Absorption population kinetics

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. 

Returning to the kinetic equations that govern the time evolution of the populations of two levels connected by photon absorption and emission, and adding in the term needed for spontaneous emission, one finds (with the initial level being of the lower energy) ... 

Experimentally the key to the method is determining the relative population of triplet molecules by the optical density of triplet-triplet absorption following flash excitation. To derive the kinetic expressions needed to determine O , the scheme is the same as that written previously with the additional proviso that the singlet interaction lead to intersystem crossing by the sensitizer. 

If high temperatures eventually lead to an almost equal population of the ground and excited states of spectroscopically active structure elements, their absorption and emission may be quite weak, particularly if relaxation processes between these states are slow. The spectroscopic methods covered in Table 16-1 are numerous and not equally suited for the study of solid state kinetics. The number of methods increases considerably if we include particle radiation (electrons, neutrons, protons, atoms, or ions). We note that the output radiation is not necessarily of the same type as the input radiation (e.g., in photoelectron spectroscopy). Therefore, we have to restrict this discussion to some relevant methods and examples which demonstrate the applicability of in-situ spectroscopy to kinetic investigations at high temperature. Let us begin with nuclear spectroscopies in which nuclear energy levels are probed. Later we will turn to those methods in which electronic states are involved (e.g., UV, VIS, and IR spectroscopies). 

Comparative Toxicokinetics. A limited number of studies exist regarding the comparative toxicokinetics of orally administered silver compounds in rats, dogs, monkeys, and humans. A more complete comparison of the absorption and elimination of silver in humans and rats may be warranted given that much of the toxicokinetic data comes from rats. It would also be useful to acquire data on the comparative toxicokinetics of various silver compounds in several species of experimental animals and in humans following inhalation and dermal exposure in order to model the kinetics of silver deposition across different exposure scenarios and within sensitive populations. 

Primary steps of photoinduced electron transfer have been studied in plant reaction centers (PS-I and PS-II), by flash absorption and EPR. In PS-I two questions wereinvestigated i) the properties of the primary radical pair P-700+, A0 (kinetics of decay nature of A0, presumably a specialized chlorophyll a decay by recombination to populate the P-700 triplet state) and ii) the nature of the secondary acceptor A,. Extraction-reconstitution experiments indicate that A, is very probably a molecule of vitamin K,. 

The core of the PS-II reaction center has been prepared. Flash absorption showed that this core is able of efficient charge separation to form the primary radical pair which decays in about 30ns. The recombination populates the P-680 triplet state, which does not transfer to beta-carotene and can be detected by spin-polarized ESR. The yield of formation and kinetics of decay of the radical-pair have been measured in various PS-II preparations. The data are in favor of an equilibrium between the radical-pair and chlorophyll excited state in the antenna. 

An ideal photosensitizer must satisfy several stringent requirements (Balzani et. al., 1986) 1) stability towards thermal and photochemical decomposition reactions 2) sufficiently intense absorption bands in a suitable spectral region 3) high efficiency of population of the reactive excited state 4) long lifetime in the reactive excited state 5) suitable ground state and excited state potentials 6) reversible redox behavior 7) good kinetic factors for outer sphere electron transfer reactions. 

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