Luminescence excited electrons

The light of a fire comes from the incandescence of heated soot particles and from chemical luminescence—excited electrons changing energy levels in the carbon and hydrocarbon molecules. (unlit/Shutterstock)... 

Figure 19 Excimer emission of pyrene. The upper curves show the potential surfaces of the ground and the luminescent excited electronic state (Reproduced with permission from Turro NJ (1978) Modem Molecular Photochemistry, p 141. Menlo Park Benjamin/Cummings.

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

The luminescent excited state of MogClii reacts rapidly with electron acceptors(24). The powerfully oxidizing MogCli is produced in these reactions. Experiments with BSEP as acceptor in... 

Surprisingly it was found that PS luminescence excited by polarized light emerges from the sample preferentially with the same sense of polarization. This memory effect has been observed despite the fact that the electron-hole pair loses energy in the order of 1 eV in elastic processes with lifetimes in the order of... 

Rehm-Weller method (21) for estimating the reduction potentials of excited molecules As our homologous series of reductants, we have used the RuL3 +/ RuL32+ couples (22,23), where RuL32+ is the luminescent excited state of Rul 7 The electron exchange rate constants for these couples are very large... 

The excited electron may also recombine with an activator, with the following luminescence, or a trap, with the following electron capturing, within the forbidden gap. Traps and activator energy levels are caused by defects in the crystal lattice (Fig. 2.6b). 

Stimulated release from an electron from the trap to the collection band, followed by emissive recombination with an activator. This process is called thermoluminescence (electron release stimulated by heating) and optically stimulated luminescence (electron release stimulated by light) (Fig. 2.6d) Extrinsic luminescence, where after being excited, electrons of defect ions recombine with the ground state with luminescence emission (Fig. 2.6e) ... 

A remarkable number of organic compounds luminesce when subjected to consecutive oxidation-reduction (or reduction-oxidation) in aprotic solvents1-17 under conditions where anion radicals are oxidized or cation radicals are reduced. In many instances, the emission is identical with that of the normal solution fluorescence of the compound employed. In these instances the redox process has served to produce neutral molecules in an excited electronic state. These consecutive processes which result in emission are not special examples of oxidative chemiluminescence, but are more properly classified as electron transfer luminescence in solution since the sequence oxidation-reduction can be as effective as reduction-oxidation.8,10,12 A simple molecular orbital diagram, although it is a zeroth-order approximation of what might be involved under some conditions, provides a useful starting... 

Light is absorbed by two basic mechanisms electronic polarization and electronic excitation. Electronic polarization and its effect on refractive index were described in the previous section and will not be elaborated npon here. The process of electronic excitation is an important one, however, and has implications to a nnmber of optical phenomena such as lasing and luminescence. 

Semiconductor band-gap luminescence results from excited electrons recombining with electron vacancies, holes, across the band gap of the semiconductor material. Electrons can be excited across the band gap of a semiconductor by absorption of light, as in photoluminescence (PL), or injected by electrical bias, as in electroluminescence (EL). Both types of luminescence have been used in chemical sensing applications [1,3]. 

Certain aromatic hydrocarbons luminesce when raised to an excited electronic state by electrochemical energy. This phenomenon is called electroluminescence (eel) and is shown by some benzo[c]furans. The eel emission was examined in V,V -dimethylformamide as solvent with tetra-n-butylammonium perchlorate as electrolyte. - The emission was identical with the normal fluorescence emission. Cyclic voltammograms were measured under the same conditions as used for the eel studies slowest scan rates at which rereduction of the cation or reoxidation of the anion... 

If the recombination is delayed, e.g., by migration of excited electrons, luminescence takes place by a second-order bimolecular reaction. The probability of a luminescent recombination of the excited electron with the holes is then proportional to the product of the concentration of electrons and the concentration of holes. The lower the initial intensity is, and the further the decay has progressed, the slower the decay to the half value is. This hyperbolic decay law is only of limited validity. If the excited electron is momentarily trapped before recombination, very complex interactions can arise. 

Phosphors for cathode-ray tubes, television screens, monitor screens, radar screens, and oscilloscopes are tested under electron excitation. Electron energy and density should be similar to the conditions of the tube in which the screen will be used. The phosphors are sedimented or brushed onto light-permeable screens and coated with an evaporated aluminum coating to dissipate charge. The luminescence brightness and color of the emitted light are measured with optical instruments such as photomultipliers or spectrophotometers. 

The other way to study the "conductivity of protein molecules towards electron tunneling is to investigate the quenching of luminescence of electron-excited simple molecules by redox sites of proteins [95,96]. Experiments of this sort on reduced blue copper proteins have involved electron-excited Ru(II)(bpy)3, Cr(III)(phen)3, and Co(III)(phen)3 as oxidants. The kinetics of these reactions exhibit saturation at protein concentrations of 10 3 M, suggesting that, at high protein concentrations, the excited reagent is bound to reduced protein in an electron transfer precursor complex. Extensive data have been obtained for the reaction of reduced bean plastocyanin Pl(Cu(I)) with Cr(III)(phen)3. To analyze quenching experimental data, a mechanistic model that includes both 1 1 and 2 1 [Pl(Cu(I))/ Cr(III)(phen)3] complexes was considered [96]... 

For systems in which no electron migration or stabilization can occur after excitation, the excited electron returns to its ground state by either luminescence or radiationless transition. Since most charge transfer transitions occur with very high probability, the excited state persists for only about 10 8 sec. Therefore, the secondary process must be extremely fast to compete with spontaneous emission of the excited state. 

Fig. 14. Origin region of the low temperature excitation (luminescence monitored broad band below 20 200 cm 1) and luminescence (excitation at 457.9 nm with an Ar laser) spectra of [lr(ppy)2bpy]PF5 in [Rh(ppy)2bpy] PF6. M, C and D label electronic origins (from Ref. [45])...

The different ratio of usual and additional luminescent bands integral intensities for different exciting laser wavelengths may partially be caused by influence of large reabsorption of light inside of a sample (see Fig. 3). Another cause, which may be responsible for this effect, is different excitation spectra for the luminescence of two types. Moreover, if energy relaxation pathways, which lead to occupation of excited electronic luminescent levels of two types, are not independent, this may cause the observed effect also. 

The phosphorescence decay kinetics of the triplet excited states of CuP molecules (Fig. 14) is adequately described by Eq. (16). Using this equation one can obtain the values of the parameter p = (Tra /2) In2 veT from the initial non-exponential part of the phosphorescence decay curves and the values of t = l/ k, i.e. the characteristic time of phosphorescence decay, from the final exponential part. Then the data on the dependence of the quantum yield of CuP phosphorescence on the concentration of C(N02)4 have been used to estimate the effective radii of electron tunneling from triplet excited copper porphyrins to C(N02)4 within the time x R, = (ac/2) In vet (Table 3). In doing so, the quenching of CuP luminescence by electron abstraction was assumed to be the only process leading to a decrease in the quantum yield of CuP phosphorescence in the presence of C(N02)4. From Table 3 an electron is seen to tunnel, within the lifetime of triplet excited states x at 10-4s, from CuP particles to C(N02)4 molecules over the distance R, 11 A. Further, the parameter vc and ae for different porphyrins were estimated from the values of (3, Rt, and x. These values are also cited in Table 3. 

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