Resonance dominant, lifetimes

A result of tlie relaxation processes is a shortened lifetime of the spin states giving rise to a broadening of the EPR line, which for most magnetic resonance lines dominated by homogeneous linewidth can be written as... 

Many bimolecular and unimolecular reactions are dominated by long-lived resonances. As a result, having knowledge about the positions and lifetimes of such resonance states is highly desired. Recursive calculations of resonance states have been reported for many molecular systems, including... 

INTRINSIC AND EXTRINSIC FLUORESCENCE. Intrinsic fluorescence refers to the fluorescence of the macromolecule itself, and in the case of proteins this typically involves emission from tyrosinyl and tryptopha-nyl residues, with the latter dominating if excitation is carried out at 280 nm. The distance for tyrosine-to-tryp-tophan resonance energy transfer is approximately 14 A, suggesting that this mode of tyrosine fluorescence quenching should occur efficiently in most proteins. Moreover, tyrosine fluorescence is quenched whenever nearby bases (such as carboxylate anions) accept the phenolic proton of tyrosine during the excited state lifetime. To examine tryptophan fluorescence only, one typically excites at 295 nm, where tyrosine weakly absorbs. [Note While the phenolate ion of tyrosine absorbs around 293 nm, its high pXa of 10-11 in proteins typically renders its concentration too low to be of practical concern.] The tryptophan emission is maximal at 340-350 nm, depending on the local environment around this intrinsic fluorophore. 

Within the harmonic approximation, we observe that the lifetimes of the dominant resonances (with Vj = 0 for the unstable modes j) are therefore directly related to the sum of positive Lyapunov exponents,... 

At 2 V/cm (Fig. 1 e) the spectrum is dominated by hydrogenic manifold structure and there is strong overlap between many manifolds. Other calculations (not shown) indicate that there is no major qualitative change in average lifetimes between 1 and 5 V/cm, although the resonances become... 

Fig. 8. Depletion-recovery spectra of the 597.26-nm (left) and 470.72-nm (right) resonances at various target densities. The lifetimes of the radiation-dominated parent states (n, V) = (39,35) and (37,34) axe plotted as a function of density. The (37,34) state becomes short lived with increased density, while the higher-lying (39,35) remains unaffected. Theoretical radiative rates and the sum of radiative and Auger rates axe also shown [27]. From Hori et al. [26]...

Fig. 3 shows the combined Mg10+ data, and N5+ resonances obtained with the same 4 pg cm-2 foil. The magnesium resonance width is dominated by the natural width due to the 29.5 ps mean lifetime of the 2 3Pi state [24], A Lorentzian fit gave a FWHM in beam energy of 315(24) keV, consistent with the 326 keV expected from the natural width. The widths of the much narrower nitrogen... 

The lifetimes of molecular fluorescence emissions are determined by the competition between radiative and nonradiative processes. If the radiative channel is dominant, as in the anthracene molecule, the fluorescence quantum yield is about unity-and the lifetime lies in the nanosecond range. In molecular assemblies, however, due to the cooperative emission of interacting molecules, much shorter lifetimes—in the picosecond or even in the femtosecond range—can theoretically be expected an upper limit has been calculated for 2D excitons [see (3.15) and Fig. 3.7] and for /V-multilayer systems with 100 > N > 2.78 The nonradiative molecular process is local, so unless fluorescence is in resonance by fission (Section II.C.2), its contribution to the lifetime of the molecular-assembly emission remains constant it is usually overwhelmed by the radiative process.118121 The phenomenon of collective spontaneous emission is often related to Dicke s model of superradiance,144 with the difference that only a very small density of excitation is involved. Direct measurement of such short radiative lifetimes of collective emissions, in the picosecond range, have recently been reported for two very different 2D systems ... 

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