Radiative deactivation

In the absence of triplet quenchers two processes compete for triplet deactivation radiative (phosphorescence) and nonradiative decay of the triplet to the ground state ... 

How can we interpret this parameter for excitons in J-aggregates There is a competition between two processes exciton radiative relaxation and exciton trapping and even if an exciton forms near the trap there is a possibility that it is deactivated radiatively before trapping due to a very short radiative lifetime of J-aggregates (hundreds picoseconds and less) [3,5]. 

The excited state undergoes two competitive processes of rapid decay deactivation (radiative or not) and a reaction leading to the formation of the final product B. Under low-power continuous irradiation, the concentration of the excited state [A ], which is a species of shortlifetime (10-10—10 8 s), remains low and changes little. This is thus compatible with Bodenstein s assumption (quasi-stationary state) ... 

The efficiency of these radiative processes often increase at low temperatures or in solvents of high viscosity. Consequently emission spectra are generally run in a low-temperature matrix (glass) or in a rigid polymer at room temperature. The variation in efficiency of these processes as a function of temperature and viscosity of the medium indicates that collisional processes compete with radiative and unimolecular nonradiative processes for deactivation of the lowest singlet and triplet states. 

A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). 

As seen from (1) and (2), intermolecular processes may reduce essentially the lifetime and the fluorescence quantum yield. Hence, controlling the changes of these characteristics, we can monitor their occurrence and determine some characteristics of intermolecular reactions. Such processes can involve other particles, when they interact directly with the fluorophore (bimolecular reactions) or participate (as energy acceptors) in deactivation of S) state, owing to nonradiative or radiative energy transfer. Table 1 gives the main known intermolecular reactions and interactions, which can be divided into four groups ... 

Figure 2. Principles of reversible luminescence sensing using photochemical quenching processes (electron, energy or proton transfer). Dye = luminescent indicator Q = quencher species dotted arrow non-radiative deactivation processes. The luminescence intensity (and excited state lifetime) of the indicator dye decreases in the presence of the quencher. The indicator dye is typically supported onto a polymer material in contact with the sample. The quencher may he the analyte itself or a third partner species that interacts with the analyte (see text).

By absorption of light a molecule is promoted to a higher electronic state. The monomolecular physical processes for the dissipation of the excess energy are outlined in Fig. 5 in a so called Jablonski diagramm. In principle one has to differentiate between radiative and non-radiative deactivation on the one side and on the other side one has to consider if the multiplicity of the system is conserved or not. Radiative deactivation, i.e. deactivation accompanied by emission of light, is termed fluorescence if the transition occurs with spin conservation and phosphorescence, if spin inversion occurs. 

Chemiluminescence is believed to arise from the 2Bj and the 2B2 electronic states, as discussed above for the reaction of NO with ozone [17]. The primary emission is in a continuum in the range =400-1400 nm, with a maximum at =615 nm at 1 torr. This emission is significantly blue-shifted with respect to chemiluminescence in the NO + 03 reaction (Xmax = 1200 nm), as shown in Figure 2, owing to the greater exothermicity available to excite the N02 product [52], At pressures above approximately 1 torr of 02, the chemiluminescence reaction becomes independent of pressure with a second-order rate coefficient of 6.4 X 10 17 cm3 molec-1 s-1. At lower pressures, however, this rate constant decreases and then levels off at a minimum of 4.2 X 1(T18 cm3 molec-1 s-1 near 1 mtorr, and the emission maximum blue shifts to =560 nm [52], These results are consistent with the above mechanism in which the fractional contribution of (N02 ) to the emission spectrum increases as the pressure is decreased, therefore decreasing the rate at which (N02 ) is deactivated to form N02. Additionally, the radiative lifetime and emission spectrum of excited-state N02 vary with pressure, as discussed above for the NO + 03 reaction [19-22],... 

As illustrated in Fig. 7.15, the electromagnetic radiation measured in an XRF experiment is the result of one or more valence electrons filling the vacancy created by an initial photoionization where a core electron was ejected upon absorption of x-ray photons. The quantity of radiation from a certain level will be dependent on the relative efficiency of the radiationless and radiative deactivation processes, with this relative efficiency being denoted at the fluorescent yield. The fluorescent yield is defined as the number of x-ray photons emitted within a given series divided by the number of vacancies formed in the associated level within the same time period. 

The intramolecular processes responsible for radiative and radiationless deactivation of excited states we have considered so far have been uni-molecular processes that is, the processes involve only one molecule and hence follow first-order kinetics. 

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