When photons interacts with matter, specifically molecules, there are a few events that can take place. One of them is the inelastic scattering of the photon , or Raman scattering, that happens when the incoming photon changes direction and energy (or equivalently, color) after the interaction. How much the photon's energy changes corresponds to the energy of a vibrational level in the molecule. Not all vibrational levels in a molecule are "Raman active", there are quantum rules that help determine which ones are or aren't active, but a mapping of those that are results in a spectrum like the one shown in Figure 1. A spectrum like that is achieved by illuminating the sample with a single color light source, in other words a laser, and collecting the photons after they have interacted with the molecules in the sample, and thus changed energy. Each peak in the spectrum is generated by photons which energy changed by the same amount, and together all the peaks in the spectrum create a unique fingerprint for a molecule.
Figure 1. Raman spectrum of polystyrene acquired with a 785 nm excitation laser. Relative wavenumbers are a measure of the change in the photons energy with respect to their initial value.
Raman scattering as an spectroscopy and imaging tool
Since each molecule's Raman spectrum is unique, it becomes a very powerful tool to do chemical identification. Not only that, but Raman scattering is proportional to the concentration of molecules in the volume probed, so it also is a quantitative tool. In many instances a spectrum is all is needed to characterized a sample, for example when the sample is solution, or homogeneous as the polystyrene in Figure 1. In other cases, when the sample is not uniform or more information about th different components distribution is required, Raman spectra collected on different spots on the sample can be used to reconstruct a Raman map or image. Figure 2 shows how Raman scattering can be used as an imaging tool.