Raman Spectroscopy v Infra-red Absorption Spectroscopy
In Raman spectroscopy, a single frequency of light is used to irradiate a sample, and radiation scattered from the molecules whose frequency is shifted from that of the incident beam is detected. Unlike infrared absorption spectroscopy, it does not require that the frequency of this incident radiation ever match the energy difference between the ground and excited states of a molecule. Since the virtual energy state excited in Raman spectroscopy is created by the photon induced polarisation, the energy of this state is determined by the frequency of the incident light, and can be chosen. This means that the frequency of the incident light can be selected to optimise detection or accommodate other features of the sample. For instance, it is possible to choose an excitation frequency which minimises absorption in an aqueous sample. The fact that it only requires a single non-specific incident frequency is a comparative advantage of Raman spectroscopy over infrared absorption spectroscopy. It should be noted, however, that as the difference in energy between the virtual state and the excited state of the molecule increases, the scattering rate decreases, leading to a weaker signal.
The main advantage of infrared absorption spectroscopy over Raman spectroscopy is precisely that it yields a much stronger signal. Infrared absorption spectroscopy is also comparatively inexpensive. The downside of infrared absorption spectroscopy is that the absorption of water increases as you move further into the infrared, so it is not as suitable for the analysis of aqueous samples.