Parallel and perpendicular bands

Vibrational Assignments and Fundamental Frequencies. Examine your survey spectra for C2H2 and C2D2 and note the striking difference between parallel and perpendicular bands. Determine the frequencies of as many of the transitions shown in Fig. 1 as your data allow. In doing this take the Q branch maximum of each expanded spectrum of the v, vi V2 - v, and perpendicular bands as a measure of these... 

Finally, it should be noted that certain overtone and combination bands, or the spectrum of an accidentally symmetric rotor may give rise to hybrid bands with both parallel and perpendicular band features. 

Parallel and perpendicular bands in linear molecules have identical P and R branches with a spacing of 2B between the rotational bands according to... 

Fig. 1.28. Gas phase contours of absorption bands of acetylene, acetonitrile, tri-methylamine, and benzene measured in a 10-cm cell with a rock salt prism, (a) Acetylene at 81 mm pressure. The parallel and perpendicular bands are marked. The perpendicular band has a central peak which is absent in the parallel bands. There is partial resolution of some rotational fine structure, (b) Acetonitrile at 67 mm Hg pressure. The moment of inertia about the symmetry axis is relatively small. The perpendicular band has a round contour with some fine structure resolved. The parallel band is a triplet, (c) Trimethylamine at 27 mm Hg pressure. The parallel bands are a little wider than the perpendicular bands and show a more distinct central peak. The triplet bands are asymmetrical because of centrifugal distortion, (d) Benzene at 26 mm Hg pressure. The parallel and perpendicular bands are not very different. The references cited refer to the band assignments the spectra shown were obtained by the authors of this text.

As can be seen from (9), the energy depends only on the magnitude of AT, so there are J + 1 different values. In considering selection rules we have to distinguish between parallel and perpendicular bands, which may be defined according to whether the quantum number K changes ... 

The ctmtribution of rotational quantization to the integrated absorption coefficient has been treated for symmetric rotor molecules [8]. Summation over rotational quantum numbers for parallel and perpendicular bands introduces a correction factor in the expression for the absorption... 

For films on non-metallic substrates (semiconductors, dielectrics) the situation is much more complex. In contrast with metallic surfaces both parallel and perpendicular vibrational components of the adsorbate can be detected. The sign and intensity of RAIRS-bands depend heavily on the angle of incidence, on the polarization of the radiation, and on the orientation of vibrational transition moments [4.267]. 

In the optical absorption, two different polarisations of light should be considered the electric field is along (parallel or y polarisation) and perpendicular (perpendicular or x) to the axis. Figure 5 shows the energy band of a metallic CNT for flux < )/< )o =0, 1/4 and 1/2 and the process of optical transitions for the parallel and perpendicular polarisations. Some examples of calculated absorption... 

Fig. 24. The computed valence band dispersion E(k) computed from the 6 x 6 Luttinger model for the wave vector parallel and perpendicular to the Mn spin magnetization in (Ga,Mn)As. assuming that the spin splitting of the heavy-hole band at the f point is 0.15 eV.

Similar curves are obtained with other synthetic polypeptides, and in most cases they are reasonably independent of the nature of the amino acid side chains. In synthetic polypeptides and proteins the observed Cotton effects do not arise from isolated chromophores but are composite curves resulting from several transitions assigned to the amide bonds in the 200-m/x region. The a-helical curve, for example, results from three optically active absorption bands. One around 222 m/ arises from an n — 7T transition of nonbonding electrons, and the other two at 208 and 191 m/ji are attributed to w — tt transitions parallel and perpendicular to the axis of the helix. These transitions of the a-helix and the resulting Cotton effects characteristic of the a-helix are at present of great interest in interpreting ORD curves of membranes. 

Fig. 6.6 Gas-phase parallel and perpendicular IR bands of a linear molecule for low resolution.

Palladium black is another transition metal subsTrate on which hydrogen chemisorption has been studied by inelastic neutron scattering (33). Two bands were observed at 916 cm-1 and 823 cm"1 which were assigned to vibrations of the hydrogen parallel and perpendicular to the surface, respectively. A bridge site was inferred based on the number of modes observed without making any assumptions about the expected excitation frequencies for the different site geometries. 

To add further credence to these critical assignments, the polarization-depen-dent OODR(Si) spectrum was recorded for the Ojj and 6j bands with implement of photoelastic modulator (PEM). This device alters the polarizations of the probe and pump beams with each laser pulse in a shot-by-shot fashion. For the 0q transition, the two beams would have parallel polarizations (pump and probe transition are both y (B2) polarized). For the 6j transition, the two photons would be perpendicularly polarized to each other, with y and z polarization, respectively. The temporal profiles of polarization-dependent OODR spectra in Figure 2.25 show the expected behavior, where the signals of the parallel and perpendicular polarizations for the S2(0°) < S [ (41) < S0(Oo) and S2(61) <— SjtT1) <— So(0o) are plotted against the delay time. The effects of reversing the polarizations is not too pronounced in these experiments, as the rotational coherence lifetimes are very short and the overall rotation of the molecule quickly scrambles the polarizations of the signals. At very short delay... 

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... 

Scan the Stokes Raman spectrum of CCI4 for shifts of 150 to 1000 cm from the exciting line.t Note that the frequency in wavenumbers is given by v (cm ) = 1/A = vie, where c is the speed of light in cm s Record both parallel and perpendicular polarization scans so that you can determine the depolarization ratio of aU bands. Indicate the spectrometer wavelength or wavenumber reading on the chart at several points in the scan to provide reference points for the determination of the Raman shifts for all bands. 

Fig. 8. Allowed transitions for the parallel ) and perpendicular (i) vibration-inversion-rotation bands in NH3. System of levels on the right side of the figure illustrates a vibration-rotation mixing of the ground vibrational state with the U4 degenerate state...

Figure 9 Parallel ( ) and perpendicular (J.) scattered Raman spectra from liquid CCI4 showing the polarized (vi) and depolarized (v2, V3, and V4) bands associated with totally symmetric (Ai) and nontotally symmetric (E, T)) vibrations, respectively...

Figure 10 Parallel ( ) and perpendicular ( ) scattered resonance Raman spectra from NiP in CS2 showing the anomalously polarized bands (labeled) associated with the ring A2g vibrations...

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