This is a form of
vibrational spectroscopy for investigating the
structure and therefore the
identity of
molecular and
crystalline compounds. It is commonly referred to as IR spectroscopy.
A beam of infrared light is shone through the material and the intensity compared with the intensity of a reference beam from the same source that does not pass through the material (to allow for source fluctuation etc.). A spectrometer measures the relative intensities at different wavelengths and a spectrum obtained.
Materials are normally held in a matrix of an infrared inactive material. These are normally salts such as potassium chloride and sodium bromide. The sample can also be ground into a paste in a hydrocarbon such as Nujol and then placed between salt disks (which is easier) though the Nujol will cause some bands of its own.
To be active in IR spectroscopy a vibration must cause a change in the dipole of the molecule.