What is Spectroscopy?

Spectroscopy in optics is the study of the different frequency or wavelength components (colors) of a light source, and using this information to deduce information about the material samples with which the collected light interacted. Variations in molecular structure correspond to different optical properties, creating unique spectroscopic “signatures” used to identify the materials under interrogation. The devices used to collect and identify the intensity of each wavelength of light in these signatures are called optical spectrometers.

image showing how a spectrometer works

There are several types of optical spectroscopy, including Emission, Absorption, and Raman.

Emission spectroscopy – One way to determine the composition of a material is to taste it. Another is to smell it. These are not recommended methods for quantitative analysis. Emission spectroscopy is one such quantitative method. A hot or energized source will emit certain colors based on the composition of the material. A good example is high-pressure sodium streetlights that glow a characteristic yellow color due to the presence of sodium. Replacing the sodium with mercury vapor creates a purple glow characteristic of mercury.

Absorption Spectroscopy – Cool material in front of a hot continuum source will remove specific colors from the broadband spectrum based on the composition of the material. A good example is the (cool) atmosphere of the sun, which generates absorption lines that can be attributed to individual elements. The yellow features from sodium, for example, are seen in absorption from the sun due to the presence of cool sodium gas in the solar atmosphere.

Raman spectroscopy exploits an effect where a small fraction of the laser light striking a material will scatter at different wavelengths (the Raman effect) than the incoming laser wavelength, while the rest of the light is scattered with no change in wavelength. This Raman technique has shown great promise for high accuracy in differentiating materials using portable and robust spectrometer configurations outside of the lab. Despite the growing use of the technique, until now its usefulness has been limited by the faintness of the Raman-shifted signal.

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