Rate of excitation

Here t. is the intrinsic lifetime of tire excitation residing on molecule (i.e. tire fluorescence lifetime one would observe for tire isolated molecule), is tire pairwise energy transfer rate and F. is tire rate of excitation of tire molecule by the external source (tire photon flux multiplied by tire absorjDtion cross section). The master equation system (C3.4.4) allows one to calculate tire complete dynamics of energy migration between all molecules in an ensemble, but tire computation can become quite complicated if tire number of molecules is large. Moreover, it is commonly tire case that tire ensemble contains molecules of two, tliree or more spectral types, and experimentally it is practically impossible to distinguish tire contributions of individual molecules from each spectral pool. 

Being applied for the relaxation of populations (k = 0), this equality expresses the demands of the detailed balance principle. This is simply a generalization of Eq. (4.25), which establishes the well-known relation between rates of excitation and deactivation for the rotational spectrum. It is much more important that equality (5.21) holds not only for k = 0 but also for k = 1 when it deals with relaxation of angular momentum J and the elements should not be attributed any obvious physical sense. The non-triviality of this generalization is emphasized by the fact that it is impossible to extend it to the elements of the four-index... 

The pK of tyrosine explains the absence of measurable excited-state proton transfer in water. The pK is the negative logarithm of the ratio of the deprotonation and the bimolecular reprotonation rates. Since reprotonation is diffusion-controlled, this rate will be the same for tyrosine and 2-naphthol. The difference of nearly two in their respective pK values means that the excited-state deprotonation rate of tyrosine is nearly two orders of magnitude slower than that of 2-naphthol.(26) This means that the rate of excited-state proton transfer by tyrosine to water is on the order of 105s 1. With a fluorescence lifetime near 3 ns for tyrosine, the combined rates for radiative and nonradiative processes approach 109s-1. Thus, the proton transfer reaction is too slow to compete effectively with the other deactivation pathways. 

A similar but smaller intramolecular quenching effect was seen by Phillips and co-workers 44,4S) for 1-vinylnaphthalene copolymers incapable of excimer fluorescence. The monomer fluorescence lifetime of the 1-naphthyl group in the methyl methacrylate copolymer 44) was 20% less than the lifetime of 1-methylnaphthalene in the same solvent, tetrahydrofuran. However, no difference in lifetimes was observed between the 1-vinylnaphthalene/methyl acrylate copolymer 45) and 1-methylnaphthalene. To summarize, the nonradiative decay rate of excited singlet monomer in polymers, koM + k1M, may not be identical to that of a monochromophoric model compound, especially when the polymer contains quenching moieties and the solvent is fluid enough to allow rapid intramolecular quenching to occur. 

The sky emits radiation, known as the airglow, which during both day and night contains components resulting from optical transitions of O Ag) and 02(1Se+), and it is important to assess the possible mechanisms by which Oa may be excited to see whether they can account for the observed concentrations of singlet 02. The concentrations of possible precursors, such as 0(3P), (X1/)), and 03, probably undergo a diurnal and a seasonal variation, and it should be possible to relate the changes in [O2(1A0)] and [02(1E J+)] to the rates of excitation and loss processes. Several problems arise in these studies, since, for many of the potentially... 

Three limiting dynamic situations can be envisioned in Scheme 3 (1) the rate of excited state reaction is slower than the rate of interconversion between A and A (equilibrium is established between A and A before... 

The rate of diffusive separation, k, was determined from separate experimental measurements of iodine radical diffusion rates in the high pressure diffusion limited regime (19). The rate of excited state deactivation, k i, was calculated from the measured quantum yields at high densities where G> = kd/k i (18). It was assumed that k i is proportional to the inverse diffusion coefficient, D 1 (19,23) as both properties are related to the collision frequency. 

P. Kolorenc, V. Averbukh, K. Gokhberg, L.S. Cederbaum, Ab initio calculation of interatomic decay rates of excited doubly ionized states in clusters, J. Chem. Phys. 129 (2008) 244102. 

At s = 0 the concentration corrections in Eq. (3.667) become the rates of excitation quenching by any partner that does not belong to a given couple (reactant pair). These bachelors compete for an excitation with the reactants in a couple when they move apart for a while between successive recontacts. Similar results were obtained with the many-particle theory of diffusion-influenced reactions based on the revised superposition approximation and became known as MPK1 [51]. The authors were the first who managed to obtain concentration corrections to the IET result for the kinetics of reversible energy transfer. In a subsequent modification of their theory, named MPK3 [126], the same authors reached the full correspondence with MET. 

In the case of irreversible energy transfer, such a dependence is given by Eq. (3.676). An increase of x, linear in quencher concentration NB, accelerates the rate of excitation decay, thus enhancing the role of nonstationary quenching. This well-known effect exists only in the limit of diffusion-controlled transfer... 

Three limiting dynamic situations can be envisioned for Sch. 20 (I) the rate of excited state reaction is slower than the rate of interconversion between Ax and A (equilibrium is established between Ax and A Y before decay kXY and kYX >> k Ax and kAr) (II) the rate of excited state reaction is faster than the rate of interconversion between Ax and AY (III) the rate of interconversion between Ax and AY and the rates of reaction (decay) of excited guest molecules at these sites are comparable (assume for instance,... 

Photo-induced electron transfer between [Ru(bpy)3]2+-like centres covalently bound to positively-charged polymers (N-ethylated copolymers of vinylpyridine and [Ru(bpy)2(MVbpy)]2+) and viologens or Fe (III) has been studied using laser flash photolysis techniques. It is found that the backbone affects the rates of excited state quenching, the cage escape yield, and the back electron transfer rate because of both electrostatic and hydrophobic interactions. The effect of ionic strength on the reactions has been studied. Data on the electron transfer reactions of [Ru(bpy)3]2+ bound electrostatically or covalently to polystyrenesulphonate are also presented. 

The photochemistry of alkanones has been reviewed recently 2c>. It would appear that rate constants for the physical decay processes of both singlets and triplets vary only slightly among a wide collection of structurally varied acyclic, cyclic, and polycyclic alkanones. Consequently, relative triplet yields are determined primarily by relative rates of excited singlet chemical reactions. As in most triplet reactions, overall quantum yields are usually determined primarily by the partitioning of metastable primary photoproducts. 

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