Mean quenching rate

After the excitation the population N2 decreases and the whole fluorescence decay, if exponential, gives a mean quenching rate Q according to ... 

Other groups may cause shortening of the lifetime. The phosphorescence of parvalbumin is quenched by free tryptophan with a quenching rate constant of about 10s M i s l (D. Calhoun, unpublished results). A more extensive survey of proteins or model compounds with known distances between tryptophans is needed to study how adjacent tryptophans affect the lifetime. It should be noted that at low temperature the phosphorescence lifetime of poly-L-tryptophan is about the same as that of die monomer.(12) This does not necessarily mean that in a fluid solution tryptophan-tryptophan interaction could not take place. Thermal fluctuations in the polypeptide chain may transiently produce overlap in the n orbitals between neighboring tryptophans, thus resulting in quenching. 

Information extracted from a meticulous library search, however, invalidates the radical mechanism.In a recent short communication researchers report that the quenching rate of the fluorescence of compound I with electron-rich alkenes is extremely fast, close to the diffusion control limit. Conversely, fluorescence quenching by methanol is 100 times slower. In the present case, this means that photoexcited I would be expected to react much faster with isopropylene than with methanol to form the methoxyl radical required by our hypothetical sequence. 

HAS are weaker quenchers of 02 than aromatic amines [208]. Extensive quenching studies were performed with aliphatic and heterocyclic amines [209]. Data obtained in the inhibition of the self-sensitized photo-oxidation of rubrene revealed that the quenching efficiency of unhindered amines may be correlated with their ionization potentials. A low ionization potential means a better quenching activity. Substitution on the a-carbon atom to nitrogen by alkyls reduces the quenching rate predicted from the ionization potential [208, 209]. The sensitivity of 02 quenching to steric effects may be exemplified with quenching rate constants kq(in dm3 mol was used as a standard. 

Whereas the observed decay profile no longer is characterized by a single decay rate, the steady-state fluorescence intensity becomes dependent on both 7obs and fc>bs. The typical Stern-Volmer plot is no longer represented by equation 7a, but rather by equation 7b, where fcobs is defined by equation 6b, fc q is the bimolecular quenching rate constant, fco is the probe s mean excited-state unimolecular decay rate constant, fcobs is the mean observed decay rate constant, 70 is the distribution parameter of the Gaussian for the unimolecular decay, and 7obs is the distribution parameter for the observed unimolecular decay rate. 

Ab-initio MD (AIMD) can usually target systems of few hundreds atoms and time scales of few tens of picoseconds [26-31], In the case of glasses, this means that it is possible to perform an AIMD melt-and-quench simulation of a system around 100-200 atoms, using quenching rates around 20-100 K/ps [31-34]. The small system size in AIMD prevents to extract a full description of medium-range order features such as the network connectivity (NQ of a glass. However, the AIMD models provide the most accurate description of local structure, such as the coordination of key ions, unbiased from any potential bias introduced by an empirical force field. This could be particularly important in order to target the local stmcture... 

What is quite surprising in these plots is not the (solution-like) Stem-Volmer behavior that is found, but the diffusion coefficients which can be calculated for MV2+. From the luminescence lifetime of Ru(bpy)32+ in the absence of quencher, the kgv values can be converted to bimolecular quenching rate constants kq. The diffusion coefficient is derived from kq by means of a modified Smoluchowsky equation (equation 1), where Dq is the diffusion coefficient of MV2+, Rq is its radius (taken to be 6.7 A), R is the average... 

Thermal spray processing A coating process where material (wire, rod, powder) is melted by a flame, plasma, electric arc, or some other means and the molten particles are propelled in a high velocity gas stream to the substrate surface, where they are splat cooled at a high quench rate. See also Arc-wire spray Detonation flame spraying Flame spray High velocity oxygen fuel (HVOF) spray Plasma spray. 

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