Luminescence center

In thermoluminescence dating, a sample of the material is heated, and the light emitted by the sample as a result of the de-excitations of the electrons or holes that are freed from the traps at luminescence centers is measured providing a measure of the trap population density. This signal is compared with one obtained from the same sample after a laboratory irradiation of known dose. The annual dose rate for the clay is calculated from determined concentrations of radioisotopes in the material and assumed or measured environmental radiation intensities. 

The Configuration Coordinate Model. To illustrate how the luminescent center in a phosphor works, a configurational coordinate diagram is used (2) in which the potential energy of the luminescent or activator center is plotted on the vertical axis and the value of a single parameter describing an effective displacement of the ions surrounding the activator, is plotted on the horizontal axis (Fig. 2). At low temperatures, near room... 

Obtaining information on a material s electronic band structure (related to the fundamental band gap) and analysis of luminescence centers Measurements of the dopant concentration and of the minority carrier diffusion length and lifetime... 

The luminescent centers require a range of properties that include a large cross-section for the collision excitation to occur, an ionic radius and valency to fit the lattice and be stable under the applied high electronic fields, and the capability to display high luminous efficiency when excited.11 Metal ions suitable for EL devices include Mn, Tb, Sm3+, Tm3+, Pr3+, Eu2+, and Ce3+.12-17 ZnS lattices doped with Mn2+ (yellow-orange emission at ca. 585 nm) have proved to be one of the best phosphors for EL devices. 

Another open question is the relationship between the H-induced radiative recombination centers and the H-induced platelets. Controlled layer removal of the plasma-processed silicon surface reveals that the density of luminescence centers decays nearly exponentially with a decay length that is comparable to the depth over which the platelets form (Northrop and Oehrlein, 1986 Jeng et al., 1988 Johnson et al., 1987a). However, the defect luminescence has also been obtained from reactive-ion etched specimens in which platelets were undetectable (Wu et al., 1988). Finally, substantial changes in the luminescence spectra occur at anneal temperatures as low as 250°C (Singh et al., 1989), while higher temperatures... 

Because of the high sensitivity of Ti-containing luminescence centers to their local environments, photoluminescence spectroscopy can be applied to discriminate between various kinds of tetrahedral or near-tetrahedral titanium sites, such as perfectly closed Ti(OSi)4 and defective open Ti(OSi)3(OH) units. Lamberti et al. (49) reported an emission spectrum of TS-1 with a dominant band at 495 nm, with a shoulder at 430 nm when the sample was excited at 250 nm. When the excitation wavelength was 300 nm, the emission spectrum was characterized by a dominant band at 430 nm with a shoulder at 495 nm. These spectra and their dependence on the excitation wavelength clearly indicate the presence of two slightly different families of luminescent Ti species, which differ in their local environments, in agreement with EXAFS measurements carried out on the same samples. 

The photoluminescence of these nanoparticles has very different causes, depending on the type of nanomaterial semiconductor QDs luminescence by recombination of excitons, rare-earth doped nanoparticles photoluminescence by atom orbital (AO) transitions within the rare-earth ions acting as luminescent centers, and metallic nanoparticles emit light by various mechanisms. Consequently, the optical properties of luminescent nanoparticles can be very different, depending on the material they consist of. 

Bilan ON, Tyul nin VA, Cherenda NG, Shendrik AV, Yudin DM (1980) Radiation paramagnetic centers and luminescence centers in silver-doped quartz glasses. J Appl Spectrosc 33 717-720... 

In addition, the pure solid (N(C2Hs)4)2MnBr4 is recommended as standard. Its phosphorescence yield is high, , = 0.8, the material has only medium absorption despite its high concentration, and there is no long-distance impurity quenching because the luminescence centers are electronically isolated from one another by the large counter ions. 

Comparing this value to the typical sensitivity provided by a spectrophotometer, (OT>)niin = 5 X 10 (see Section 1.4), we see that the luminescence technique is much more sensitive than the absorption technique (about 10 times for this experiment). Although this large sensitivity is an advantage of photoluminescence, care must be taken, as signals from undesired trace luminescent elements (not related to our luminescent center) can overlap with our luminescent signal. 

In principle, an increase in the concentration of a luminescent center in a given material should be accompanied by an increase in the emitted light intensity, this being due to the corresponding increase in the absorption efficiency (see expression (1.15)). However, such behavior only occurs up to a certain critical concentration of the luminescent centers. Above this concentration, the luminescence intensity starts to decrease. This process is known as concentration quenching of luminescence. 

The fluorescence spectra exhibit features common to all compounds besides the two bands of the main product there Is a new system of bands which corresponds to the additional absorbing center (Fig. 3,5). The relative Intensity of these new bands Is strongly dependent on the exciting light wavelength. It should be noted that the fluorescence lifetimes of the 0-0 bands for the two luminescent centers, measured at 77 K, are rather close (for... 

The luminescent mineral consists of a host lattice and a luminescent center, often called an activator. The determination of the nature of the center responsible for luminescence is not generally a trivial task. Correlation of the observation of the specific luminescence with a particular impurity concentration may give an indication of the source of the emission but it is not proof of the origin, and can sometimes be misleading. Furthermore, it does not give any details about the precise nature of the center. Spectroscopic studies may... 

In many luminescence centers the intensity is a function of a specific orientation in relation to the crystallographic directions in the mineral. Even if a center consists of one atom or ion, such luminescence anisotropy may be produced by a compensating impurity or an intrinsic defect. In the case of cubic crystals this fact does not disrupt optical isotropy since anisotropic centers are oriented statistically uniformly over different crystallographic directions. However, in excitation of luminescence by polarized fight the hidden anisotropy may be revealed and the orientation of centers can be determined. 

Table 1.3. Luminescence centers found in steady-state spectra of minerals ...

Luminescence center Transi- tion Decay time... 

Time-resolved luminescence spectroscopy may be extremely effective in minerals, many of which contain a large amount of emission centers simultaneously. With the steady state technique only the mostly intensive centers are detected, while the weaker ones remain unnoticed. Fluorescence in minerals is observed over time range of nanoseconds to milliseconds (Table 1.3) and this property was used in our research. Thus our main improvement is laser-induced time-resolved spectroscopy in the wide spectral range from 270 to 1,500 nm, which enables us to reveal new luminescence centers in minerals previously hidden by more intensive centers. 

As we will demonstrate, luminescent properties, radiative transition characteristics as well as emission under site selective excitation depend on the local environment s)rmmetry of the luminescent center. Therefore it is necessary to take into account and to describe the different local symmetry. There are two systems commonly used in describing symmetry elements of punctual groups ... 

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. 

There are a large number of cases where the spectra of luminescence centers remain broad up to helium temperatures. In certain cases, this is explained by a strong electron-phonon interaction, but more often the inhomogeneous broadening, connected with several types of the same center presence, causes this. In such cases it is possible to simplify the spectrum by selective excitation of specific centers. 

In most luminescence experiments, at least in the mineral luminescence field, excitation is due to absorption of a single photon. However, it is also possible for a luminescence center to absorb two or more long-wavelength photons to reach the excited state. Two-photon excitation occurs by the simultaneous absorption of two lower-energy photons. Such excitation requires special conditions including high local intensities, which can only be obtained from laser sources. 

Most of the electronic spectroscopy in minerals can be interpreted by the well-known ligand field theory. The main luminescence centers in minerals are transition and rare-earth elements. The ground and excited levels in these cases are d and / orbitals, while d-d and d-f emission transitions are subjected to strong influence from nearest neighbors, so called ligands. The basic notion of... 

The concentration of luminescence centers is usually extremely low and we need high-energy sources. 

Several luminescence centers often present simultaneously in one mineral and we need selective monochromatic excitation in a broad spectral range. 

Decay time of many luminescence centers in minerals is extremely short and for time-resolved spectroscopy we need the pulsed source with a very short pulse width. 

Natural minerals may contain simultaneously up to 20-25 luminescence centers, which are characterized by strongly different emission intensities. Usually one or two centers dominate, while others are not detectable by steady-state spectroscopy. In certain cases deconvolution of the liuninescence spectra may be useful, especially in the case of broad emission bands. It was demonstrated that for deconvolution of luminescence bands into individual components, spectra have to be plotted as a function of energy. This conversion needs the transposition of the y-axis by a factor A /hc (Townsend and Rawlands 2000). The intensity is then expressed in arbitrary imits. Deconvolution is made with a least squares fitting algorithm that minimizes the difference between the experimental spectrum and the sum of the Gaussian curves. Based on the presumed band numbers and wavelengths, iterative calculations give the band positions that correspond to the best fit between the spectrum and the sum of calculated bands. The usual procedure is to start with one or... 

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