at 692 and 694 nm (Fig. 4.52) with a mutual decay time of 100 ps in the laser-induced time-resolved luminescence spectrum of beryl is not similar to the Cr emission in emerald. Thus we suppose that such typical emission may be connected with the center. 

Broad bands at 525 and 575 nm in the time-resolved luminescence spectra of hardystonite under 355 nm excitation (Fig. 4.20d) with very long decay time of several ms may be ascribed to strongly forbidden d-d transitions in the Mn " " luminescence center. Two bands may be connected with isomorphous substitutions on Ca in Zn structural positions. The spectriun of a famous yellow-green esperite luminescence (Fig. 4.21a) consists of a narrow band peaking at 545 nm with a very long decay time of 9 ms. Such parameters together with the typical excitation spectrum (Fig. 4.2 lb) enable confident identification of the luminescence center as Mn +. The orange emission near 600 nm of apophyllite is also evidently connected with the Mn center (Fig. 4.19c,d). 

Figure 4.14c demonstrates time-resolved luminescence spectra of caldte, Franklin, NJ, under 266 nm laser excitation. A very intensive UV band at 312 nm with a short decay time of 120 ns is detected. It may not be connected with Ce emission, because its spectrum is situated at a substantially longer wavelength near 400 nm (Fig. 4.14e). The excitation spectrum of the band at 312 nm consists of one band at 240 nm (Gaft et al. 2003a).

Figure 4.37a represents the time-resolved luminescence spectrum of a hydrozincite under 266 nm laser excitation. A relatively broad band is detected at 430 nm, which is responsible for the well-known blue hydrozindte luminescence. Its spectral position and decay time of approximately 700 ns are typical for Eu luminescence. However, the excitation spectrum of this band consists of one narrow band at 240 nm (Fig. 4.37b), which does not correspond to an Eu " excitation spectrum. Two bands usually characterize the latter with relatively small Stokes shifts of 30-50 nm caused by crystal field splitting of the 4/ 5d-levels. Moreover, the measured Eu concentrations in the hydrozincite samples under investigation are very low (less than 0.5 ppm) and they do not correlate with the intensity of the blue luminescence, i.e. the band at 430 nm. 

Time-resolved luminescence spectroscopy of sodalite evidences that the vibration structure has a very short decay time and disappears after a delay of 250 ns. Such structure is superimposed on the very broad IR band (Fig. 4.65). 

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). 

Our study of time-resolved luminescence of diamonds revealed similar behavior (Panczer et al. 2000). Short-decay spectra usually contain N3 luminescence centers (Fig. 4.71d 5.69a,b) with decay time of r = 30-40 ns. Despite such extremely short decay, sometimes the long-delay spectra of the same samples are characterized by zero-phonon lines, which are very close in energy to those in N3 centers. At 77 K Aex = 308 nm excitation decay curve may be adjusted to a sum of two exponents of ti = 4.2 ps and i2 = 38.7 ps (Fig. 5.69c), while at 300 K only the shorter component remains. Under Aex = 384 nm excitation an even longer decay component of 13 = 870 ps may appear (Fig. 5.69d). The first type of long leaved luminescence may be ascribed to the 2.96 eV center, while the second type of delayed N3 luminescence is ascribed to the presence of two metastable states identified as quarfef levels af fhe N3 cenfer. 

Fig. 5.69. a-d Laser-induced time resolved luminescence of N3 center in diamond (a, b) and decay times (c, d)... 

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... 

Table I shows examples of the steady-state and time-resolved emission characteristics of [Ru(phen)2(dppz)]2+ upon binding to various DNAs. The time-resolved luminescence of DNA-bound Ru(II) is characterized by a biexponential decay, consistent with the presence of at least two binding modes for the complex (47, 48). Previous photophysical studies conducted with tris(phenanthroline)ruthenium(II) also showed biexponential decays in emission and led to the proposal of two non-covalent binding modes for the complex (i) a surface-bound mode in which the ancillary ligands of the metal complex rest against the minor groove of DNA and (ii) an intercalative stacking mode in which one of the ligands inserts partially between adjacent base pairs in the double helix (36, 37). In contrast, quenching studies using both cationic quenchers such as [Ru(NH3)6]3+ and anionic quenchers such as [Fe(CN)6]4 have indicated that for the dppz complex both binding modes...

---