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Raman spectroscopy in more detail

For a more basic introduction to Raman spectroscopy see Basic overview of Raman spectroscopy.

Raman spectroscopy reveals the chemical and structural composition of samples. Generally, all materials produce Raman spectra, with the exception of pure metals.

Raman scattering

Raman scattering occurs when light interacts with molecular vibrations. This is similar to the more widely known infrared absorption spectroscopy, but different rules apply. A change in molecular polarisability is required during the vibration for the Raman effect to occur.

You will see some vibrations in the Raman spectrum that are not visible in the infrared spectrum, and vice-versa, because of the different selection rules. For example, Raman spectroscopy is superb for studying the carbon atoms that make up the structure of diamond, unlike infrared absorption spectroscopy.

Scattered light

The first step in producing a Raman spectrum is to illuminate your sample with a monochromatic light source, such as a laser.

Most of the light that scatters off is unchanged in energy ('Rayleigh scattered'). A minute fraction—perhaps 1 part in 10 million—has lost or gained energy ('Raman scattered'). This Raman shift occurs because photons (particles of light) exchange part of their energy with molecular vibrations in the material.

Where energy is lost the Raman scattering is designated as 'Stokes'; where energy is gained the Raman scattering is designated as 'anti-Stokes'. We rarely use anti-Stokes Raman light as it is less intense than the Stokes, however it does represent equivalent vibrational information of the molecule.

Vibrating atoms

The change in energy depends on the frequency of vibration of the molecule. If it is very fast (high frequency)—light atoms held together with strong bonds—the energy change is significant. If it is very slow (low frequency)—heavy atoms held together with weak bonds—the energy change is small.

Raman spectrometers

Renishaw inVia systems consist of:

  • single or multiple lasers, from UV (244 nm) to IR (1064 nm) – switch with a single click
  • high quality objective lenses, from high confocal 100× to long working distance and immersion options
  • custom designed motorised spectrometer lenses­ - automatically align for each configuration
  • laser-line-specific Rayleigh filters with a dual filter arrangement to optimise sensitivity
  • highest quality master diffraction gratings provide exceptional dispersion and longevity
  • thermoelectrically cooled (- 70 ºC) CCD detector – stable and sensitive
  • high specification multi-core PC for data collection and analysis

Download our Raman spectroscopy explained booklet