Energy Transfer Phenomena

Lanthanide and Transition Metai Doped inorganic Lattices 

We have recently considered energy transfer in the Cr + Tm + YAG system [267,268]. At ambient pressure, energy transfer occurs from the thermally and spin-orbit coupled E and T2 states of Cr + to the overlapping p2, p3 states of Tm (Figs. 28 and 29). Once excited, Tm + decays non-radiatively and subse- 

Our experiments consisted of measuring the decay curves of Cr + in 0.2% Cr + 0.7%Tm + YAG as a function of temperature and pressure. Representative pressure dependent decay curves at room temperature are shown in Fig. 30. The decay curves indicate a decrease in the decay rate of Cr + with increasing pressure. The rate of decrease, however, differs from that observed for Cr + in Cr3+ YAG (Fig. 14) where no energy transfer occurs. In order to separate the contributions of intracenter ( E, T2 radiative and non-radiative decay to the A2 ground state) and intercenter ( E, T2 energy transfer to Tm +) processes to the decay, we analyzed the decay curves using a rate equation model. According to the model, the time dependence of the excited state population of Cr + is 

Equations (51) and (52) were used to fit the pressure dependent decay curves at room temperature. In the fits we considered the first eight acceptor coordination shells, assumed a dipole-dipole energy transfer mechanism (m = 6) and al- 

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In the current study and similar studies reported by Schumacher and coworkers, the ratio of sodium atoms to sodium trlmers is thought to be between 10 and 40 to one as a function of experimental conditions (a more exact estimate will require more certain measurement of the cross sections for electron impact and/or photoionization cross sections). The difference in monomer and trimer abundance can easily be overcome by the density of effective states in Nas which significantly exceeds those in atomic sodium. Because of the Increased density of states, it would not be surprising if the cross section for Na2 -Na3 energy transfer far exceeded that for Na2 -Na. The energy transfer phenomena is currently the subject of further investigation in our laboratory. 

Hopefully, the ongoing work will enable us to complete our knowledge of radiolytic yields in confined media. But their proper interpretation relies on a profound understanding of the energy transfer phenomena occurring between phases and of the kinetic perturbation induced by confinement. Simulations already allow to make assumptions on these perturbations, but only time-resolved experiments can determine the dominant ones and put numbers on the time scale involved. 

Comparison of the results discussed above for the P-Q dyads and the C-P-Q triad molecules shows that although the triads are more complex molecular devices, and therefore more difficult to prepare and study, this very complexity makes possible interesting photochemistry which is precluded in the simpler systems. It is reasonable to suppose, then, that still more complex molecular species might demonstrate other novel electron and energy transfer phenomena which cannot occur in triads. This has proven to be the... 

At the present time, the existence of energy transfer phenomena appears to be clearly demonstrated in an experimental way. However, the transfer mechanisms we propose, even if they are plausible, nevertheless should be looked upon as different hypotheses that cannot be, as yet, either confirmed or invalidated. It is probable that, in certain cases, only one particular transfer mechanism is possible, but it is also quite plausible that, in other cases, the three transfer mechanisms exist simultaneously. 

For tungstates more values of the Stokes shift are known and these confirm quantitatively the statement made above for the titanate group (see Sect. 3.7). Due to the smaller Stokes shift in the case of the octahedral complexes, energy transfer between octahedral complexes is more efficient than between tetrahedral complexes. This will be discussed further in Chap. 2. At this stage of the discussion this fact is nevertheless important to realize, because the occurrence of energy transfer phenomena complicates the study of isolated centres considerably. 

In this section we will review experimental results which have contributed to our knowledge of host-sensitized energy transfer. A large part of these results is of a qualitative nature only. Nevertheless this is instructive and often of large value in predicting energy transfer phenomena in new materials. On the other hand the number of reliable, quantitative results is increasing, especially after the introduction of laser spectroscopy in this field of research. 

In this chapter results of the picosecond laser photolysis and transient spectral studies on the photoinduced electron transfer between tryptophan or tyrosine and flavins and the relaxation of the produced ion pair state in some flavoproteins are discussed. Moreover, the dynamics of quenching of tryptophan fluorescence in proteins is discussed on the basis of the equations derived by the present authors talcing into account the internal rotation of excited tryptophan which is undergoing the charge transfer interaction with a nearby quencher or energy transfer to an acceptor in proteins. The results of such studies could also help to understand primary processes of the biological photosynthetic reactions and photoreceptors, since both the photoinduced electron transfer and energy transfer phenomena between chromophores of proteins play essential roles in these systems. 

Studies of vibrational relaxation using lasers have expanded dramatically in the past fourteen years due largely to the increased availability of versatile, high-power, pulsed-laser devices. Increased demand for energy-transfer parameters has resulted from attempts to develop new lasers and to push existing systems to their practical limits. Thus the development of lasers and the study of energy-transfer phenomena have been mutually synergistic processes. 

I igure 6.14 shows the emission spectra of the three green phosphors. In all of them some ultraviolet emission originating from Ce " or Gd + is present. The energy transfer phenomena are different as will be discussed now. 

The energy transfer phenomena have been well known for several decades. Two mechanisms are known (1) Forster (the most common) and (2) Dexter [49-51]. The processes are shown in Figure 4.12. 

Hirayama (1965). Energy transfer phenomena relevant to rare earths have been reviewed recently by Kushida (1973) and Watts (1975). 

We then introduce a few examples for the use of SPFS, first in surface hybridization studies and then for antigen-antibody interaction assays. We will give, in particular, examples for different versions of fluorescence spectroscopy making use of, e.g., donor-acceptor energy transfer phenomena between correspondingly labeled binding partners. 

These structures provide an excellent platform for the study of novel molecule-superconductor electron and energy transfer phenomena. 

Similar examples for energy transfer from ligand localized levels to highly localized 4f levels are represented by the rare-earth chelates. Voloshin and Savutskii (1976) studied europium benzoylacetonate imder pressures up to 6 GPa. Exciting the triplet level they could observe the luminescence from the Eu " " ion. It was possible to describe the observed initial increase in the quantum yield of the Eu " " luminescence up to 2.5 GPa and the following decrease by the exchange resonance theory (Dexter, 1953). A more detailed study on different Tris chelates of Sm +, Eu +, Gd " ", and Tb " with -diketonates was performed by Hayes and Drickamer (1982), where the most dramatic effects of pressure on energy transfer phenomena were found for the Eu + chelates. 

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