Off-axis ellipsoid

Schematic optical layout of FT-IR spectrometer. A = Source, B = Off-axis paraboloid, C = Beamsplitter, D = Fixed mirror, E = Movable mirror, F = Off-axis paraboloid, G = Sample point, H = Off-axis ellipsoid, I = Infrared detector, J = Visible beamsplitter, K = Fixed mirror, L = He Ne and white light sources, M = He Ne and white light detectors.

Diffuse reflectance can be used as a trace analysis method. In this case, the material to be studied is applied as a small quantity of a solution in a volatile solvent to a powdered KBr matrix. The solvent is evaporated and the sample is allowed to coat the KBr crystals. The normal sampling cup dimensions are approximately 13 mm in diameter x 3 mm deep however, smaller diameter cups are available with diameters of 1.5-2 mm. These are closely matched to the typical instrument beam diameter after imaging with the diffuse reflectance accessory primary focusing mirror (typically a 6 1 off-axis ellipsoid). With this microsampling extension, it is possible to obtain good spectra from sample loadings as low as a few 100 ng. 

Fig. 4.1. Schematic of an experimental set-up for absorption measurements at low temperature incorporating a Perkin-Elmer Model 99G monochromator. Si, S2 and S3 are IR sources selectable with plane mirrors Mi and M2. FM focusing spherical mirrors. Ei and E2 entrance and exit slits. CM off-axis paraboloid collimating mirror. G plane reflection grating. Beam 1 from Si is converted by CM into a parallel beam dispersed by G. One wavelength is diffracted in a direction where it can be intercepted by first mirror M as beam 2 and focused on the internal chopper Ch. Modulated beam 2 is redirected toward G as beam 3 and re-dispersed a second time as beam 4. Beam 4 intercepted by IM is focused on E2 and re-focused on the sample by FM. The divergent monochromatic beam is finally focused on thermocouple D by ellipsoidal mirror EFM. Fi, F2 and Pol are locations for transmission filters and a polarizer. Beam 1 can be blocked by shutter Sh (after [37]). With permission from the Institute of Physics...

In the MIR, diffuse reflectance is very weak and could only be measured after routine FT-IR spectrometers became available (DRIFT spectroscopy, diffuse reflectance infrared Fourier-transform spectroscopy). Due to the lack of ideal non-absorbing scattering substrates in the MIR, the diffusely reflected MIR radiation is generally collected by large ellipsoidal mirrors, which cover as much area above the sample as possible. Two optical configurations are commercially available, on-axis and off-axis designs (Fig. 5.9). 

Figure 4.28. Optical diagram of Praying Mantis (Harrick Scientific Inc.) diffuse-reflectance accessory Mi, M2, M5, Me — plane mirrors Ms, M4 — 6 1 90° off-axis focusing and collecting ellipsoids. Sample is placed at position S. Adapted, by permission, from K. Moradi, C. Depecker, and J. Corset, Appl. Spectrosc. 48,1491 (1994), p. 1492, Fig. 2. Copyright 1994 Society for Applied Spectroscopy.

Let us now consider the case of a polarized-distorted sphere such as the one in Figure 8. We would expect that associated with the polarization process there is a distortion process because dipoles are associated with some real molecular group that has a shape. In other words, when the dipoles orient in the electric field, there is an average projection in the field direction that is different from the average when the field is turned off. At this point it is necessary to postulate a molecular mechanism for the distorted sphere. Consider the sphere to consist of N ellipsoids with a major (2a) to minor (a) axis ratio of 2. The dipole moment = 4D units, which was computed from Eq. (12), , = 1) 7 = room... 

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