Polarization emitted light

Pulsed method. Using a pulsed or modulated excitation light source instead of constant illumination allows investigation of the time dependence of emission polarization. In the case of pulsed excitation, the measured quantity is the time decay of fluorescent emission polarized parallel and perpendicular to the excitation plane of polarization. Emitted light polarized parallel to the excitation plane decays faster than the excited state lifetime because the molecule is rotating its emission dipole away from the polarization plane of measurement. Emitted light polarized perpendicular to the excitation plane decays more slowly because the emission dipole moment is rotating towards the plane of measurement. 

The polarization P of a molecule has three frequency components v and v v and the vibrational polarization emits light with its frequency. Therefore in general light scattering experiments, scattered light with these frequencies can be observed. In contrast a Raman spectrum can be measured only in the case where there are changes in molecular polarizability in the various vibrational modes, since Eq. (4.16) is expected to hold at dA/dQ 0 (Raman active). 

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... 

The simplest fluorescence measurement is that of intensity of emission, and most on-line detectors are restricted to this capability. Fluorescence, however, has been used to measure a number of molecular properties. Shifts in the fluorescence spectrum may indicate changes in the hydrophobicity of the fluorophore environment. The lifetime of a fluorescent state is often related to the mobility of the fluorophore. If a polarized light source is used, the emitted light may retain some degree of polarization. If the molecular rotation is far faster than the lifetime of the excited state, all polarization will be lost. If rotation is slow, however, some polarization may be retained. The polarization can be related to the rate of macromolecular tumbling, which, in turn, is related to the molecular size. Time-resolved and polarized fluorescence detectors require special excitation systems and highly sensitive detection systems and have not been commonly adapted for on-line use. 

A competitive fluorescence-polarization immunoassay method was described for the monitoring of 12 drugs including valproic acid [18]. Samples (serum or plasma) were deproteinated. Fluorescence from the fluorescein-labeled analyte used as tracer was excited at 488 nm and polarization of light emitted at 531 nm was measured. The calibration was stable for 4 weeks and the coefficient of variation was below 10%. A single measurement took 8-10 min. 

Polythiophenes for Structured and Polarized Polymer Light-Emitting Diodes... 

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. 

Polarization effects The transmission efficiency of a monochromator depends on the polarization of light. This can easily be demonstrated by placing a polarizer between the sample and the emission monochromator it is observed that the position and shape of the fluorescence spectrum may significantly depend on the orientation of the polarizer. Consequently, the observed fluorescence intensity depends on the polarization of the emitted fluorescence, i.e. on the relative contribution of the vertically and horizontally polarized components. This problem can be circumvented in the following way. 

Figure 1.11. Schematic view of a zeolite L crystal loaded with type 1 (Fig. 1.8) dyes with electronic transition moments aligned along the axis of the channels. Left Side view of the morphology, size, and optical anisotropy of the material. Right Front view of a few individual dye-filled channels. The polarization of absorbed and emitted light is indicated.

Usually, the most general nonspecific effects of dipole-orientational and electronic polarization of the medium are discussed, and the results of the theory of relaxational shifts developed under the approximation of a continuous dielectric medium may be used.(86 88) The shift of the frequency of the emitted light with time is a function of the dielectric constant e0, the refractive index n, and the relaxation time xR ... 

Thus, the electric dipole a polarized emissions departing from the Ai level (in which the electric field of the emitted light is parallel to x or y) are those defined by the direct product Ai x L. An inspection of Table 7.6 shows that Ai X E = E, so that only A E emissions are allowed for a emitted radiation (as shown in Figures 7.7 and 7.8). 

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