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Why spectroscopy?

And, in particular, why optical emission spectroscopy?

Sometimes it may happen to wonder why the sky looks blue and what is the answer? It may be already known that sunlight is made up of all the colours of the rainbow: red, orange, yellow, green, blue, and violet. It may be also known that sunlight has to pass through our atmosphere before it reaches our eyes. The gas molecules in the atmosphere break up, or "scatter" the sunlight into its many parts. But they scatter some parts more effectively than others. Different colours of light have different energies or wavelengths. Red light has a long wavelength and a lower energy, blue light has a short wavelength and a higher energy. The gas molecules in the atmosphere scatter the higher-energy blue wavelengths better than the red wavelengths. So the sky looks blue.
As mentioned before, red and blue are only two of the colours which make up the light coming from the sun. This light can be seen as a source and as such it can be analyzed, but how can this be done? As a source, light has a spectral range which can be totally or selectively transmitted to an imaging system which transfers this range to a spectrometer. This spectrometer, on receiving the spectral range transmits it to a detector which eventually elaborates the data.

Spectroscopy is the study of spectra, ie characteristic wavelengths or colours. Optical emission spectroscopy (OES) comprises several techniques that form the most important means we have for chemical analysis.

In OES, we measure spectra emitted by atoms and ions with optical transitions in the wavelength range from about 100 nm to 900 nm. This range includes the ultraviolet, and visible light (from violet at 380 nm to red at 760 nm), and the near infra-red.

With OES, we can:

determine the chemical composition of solids, like steel or wood or paint, and liquids, like oil or milk, and gases, like air or car exhausts. Knowing the content of materials, we can monitor changes in the environment, and improve production processes and product properties, such as strength, corrosion resistance and appearance.
monitor the deposition of layered devices used in the semiconductor industry, and the deposition of hard coatings on tools.
study the inner workings of atoms and ions and molecules. Knowing their electronic structure, we can test the predictions of quantum theory and better understand the behaviour of the chemicals that compose our bodies, our food and the world we live in.
With spectroscopy we can determine the elements that make up the sun and the stars. Knowing the elements in stars, we can learn much about their age and their origins, and  this helps understanding the nature of the universe.

First published on the web: 08.12.2007

Authors: Aranka Derzsi and Giovanni Lotito. The text is based on a lecture given by Thomas Nelis at the first Gladnet training course in Antwerp Sept. 2007