Energy transfer radiative

Radiative transfer is a two-step process a photon emitted by a donor D is absorbed by an acceptor that is chemically different (A) or identical (D) 

Characteristics of the donor emission Radiative transfer Non-radiative transfer 

Fluorescence spectrum Modified in the region of spectral overlap Unchanged 

Steady-state fluorescence Decreased in the region of Decreased by the same factor 

Fluorescence spectrum Steady-state fluorescence intensity Fluorescence decay Modified in the region of spectral overlap Decreased in the region of spectral overlap Unchanged Unchanged Decreased by the same factor whatever Xem Shortened 

Steady-state emission Decreased Strongly decreased 

This process is often called trivial transfer because of the simplicity of the phenomenon, but in reality the quantitative description is quite complicated because it depends on the size of the sample and its configuration with respect to excitation and observation. 

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In the case of polarized, but otherwise incoherent statistical radiation, one finds a rate constant for radiative energy transfer between initial molecular quantum states i and final states f... 

More recently Andrews and Juzeliunas [6, 7] developed a unified tlieory that embraces botli radiationless (Forster) and long-range radiative energy transfer. In otlier words tliis tlieory is valid over tire whole span of distances ranging from tliose which characterize molecular stmcture (nanometres) up to cosmic distances. It also addresses tire intennediate range where neitlier tire radiative nor tire Forster mechanism is fully valid. Below is tlieir expression for tire rate of pairwise energy transfer w from donor to acceptor, applicable to transfer in systems where tire donor and acceptor are embedded in a transparent medium of refractive index ... 

Noncontact Interactions (Nonradiative and Radiative Energy Transfer).197... 

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 ... 

Thus, this mechanism requires that A must be capable of absorbing the photon emitted by D that is, the acceptor absorption spectrum must overlap with the donor emission spectrum. Radiative energy transfer can operate over very large distances because a photon can travel a long way and A simply intercepts the photon emitted by D. ... 

Since the photon emitted by D is absorbed by A, the same rules will apply to radiative energy transfer as to the intensity of absorption. Because singlet-triplet transitions are spin-forbidden and singlet-triplet absorption coefficients are usually extremely small, it is not possible to build up a triplet state population by radiative energy transfer. For this... 

Finally, there is a specific red-edge effect related to non-radiative energy transfer between a donor fluorophore whose emission spectrum overlaps the absorption spectrum of an acceptor fluorophore in rigid polar solutions, there is a lack of energy transfer upon excitation at the red-edge. This effect, called Weber s effect, will be described in Section 9.4.3. 

Perrin s model has been used in particular for the interpretation of non-radiative energy transfer in rigid media (see Chapter 9). 

The use of Forster non-radiative energy transfer for measuring distances at a supramolecular level (spectroscopic ruler) will be discussed in detail in Chapter 9. 

Further details of non-radiative energy transfer will be presented in Chapter 9, together with various applications. 

We have considered so far non-radiative energy transfer from a donor to an acceptor that is a different molecule (heterotransfer). Energy transfer between like molecules is also possible (homotransfer) if there is some overlap between the absorption and fluorescence spectra. 

RADIATIVE ENERGY TRANSFER SPECTRAL OVERLAP ENHANCER ENOLASE... 

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. 

Fig. 2.6. Simplified sketch of electron band structme of a semiconductor mineral, showing the processes of excitation (energy absorption), non-radiative energy transfer and generation of luminescence (after Nasdala et al. 2004)...

The decay time of the Cr " band of approximately 150 ns is very short for such emission. Radiative energy transfer may not explain it because in such a case the decay curves of each of the ions are independent of the presence of the other. Thus non-radiative energy transfer may also take part, probably via multipolar or exchange interactions. In such cases the process of luminescence is of an additive nature and the lifetime of the sensitizer from which the energy is transferred is determined, apart from the probability of emission and radiationless transitions, by the probability of the energy transfer to the ion activator. 

Morawetz et al. [105,106] were the first to use non-radiative energy transfer (NRET) fluorospectroscopy for exploring polymer-polymer miscibility. The basic principle is as follows. In a system containing two kinds of fluorescence chromophore, if the emission spectrum of one (donor D) overlaps the absorption spectrum of the other (acceptor A), a non-radiative energy transfer from the former to the latter may occur when the system is excited by irradiation that the former selectively absorbs. The efflciency of energy transfer (E) inversely proportional to Icj/Ia> where Id and la denote the emission intensities of D and A, respectively, depends on the average distance r between D and A according to the relationship ... 

The primary objective of our investigations in this area has therefore been to investigate the relative importance of direct and radiative energy transfer processes in the interaction of various polymers with plasmas excited in inert gases2 8 ... 

V.L. Ermolaev, E.N. Bodunov, E.B. Sveshnikova and T.A. Shahverdov, Non-Radiative Energy Transfer (Nauka, Moscow, 1977). 

A non-radiative energy transfer solely by dipole-dipole interactions - without recourse to electron exchange - from energy donor to an acceptor (Fig. 5.4) is described by the Forster mechanism. 

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