Monochromators and Interferometers

In between the source and detector, the spectrometer must have some means of analyzing the radiation so that an intensity can be deduced for each wavelength resolution element. Two completely different types of devices are used, namely, monochromators and interferometers. Monochromators with gratings or prisms are used in dispersive instruments, and interferometers are used in Fourier transform instruments. 

We have performed optically heterodyne-detected optical Kerr effect measurement for transparent liquids with ultrashort light pulses. In addition, the depolarized low-frequency light scattering measurement has been performed by means of a double monochromator and a high-resolution Sandercock-type tandem Fabry-Perot interferometer. The frequency response functions obtained from the both data have been directly compared. They agree perfectly for a wide frequency range. This result is the first experimental evidence for the equivalence between the time- and frequency-domain measurements. 

Figure 1 a) The temporal profile of OHD-OKE on nitrobenzene. In the insert are shown the short-time behavior of the OKE transient solid line) and the intensity autocorrelation function of the laser pulse (dashed line) b) The light scattering spectrum of nitrobenzene measured by the double monochromator and the tandem interferometer (insert). 

Extremely quiet and rapid monochromators are now available from numerous vendors. As covered in detail in Chapter 2, top-notch monochromators (grating and interferometer), diode arrays, accousto-optic tunable filters, as well as modern interference-filter instruments now exist. Fiber optic probes, multiple detector modules, and transmission attachments all lend themselves to superior sample handling of powders and solid dosage forms. 

Fourier transform IR instruments contain no dispersing element, and all wavelengths are detected and measured simultaneously. Instead of a monochromator, an interferometer is used to produce interference patterns that contain the infrared spectral information. The. same types of sources used in dispersive instruments are used in FTIR spectrometers. Transducers are typically triglycine sulfate—a pyroelectric transducer—or mercury cadmium telluride—a photoconductive trans-... 

The older, conventional instruments are known as dispersive spectrometers, where the infrared radiation is divided into frequency elements by the use of a monochromator and slit system. Although these instruments are still in use today, the recent introduction of Fourier transform infrared (FT-IR) spectrometers has revitalized the field (4). The FT-IR system is based on the Michelson interferometer. The total spectral information is contained in an interferogram from a single scan of a movable mirror. There are no slits, and the amount of infrared energy falling on the detector is greatly enhanced. Together with the use of modem computer techniques, an entirely new breed of instrument has been created. 

Dispersive elements and interferometers are widely used in vibrational microspectroscopy. As in bulk measurements, microscopic Raman studies are carried out with grating monochromators, spectrographs, or Fourier transform spectrometers, although Fourier transform instruments are usually limited to applications in the near-infrared spectral region. Infrared microspectroscopy, by contrast, is almost exclusively a Fourier transform technique. 

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