Linearly depolarization

In analogy with the depolarization ratio for linearly polarized light, the ratio of the two above quantities is known as the reversal coefficient, R(Q, given by... 

A depolarization measurement consists of exciting a fluorescent sample with linearly polarized light and measuring the polarization of emitted light at right angles to the plane of excitation. The polarization of the emitted light is defined as... 

Steady-State Fluorescence Depolarization Spectroscopy. For steady state depolarization measurements, the sample is excited with linearly polarized lig t of constant intensity. Observed values of P depend on the angle between the absorption and emission dipole moment vectors. In equation 2 (9), Po is the limiting value of polarization for a dilute solution of fluorophores randomly oriented in a rigid medium that permits no rotation and no energy transfer to other fluorophores ... 

It is important from a practical viewpoint to predict the shear viscosity of mixtures from those of pure melts. For alkali nitrate melts, a linear dependence has been found between the reorientational line width obtained by Raman measurements and the ratio of temperature divided by shear viscosity.For NO3 ions, the depolarized Raman scattering from 1050cm" total stretching vibrational mode (Al) has a contribution to the line width L, which is caused by the reorientational relaxation time of the Csv axis of this ion. The Stokes-Einstein-Debye(SED) relation establishes a relation between the shear viscosity r of a melt and the relaxation time for the reorientation of a particle immersed in it ... 

The fluorescence depolarization technique excites a fluorescent dye by linearly polarized light and measures the polarization anisotropy of the fluorescence emission. The fluorescence anisotropy, r, is defined as... 

Figure 4.6 shows an apparatus for the fluorescence depolarization measurement. The linearly polarized excitation pulse from a mode-locked Ti-Sapphire laser illuminated a polymer brush sample through a microscope objective. The fluorescence from a specimen was collected by the same objective and input to a polarizing beam splitter to detect 7 and I by photomultipliers (PMTs). The photon signal from the PMT was fed to a time-correlated single photon counting electronics to obtain the time profiles of 7 and I simultaneously. The experimental data of the fluorescence anisotropy was fitted to a double exponential function. 

Depolarization of the synaptosomes with Ca-free media containing lOOmM K increased 86Rb efflux (figure 1, open squares) two kinetically and pharmacologically distinct K conductances could be discerned. Between 1 and 4 seconds, Rb efflux was linear and was 2.2 to 2.4%/sec (component "S"). Extrapolation of Rb efflux to the ordinate ("zero time") exposed an additional, rapid component of 86Rb efflux (component "T"). Component T reflects a distinct K channel that, unlike component S, appeared to inactivate in less than 1 second (Bartschat and Blaustein 1985a). 

Another important linear parameter is the excitation anisotropy function, which is used to determine the spectral positions of the optical transitions and the relative orientation of the transition dipole moments. These measurements can be provided in most commercially available spectrofluorometers and require the use of viscous solvents and low concentrations (cM 1 pM) to avoid depolarization of the fluorescence due to molecular reorientations and reabsorption. The anisotropy value for a given excitation wavelength 1 can be calculated as... 

Thus, when a population of fluorophores is illuminated by a linearly polarized incident light, those whose transition moments are oriented in a direction close to that of the electric vector of the incident beam are preferentially excited. This is called photoselection. Because the distribution of excited fluorophores is anisotropic, the emitted fluorescence is also anisotropic. Any change in direction of the transition moment during the lifetime of the excited state will cause this anisotropy to decrease, i.e. will induce a partial (or total) depolarization of fluorescence. 

Figure 7.2. (A) The greased-gap technique for recording depolarizations of the rat isolated vagus nerve. (B) The effect of ondansetron on depolarizations of the rat isolated vagus nerve induced by 5-HT. Symbols indicate controls (9) or in the presence of ondansetron at I x 10 M (O), 3 X 70" M (M), I X 10 M (O) or 3 x 10 M (A). Results are the mean + S.E.M. of at least four determinations. Experiments were performed as described by Ireland and Tyers [21J. (C) Data from the experiments illustrated in B plotted according to Arunlakshana and Schild [22]. Each point is the result from a separate tissue. The gradient of straight line (95 Vo confidence limits) was determined using linear regression analysis.

One can employ linearly polarized light to excite selectively those fluorophores that are in a particular orientation. The difference between excitation and emitted light polarization changes whenever fluorophores rotate during the period of time between excitation and emission. The magnitude of depolarization can be measured, and one can therefore deduce the fluorophore s rotational relaxation kinetics. Extrinsic fluorescence probes are especially useful here, because the proper choice of their fluorescence lifetime will greatly improve the measurement of rotational relaxation rates. One can also determine the freedom of motion of the probe relative to the rotational diffusion properties of the macromolecule to which it is attached. When held rigidly by the macromolecule, the depolarization of a probe s fluorescence is dominated by the the motion of the macromolecule. 

If one follows the solution viscosity in concentrated sulfuric acid with increasing polymer concentration, then one observes first a rise, afterwards, however, an abrupt decrease (about 5 to 15%, depending on the type of polymers and the experimental conditions). This transition is identical with the transformation of an optical isotropic to an optical anisotropic liquid crystalline solution with nematic behavior. Such solutions in the state of rest are weakly clouded and become opalescent when they are stirred they show birefringence, i.e., they depolarize linear polarized light. The two phases, formed at the critical concentration, can be separated by centrifugation to an isotropic and an anisotropic phase. A high amount of anisotropic phase is desirable for the fiber properties. This can be obtained by variation of the molecular weight, the solvent, the temperature, and the polymer concentration. 

Equation 3.13 predicts a linear dependence of In / on E whose slope depends on the coefficient ana, while the ordinate at the origin depends on the electrochemical rate constant and the net amount of depolarizer deposited on the electrode. Accordingly, both the slope and the ordinate at the origin of Tafel plots become phase-dependent [133, 183]. Since the quantity of depolarizer varies from one... 

For most experiments on nonisothermal TSR, simple cooling of the sample to the desired initial temperature and a linear increase in T after excitation are sufficient to obtain TSC and TSL glow curves. Some techniques require more elaborate heating cycles, the details of which depend on the relaxation mechanism under study and on whether it is necessary to discriminate between simnltaneously occurring processes, e.g., thermally stimulated depolarization and thermally stimulated conductivity (see Chapter 2). 

The decomposition of linearly polarized wave is the reverse of compounding of two plane polarized waves of the same phase angle (8 = 0). Depending on the slope tan-1 (b/a), the amplitudes a and b of the two waves, will differ and can be computed. For fluorescence depolarization studies, these amplitudes will correspond to Ij. and Ig components of the emitted radiation. 

Typical curves are shown in Fig. 22 a and b for current and potential steps respectively. For the short times (trate determining step is due to a concentration polarization [78], The variation of the transient current versus ]fi is linear, described satisfactorily by the classical Cottrel-type relation (see Fig. 23). After a certain time which depends on the experimental conditions, mainly the oxygen partial pressure in the surrounding atmosphere, a depolarization effect is observed the V (t) curve shows an overshoot while the I(t) curve shows a net enhancement of current. 

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