Multiple reflection arrangements

Absorption of infrared radiation by characteristic vibrations of a surface can be used to obtain information about that surface, by comparison with known absorption frequencies in molecules of known structure. Surface sensitivity is obtained by using small particles ) and thin films or, better, a multiple-reflection arrangement with optimized angles of incidence and reflection in particular making work on single-... 

Thus, multiple reflection arrangements may have a high efficiency, which is nearly independent of the size of the grains ... 

In the case of relatively high absorption coefficients, as in NIR-excited Raman spectroscopy, coarse powders should be investigated with a back-scattering (180°) multiple reflection arrangement. 

For nonabsorbing crystal powders, the 0° multiple reflection arrangement shown in Fig. 3.5-8 g has proven to be superior to other arrangements, because it combines a high intensity of the Raman radiation with the maximum ratio of Ram an/exciting radiation, Ir/Ip- All of the Raman spectra of crystal powders reproduced in the Raman/ Infrared Atlas (Schrader, 1989) have been recorded with an arrangement according to this principle. 

Figs. 3.5-8 e - h show sample facilities for solids and powders which employ different multiple reflection arrangements. Conical arrangements of powders were used for Raman spectroscopy long ago (Brandmiiller and Moser, 1962). These devices make use of the advantages of multiple reflection and refraction at the surfaces of the grains in a powder. A multiple reflection system may be equipped with a metal mirror, like the cone-shaped indentation of the metal block shown in Fig. 3.5-8 e. Any flat sample whose surface is to be investigated may be placed on top of the mirror. Fig. 3.5-8 f shows a tablet with a conical or cylindrical bore in which multiple reflection at the internal surface of the sample occurs. 

For certain applications the polarization of the x radiation is important. As follows from theory, synchrotron radiation is perfectly linearly polarized in the plane of the electron orbit and elliptically polarized outside the plane (Figure 4). However, in practice one has to take into account the finite size and position stability of the radiation source as well as the polarizationchanging properties of monochromators. Therefore it is desirable to determine the actual polarization experimentally. The linear polarization can be measured by diffraction methods, such as, e.g., by Bragg reflection at 2d = 90° or by observing the high Laue transmission for the polarization component with electric vector parallel to the crystal lattice (Borrmann effect). By employing multiple reflection arrangements polarization ratios can be determined even at the level of A simple and fast method... 

Nacreous and Interference Pigments. The optical principles of nacreous (interference) pigments are shown in Figure 72 for a simplified case of nearly normal incidence without multiple reflection and absorption. At the interface P, between two materials with refractive indices n t and n2, part of the beam of light L, is reflected (L,) and partially transmitted (i.e., refracted) (L2). The intensity ratios depend on n, and n2. In a multilayer arrangement, as found in pearl or pearlescent and iridescent materials (Fig. 71 C), each interface produces partial reflection. After penetration through several layers, depending on the size and difference between n1 and n2,... 

See reference (7) for optical arrangement and reference (13) for multiple reflection cell details. 

This is a useful prerequisite for the optimization of sample arrangements the low intensity of the Raman radiation can be considerably enhanced by utilizing multiple reflections of the exciting and the emerging Raman radiation at the sample and an external spherical mirror. 

Fig. 3.5-10 c shows another sample arrangement which makes use of a fiber-optical connection from the laser to the sample and back to the spectrometer. It is specially designed for the scanning of surface layers, e.g., of precious prints or paintings. The half spheric concave mirror reflects the portion of exciting radiation and Raman radiation back to the sample which has been. scattered by the sample and is not collected by the optical fiber. Thus the mirror as a component of a multiple reflection system enhances the observed intensity of the Raman lines by a factor of 2 to 8, depending on the properties of the sample. 

Fig. 3.5 Experimental arrangement for intracavity Raman spectroscopy with an argon laser CM, multiple reflection four-mirror system for efficient collection of scattered light LM, laser-resonator mirror DP, Dove prism, which turns the image of the horizontal interaction plane by 90° in order to match it to the vertical entrance slit S of the spectrograph FPE, Fabry-Perot etalon to enforce single-mode operation of the argon laser LP, Littrow prism for line selection [315]...

The infrared spectra were obtained with a BRUKERIFS-113v Fourier Transform Spectrometer. All experiments were carried out at a constant resolution of 4cm . Figure 2 shows the mirror arrangement used for the IRAS experiments. An additional metal mirror was mounted in front of the sample substrate when multiple reflection spectra were recorded. A ZnSe wire grid polarizer was used to remove the perpendicular part of the infrared radiation. 

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