Transmission Surface selection rule

RAIRS results in a spectrum that is similar to that of a transmission measurement For grazing incidence geometry, only molecular vibrations giving rise to a dynamic dipole perpendicular to the surface are infrared active this is termed the surface selection rule for surface vibrational spectroscopy. 

Fourier transform infrared microscopes are equipped with a reflection capability that can be used under these circumstances. External reflection spectroscopy (ERS) requires a flat, reflective surface, and the results are sensitive to the polarization of the incident beam as well as the angle of incidence. Additionally, the orientations of the electric dipoles in the films are important to the selection rules and the intensities of the reflected beam. In reflectance measurements, the spectra are a function of the dispersion in the refractive index and the spectra obtained are completely different from that obtained through a transmission measurement that is strongly influenced by the absorption index, k. However, a complex refractive index, n + ik can be determined through a well-known mathematical route, namely, the Kramers-Kronig analysis. 

The important information that can be provided by IR spectra is the molecular orientation in/on polymer films, which include SAMs as the specific case (Section 3.11). In the case of self-supporting anisotropic films, the linear dichro-ism is usually calculated from normal-incidence transmission spectra measured at two mutually perpendicular positions of the polarizer [684], Obviously, this approach is insensitive to the modes perpendicular to the film surface. This problem is circumvented by using a combination of the normal-incidence transmission with metallic IRRAS [727], since these methods have complimentary selection rules — the modes whose TDMs are parallel or perpendicular to the surface are active in transmission or IRRAS, respectively. This technique was used to study the MO in ultrathin n-aUcylacrylamide LB films [727, 728]. A strong biaxial distribution was found in these LB films in which the carbon-hydrogen chains are inclined in the dipping direction [727]. 

Since both HREELS and RAIRS are vibrational spectroscopies, and the same selection rules apply, their information contents must overlap. This is demonstrated in Fig. 8 [2], in which the HREELS and RAIRS spectra from a Cu(l 11) surface covered with about 10 molecular layers of cyclohexane at low temperature are shown. The vibrational spectra appear at the same energetic positions in both techniques, but it should be noted that whereas RAIRS has the advantage of better energy resolution HREELS is able to record spectra down to losses close to 0 cm. For reasons of IR transmission of window materials, the cutoff in RAIRS is in the region 400-800 cm". ... 

The work dealt with phase and amplitude-phase transmission hologram-gratings with different thicknesses and constants, constructed, as a rule, in a symmetrical (or close to that) configuration of interfering beams relative to the sample surface. Main measured holographic parameters were the diffraction efficiency (DE) and selectivity contour of holograms, constructed under different recording and post-exposure treatment conditions. 

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