Lifetime parameters spectra

The reciprocal of the FRET-unperturbed donor lifetime, td, is given by the sum of all rate constants for deactivation. These parameters have been extensively discussed in earlier chapters. We note in passing that the constants with extreme values in Eq. (12.1) disappear if one expresses the absorption (excitation) spectrum of the acceptor in terms of the molecular absorption cross-section, o (2) = 1017ln[10] Njy x e (2)(nnr/moleculc). 

The dyes exhibiting response manifested in change in one or several parameters of their fluorescence spectrum (quantum yield, maximum wavelength, excited state lifetime, etc.) are widely used in sensing. This response can be caused by various reasons. Here, one kind of dye response will be discussed, i.e., the formation of their associates or aggregates that often leads to drastic changes in the spectroscopic properties. 

An excitation-wavelength dependence at the longwave edge of the absorption spectrum has been observed not only for spectral displacement but also for other parameters such as lifetime, quantum yield and apparent rotational rate. Applications to the investigation of polymer rigidity and/or free volume, and to the study of biological systems and excited-state reactions have been developed. 

The concept of polarity covers all types of solute-solvent interactions (including hydrogen bonding). Therefore, polarity cannot be characterized by a single parameter. Erroneous interpretation may arise from misunderstandings of basic phenomena. For example, a polarity-dependent probe does not unequivocally indicate a hydrophobic environment whenever a blue-shift of the fluorescence spectrum is observed. It should be emphasized again that solvent (or microenvironment) relaxation should be completed during the lifetime of the excited state for a correct interpretation of the shift in the fluorescence spectrum in terms of polarity. 

Observation of reorientational dynamics of dipolar groups surrounding the fluorophore in response to changes in the dipole moment of the fluorophore occurring upon electronic excitation. Such dynamics result in the appearance of spectral shifts with time,(1 ) in changes of fluorescence lifetime across the fluorescence spectrum,(7,32) and in a decrease in the observable effects of selective red-edge excitation.(1,24 33 34) The studies of these processes yield a very important parameter which characterizes dynamics in proteins— the reorientational dipolar relaxation time, xR. 

Time-resolved emission spectra were reconstructed from a set of multifrequency phase and modulation traces acquired across the emission spectrum (37). The multifrequency phase and modulation data were modeled with the help of a commercially available global analysis software package (Globals Unlimited). The model which offered the best fits to the data with the least number of fitting parameters was a series of bi-exponential decays in which the individual fluorescence lifetimes were linked across the emission spectrum and the pre-exponential terms were allowed to vary. 

We now consider the parameters, listed below as (i)-(x) (Heyland et al., 1982), which can be derived from analysis of a gas lifetime spectrum. 

The radiative lifetime may be calculated from the Einstein B coefficient as determined from the integrated absorption spectrum. The absolute intensity of electronic transitions is usually determined from the absorption spectrum since for emission it is difficult to determine the number of molecules in the excited state. The parameter measured experimentally is the absorption coefficient, kv, which is defined by the relation... 

Several molecules with 3 A. ground states have been studied by both microwave and far-infrared laser magnetic resonance they include O2, SO and SeO. In O2 the observed transitions are necessarily magnetic dipole, and they are frequently used to calibrate the sensitivity of a FIR laser magnetic resonance spectrometer. The other species have electric dipole transitions, and we shall illustrate the situation by describing the studies of SO carried out by Carrington, Levy and Miller [56], SO was also one of the first free radicals to be studied by pure microwave methods, which we will describe in chapter 10. The analysis of the magnetic resonance spectrum actually made use of the parameters determined earlier by pure microwave studies. SO is an easy radical to study experimentally since it is relatively unreactive and has a lifetime of several... 

Kottis and Lefebvre (322) have suggested that if polarized light is used to excite randomly oriented molecules to the triplet state, observation of the changes in the AMg = +1 ESR spectrum can reveal the correlation of the polarization properties of the excitation with the principal axis system of the triplet zero-field tensor. Such photoselection experiments have been carried out successfully by Lhotse and coworkers (323) and El-Sayed and Siegel (324) on a number of aromatic systems. Piette and collaborators (325) have studied the effect of metal complexation on the zero-field parameters and lifetimes of the phosphorescent triplet of aromatic-metal complexes with similar photoselection technique. The changes in... 

The hydrogen bond lifetime tjq, a fundamental parameter in our picture for water, has also been considered by other authors, some of whonf suggested the use of depolarized Rayleigh scattering eiqiaiments to measure this quantity. The observed frequency spectrum consists of two distinct Lorentzians, the linewidth of the broader one being interpreted by these authors as the inverse of Tiq. 

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