A basic overview of Raman spectroscopy
Spectroscopy and light
Light interacts with matter in different ways, transmitting through some materials, while reflecting or scattering off others. Both the material and the colour (wavelength) of the light affect this interaction. We call the study of this light ‘spectroscopy'. Which parts of the visible spectrum enter our eyes determines which colours we perceive.
A substance might appear blue, for example, if it absorbs the red parts of the spectrum of light falling upon it, only reflecting (or scattering) the blue parts into our eyes.
Raman spectroscopy looks at the scattered light
If you were to shine blue light—from just one part of the spectrum—onto the material, you might expect to just see blue light reflected from it, or no light at all if it is completely absorbed (i.e. a black material).
However, by using a Raman spectrometer, you can see that often a very tiny fraction of the scattered light has a different colour. It has changed frequency because, during the scattering process, its energy changed by interacting with molecular vibrations. This is the Raman scattering process, named after its discoverer, the famous Indian physicist C.V. Raman. He was awarded the 1930 physics Nobel Prize for this great discovery.
By studying the vibration of the atoms we can discover the chemical composition and other useful information about the material.
The Raman effect is very weak; only about 1 part in 10 million of the scattered light has a shifted colour. This is too weak to see with the naked eye, so we analyse the light with a highly sensitive spectrometer.
These systems consist of:
- one or more single coloured light sources (lasers)
- lenses (both to focus the light onto the sample and to collect the scattered light)
- filters (to purify the reflected and scattered light so that only the Raman light is collected)
- a means of splitting the light into its constituent colours (normally a diffraction grating or prism)
- a very sensitive detector (to detect the weak light)
- a device such as a computer to control the whole system, display the spectrum and enable this information to be analysed
Raman scattering offers significant advantages for the investigation of materials over other analytical techniques, such as x-raying them or seeing how they absorb light (e.g. infrared absorption or ultraviolet absorption).
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Discover more about Raman spectroscopy, what it can tell you and why we use it.