Emission or luminescence

The jump of an electron from one orbital to another is a transition between two energy states, such that an upward transition requires an input of energy, in the form of a photon of light for example. This is the absorption of light by the atom. A downward transition is accompanied by the release of energy, for instance in the shape of a photon of light. This is the process of emission or luminescence. 

One very important technique for probing excited states of products of charge transfer and for determining their populations is that of light emission or luminescence measurements. These methods are discussed in greater detail in Section IV.A.l. 

Emission or luminescence is referred to as fluorescence or phosphorescence, depending on whether it corresponds to a spin-allowed or a spin-forbidden transition, respectively. Similarly, radiationless transitions between states of the same multiplicity and of a different multiplicity are known as internal conversion (1C) and intersystem crossing (ISC), respectively. 

Spectroscopy A general term used to describe techniques based on the measurement of absorption, emission, or luminescence of electromagnetic radiation. 

Resonant interactions are ordinarily much stronger than non-resonant ones. Each characteristic frequency is actually a band of frequencies of width Av more or less symmetrically distributed around the same center vq. The resonant transfer of energy from a radiation field to matter is called absorption. The absorption process ereates also an induced dipole moment, but a larger one than in the case of polarization. The transfer of energy from matter to the radiation field is termed emission (or luminescence). We... 

For emission (or luminescence) spectra, the spontaneous emission coefficient (also called probability for spontaneous emission or the Einstein coefficient for spontaneous emission) A WJ, WJ ) is often reported instead of the oscillator or dipole strength ... 

All forms of spectroscopy require a source of energy. In absorption and scattering spectroscopy this energy is supplied by photons. Emission and luminescence spectroscopy use thermal, radiant (photon), or chemical energy to promote the analyte to a less stable, higher energy state. 

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. 

A simplified schematic diagram of transitions that lead to luminescence in materials containing impurides is shown in Figure 1. In process 1 an electron that has been excited well above the conduction band et e dribbles down, reaching thermal equilibrium with the lattice. This may result in phonon-assisted photon emission or, more likely, the emission of phonons only. Process 2 produces intrinsic luminescence due to direct recombination between an electron in the conduction band... 

Bioluminescence results from a chemical reaction, so it is more strictly termed chemiluminescence. Biochemical energy is converted directly to radiant energy. The process is virtually 100 per cent efficient, so remarkably little heat is generated during emission. For this reason the emission is often called cold light , or luminescence. 

Molecular spectroscopy is a key method in almost all fields of ILs research. Starting with the assessment of the purity of ILs and study of their properties using different spectroscopic probes and their absorption and emission spectra, the reactions taking place in ILs are almost impossible to be studied without using molecular spectroscopy. Recording the UV-Vis or luminescence spectra is a commonly used technique for the detection of compounds by chromatography and electrophoresis, and ILs are more widely used in the respective studies. So, it is important to further investigate the applicability of ILs to molecular spectroscopy. 

Deep state experiments measure carrier capture or emission rates, processes that are not sensitive to the microscopic structure (such as chemical composition, symmetry, or spin) of the defect. Therefore, the various techniques for analysis of deep states can at best only show a correlation with a particular impurity when used in conjunction with doping experiments. A definitive, unambiguous assignment is impossible without the aid of other experiments, such as high-resolution absorption or luminescence spectroscopy, or electron paramagnetic resonance (EPR). Unfortunately, these techniques are usually inapplicable to most deep levels. However, when absorption or luminescence lines are detectable and sharp, the symmetry of a defect can be deduced from Zeeman or stress experiments (see, for example, Ozeki et al. 1979b). In certain cases the energy of a transition is sensitive to the isotopic mass of an impurity, and use of isotopically enriched dopants can yield a positive chemical identification of a level. 

The inelastically scattered electrons and the secondary electrons produce charge carriers in the phosphor, which then recombine with emission of luminescent radiation, either directly or after traveling in the lattice. 

The simplest case of reaction kinetics occurs when the excitation and emission occur in the same atom, molecule, or luminescence center. The recombination can then be treated as a first-order unimolecular reaction. The decay time is independent of the number of other similarly excited atoms or molecules. 

It is discussed how the primary processes of defect formation during irradiation occur via electronic excitation. This can take the form of either the creation of electron-hole pairs, followed by trapping into localized energy states, or of exciton creation leading to the formation of stable vacancy and interstitial defects. Heating the sample after the irradiation causes the release of this stored energy in the form of phonons or photons. Photon emission, ie. luminescence, results from either electron-hole recombination or from vacancy-interstitial recombination. Several examples of both types are discussed for crystalline CaF and SiC. ... 

The sample is purified by distillation to separate the tritium-containing water from both non-radioactive and radioactive impurities. Various substances can cause scintillations by means other than radionuclide emission - by chemical fluorescence or luminescence - or interfere with ( quench ) detection of scintillations due to radionuclides. Even after purification, both processes are inevitable, but to a limited extent. Luminescence due to visible light will decay when the sample is stored in a darkened region of the LS system before the sample is counted. The degree of quenching, notably due to water in the sample, is determined instrumentally by reference to comparison sources and recorded, so that any deviation from the quenching observed for the tritium standard can be taken into account. 

One advantage of a spectral radiation pyrometer is that the emissivity or emittance at only a specific wavelength (e.g. 0.653 pm) is of importance. A non-blackbody source will be less luminescent than a blackbody source at the same temperature. Thus, a falsely low temperature will be determined by sighting a calibrated disappearing filament pyrometer on the non-blackbody. This temperature has been referred to as the brightness temperature . 

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