Spectra, "tilting perpendicular

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. 

Fig. 10 Estimation of the tilt angle for an alkane between gold electrodes, determined by fitting the computed IETS spectrum with the experiment (panel b below). Result is a 40 degree tilt angle perpendicular to the plane of the carbon chain, as illustrated in the lighter shade structure in the sketch (b) above. Sketch (a) above and panel (a) below refer to the alkane tilted in the plane of the carbon chain. The structures in sketch (a) do not fit so well an those in (b), suggesting the methyl group position shown in (b) above. From [107], Reproduced by permission of the PCCP Owner Societies...

Pd(110), on the other hand, implies a species with the HCCH plane essentially perpendicular to the (110) surface. The latter spectrum has also a prominent soft rCH mode at 2820 cm"1, indicating an interaction between one of the CH bonds and a surface metal atom. The suggested structure—an HCCH plane perpendicular to an M-M bond of the ridge but tilted down toward the bottom of the trough (30)—is consistent with these spectroscopic criteria. 

It is noted in Fig. 4 that the intensities of the bands at 2222, 1547, and 1529 cm-1 due to in-plane TCNQ modes are much stronger in the RA spectrum than in the transmission spectrum. Therefore, it seems that the TCNQ plane is nearly perpendicular to the substrate surface. Transition moments of the two C=C stretching bands at 1547 and 1531cm-1 are perpendicular to the molecular axis of the TCNQ chromophore [20]). It should be noted that both have comparable intensities in the transmission and RA spectra (Fig. 4). Therefore, it may be concluded that the molecular axis of the TCNQ chromophore is neither parallel nor perpendicular to the surface, being in an intermediate direction. The intensities of CH2 antisymmetric and symmetric stretching bands are also comparable between the two spectra, suggesting that the alkyl chain is tilted considerably with respect to the surface normal. 

Fig. 6.10. Tilting effects of doubly oriented films showing settings for observing (A) parallel tilting spectrum, and (B) perpendicular tilting spectrum. The vectors are E.V., the electrical vector (perpendicular to slit) M, the transition moment vector parallel to chain axis Mj, the vector perpendicular to doubly oriented film surface and M3, the vector perpendicular to chain axis and parallel to film surface. (Marchessault et al., 1960.)...

For partially oriented uniaxial samples, two linearly independent spectra can be obtained from an experiment, irrespective of the mode of measurement and the polarization state of the absorbed light. The obvious procedure is to record two absorption spectra, one with the electric vector of light parallel to U, the unique axis of the sample Ei), the other with the electric vector perpendicular to the unique axis (Ey). The spectra must be baseline-corrected by subtracting the curves recorded on pure solvents. This yields the largest difference between the tow linearly independent spectra. Other alternatives are also possible. A combination of Ey or Ey with the spectrum of an isotropic sample, E =[E + 2Ey]/3) is easily obtained in liquid crystal. In cases where the sample geometry precludes the measurement of both Ey and the isotropic spectrum (membranes), the two spectra are Ey and the latter recorded with the incidence angle nlZ-co, and with the plane of polarization containing the U axis (the tilted plate method ). 

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