Energy transfer stepwise

Kr. In the B-emitting states, a slower stepwise relaxation was observed. Figure C3.5.5 shows the possible modes of relaxation for B-emitting XeF and some experimentally detennined time constants. Although a diatomic in an atomic lattice seems to be a simple system, these vibronic relaxation experiments are rather complicated to interiDret, because of multiple electronic states which are involved due to energy transfer between B and C sites. 

Ehrenfest dynamics with the MMVB method has also been applied to the study of intermolecular energy transfer in anthryl-naphthylalkanes [85]. These molecules have a naphthalene joined to a anthracene by a short alkyl —(CH)n— chain. After exciting the naphthalene moiety, if n = 1 emission is seen from both parts of the system, if n = 3 emission is exclusively from the anthracene. The mechanism of this energy exchange is still not clear. This system is at the limits of the MMVB method, and the number of configurations required means that only a small number of trajectories can be run. The method is also unable to model the zwitterionic states that may be involved. Even so, the calculations provide some mechanistic information, which supports a stepwise exchange of energy, rather than the conventional direct process. 

Nonradiative energy transfer has a major role in the process of photosynthesis. Light is absorbed by large numbers of chlorophyll molecules in light-harvesting antennae and energy is transferred in a stepwise manner to photosynthetic reaction centres, at which photochemical reactions occur. This fundamental energy-transfer process will be considered in more detail in Chapter 12. 

The processes involved are stepwise energy transfer, cooperative sensitization of luminescence and cooperative luminescence. As an example of stepwise energy transfer a system containing Er and Yb may be considered. The latter ion absorbs at 970 nm (10,300 cm ) and in phonon assisted an Er ion... 

This formula was first derived in ref. 6 when calculating the kinetics of donor luminescence decay in the presence of the randomly, i.e. chaotically, located acceptors under the condition n N and on the assumption of the resonance exchange mechanism of energy transfer. Similar equations were later used for the analysis of experimental data on the kinetics of electron tunneling reactions obtained under conditions of the chaotic distribution of the reagents and at n < N. As a rule, only the first term of the exponent in eqn. (23) has been taken into account, which is equivalent to employing the previously mentioned (see Sect. 2.1) stepwise approximation of the function 0(R,t) = exp[- 1V(jR)(]. In this case, one obtains... 

Looking at this summary reaction, you may wonder why it doesn t reflect the substantially greater ATP yield that is supposed to be characteristic of aerobic metabolism. The answer is that energy is stored in the reduced coenzyme molecules on the right-hand side of the reaction. Reoxidation of these compounds liberates a large amount of free energy. It is only as the electrons are transferred stepwise from the coenzymes to molecular oxygen that the coupled... 

The other complexes were more complicated and involved stepwise energy transfer from the 3MLCT state of the Ru(II) chromophore to the Os(II) chro-mophore. For the case of the banb bridging ligand, the bridge anthracene 3(n-n ) state is the lowest energy excited state and both the Ru chromophore... 

We studied, at the B3LYP/6-31+G theoretical level, four monomers and 12 NH-pyrazole cyclamers, C-unsubstituted or bearing fluoro, chloro and bromo substituents at positions 3 and 5 [100], Two mechanisms of proton transfer, stepwise and synchronous, were calculated for dimers, trimers, and tetramers. The set of values of energies and geometries thus obtained provide useful insights about the dynamics of NH-pyrazoles in the solid state. It has been shown that pyrazole cyclamers exist not only in condensed phases but in the gas phase as well [101], thus our gas-phase calculations will provide information about the solid state. 

Atmospheric pressure plasmas, just like most other plasmas, are generated by a high electric field in a gas volume. The few free electrons which are always present in the gas, due to, for example, cosmic radiation or radioactive decay of certain isotopes, will, after a critical electric field strength has been exceeded, develop an avalanche with ionization and excitation of species. Energy gained by the hot electrons is efficiently transferred and used in the excitation and dissociation of gas molecules. In a nonequilibrium atmospheric pressure plasma, collisions and radiative processes are dominated by energy transfer by stepwise processes and three-body collisions. The dominance of these processes has allowed many... 

The approximate treatment of these systems composed of an electron donor and an electron acceptor as two separate entities appears to be best justified for the van der Waals adducts. As the interaction between the two components becomes stronger, the distinction between two-component systems and two-state systems becomes hazy. In particular, the rigid systems studied by Verhoeven, Paddon-Row and their co-workers are most interesting. The very strong interaction does not express itself in rapid electron transfer only, but also in energy transfer [37]. In these systems, the concept of stepwise electron transfer between different parts of the molecule may be inappropriate. When the interaction between donor and acceptor states becomes appreciable, the system has to be treated as a single quantum me-... 

Figure 8. Energy level scheme for 33 and 34. Excitation with 226 nm light populates BP, which undergoes efficient energy transfer to PE and NP. Subsequent energy transfer and intersystem crossing populate the triplet state of the phenanthrene, PE. Excitation to the PE state with 488 nm light results in stepwise energy transfer to BP and NP, the latter of which acts as an energy well.

Substantial cross interaction between effects on forming and breaking bonds ( ,., ) is consistent with a concerted mechanism. A stepwise process, where there is no substantial energy transfer between bond formation and bond fission, would give only small cross-interaction coefficients. 

The incident energy is selected stepwise across the spectrum and, since the final energy is fixed below 40 cm, the energy transfer is obtained. This is the working principle of the spectrometer INlBeF [16] at the ILL, which was, for many years, the best spectrometer for neutron vibrational speetroscopy. 

Siegel and Judeikis [124] have shown that the photodegradation of polydimethylsiloxane is sensitized by naphthalene. A stepwise biphotonic absorption process, involving the first triplet state of naphthalene as an intermediate, followed by energy transfer, has been proposed. It seems that the decomposition of the polymer occurs via Si—CH3 bond rupture and abstraction of hydrogen atoms by the methyl radicals to produce —Si—CHj radicals. Breaking of the — Si—CH3 bond has also been proposed as the most probable reaction in the vacuum ultraviolet photolysis of polydimethylsiloxane at 123.6 and 147 nm [125]. 

P. Graber, Stepwise rotation of the y-subunit of EFoFi-ATP synthase observed by intramolecular single-molecule fluorescence resonance energy transfer, FEBS Lett. 2002, 527, 147-152. 

---