Polarized infrared absorption

The differently produced conductive polymer structures described above all have enhanced conductivity, which can be employed in microelectronics [44] and as sensors using immobilized enzymes [46, 47[. Martin and coworkers used polarized infrared absorption spectroscopy to access the alignment of the polymer fibers on the outer surface of the nanotubes [48[. The study showed that the enhancement of the conductivity is due to the alignment of the polymer fibers on the outer surface of the tubes. 

Fig. 25. Polarized infrared absorption spectra of the mixed LB films of the MS-C20 binary and MS-C20-AL18 ternary systems with the molar mixing ratios of [MS] [C2o] [ALi8] = 1 2 .v (x — 0 and 1). The thick and thin lines refer to spectra A and A measured by linearly polarized light with the electric vector parallel and perpendicular to the dipping direction of substrates, respectively. (Reproduced with permission from Ref. [84]. Copyright (2006) Elsevier.)...

Fig. 28(b) depicts polarized infrared absorption spectra of 10-layer LB films of pure CdC2()-r/, and the MS-C2o binary and MS-C20-AL18-r/ ternary systems [80,82]. In the case of CdC20-r/ LB film, peaks at 2193 and 2089 cm-1 are assigned to the CD2 antisymmetric and symmetric stretching modes of the hydrocarbon chain, respectively, and their peak positions are characteristic of the stW-trans conformation of the hydrocarbon chain. The LB film of the MS-C20 binary system does not show the peak, as a matter of course. The film of the MS-C20-AL18 ternary system yields the peaks at 2194 and 2089 cm suggesting the presence of AL 18-d with the all-/ra//,v conformation. 

Parthasarathy and Martin [104] synthesized PAn microtubules within the pores of polycarbonate template membranes and reported that the conductivity increases drastically as the tubule diameter decreases. Polarized infrared absorption spectroscopy (PIRAS) data showed that the polymer deposited directly on the pore wall is highly ordered, which is believed to be responsible for the enhancement in conductivity. The PIRAS data also showed that the polymer chains are preferentially aligned perpendicular to the tubule axis. 

Fig. 19.12. Polarized infrared absorption spectra of longitudinal sections of human enamel. (Elliott, 1965.)...

Carbonate tons and Enamel. Elliott (1965) has used polarized infrared absorption spectra to study longitudinal sections of human tooth enamel in order to determine whether carbonate ions can substitute for hydroxyl ions in the enamel. His conclusion was that carbonate ions substitute to a very limited extent for hydroxyl ions. The evidence consisted of certain bands in the infrared spectra of enamel (Fig. 19.12) which coincide with those of the synthetic apatite in which this substitution is known to have taken place (Fig. 19.13). In a hydroxyapatite that had been reacted with carbon dioxide at 1000°C, carbonate ion had absorption bands at 878, 1463, and 1528cm , and the hydroxyapatite 3570 cm band (OH ) had disappeared (Fig. 19.13). Elliott examined enamel which had been heated at 1000°C in carbon dioxide and measured the dichroism of the out-of-plane deformations at the 879 cm" mode (Fig 19.14). From the dichroic ratio he was able to calculate that the plane of the carbonate ion is nearly parallel to the c-axis of the apatite. Elliott et al. (1948) have given the dichroic ratio applicable to this case as... 

Fig. 19.14. Polarized infrared absorption spectrum ofthe carbonate ion that has replaced hydroxyl ions in the apatite lattice. (A) A 100- i longitudinal section of enamel heated at 1100°C in air for 2 hr. (B) A 50-/t longitudinal section of enamel heated at 900 C in CO2 for 30 min. (Elliott, 1965.)...

Although a transmittance IR absorption spectrum measurement evaluates the bulk of the alignment material, the distribution of the alignment of the molecules can he determined hy a qualitative analysis by changing the polar and azimuthal angles of the sample. The relationship between the dichroism of the polyimide and its thickness was studied using polarized infrared absorption measurements, and it was found that the dichroism was consistently less than 17 nm and decreased when the polyimide becomes thicker [22]. From these results and retardation measurements, a molecular distribution model was proposed which shows uniform anisotropy to some depth, lower anisotropy in deeper areas, and isotropy in areas which are even deeper [22]. 

We have used X-ray diffraction and polarized infrared absorption spectroscopy (PIRAS) to prove that the tem-... 

The 4-Hydroxy-thiazoles are characterized by infrared absorption near 1610 cm (KBr) (3) or 1620 to 16.S0cm (CCI4) (8), indicating a strongly polarized carbonyl group. H-5 resonates near 5.6 ppm in the NMR spectrum like similar protons in other mesoionic compounds (3). Two fragmentations of the molecular ion are observed in the mass spectra. The first involves rupture of the 1,2 and 3,4 bonds with loss of C2R 0S . In the second, the 1,5 and 3,4 bonds are cleaved with elimination of C2R 0. ... 

A powerful characteristic of RAIR spectroscopy is that the technique can be used to determine the orientation of surface species. The reason for this is as follows. When parallel polarized infrared radiation is specularly reflected off of a substrate at a large angle of incidence, the incident and reflected waves combine to form a standing wave that has its electric field vector (E) perpendicular to the substrate surface. Since the intensity of an infrared absorption band is proportional to / ( M), where M is the transition moment , it can be seen that the intensity of a band is maximum when E and M are parallel (i.e., both perpendicular to the surface). / is a minimum when M is parallel to the surface (as stated above, E is always perpendicular to the surface in RAIR spectroscopy). 

The difficulty of assigning a formal oxidation state is more acutely seen in the case of 5-coordinate NO adducts of the type [Co(NO)(salen)]. These are effectively diamagnetic and so have no unpaired electrons. They may therefore be formulated either as Co -NO or Co -NO+. The infrared absorptions ascribed to the N-O stretch lie in the range 1624-1724 cm which is at the lower end of the range said to be characteristic of NO+. But, as in all such cases which are really concerned with the differing polarities of covalent bonds, such formalism should not be taken literally. 

Bonamy L., Nguyen Minh Hoang P. Far infrared absorption of diatomic polar molecules in simple liquids and statistical properties of the interactions. I. Spectral theory, J. Chem. Phys. 67, 4423-30 (1977) ... 

This two-phonon transition was subsequently observed as predicted in infrared-absorption (Stavola et al., 1989a). Furthermore, its polarization is the same as the fundamental one-phonon (111) transition from which it gains its oscillator strength, providing further support for the BC model. Finally, a considerably weaker two-phonon infrared absorption was observed for nB, also consistent with predictions based on this model. 

The second problem of interest is to find normal vibrational frequencies and integral intensities for spectral lines that are active in infrared absorption spectra. In this instance, we can consider the molecular orientations, to be already specified. Further, it is of no significance whether the orientational structure eRj results from energy minimization for static dipole-dipole interactions or from the competition of any other interactions (e.g. adsorption potentials). For non-polar molecules (iij = 0), the vectors eRy describe dipole moment orientations for dipole transitions. 

Osawa, M., Surface enhanced infrared absorption, In Near Field Optics and Plasmon Polar itons Kawata, S., Ed. Springer, Berlin, 2001, 163 187... 

In the normal-incident transmission measurements of LB films deposited on transparent substrates, the electric vector of the infrared beam is parallel to the film surface (Figure 5A). Therefore, only absorption bands which have the transition moments parallel to the film surface can be detected by this method. On the other hand, in the above-mentioned RA measurements, in which the p-polarized infrared beam is incident upon the LB film prepared on Ag-evaporated substrates at a large angle of incidence, we have a strong electric field perpendicular to the film surface as shown in Figure 5B. Therefore, in this case, only absorption bands which have the transition moments perpendicular to the film surface can be detected with a large intensity enhancement. Thus, if the molecules are highly oriented in the LB films, the peak intensities of particular bands should be different between the transmission and RA spectra. 

Although by now a large number of electrochemical systems have been examined using both SERS and IRRAS, including some common to both techniques (2b), the conditions employed are usually sufficiently different (e.g. disparate surface state, adsorbate concentrations) so to preclude a quantitative comparison of the spectral responses. One further hindrance to such comparisons is that it usually is difficult to remove entirely the contribution to the infrared spectra from solution-phase species. Two types of approaches are commonly used in IRRAS with this objective in mind. Firstly, modulating the infrared beam between s- and p-polarization can achieve a measure of demarcation between surface and bulk-phase components since considerably greater infrared absorption will occur for the former, but not the latter, species for p- versus s-polarized light (2.81. However, a complication is that the "surface... 

The fact that the molecules are adsorbed on a solid surface giv rise to a number of new effects compared to the gas phase situation. The experimental situation consists of a monolayer of molecules adsorbed on a metal surface, on which we shine infrared radiation and then detect the reflected light. The macroscopic theory for the electromagnetic response of such a system is reproduce in the previous reviews. A more microscopic treatment has been given by Persson, showing that the integrated infrared absorptance for p-polarized light is given by ... 

One must account for the screening of the external field by the electronic polarization of the adsorbed molecules. This screening gives rise to a reduced infrared absorption. Taking it into account in a proper way shows... 

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