Polarized infrared reflectance spectra

The aim of this article is to show that the new quasi-two-dimensional organic conductor p -(BEDO-TTF)5[CsHg(SCN)4]2 [hereafter called (BEDO)CsHg] (BEDO-TTF - bis-(ethylenedioxy)tetrathiafulvalene) which contains closed and open orbits displays rather complicated oscillatory spectra associated with magnetic breakdown (MB) and quantum interference (QI) effects. Tight binding band structure calculations for this compound are proposed to characterise its Fermi surface. The aim of the article includes also an investigation of the optical conductivity anisotropy with polarized infrared reflectance spectra. 

The considerable electrical anisotropy of this salt is made still more evident on the room-temperature polarized infrared reflection spectra [30] (see also Chapter 6 and Fig. 1 of this chapter). In addition, these reflection spectra give a remarkably clear information on the existence of strong coupling between the quasi-one-dimensional electronic system and the symmetric ag vibrational modes of the TCNQ molecules. More precisely, under the effect of this coupling, these particular modes, which are normally Raman active but infrared nonactive, acquire an anomalous intense infrared activity together with an anomalous polarization [20,30,31]. 

FIG. 6. Upper panel polarized infrared reflectance spectra of a mosaic of single crystals of M2P-TCNQ. Electric field vector parallel (full line) and perpendicular (broken line) to the stack axis. Lower panel vibronic reflectance spectrum calculated on the basis of the dimer model. (From Ref. 27)... 

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. 

Figure 10.1 Comparisons of visible to near-infrared spectra of calcic pyroxene in transmitted and reflected light. Polarized absorption spectra of single crystals are correlated with the reflectance spectrum of a powdered sample of the same mineral (cf. fig. 5.14).

In PM IRRAS, the incident beam polarization is modulated between two orthogonal directions (a) perpendicular (s-polarization) and (b) parallel (p-polar-ization) to the plane of incidence of the infrared beam. At the metal surface, the electric field of the s-polarized infrared beam almost vanishes, while the electric field of the p-polarized beam is enhanced (see Fig. 9.3). Thus, the first component of the modulated signal is insensitive to the presence of the film of adsorbed molecules and can be used to obtain the background spectrum, while the other component can be utilized to obtain the spectrum of the film. Modulation between s- and p-polarizations is carried out at a high frequency (50 kHz), and the reflection absorption spectrum AR/(R) is obtained as... 

Fine structure of infrared bands Intensity of infrared bands Intensity of Raman bands Polarization of infrared bands Reflection spectrum in the infrared region Resonance Raman effect... 

Infrared spectroscopy can provide a great deal of information on molecular identity and orientation at the electrode surface [51-53]. Molecular vibrational modes can also be sensitive to the presence of ionic species and variations in electrode potential [51,52]. In situ reflectance measurements in the infrared spectrum engender the same considerations of polarization and incident angles as in UV/visible reflectance. However, since water and other solvents employed in electrochemistry are strong IR absorbers, there is the additional problem of reduced throughput. This problem is alleviated with thin-layer spectroelectro-chemical cells [53]. 

Energies in the infrared spectrum are conventionally expressed in wave numbers, which are defined as the number of waves per centimeter, i.e., the reciprocal of the wavelength measured in centimeters. The infrared spectrum extends from 12,500 to 50 cm (i.e., a wavelength of 0.8-200 fjLia.) and the far infrared from 40-10 cm (260 p.m-1 mm), but the upper limit of most commercial instruments is about 200 cm (50 ixm). Spectra are most frequently obtained by absorption and reflection techniques, but polarization, emission, and luminescence are also used (C26). Similar components are used in all types of instrument. Reflection measurements of samples with low transmission are made in the near infrared with a conventional spectrophotometer fitted with a reflec-... 

The only piece of experimental evidence for the orientation of apoA-I helices on DMPC discoidal complexes has come from the use of polarized attenuated total reflection infrared (ATR-IR) spectroscopy (Brasseur et al. (1990 Wald et al., 1990). Although internal reflection spectroscopy is not new (Harrick, 1967), its combination with polarized IR measurements of oriented biological membranes is a more recent application, which allows the spectrum to be taken in the presence of water. The technique has been used most frequently to study the orientation of lipid functional groups relative to the membrane plane in pure lipid bilayers (Fringeli and Gunthard, 1981 Holmgre et al., 1987 Okamura et al., 1990 Hubner and Mantsch, 1991), but has also been applied to studies of... 

The properties of the dual-film electrode were characterized by in situ Fourier transform infrared (FTIR) reflection absorption spectroscopy [3]. The FTIR spectrometer used was a Shimadzu FTIR-8100M equipped with a wide-band mercury cadmium teluride (MCT) detector cooled with liquid nitrogen. In situ FTIR measurements were carried out in a spectroelectro-chemical cell in which the dual-film electrode was pushed against an IR transparent silicon window to form a thin layer of solution. A total of 100 interferometric scans was accumulated with the electrode polarized at a given potential. The potential was then shifted to the cathodic side, and a new spectrum with the same number of scans was assembled. The reference electrode used in this experiment was an Ag I AgCl I saturated KCl electrode. The IR spectra are represented as AR/R in the normalized form, where AR=R-R(E ), and R and R(E ) are the reflected intensity measured at a desired potential and a base potential, respectively. 

Microwaves (0.3-300 GHz) lie between radiowave (RF) and infrared (IR) frequencies in the electromagnetic radiation spectrum. Microwaves can be reflected, absorbed and/or transmitted by materials. Microwaves can interact with materials through either polarization or conduction processes. Polarization involves the short-range displacement of charge through... 

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