Spectrographs and Monochromators

Spectrographs were the first instruments for measuring wavelengths and they still hold their position in spectroscopic laboratories, particularly when equipped with modern accessories such as computerized microdensitometers. Spectrographs are optical instruments which form images S2(x) of an entrance slit which are laterally separated for different wavelengths X of the incident radiation (see Fig.2.10). This lateral dispersion is achieved either by spectral dispersion in prisms or by diffraction on plane or concave reflection gratings. 

In spectrographs a photoplate or photographic film is placed in the focal plane of L2 or M2. The whole spectral range AX = x (Xj ) - covered by 

Monochronators, on the other hand, use photoelectric recording. An exit slit 2, selecting an interval focal plane B, lets only a limited 

Whereas the spectrograph allows the simultaneous measurement of a large spectral region with moderate time resolution, photoelectric detection allows 

In spectroscopic literature [4.1-5] the name spectrometer is often used for both types of instruments. The introduction of optical multichannel analyzers (see Sect.4.5.9) as detectors behind spectrographs combines the advantages of high spectral and time resolution with simultaneous detection of extended spectral ranges. 

At first we discuss the basic properties of spectrographs and monochromators. Although for many experiments in laser spectroscopy these instm-ments can be replaced by monochromatic tunable lasers (Chap. 5 and Vol. 2, Chap. 1), they are still indispensible for the solution of quite a number of problems in spectroscopy. 

Probably the most important instmments in laser spectroscopy are interferometers, which are applicable in various modifications to numerous problems. We therefore treat these devices in somewhat more detail. Recently, new techniques of measuring laser wavelengths with high accuracy have been developed they are mainly based on interferometric devices. Because of their relevance in laser spectroscopy they will be discussed in a separate section. 

In spectrographs a charge-coupled device (CCD) diode array is placed in the focal plane of L2 or M2. The whole spectral range AX = Xi(xi) — A.2(x2) covered by the lateral extension Ax =xi — X2 of the diode array can be simultaneously recorded. The cooled CCD array can accumulate the incident radiant power over long periods (up to 20 h). CCD detection can be employed 

Great progress has also been achieved in the field of low-level signal detection. Apart from new photomultipliers with an extended spectral sensivity range and large quantum efficiencies, new detection instruments have been developed such as image intensifiers, infrared detectors, or optical multichannel analyzers, which could move from classified military research into the open market. For many spectroscopic applications they prove to be extremely useful. 

Spectrographs were the first instruments for measuring wavelengths and they still hold their position in spectroscopic laboratories, particularly when equipped with modern accessories such as computerized microdensitometers or optical multichannel analyzers. Spectrographs are optical instruments 

For all these reasons diffraction gratings are used in most dispersive optical instruments. It is however essential to bear in mind the artefacts that can arise from harmonic transmissions. 

From the point of view of the photochemist there are three important features in the choice or design of a monochromator. 

1 Double and Multiple Monochromators. If monochromatic light of very high purity is required, it is often necessary to use two or several monochromators in series, such that the exit light of the first one becomes the entrance light of the second. The reasons for using such complex optical arrangements are two-fold. 

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The contents of Chap. 4, which covers spectroscopic instrumentation and its application to wavelength and intensity measurements, are essential for the experimental realization of laser spectroscopy. Although spectrographs and monochromators, which played a major rule in classical spectroscopy, may be abandoned for many experiments in laser spectroscopy, there are still numerous applications where these instruments are indispensible. Of major importance for laser spectroscopists are the different kinds of interferometers. They are used not only in laser resonators to realize single-mode operation, but also for line-profile measurements of spectral lines and for very precise wavelength measurements. Since the determination of wavelength is a central problem in spectroscopy, a whole section discusses some modern techniques for precise wavelength measurements and their accuracy. 

The contents of Chap. 4, which covers spectroscopic instrumentation and its application to wavelength and intensity measurements, are essential for the experimental realization of laser spectroscopy. Although spectrographs and monochromators, which played a major rule in classical spectroscopy, may be abandoned for... 

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