Jacquinot stop

Figure 3.1-10 The significant features of an interferometer A/ displacement of the moving mirror, 2r diameter of the Jacquinot stop.

Here, r is the radius of the Jacquinot stop, F] stands for the area of the beam at the beam splitter, and / o represents the resolving power, determined by the displacement of the moving mirror. For the optical conductance per unit bandwidth G or Gp, we obtain ... 

For all applications in spectroscopy it is necessary to adapt the optical conductance of the fiber to the other elements of the instrument. Laser radiation can be transported easily by one fiber, but its connection to a spectrometer principally needs a bundle of fibers, because spectrometers have an optical conductance which usually exceeds that of one fiber. Besides, irradiation of the entrance slit of a grating spectrometer requires the fibers to be arranged in a row, while an interferometer requires a circular arrangement at an image of the Jacquinot stop. The optical conductance of one fiber may be approximated by... 

The connection to the other elements of the instrument should be designed such as to fully exploit the available fiber aperture and to find the best possible match to the optical properties of radiation sources, of emitting, absorbing or scattering samples as well as with the images of slits, Jacquinot stops, and gratings or beam splitters. 

Michelson interferometers also have the advantage of being much more tolerant to misadjustment of the sample arrangement, since they have circular Jacquinot stops, compared to the straight small entrance slits of grating spectrometers (Hirschfeld, 1977 b). 

Beam condensers, by using a pair of ellipsoid mirrors, produce very small images of the Jacquinot stop or the entrance slit at the sample position. The size of these images may be even further reduced by making use of a Weierstrass sphere. Weierstrass (1856) showed that a spherical lens has two aplanatic points . If a sphere of a glass with a refractive index n is introduced into an optical system which has a focus at a distance of r n from its center, then the beam is focused inside the sphere at a distance of r/n from the center (Fig. 3.5-9). In this case the angle O in Eq. 3.4-5 may approach 90°. Thus, a sample with a very small area can fully fit the optical conductance of the spectrometer (Fig. 3.4-2d). Microscopes usually have an optical conductance which is considerably lower than that of spectrometers. In this case, the sample and the objective are the elements limiting the optical conductance (Schrader, 1990 Sec. 3.5.3.3). 

From Eq. 2.41 it can be seen that the solid angle of the beam passing through the interferometer must be reduced every time the resolution is increased (i.e., Av is decreased). This is usually accomplished by reducing the diameter of the Jacquinot stop that is mounted at the beam focus. 

Figure 6.8. Optics of the Nicolet Model 60-SX FT-IR spectrometer (now obsolete). Sources can be located at Si, S2, and/or S3 detectors can be located at Di, D2, and/or D3 M signifies a fixed mirror MF signifies a flip mirror Ai is a Jacquinot stop.

Spectrometers capable of higher resolution (Av < 1 cm ) are usually equipped with a Jacquinot stop. The J-stop is located at an intermediate focus in the source optics (see Figures 6.9 and 6.10) to reduce the solid angle of the beam to the value given by Eq. 7.10. Measurements at high resolution are therefore measured with variable throughput, 0/, which must be decreased as the spectral resolution increases (i.e., Av decreases). 

Under carefully controlled conditions, wavenumber measurements may be precise to 0.01 cm (discussed below), but usually only when a sample is left undisturbed in the sample compartment. Even if the sample is simply removed and reinserted between measurements, the repeatability is often worse than 0.01 cm . Several reasons can be advanced to explain why band shifts occur. First, the temperature of the sample may change between measurements, which leads to small spectral shifts. Second, it was noted in Section 2.6 that changes in the effective solid angle of the beam through the interferometer can lead to small wavenumber shifts. Because the cell may represent a field (Jacquinot) stop, if a cell is not placed in exactly the same position for successive measurements, bands will appear to shift from one measurement to the next. Furthermore, if the cell is slightly tilted and the angle changes appreciably from one measurement to the next, the beam may be refracted to a different position on the detector, which also shifts the wavenumber scale. Loose or insecure sample mounts should be avoided if users require the wavenumbers of absorption band maxima to be repeatable to better than 0.1 cm . ... 

To avoid a large decrease in wavenumber resolution arising from oblique rays, a circular aperture, called the Jacquinot stop (J-stop), is placed in the focal plane of the collimator as shown in Figure 4.12a. The optimal diameter of the J-stop is determined in order to make the wavenumber shift due to oblique rays smaller than the wavenumber resolution 8v determined by the OPD for the beam parallel to the optical axis. 

Figure 4 The significant features of an interferometer A/ displacement of the moving mirror, 2r diameter of the Jacquinot stop. Reproduced from Schrader B (ed) (1995) Infrared and Raman Spectroscopy. Weinheim VCH Publishers, with permission of VCH.

In an interferometer the total radiation power on the detector is much higher than that in a grating instrument. In practice, the FT-IR spectrometer has an approximately 200 times larger S/N than the grating spectrometer with the same resolution and recording time. The reason is that at the optimum resolution the area of the circular aperture (the Jacquinot stop) in the FTTR spectrometer is 200 times larger than the slit in the grating spectrometer. 

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