How Does it Work?
Infrared and Raman spectroscopy provide complementary information about molecular structure. In infrared spectroscopy the sample is irradiated with polychromatic light and a photon of light is absorbed when the frequency (energy) of the absorbed light matches the energy required for a particular bond to vibrate within the sample. In order for a vibration to be infrared active the molecular dipole moment must change during the vibration.
In Raman spectroscopy the sample is irradiated with monochromatic light and the photons are either inelastically or elastically scattered. The inelastically scattered light, known as Raman scatter, has lost (Stokes) or gained (Anti–Stokes) energy during this interaction and the emitted photon contains information about the molecular structure of the sample. The elastically scattered light has the same energy as the incident laser light and is called Rayleigh scatter. Modern Raman instruments are designed to filter out the Rayleigh light as only one in every million photons will be Raman scattered! There is one other requirement for a vibration to be Raman active – when the molecule vibrates there must be a change in polarisability i.e., a change in the shape, size or orientation of the electron cloud that surrounds the molecules (1).
(1) P. Hendra, Int. J. Vib. Spec.,Vol 1, (1998) 6-16