Spectroscopic measurements require a light source, a sample exposed to light emanating from the light source, and a detector. These three components may be arranged in linear fashion or in the form of a right angle, depending on whether the detector will examine the remnants of the incident beam, or will instead look at photons emanating from the sample.
A linear arrangement is used in absorption spectroscopy, where the measurement determines what fraction (T = I/I0) of the intensity of the incident beam (I0) is transmitted through the sample (I).
Light source --> sample --> detector
T, the transmittance, is related to the absorbance, A, by
A = - log T
An example of this arrangement is depicted in PC Fig. 6.2.
Some spectroscopic techniques look for photons that originate in the sample, as a consequence of its interaction with the incident beam. In these techniques, we want to place the detector so that it will not see photons from the incident beam that pass without change through the sample, but will see photons that emanate from the sample itself. This objective is achieved by rotating the detector away from the extension of the line from the light source to the sample. In some techniques (such as fluorescence or Raman spectroscopy) the detector is placed at 90 deg to this line, so that the three components make a right angle. In other techniques using an incident beam (such as light scattering), one might be interested in how the signal at the detector depends on the angle.
The light source may be monochromatic (as with a laser) or polychromatic (as with a xenon or tungston lamp). If it is polychromatic, arrangement is usually made for selection of photons of a particular energy, using filters, a prism, or a diffraction grating. In the example depicted in PC Fig. 6.2, the light source is polychromatic, and a dispersive element is placed between the sample and the detector. Other absorption experiments may place the monochromator between the light source and the sample. Both arrangements yield equivalent information in the linear arrangement.
If the experiment uses the right-angle arrangement, fundamentally different information is obtained with the two ways of positioning the monochromator.
The experiment may also require light of a specific polarization, which will introduce additional components into the optical system. For example, the unpolarized light from the sample may be rendered plane polarized (in optical rotatory dispersion and in some scattering measurements) or circularly polarized (in circular dichroism) before it reaches the sample.`
Spectra are usually recorded as absorbance, transmittance, or intensity as a function of wavelength, frequency, or wavenumber.
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