Excited quenching

By criterion (3.151), the region is established where the energy transfer results in excitation quenching. At the border of this region, at xB = xA, the stationary constant (3.150) reaches the minimal value at given ka and kb... 

Figure 3.52. The kinetics of excitation quenching followed by delayed decay [solid line in (a)] in comparison to the free exponential decay with time zD [long dashed line in (a)]. The short dashed horizontal line in (a) indicates the amplitude of the delayed fluorescence yNo, whose free energy dependence is outlined in (b). The initial concentration of the excitations Nq — 0.01 M, zD = 1 us. The other parameters are D = D = 10 7 cm2/s, rc =0, Xc = 0.4 eV, c = 0.1M, A q = 10 4 M, Wi = 103 ns-1, L = 1.4 A, ct = 10A, T = 293 K. (From Ref. 189.)...

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. 

Figure 4.4 Excitation quenching by electron transfer between excited 1AE> molecule and a quencher in its singlet ground state, 1Q...

In the case of a metal substrate, the experimental evidence shows that metal excitation is dominated by surface photon absorption. Optical radiation excites surface charge carriers, usually free or sub-vacuum-level electrons that can efficiently couple to the adsorbate. This often leads to enhanced photolysis cross sections or altered product distributions. Excitation localized on the adsorbed molecule in close proximity to a metallic solid may efficiently couple to the electronic states of the surface, leading to excitation quenching. When light-absorbing molecules are separated from the surface by spacer molecules, the influence of the surface on molecular excitation and relaxation decreases [4,21],... 

Photocurrent response in the near-infrared region up to 1600 nm, related to absorption features of semiconducting SWNTs in blends with MEH-PPV and P30T, offers principally the operation of infrared sensitive photodetectors with these materials [318]. To enable the photon harvesting in this spectral region, the SWNTs needed to be finely dispersed within the polymer matrices, thereby switching off the excitation quenching observed within CNT bundles. 

Kyle DJ (1987) The biochemical basis for photoinhibition of Photosystem II. In Kyle DJ, Osmond CB and Arntzen CJ (eds) Photoinhibition. Photoinhibition, Topics of Photosynthesis, pp 197-226. Elsevier, Scientific Press, Amsterdam KyleDJ, Ohad I and Arntzen CJ (1985) Molecular mechanisms of compensation to light stress in chloroplast membranes. In Key JL and Kosuge T (eds) Cellular and Molecular Biology of Plant Stress, pp 51-69. Liss, New York Laisk A, Oja V, Rasulov B, Eichelmann H and Sumberg A (1997) Quantum yields and rate constants of photochemical and non-photochemical excitation quenching—Experiment and model. Plant Physiol 115 803-815 Lancaster CRD and Michel H (1996) Three-dimensional stmetures... 

STL has been widely applied to inorganic semiconductor surfaces and thin-film semiconductors especially those with grain boundaries and nanoscale impurities that lead to excitation quenching. It has also been applied to thin films of organics where properties such as the exdton diflusion length can be studied. " " ... 

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

The interpretation of emission spectra is somewhat different but similar to that of absorption spectra. The intensity observed m a typical emission spectrum is a complicated fiinction of the excitation conditions which detennine the number of excited states produced, quenching processes which compete with emission, and the efficiency of the detection system. The quantities of theoretical interest which replace the integrated intensity of absorption spectroscopy are the rate constant for spontaneous emission and the related excited-state lifetime. 

Typical singlet lifetimes are measured in nanoseconds while triplet lifetimes of organic molecules in rigid solutions are usually measured in milliseconds or even seconds. In liquid media where drfifiision is rapid the triplet states are usually quenched, often by tire nearly iibiqitoiis molecular oxygen. Because of that, phosphorescence is seldom observed in liquid solutions. In the spectroscopy of molecules the tenn fluorescence is now usually used to refer to emission from an excited singlet state and phosphorescence to emission from a triplet state, regardless of the actual lifetimes. 

The data obtained in tlie infrared-diode-laser-probe studies described above provides quenching infonnation at a given substrate donor energy E. By varying tlie laser excitation wavelengtli for production of vibrationally hot... 

Michaels C A, Lin Z, Mullin A S, Tapalian H C and Flynn G W 1997 Translational and rotational excitation of the C02(00°0) vibrationless state in the collisional quenching of highly vibrationally excited perfluorobenzene evidence for impulsive collisions accompanied by large energy transfers J. Chem. Phys. 106 7055-71... 

Agranovich V M, Efremov N A and Kirsanov V V 1980 Computer simulation of kinetics of excitation bimolecular quenching by Monte-Carlo method Fiz. Tverd. Tela 22 2118-27... 

Hydroxybenzophenones represent the largest and most versatile class of ulbaviolet stabilizers that are used to protect materials from the degradative effects of ulbaviolet radiation. They function by absorbing ultraviolet radiation and by quenching elecbonically excited states. 

Hindered amines, such as 4-(2,2,6,6-tetramethylpiperidinyl) decanedioate, serve as radical scavengers and will protect thin Aims under conditions in which ultraviolet absorbers are ineffective. Metal salts of nickel, such as dibutyldithiocarbamate, are used in polyolefins to quench singlet oxygen or elecbonically excited states of other species in the polymer. Zinc salts function as peroxide decomposers. 

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