Luminescent properties: decay time

Luminescent Pigments. Luminescence is the abihty of matter to emit light after it absorbs energy (see Luminescent materials). Materials that have luminescent properties are known as phosphors, or luminescent pigments. If the light emission ceases shortly after the excitation source is removed (<10 s), the process is fluorescence. The process with longer decay times is referred to as phosphorescence. 

The surface-state model, in which the luminescent recombination occurs via surface states, was proposed to explain certain properties of the PL from PS, for example long decay times or sensitivity of the PL on chemical environment. In the frame of this model the long decay times are a consequence of trapping of free carriers in localized states a few hundred meV below the bandgap of the confined crystallite. The sensitivity of the PL to the chemical environment is interpreted as formation of a trap or change of a trap level by a molecule bonding to the surface of a PS crystallite. The surface-state model suffers from the fact that most known traps, e.g. the Pb center, quench the PL [Me9], while the kinds of surface state proposed to cause the PL could not be identified. 

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... 

Figures 4.31a,b represent narrow luminescence hnes detected in barite by time-resolved spectroscopy. Much weaker lines at 446 and 672 nm accompany the strongest one at 588 nm. They have a relatively short decay time of 5 ps and emphasized in the spectrum with short gate. Such a combination of spectral and kinetic properties is not suitable for any trivalent REE besides P j2 f9/2...

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.

Nevertheless, in certain cases anomalous liuninescence may be possible, identification of which may be based on the following aspects an abnormally large Stokes shift and width of the emission band a wavelength of emission that is not consistent with the wavelength anticipated from the properties of the compound an anomalous decay time and thermal behavior (Dorenbos 2003). Such luminescence may be red, for example at 600 nm in Bap2, with a decay time of about 600-800 ns. This is due to the fact that the emitting level contains spin octets and sextets, whereas the ground state level is an octet, so that the optical transition rate is slower because of spin selection rule (Dorenbos et al. 2003). 

Spectra with narrow gates where the centers with a short decay time are emphasized enables us to detect broad bands at 794 nm with a decay time of 5 ps and broad band at 840 nm with a decay time of 190 ps (Fig. 4.42). These bands maybe ascribed to Cr luminescence centers, in addition to Cr with narrow -lines at 720 nm, detected by steady-state spectroscopy (Min ko et al. 1978). Wollastonite structure has three different types of six-coordinated calcium-oxygen groups, which enables the formation of several types of Cr " limiinescence centers. Nevertheless, luminescence of Cr " as result of Ca substitution has not been confidently found yet and another interpretation is also possible. For example, ions of V maybe considered, which have similar luminescence properties with Cr + and may substitute in Ca + sites. 

Emerald, Cr " doped beryl, has a beryl structure with the Cr " impurity ions in highly distorted octahedron sites. The discovery of lasing action in emerald stimulated investigation of its luminescence properties. It was established that its tuning range is approximately 730-810 nm, while luminescence consists of a narrow line at 684 nm and a band peaking at 715 nm with similar decay times of 62 ps. The relative intensities of those line and band are different in a- and 7T-polarized spectra (Fabeni et al. 1991). 

Time-resolved emission spectroscopy is gaining importance in the study of various chemical aspects of luminescent lanthanide and actinide ions in solution. Here, the author describes the theoretical background of this analytical technique and discusses potential applications. Changes in the solution composition and/or in the metal-ion inner coordination sphere induce modifications of the spectroscopic properties of the luminescent species. Both time-resolved spectra and luminescence decays convey useful information. Several models, which are commonly used to extract physico-chemical information from the spectroscopic data, are presented and critically compared. Applications of time-resolved emission spectroscopy are numerous and range from the characterization of the... 

As reviewed in the introduction, the luminescence from polymers and biopolymers may be described in terms of spectral shape, quantum yield of emission, decay time characteristics and polarization properties. The recent rapid increase in interest in the usefulness of luminescence techniques to study the structure and prtqwrties of molecular systems is partly due to the now ready availability of reliable instrumentation. Although the apparatus necessary for studying the spectral characteristics of luminescence is well established and has been r iewed in detail by several authors there have been recent rapid developments in the techniques available for time-... 

For concentrated Bi compounds another model yields a similar temperature dependence, viz. mobile excitons with concentration nj and self-trapped excitons with concentration n and an energy difference AE, representing the thermal activation energy for exciton migration through the lattice. Unfortunately it is seldom checked whether the temperature dependence of the decay time in concentrated systems refers to an intrinsic property of an isolated luminescent centre or to an activation energy for migration. This, by the way, holds also for other compounds which have been discussed above. ... 

The luminescence from octahedral uranate groups has also been reported for other uranium-doped oxidic compounds (see e.g. Ref. 7). Like in uranium-doped compounds with ordered perovskite structure isolated UOg octahedra are present in several other host lattices. In this type of compounds e.g. Y3Li3Te20i2-U" LigWOs-LT I and Mg3TeOg—, the luminescence properties of the octahedral uranate group are similar to the properties which have been observed for uranium-doped ordered per-ovskites. Due to symmetry lowering the vibrational structure in the luminescence spectra is more complicated, and also the luminescence decay time is shorter than in ordered perovskite systems (c.f. Sect. 2.1). 

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