Dichroic ratio polarized

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

Intermediate methods include the earliest procedure based on Stein s equation [33] and one based on Samuels equation [34]. Among the direct methods is an IR spectroscopic method based on the measurement of the dichroic ratio (R), of amorphous absorption bands. In the investigations [35], the amorphous bands 898 cm" and 1368 cm", for which the angles of transition moment are a898 = 39 and aneg = 80 , respectively, were used. Other methods are spectroscopy of polarized fluorescent radiation [35,36], measurement of color di-... 

Normal incidence transmission IRLD measurements are used to study thin films (typically 100 pm thickness and less, depending on the molar extinction coefficient of the bands) with in-plane uniaxial orientation. Two spectra are recorded sequentially with the radiation polarized parallel (p) and perpendicular (s) to the principal (machine) direction of the sample. The order parameter of the transition moment of the studied vibration is calculated from either the dichroic ratio (R — Ap/As) or the dichroic difference (AA = Ap—As) as ... 

The parameters K1/ K2/ and K3 are defined by the refractive indices of the crystal and sample and by the incidence angle [32]. If the sample has uniaxial symmetry, only two polarized spectra are necessary to characterize the orientation. If the optical axis is along the plane of the sample, such as for stretched polymer films, only the two s-polarized spectra are needed to determine kz and kx. These are then used to calculate a dichroic ratio or a P2) value with Equation (25) (replacing absorbance with absorption index). In contrast, a uniaxial sample with its optical axis perpendicular to the crystal surface requires the acquisition of spectra with both p- and s-polarizations, but the Z- and X-axes are now equivalent. This approach was used, through dichroic ratio measurements, to monitor the orientation of polymer chains at various depths during the drying of latex [33]. This type of symmetry is often encountered in non-polymeric samples, for instance, in ultrathin films of lipids or self-assembled monolayers. 

The optical transition moments for vibrational or electronic transitions between defect states have specific orientations with respect to the defect coordinates. The absorption strength of polarized light for each of the differently oriented centers is proportional to the square of the component of the transition moment that is along the polarization direction. Hence, a stress-induced redistribution of the defects among their different orientations will be detected as an anisotropy in the polarized optical absorption. A convenient measure of the anisotropy is the dichroic ratio, defined as... 

The out-of-plane orientation of chromophores can be more easily controlled in LB films as compared with the in-plane orientation. Many chromophores are known to show anisotropic orientation in the surface normal direction. The molecular structure of chromophores and their position in amphiphile molecules, the surface pressure, the subphase conditions are among those affect their out-of-plane orientation. The out-of-plane orientation has been studied by dichroic ratio at 45° incidence, absorbance ratio at normal and 45° incidence, and incident angle dependence of p-polarized absorption [3,4,27,33-41]. The evaluation of the out-of-plane orientation in LB films is given below using amphipathic porphyrin (AMP) as an example [5,10,12]. 

Further, planar orientation of PPV chains in the film plane was demonstrated from the linear dichroism dichroic ratio Ap/As decreased from 2.1 to 1.2 when the incident angle of polarized light increased from 0° to 45°, where Ap and As... 

Dichroic ratios are very useful to describe the PL emission from a physiological point of view (e.g., brightness of a device as perceived by the human eye), and we have attempted to consistently employ this representation throughout this text. Most authors express DRE as a ratio of intensities at a given wavelength, usually at a maximum of the emission spectrum, and, unless otherwise stated, we here follow this convention. However, quite frequently the emission spectra of p- and s-polarized are not identical, so that it would be more appropriate to compare the integrated emission spectra in order to allow for an apple-to-apple comparison. 

Most of the LC polymer systems discussed so far contain the LC moiety incorporated in the polymer main chain. One of the few examples of polarized emission of light using a side-chain LC system was reported by Chang et al. [107]. A polyacrylate was used as the polymer backbone. The nematic LC side-chains consisted of ethyl- and propyl-substituted bis-tolan units. When deposited on a rubbed PEDOT film, polarized EL dichroic ratios were reported to be around 6. 

When a polarized spectrum of the film was taken in the dry state, a small dichroic ratio was observed (A L/A// = 1.3, see Fig. 9c). This is due to the slant structures of base pairs and intercalated dyes to the axis of the strands, since the DNA strands were confirmed to be aligned along the stretching direction even in the dry film (see Fig. 7c). These changes of polarized absorption spectra dependent on water moisture were reversible at least 10 times. 

IR spectroscopy is not only useful for determining the chemical constitution of polymers. It additionally provides profound information on chain orientation and on the orientation of attached lateral substituents of polymers. In this case, polarized IR radiation is applied which is only absorbed by an IR-active bond if the plane in which the electrical field vector E of the IR beam oscillates is parallel to the transition dipole moment p of the vibration to be excited. If, on the other hand, the transition dipole moment p is perpendicular to the electrical field vector E of the IR beam no absorption is observed. Using this effect, the degree of orientation of a polymer sample (film, fiber) can be estimated by comparing the intensity at maximum /(11) and at minimum I ) absorption, i.e., the dichroic ratio. 

Mechanical deformation induces orientation into the polymer samples and polarized infrared can be used to characterize this orientation either by direct measurement of the dichroic ratio 287,289) or by spectral subtraction 286), three dimensional sample tilting 68,286), or internal reflection spectroscopy 130). 

In the above instances the qualitative polarization characteristics of a band served to provide information of importance in making assignments. If we make use of the quantitative aspects of the dichroic ratios of absorption bands then we can in addition obtain structural information about the polymer. In particular, it becomes possible to determine in many cases the direction which transition moments make with the fiber axis, which is often significant in establishing structural parameters. It must be noted that transition moment directions do not by themselves always serve to specify the orientation of chemical groups. In cases where... 

In such instances no knowledge of the orientation distribution or of / is required to compute 6V Once 6X is obtained, / can be calculated from equation (49), and using this / the 6 values for other moments can be obtained directly from the observed dichroic ratios. These relations demonstrate the potentialities of polarized radiation studies in providing structural information on high polymers. 

Figure 5.17 Images of a PEG-PVAc blend system showing the typical behavior for (a) parallel polarized, (b) perpendicular polarized and (c) dichroic ratio images. The bottom images generated by plotting the 1721 cm- carbonyl stretching band of PVAc for both (d) parallel and (e) perpendicular polarization show that PVAc is located mainly in the interspherulite regions. Reproduced from figure 3 of Ref. 31, with permission.

Light polarized parallel to the incident plane (p-polarized) has components in the x and z directions, while light polarized perpendicular to the incident plane (s-polarized) has only a y-component. Therefore, the dichroic ratio is given by equation 6. 

Polarized absorption spectra are often expressed in terms of the dichroic ratio,... 

Assuming that the transition dipole does not absorb light polarized perpendicular to its principal axis ( a2 = 0 ), the dichroic ratio takes on the simple form,... 

Fig. 3.3 Infrared spectra with attenuated total reflectance using polarized light for lipid A from E.coli and from Rhodobacter capsulatus. The two lipid A have identical backbone structures, but differences in the acylation pattern. As can be seen, the vibrational bands corresponding to diglucosamine ring vibrations have different dichroic ratios, which is indicative for a different inclination of the backbones with respect to the membrane normal...

Fig. 22(b) shows polarized visible absorption spectra of mixed LB films of MS-C2o binary and MS-C20-AL18 ternary systems prepared by using an aqueous subphase containing Cd2+ ions, where the ratio in the mixed system is [MS] [C20] [AL18] = l 2 x (x — 0 and 1) [77-84]. The thick and thin lines in Fig. 22(b) refer to the spectra, Atl and A , measured by linearly polarized light with the electric vector parallel and perpendicular to the dipping direction of the substrate, respectively. For x = 0, a sharp absorption peak is observed at 590 nm, which is red-shifted from the MS monomer peak at around 540 nm. The dichroic ratio R of the 590 nm band is R> 1, where R is defined as A /A . For x= 1.0, on the other hand, a sharp blue-shifted band with R< 1 appears at 505 nm. Fig. 22(c) refers to the cases of the corresponding binary and ternary systems fabricated under the subphase without the Cd2+ ions [84], Unlike the results in Fig. 22(b), remarkably red- and blue-shifted bands are not observed... 

Using 34, a Type I molecule, one can orient these molecules in liquid crystal (LC) phase such as a nematic ZLI-2452 (LC phase commercially available from Merck Co). Using polarized light, a different ratio D of the absorption E y and E is possible. The dichroic ratio... 

The polymer absorption is intense ap 555 7.5 x 105 cm-1 for light polarized parallel to the PA chains and the dichroic ratio is 25. The polymer is not fluorescent. 

The maximum absorption per chain for light polarized parallel to the PDA chains is at least as intense as for PA. The measured dichroic ratio is about 120 [117,118]. It may be smaller in other PDAs for instance, about 25 in pTS [123]. As for trans-PA, solid PDAs are not fluorescent (there is a small solution fluorescence). 

---