Dispersive Raman Spectrometers

Abstract

As discussed in Chapter 5, all Raman spectrometers built before 1986 were dispersive, with the Fourier transform (FT) systems being a popular alternative since then when the laser wavelength is longer than about 1.0 Fm. Dispersive Raman designs cover a wide range of types and configurations, and only the most common will be addressed in this chapter. Until ca. 1980, dispersive spectrometers were single-channel, scanning systems based on single, double, or occasionally triple monochromators. Since 1990, multichannel dispersive systems superceded single-channel designs due to the speed and sensitivity gains noted in Chapters 3 and 4. In the current chapter, we will consider single grating dispersive spectrometers initially, in order to illustrate dispersive principles. The majority of the chapter is on multichannel systems and charge-coupled devices CCD, as this combination currently dominates dispersive designs.Dispersion of light by a diffraction grating, shown schematically in Figure 8.1, is the principle of the most common type of dispersive wavelength analyzer. As noted in Chapter 1, the wavelength analyzer is a subunit within a Raman spectrometer that allows separation of wavelengths and eventually Raman shifts. A spectrograph disperses light along a focal plane. A spectrograph with a multichannel detector at the focal plane is a key component in a multichannel spectrometer. If an exit slit is placed at the focal plane, a small range of wavelengths (the bandpass) is transmitted, and the device is a monochromator. Strictly speaking, a monochromator is used only with a single-channel detector, but in common usage, the spectrograph preceding a multichannel detector is often referred to as a monochromator.Before considering specific spectrometer designs, a few general points that apply to all dispersive systems deserve note. Nearly all dispersive Raman spectrometers are based on diffraction gratings, shown schematically in Figure 8.1. Gratings disperse the light according to wavelength, not wavenumber, resulting in a linear spread of wavelengths at the focal plane of the spectrometer. The lower portion of Figure 8.1 shows the coverage of wavelength along the focal 149 Raman Spectroscopy for Chemical Analysis. Richard L. McCreery