Polarization decay

Time Resolved Fluorescence Depolarization. In Equation 3, it is assumed that the polarization decays to zero as a single exponential function, which is equivalent to assuming that the molecular shape is spherical with isotropic rotational motion. Multiexponential decays arise from anisotropic rotational motion, which might indicate a nonspherical molecule, a molecule rotating in a nonuniform environment, a fluorophore bound to tbe molecule in a manner that binders its motion, or a mixture of fluorophores with different rotational rates. 

In principle, pulsed excitation measurements can provide direct observation of time-resolved polarization decays and permit the single-exponential or multiexponential nature of the decay curves to be measured. In practice, however, accurate quantification of a multiexponential curve often requires that the emission decay be measured down to low intensity values, where obtaining a satisfactory signal -to-noise ratio can be a time-consuming process. In addition, the accuracy of rotational rate measurements close to a nanosecond or less are severely limited by tbe pulse width of the flash lamps. As a result, pulsed-excitation polarization measurements are not commonly used for short rotational periods or for careful measurements of rotational anisotropy. 

A version of the galvanostatic method is that where the current is turned off (or a current f = 0 is applied ) and the polarization decay curve is measured. Consider an electrode which up to the time t = 0, when the current was turned off, had the potentiaf F at the net current density When the current is turned off, the ohmic voftage drop in the electrolyte gap between the electrode and the tip of the Luggin capillary vanishes, so that the potential instantaneously shifts to the value F (Fig. 12.11). After that the electrode potential returns (falls) relatively slowly to its open-circuit value, for which a certain nonfaradaic charging current is required. Since ip + =... 

A Theory of Fluorescence Polarization Decay in Membranes, Biophys. J. 20, 289-305. 

P. E. Zinsli, Inhomogeneous interior of aerosol OT microemulsions probed by fluorescence and polarization decay,/. Phys. Chem. 83, 3223-3231 (1979). 

The first observation of spin-polarization in PS I was reported by Blankenship et al.273 in 1975 (for review see reference 274). Spectra at higher mw frequencies allowed the assignment Aj to a quinone in the radical pair.275276 Interestingly, two sequential RPs were observed277,278 resulting from P q A, and P o FeS" showing that kinetic information about forward ET can be extracted in addition to relaxation and polarization decay. Furthermore, the relative orientation of AT and Aj- could be deduced, the analysis is based on the correlated coupled radical pair (CCRP) concept.279 281... 

Aryutyunan, V.M., Adonts, G.G. and Kanatsyan, E.G. (1982). Determination of atomic-level polarization decay times by a pulse delay technique, Optika i Spektroskopiya, 53, 792-795. [Opt. Spectrosc. (USSR), 53, 472-474],... 

In the frame of the Debye model, the dipolar polarization decay is given by... 

Boltzmann equation is recovered, a low concentration of Bjerrum defects provided a much too large polarization decay length (ref 13 indicated as an example the value ... 

When an external field is applied to a dielectric, polarization of the material reaches its equilibrium value, not instantaneously but rather over a period of time. By analogy, when the field is suddenly removed, the polarization decay caused by thermal motion follows the same law as the relaxation or decay function of dielectric polarization < >(t) ... 

Celaschi and Mascarenhas (1977) studied nearly dry lysozyme by elec-tret thermal depolarization, thermal-stimulated pressure, isothermal polarization decay, and thermogravimetry. For a change in temperature of the sample from 250 K to room temperature, desorption of water dipoles was the main process responsible for electrical depolarization. 

Kinosita K, Jr., Kawato S, Ikegami A. A theory of fluorescence polarization decay in membranes. Biophys. J. 1977 20 289-305. 

The transitions between levels with non-equilibrium populations will be in the direction towards restoring the normal Boltzmann population their signal intensities will depend on the extent of non-equilibrium population. The observed effects are optimal for radical pairs with lifetimes in the nanosecond range. On a shorter time-scale, hyperfine induced intersystem crossing is negligible whereas on a longer time-scale the polarization decays owing to spin-lattice relaxation in the radicals. 

Removal of the external field results in a reorientation of dipoles and the polarization decays with time according to the equation. 

Figure 8. Measured polarization decay functions at 24°C for three segmented polyether-polyurethanes. HET49-1 is tested after annealing at 80°C followed by stabilization at 24°C. UET48-1 is tested with and without annealing. The decay function is the depolarization current density normalized by the electric field value...

A treatment for analysing the excitation and fluorescence multiwavelength polarized decay surfaces has been given for the case of a mixture of noninteracting species. An improved model for analysis of fluorescence anisotropy measurements has been presented. Limitations to the use of intense excitation pulses in fluorescence and thermal lens spectrophotometers are discussed in terms of optical saturation. Such artefacts can be eliminated by reference to the fluorescence quantum yield of Rhodamine 6G. A model has been given to describe spectral diffusion in time-resolved hole-burning spectroscopy. ... 

If one of the component polymers has a third spin other than and H, several cross-relaxation experiments become possible. Since spin diffusion occurs a few 10 nm before the polarization decays, only a small amount of the spins near the interface must be detected for immiscible blends. Section... 

Fig. 5. Pulsed-nozzle FT microwave measurements. A molecule-radiation interaction occurs when the gas pulse is between mirrors forming a Fabry-Perot cavity. If the transient molecule has a rotational transition of frequency vm falling within the narrow band of frequencies carried into the cavity by a short pulse (ca. 1 (is) of monochromatic radiation of frequency v, rotational excitation leads to a macroscopic electric polarization of the gas. This electric polarization decays only slowly (half-life T2 = 100 (is) compared with the relatively intense exciting pulse (half-life in the cavity t 0.1 (is). If detection is delayed until ca. 2 (is after the polarization, the exciting pulse has diminished in intensity by a factor of ca. 106 but the spontaneous coherent emission from the polarized gas is just beginning. This weak emission can then be detected in the absence of background radiation with high sensitivity. For technical reasons, the molecular emission at vm is mixed with some of the exciting radiation v and detected as a signal proportional to the amplitude of the oscillating electric vector at the beat frequency v - r , as a function of time, as in NMR spectroscopy Fourier transformation leads to the frequency spectrum [reproduced with permission from (31), p. 5631.

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