Reflection bands

Substances formed at a given potential give rise to a diminution of the reflected intensity as compared with that at the reference potential. As a consequence, the relative reflection band presents a minimum ( negative going band ). On the contrary, the reflected intensity for substances consumed at a given potential present a maximum ( positive going band ). 

Films of pure CNLCs have a unique transmission behavior as CP light with the same sense of circular polarization as the CNLC is filtered out by reflection, while CP light of the opposite handedness as the CNLC film is transmitted. This selective optical transmission characteristic is referred to as a one-dimensional photonic stop-band or a selective reflection band. The stop-band is centered at a certain wavelength Ac, which is dependent on the pitch length p and the average refractive index n of the CNLC ... 

The CD reflection spectra are quite sharp at all temperatures, and the reflection wavelength, corresponding to the optical pitch of the TChLC phase, increased progressively with temperature from 500 nm at 70 °C to 1,000 nm at 140 °C. It was considered that the positive sign of the CD reflection band indicated M screw sense helicity of the cholesteric phase. Very recently, a smectic A-cholesteric phase transition was also observed for PDMBS.348... 

Figure 54 (a) CD reflection bands of PDMBS cholesteric phases at various temperatures (cooling run) and (b) UV and CD absorption spectra at 80°C.346 Reprinted with permission from Watanabe, J. Kamee, H. Fujiki, M. Polymer J. 2001,33, 495-497, 2001 Society of Polymer Science Japan. 

In Table I, the results of this experiment are compared to reflectance spectra measured for other simple and complex metal-TCNQ salts. We found that the CN stretching mode in reflectance measurements shifted to higher frequency by about 100 cm from absorption measurements made on the same material. The peak in the reflectance band at 2320 cm for the Cu-TCNQ film prior to the application of a field is consistent with the values measured for the simple (1 1) salts of Li+(TCNQ ) and Cu+(TCNQ") tabulated in Table I. These crystalline materials are simple salts which do not contain neutral TCNQ . On the other hand the spectra of a Cu-TCNQ film after the application of an applied field closely resembles the spectra of Cs2 (TCNQ [ )3 with two CN stretching modes separated by 20 cm. Cs2(TCNQ )3 is a complex salt which contains neutral TCNQ and radical-anion TCNQ . (11)... 

When the surfaces are highly reflecting as in the case of metals, external reflection spectroscopy (ERS) can be used with good success133). For optimum intensity of the reflection bands of thin films, angles of incidence near 88 are desirable. However, in order not to interfere with the incoming beam, angles of incidence near 80° are used. 

Cr(abt)2] is high-spin (pWK = 4.71 BM, 6 = 1°), with a broad reflectance band at 16 900 cm-1. Axial donation from adjacent [Cr(abt)2] units seems necessary to produce six-coordinate Cr" so it is surprising that the magnetic moment is not temperature-dependent. Possibly the molecule is essentially planar.296... 

Fig. 14 UV-VIS spectra of a cholesteric elastomer, based on a mixture of derivative 2 (44 wt %), monomer 1 (52wt%), and monomer 2 (3wt%), crosslinked at 32 °C. Arrows indicate the maximum position of the selective light-reflection band. (Reproduced from [233])...

C is insensitive to change in temperature and the intensity of the CD reflection band in the visible region decreases markedly. 

To determine the effect of alkali-metal promotion on the surface morphology, the catalysts were investigated by IR spectroscopy.9 DRIFT spectra revealed two intense reflectance bands at 760-680 cm-1 and 650-570 cm-1 for the sodium promoted sample. Moreover, an absorption band at 535 cm, which has previously been assigned to PdC>3,28 was observable in the DRIFT spectra of oxidized Na-PdO/Sn02. 

Figure 11. Product distribution in MoOj/SiC>2 catalysts as determined from acidimetric titrations (a) silicomolybdic acid (o) dimolybdates ( ) polymolybdates. The estimation of MoOj (i i) is derived from the intensity ratio of the reflectance bands at 360 and 440 cm-1 [66].

The BEDT-TTF trihalides and the related salts attract much attention because of a relatively high superconducting transition temperature. Figure 8 shows the polarized reflectance of a- and (3-(BEDT-TTF)2I3 crystals for two light polarizations. For both phases the electronic reflection bands with a Drude-like edge are observed in two perpendicular polarizations [47]. Drude parameters and transfer integrals of typical (BEDT-TTF)2X salts are 5000 cm-1 < top < 9600 cm-1, 500 cm-1 < y < 2000 cm-1, and 0.08 eV < t < 0.20 eV. Near isotropy of the optical properties of typical BEDT-TTF salts is confirmed by electrical transport studies. Rather small values of t are consistent with relatively low room-temperature conductivity. 

Figure 4.6-9 Induced cholesteric solutions Schematic outline of experiment and evaluation of the optical rotation p(A) related to the selective reflection band (reflection Cotton effect, RCE, centred at the wavelength A/ ) in order to characterize the chirality of the solute molecules by the helical twisting power.

Around the selective-reflection band the plane of polarization of incident radiation is strongly rotated, the spectral dependence of this optical rotation resembles a dispersion curve as outlined below, position and shape give indication of both structure parameters. 

Figure 4.6-13 Optical rotation q recorded as outlined in Fig. 4.6-12 Spectra of two differently concentrated solutions of S-tyrosine-methylester in the nematic mixture EBBA/MBBA (equimolar mixture of N-(p-ethoxybenzylidene)-p - -butylaniline and its methoxy analogue 2 of Table 4.6-1 Riedel-de Haen), left RCE (molar fraction x fa 0.024) related to the selective reflection band indicating pitch and handedness of the. structure, thus characterizing the chirality of the solute molecules by the helical twisting power right Sequence of ACE (,v se 0.0024, therefore the RCE should occur around 200 cm ) each of which indicates the induced handedness and therefore, discriminates enantiomers (Koite, 1978).

As early as in 1951 de Vries showed that a twisted stack of birefringent layers is an adequate model for a cholesteric structure in order to reproduce a principally correct spectral dependence of the optical rotation also around the selective reflection band as it was recorded in a different way for Fig. 4.6-8. Even if the layer thickness is formally reduced to zero the optical rotation and its spectral dependence is preserved. Several other approaches were reported to describe particular effects of the cholesteric structure such as the selective reflectance or the rotatory anomaly (e.g. Chandrasekhar and Prasad, 1971 Chandrasekhar and Ranganath, 1974 SchSnhofer et al., 1983 Eidner et al., 1989). 

The transfer of the nomenclature of isotropic optical activity is surely acceptable where it evidently yields a descriptive picture of the experimental observations. However, precautions are necessary as to apparently self-evident implications, this is all the more important since the anisotropic nature of the sample is by no means obvious when observing parallel to the optical and thus helical axis. An unbiased and complete record of how a cholesteric sample acts on the measuring radiation can be obtained by el-lipsometry. This method (compare Sec. 6.4) yields a comprehensive description of the state of polarization including the degree of polarization (Rbseler, 1990). An adequate simulation can be based on the Berreman formalism (1972) rendering possible a study of particularities observed, such as pronounced depolarization related to the selective reflection band (Reins et al., 1994). 

The high-wavenumber limit of such a reflectance band is clearly defined by the point where n equals unity v 1090 cm ), i.e. the refractive index of ambient air. The other occurrence of n = 1 close to o is accompanied by such high absorption, that considerable reflectance is observed at least for the s-polarized component. The reflectance band for p-polarized radiation is limited to short wavenumbers where the condition for the Brewster angle is met. This condition is met a second time within the reflection band. This does not become evident in the reflectance spectrum due to the related high value of k. 

Interaction of infrared radiation with molecules and crystals to produce absorption (or reflectance) bands can be predicted using group theory (see Orchin and Jaffe, 1971 Cotton, 1971), and the same approach is used in the prediction and interpretation of other vibrational spectra (such as Raman spectra). 

In a parallel set of discoveries, a reflectance band in the visible, similar to that for solid 02, was seen at low latitudes on Ganymede. In addition, a UV feature associated with O3 was seen on Ganymede and on the icy satellites of Saturn. Coupled with these observations was the much earlier discovery of a band indicative of SO2 in ice at Europa and Callisto and the recent discovery of CO2 trapped in the icy surfaces. The SO2 was initially assumed to be due to sulfur ions originating at lo implanted into the ice at Europa, " but the SO2 is also a radiation decomposition product like the O2, as discussed below. " The CO2 source is probably internal as carbon ions have not yet been seen in the plasma. 

In Fig. 7 the optical rotatory dispersion (ORD) as well as the circular dichroism (CD) is shown for the right-handed cholesteric liquid crystal. A right-handed helical structure reflects right circularly polarized light and it shows positive optical rotation on the short wavelength side of the reflection band. 

The very small residual increase (less than 0.2 nm/K) can be attributed to thermal expansion of the cured polymer. This was verified by measuring the coefficient of thermal expansion via the change of density by heating pieces of cured solid film from 20°C to 50°C. A value of A7JAT = (0.1 l+i.0.04) nm/K was calculated from this measurement, which is in good agreement with the observed wavelength shift. Preservation of the reflection band was observed down to -196°C. This sounds trivial for a polymer but should be mentioned for comparison with monomeric liquid crystalline materials, which tend to crystallize at low temperature. 

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