Transfer of excitation

A distinctive feature of this type of polymer is that the processes of intermolecu-lar transfer of excitation energy are also hindered to a considerable degree since they become noticeable only at very high (of the order of 10 3 mol/1) concentrations of... 

Mechanical, physical, or chemical external irritants act not only at the place of occurrence, but the excitation can be also transferred along the whole plant [3,6-21]. The speed of transfer depends on many factors, such as the intensity of the irritation, temperature, chemical treatment, or mechanical wounding it is also influenced by previous excitations. The excitation reaction travels in both directions, from the top of a stem to roots and conversely, but not always at identical rates. The transfer of excitation has a complicated character accompanied by an internal change in cells and tissues. 

Therefore, the interaction of the EEPs with the surface of sensors is a complex process that, being dependent on the nature of the surface and the nature of the active particle, results either in chemical transformation (chemisorption, for instance), or in transfer of excitation energy to a solid body, the processes that proceed at different velocities. 

This conclusion is in agreement with experiments in which a smootb quartz and cellulose were used as substrates. For above materials the transfer of excitation energy of the dye into the substrate is low which is confirmed by intensive luminescence of adsorbed tripaflavine. Note, that the activation energy of emission of singlet oxygen is close for zinc oxide oxidized by oxygen atoms, quartz and cellulose and amounts to 5-10 kcal/mol [83]. 

The decomposition of methanesulphonyl azide in isopropyl alcohol could be effected by selective irradiation of 2-acetonaphthone instead of benzophenone 21>. Since 2-acetonaphthone triplets are incapable of hydrogen abstraction from isopropyl alcohol 22>, initiation must occur via transfer of excitation energy to the azide. A marked difference was observed from benzophenone sensitization in that the reaction was extremely slow, gave a nitrogen yield of only 68%, and produced a yellow solution 21>. 

Bimolecular reactions with paramagnetic species, heavy atoms, some molecules, compounds, or quantum dots refer to the first group (1). The second group (2) includes electron transfer reactions, exciplex and excimer formations, and proton transfer. To the last group (3), we ascribe the reactions, in which quenching of fluorescence occurs due to radiative and nonradiative transfer of excitation energy from the fluorescent donor to another particle - energy acceptor. 

Nonradiative transfer of excitation energy requires some interaction between donor and acceptor molecules and occurs if the emission spectrum of the donor overlaps the absorption spectrum of the acceptor, so that several vibronic transitions in the donor must have practically the same energy as the corresponding transitions in the acceptor. Such transitions are coupled, i.e., they are in resonance, and that is why the term resonance energy transfer (RET) or electronic energy transfer (EET) are often used. 

Duysens, L.N.M. Transfer of excitation energy in photosynthesis. Thesis, Utrecht 1952... 

In the strong coupling case, the transfer of excitation energy is faster than the nuclear vibrations and the vibrational relaxation ( 10 12 s). The excitation energy is not localized on one of the molecules but is truly delocalized over the two components (or more in multi-chromophoric systems). The transfer of excitation is a coherent process9 the excitation oscillates back and forth between D and A and is never more than instantaneously localized on either molecule. Such a delocalization is described in the frame of the exciton theory10 . 

Homotransfer does not cause additional de-excitation of the donor molecules, i.e. does not result in fluorescence quenching. In fact, the probability of de-excitation of a donor molecule does not depend on the fact that this molecule was initially excited by absorption of a photon or by transfer of excitation from another donor molecule. Therefore, the fluorescence decay of a population of donor molecules is not perturbed by possible excitation transport among donors. Because the transition dipole moments of the molecules are not parallel (except in very rare cases), the polarization of the emitted fluorescence is affected by homotransfer and information on the kinetics of excitation transport is provided by the decay of emission anisotropy. 

The transfer of excitation energy present in an atom or molecular species in an excited singlet state to another atom or molecular species, generating an electronically excited species in a triplet state. 

The only recent example of Forster transfer of photochemical importance is the demonstration by Saltiel163 that the ability of azulene to increase the photostationary transjcis ratio in direct photoisomerization of the stilbenes is due entirely to radiationless transfer of excitation from traw.y-stilbene singlets to azulene. As expected for Forster transfer, this azulene effect did not depend upon solvent viscosity. The experimental value of R0, the critical radius of transfer in Forster s formula,181 was 18 A, in good agreement with the value calculated from the overlap of stilbene emission and azulene absorption. 

How are cardiac cells coupled How do cardiac cells interact electrically These are important questions to be addressed and they lead to the topic of cardiac networking. Sperelakis [1979] distinguished three forms of transfer of excitation (1) mechanical transmission (2) chemical transmission, and... 

The interactions between metastable noble-gas atoms and ground-state noble-gas atoms are relatively simple and have been investigated quite extensively. If the excitation energy is lower than the ionization potential of the collision partner, the only important inelastic process is the transfer of excitation energy.12 The excitation transfer is usually very efficient when the process is near resonant. The process that is responsible for the operation of the He-Ne laser,13... 

External Conversion.—Transfer of excitation energy to the environment. The term is now little used but was originally coined to contrast with internal conversion (vide supra). In the usual context the term has about the same meaning as energy transfer. 

The surprising occurrence of photoionization in aryl carboxylic acids may be explained by the transfer of excitation energy from the ring system to the carboxyl group to give an intermediate with charge separation at the side-chain terminus—e.g.,... 

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