Band concept luminescence

The effective masses of electrons and holes are estimated by parabolic approximation a large curvature corresponds to a small effective mass and a small curvature corresponds to a large mass. With this band concept, light absorption and luminescence are interpreted as follows Light is absorbed by the transition from valence band to conduction band. Therefore, the broadening of the absorption spectrum originates basically from the one dimensionality of the joint density of states, which is described by (E - g) . Excited electrons and holes relax to the bottom of the bands and then recombine radiatively. Therefore, the photoluminescence of the spectrum is very sharp. The energy difference between two peaks is called the Stokes shift. 

Specific examples are now used to demonstrate these concepts. First, consider the group Ru(bpy)j2+ (luminescent), Os(bpy)32+ (slightly luminescent), and Fe(bpy)32+ (nonluminescent) (Table4.1). For Fe(bpy)32+, despite an exhaustive search no emission has ever been detected even at 77K we routinely use it as a nonemissive solution filter. All three iso structural eft systems are in the same oxidation state with the same electronic configuration (ft6). The Fe(II) complex has an intense MLCT band at 510 nm, and the Ru(II) complex at 450 nm the Os(II) complex has intense MLCT bands that stretch out to 700 nm. The n-n transitions are all quite similar in all three complexes with intense absorptions around 290 nm and ligand triplet states at 450 nm (inferred from the free ligand and other emissive complexes and the insensitivity of these states to coordination to different metals). 

Luminescence spectroscopy is an analytical method derived from the emission of light by molecules which have become electronically excited subsequent to the absorption of visible or ultraviolet radiation. Due to its high analytical sensitivity (concentrations of luminescing analytes 1 X 10 9 moles/L are routinely determined), this technique is widely employed in the analysis of drugs and metabolites. These applications are derived from the relationships between analyte concentrations and luminescence intensities and are therefore similar in concept to most other physicochemical methods of analysis. Other features of luminescence spectral bands, such as position in the electromagnetic spectrum (wavelength or frequency), band form, emission lifetime, and excitation spectrum, are related to molecular structure and environment and therefore also have analytical value. 

CdS clusters of narrow size distribution were studied by Eychmuller et al. [63], In this case a rather narrow luminescence band can be observed near the absorption band. The decay kinetics of this excitonic luminescence is multiexponential with a typical lifetime on the order of nanoseconds, much longer than the expected exciton lifetime. The temperature dependence of the excitonic luminescence shows complex behavior. Again, the authors use the three-level thermal equilibrium model to explain the data. The excitonic luminescence is identified as delayed luminescence occurring by detrapping of trapped electrons. Furthermore, they invoke the concept... 

Although the appearance of red-shifted, broad-band luminescence is usually attributed to the presence of defects, this is not always so. As the size of the cluster becomes smaller, the concept of defect becomes meaningless. Red-shifted luminescence may be due to an intrinsic excited state of the cluster which is significantly distorted from the ground state. This is quite common for molecules. Since the surface structures of most of the semiconductor clusters synthesized to date are not precisely known, in most cases it is difficult to establish whether the observed red-shifted luminescence band is due to defects or the intrinsic excited state. 

Another issue regarding the use of spectrum shifting dyes is re-absorption of emitted photons before they enter the photovoltaic device (5,13,14). This occurs due to an overlap in the absorption and emission bands of a dye. While this is an enormous problem for luminescent solar concentrators, it should not be as significant for the in-line spectrum alteration concept. This is because tiie optical... 

Based on the concepts of localized recombination of close DAPs with random distribution of separation distance between pair components, postulated in [53], we propose the model of luminescence mechanism in AlN ceramics, which can explain the above-mentioned features revealed by this material in PL, AGL, TL, and OSL. The diagram of the proposed luminescence mechanism is shown in Figure 9.16. This diagram describes processes in the defects, which produce the UV emission band. We believe that luminescence mechanism of the Blue band is similar and includes localized tunnel recombination, though the responsible luminescence centers are not shown in the scheme. 

The preceding results can be compared with the selection rules for all the possible electronic transitions obtained from group theory, to give electronic assignments (term symbols) for each of the luminescence bands. The resulting electronic assignments are A2 and A2u for the HE and LE bands, respectively. A discussion of the concepts of group theory that lead... 

An alternative approach to obtain luminescent liquid crystals is to use the host-guest concept a luminescent lanthanide complex is dissolved in a hquid-crystalline matrix. This approach allows one to optimize the luminescence and mesomorphic properties of the liquid-crystal mixture independently. Boyaval et al. investigated the luminescence of lanthanide complexes in a cholesteric hquid crystal mixture (Boyaval et al., 1999, 2001). Birmemans and Moors (2002) showed that a nematic liquid crystal can be an interesting host matrix to study the spectroscopic properties of luminescent lanthanide complexes. These authors doped the /3-diketonate complex [Eu(tta)3(phen)] into the nematic liquid crystals N-(4-methoxybenzyhdene)-4-butylaniline (MBBA) and 4-n-pentyl-4 -cyanobiphet5rl (5CB) and observed narrow-band red photoluminescence with a well-resolved crystal field sphttmg. Later on, these authors extended their studies to lanthanide j8-diketonate complexes emitting in the near-infrared (Ln = Nd, Er, Yb) (Van Deun et al., 2003). 

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