Non-radiative decay

In the absence of non-radiative decay processes the experimentally observed decay time equals the radiative decay time. When non-radiative processes are present, the experimental value is reduced by a factor equal to the quantiun efficiency of the luminescence. There are many factors, which affect the decay time. One is due to competing non-radiative processes, which shorten the measured decay time. We will consider the latter first. The experimentally observed decay time of the liuninescence is given by 

Another possibility to return to the ground state is by transfer of the excitation energy from the excited center (S ) to another center (A). The energy transfer may be followed by emission from A and species S is then said to sensitize species A. However, A may also decay nonradiatively and in this case species A is said to be a quencher of the S emission. The most important quenchers in minerals are Fe, Co and which have intense charge-transfer bands. 

If the spectral overlap consists of a considerable amount of overlap of an emission band and an allowed absorption band, there can be a considerable amount of radiative energy transfer S decays radiatively and the emission band vanishes at the wavelengths where A absorbs strongly. 

If we consider now transfer between two identical ions the same considerations can be used. If transfer between S ions occurs at a high rate, in a lattice of S ions there is no reason why the transfer should be restricted to one step. This can bring the excitation energy far from the site where the absorption took place. If in this way, the excitation energy reaches a site where it is lost nonradiatively (quenching site), the luminescence will be quenched. This phenomenon is called concentration quenching. 

An additional delay between excitation and emission is often introduced by metastable electron states known as electron traps, which are filled during excitation. Once an electron has become trapped in such a state it requires certain energy to release it but this can be provided thermally. Emission, which follows the emptying of such traps at a fixed temperature, is known as phosphorescence. 

A quantum-mechanical tunnelling model of non-radiative decay 492 has been proposed. Several papers have been concerned with the choice of basis set for treatments of non-radiative decay,498 and it has been shown in the paper of Sharf that higher-order vibronic terms are comparable in magnitude with those of the first-order term. It is thus not sufficient to make the usual assumption that interactions describing non-radiative decay are independent or linearly dependent on nuclear co-ordinates. The other papers reinforce this point, and 

Some years ago it was conventional wisdom to assert that for aromatic hydrocarbons Sx - S0 internal conversion was an improbable process compared with Sx - 7i non-radiative decay. However, it has recently been demonstrated that in naphthalene and related compounds the internal conversion can be the dominant decay process, particularly for vibrationally excited species.494 It is of interest to note that calculated rates for the internal-conversion process in naphthalene could only be made to fit experimental values if some redistribution of vibrational energy occurred, but not complete statistical distribution among normal modes. 

The importance of modes which undergo large frequency and equilibrium displacements as accepting modes has been further stressed, particularly with 

Mikami, K. Mizunoya, and T. Nakajima, Chem. Phys. Letters, 1975,30,373 N. Shimakura, Y. Fujimura, and T. Nakajima, Theor. Chim. Acta, 1975, 37, 77. 

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If there are no competing processes the experimental lifetime x should equal Tq. Most connnonly, other processes such as non-radiative decay to lower electronic states, quenching, photochemical reactions or... 

R. Englman, Non-Radiative Decay of Ions and MoleciM.es in Solids, Norch-Holland, Amsterdam, 1979, p. 155. 

From the identical shape and position of the absorption spectra (not shown) in chloroform and polysulfone we conclude that the distribution of geometries of the Ooct-OPV5-CN molecules is the same in both situations. In polysulfone, the non-radiative decay channel is effectively inhibited and a normal single-exponen-... 

In electroluminescence devices (LEDs) ionized traps form space charges, which govern the charge carrier injection from metal electrodes into the active material [21]. The same states that trap charge carriers may also act as a recombination center for the non-radiative decay of excitons. Therefore, the luminescence efficiency as well as charge earner transport in LEDs are influenced by traps. Both factors determine the quantum efficiency of LEDs. 

The luminescence of an excited state generally decays spontaneously along one or more separate pathways light emission (fluorescence or phosphorescence) and non-radiative decay. The collective rate constant is designated k° (lifetime r°). The excited state may also react with another entity in the solution. Such a species is called a quencher, Q. Each quencher has a characteristic bimolecular rate constant kq. The scheme and rate law are... 

Non-Radiative Decay Channels - 1064 nm Excitation. We turn now to a comparison of the observed fluorescence photon yield defined by Equation 1 and the expected fluorescence quantum yield of the 4550 cm 1 state which indicates that several non-radiative decay channels may be open following 1064 nm excitation of PuF6(g) The following relationship between... 

At low irradiances, photosynthesis uses virtually 100% of the quanta, but in full sunlight, about 2000 imol quanta s , more quanta are available than can be used in photochemistry. Maximum rates of photosynthesis by Populus or Spinacia leaves of 15 and 70 jumol O2 m s , respectively, would require only 15 x 9 = 135 to 630 jumol quanta m s , or 10-40%. Leaves, therefore, need to be able to dissipate 60-90% of the quanta at high irradiance in an orderly manner such as non-radiative decay if they are to avoid the potentially damaging formation of oxygen radicals from reduced ferredoxin (Asada Takahashi, 1987). When plants are under a stress that restricts CO2 assimilation, excessive light will be reached at even lower irradiances. 

As mentioned in the introductory part of this section, quantum dots exhibit quite complex non-radiative relaxation dynamics. The non-radiative decay is not reproduced by a single exponential function, in contrast to triplet states of fluorescent organic molecules that exhibit monophasic exponential decay. In order to quantitatively analyze fluorescence correlation signals of quantum dots including such complex non-radiative decay, we adopted a fluorescence autocorrelation function including the decay component of a stretched exponential as represented by Eq. (8.11). 

AES was developed in the late 1960s, and in this technique electrons are detected after emission from the sample as the result of a non-radiative decay of an excited atom in the surface region of the sample. The effect was first observed in bubble chamber studies by Pierre Auger (1925), a French physicist, who described the process involved. 

Non-radiative decay. The population can switch away from level 2 to another level, level 3 in Figure 3.17b, producing a population inversion. More molecules are now in the excited state than the ground state. 

For chemical systems of interest, photolysis produces intermediates, such as radicals or biradicals, whose energetics relative to the reactants are unknown. The energetics of the intermediate can be established by comparison of the acoustic wave generated by the non-radiative decay to create the intermediate, producing thermal energy , with that of a reference or calibration compound whose excited-state decay converts the entire photon energy into heat, / (ref). The ratio of acoustic wave amplitudes, a, represents the fraction of the photon energy that is converted into heat. 

Here, is the rate constant for radiative decay (fluorescence), while k r is the combined rate constant for aU non-radiative decay processes, is virtually constant and is an inherent property of the material in question, and for this material is significantly greater than k r, given the high fluorescence efficiency. When a fluorescence quencher, such as TNT, is introduced, km increases because an additional efficient non-radiative pathway now exists. This, via Eq. (4), makes r smaller. 

The large number of electronic configuration of iron porphyrins with oxidative states of 2+ or 3+, high and low spin forms, and charge transfer states with different axial ligands offer the possibility of a number of non-radiative decay pathways ( ). In... 

The energy of an electronically excited state may be lost in a variety of ways. A radiative decay is a process in which a molecule discards its excitation energy as a photon. A more common fate is non-radiative decay, in which the excess energy is transferred into the vibration, rotation, and translation of the surrounding molecules. This thermal degradation converts the excitation energy into thermal motion of the environment (i.e., to heat). Two radiative processes are possible spontaneous emission, just like radioactivity, which is a completely random process where the excited state decays ... 

A related phenomenon is the conversion of single visible photons with the result the quantum efficiency can be higher than 100%. If, for example, 0.1% of Pr + is incorporated in YF3 and excited with the mercury spectral line at 185 nm, the electron from 4f5d states decay non-radiatively to the Sq (4p) State. This system is able to generate two visible photons by So- f6> followed by non-radiative decay to the closely adjacent Po and by transition another photon is emitted by transitions to one of the six /-levels of or F. A condition for this cascade process is that the nephelauxetic effect for inter-shell transitions is sufficiently weakly pronounced for the lowest 4/5d state to be above Sq (Reisfeld and Jbrgensen 1977). 

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