Polarizers/Polarization wavelength dependence

The analysis of cometary observations suggests the existence of very fluffy dust aggregates. Differences are observed in the light-scattering properties, e.g. stracture of the comae, polarization phase curves maxima and minima, polarization wavelength dependence. They could be a clue to the temporal evolution of the physical properties of the dust particles, with collisional processes as well as evaporation of icy mantles and organic compoimds. Table 1 presents some polarization properties of dust particles in comets, asteroids, in the interplanetary dust cloud, and on Mars, as retrieved by remote sensing. 

While s-polarized radiation approaches a phase change near 180° on reflection, the change in phase of the p-polarized light depends strongly on the angle of incidence [20]. Therefore, near the metal surface (in the order of the wavelength of IR) the s-polarized radiation is greatly diminished in intensity and the p-polarized is not [9]. This surface selection rule of metal surfaces results in an IR activity of adsorbed species only if Sfi/Sq 0 (/i = dipole moment, q = normal coordinate) for the vibrational mode perpendicular to the surface. 

Optical activity also manifests itself in small differences in the molar extinction coefficients el and er of an enantiomer toward the right and left circularly polarized light. The small differences in e are expressed by the term molecular ellipticity [9 J = 3300(el — r). As a result of the differences in molar extinction coefficients, a circularly polarized beam in one direction is absorbed more than the other. Molecular ellipticity is dependent on temperature, solvent, and wavelength. The wavelength dependence of ellipticity is called circular dichroism (CD). CD spectroscopy is a powerful method for studying the three-dimensional structures of optically active chiral compounds, for example, for studying their absolute configurations or preferred conformations.57... 

Coyne, G. V., T. Gehrels, and K. Serkowski, 1974. Wavelength dependence of polarization XXVI. The wavelength of maximum polarization as a characteristic parameter of interstellar grains, Astron. J., 79, 581-589. 

Excitation-wavelength-dependent emission polarization studies indicate the presence of an overlapping xy polarized transition in the bluer part of the 290-315-nm range, as indicated in Fig. 5. The combination of static absorption, time-resolved emission, and emission quantum yield measurements suggests that the emitting state has the same polarization (z axis, linear), but is not the same state as that giving rise to the 362-nm absorption peak. These assignments for the 3.5-nm particles are summarized in Fig. 5. 

The final remarks concern the B emission (1) compared to that of the TICT state it is only slightly dependent on the solvent polarity (the peak position varies from 29,300 cm-1 in cyclohexane to 28,200cm-1 in methanol at a temperature near to the freezing point) (2) it does not present any wavelength-dependent decay curves. This essentially proceeds from the fact that the dipole moments in the ground and in the excited B states are in the same direction and are not very different. 

Figure 3. Wavelength dependence of the absorption coefficient (measured with light polarized perpendicular to the c-axis) of undoped and doped BaTi03 crystals. Concentrations refer to dopant atoms per BaTi03 formula unit in the melt.

A / is the absorbance of the sample when the light is polarized parallel to a reference axis, and Aj is the absorbance of light which is polarized perpendicular to this axis. The strength of the absorption depends on the orientation of the electric field vector of the light and the transition moment of the chromophore - parallel orientation results in maximum absorption whereas perpendicular orientation leads to zero absorption. By dividing the LD value by the absorbance of the unoriented sample under isotropic conditions (Aiso), the reduced linear dichroism (LDr), i.e. the wavelength-dependent LD, is obtained (Eq. 7) [36]. 

When measuring absorption spectra, one records a signal that is related to the wavelength-dependent probability of making a spectroscopic transition. From the molecular point of view, this probability is proportional to the dot product jl p where (L is the molecular transition moment and p is the photon polarization direction. When the orientational distribution of the molecules is isotropic (not crystalline, liquid crystalline, or bound to a surface), its absorption spectrum represents the orientationally averaged probability of making a spectroscopic transition and the measured spectrum is independent of polarization direction. When the orientational distribution of the molecules is anisotropic, the probability of making a spectroscopic transition depends on the polarization direction, and that dependence can be exploited to deduce the direction of the transition moment relative to the laboratory frame. Because transition moments are often trivially related to the orientation of the molecule, structural information can be deduced from polarized absorption measurements on anisotropic samples. 

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