View article

Akin to IR spectroscopy, Raman spectroscopy is a branch of vibrational spectroscopy which occurs as a consequence of the interaction of light and matter. If we consider light as being composed of multiple particles termed photons, these photons can respond in a variety of ways upon interaction with physical matter; they may be absorbed, scattered, reflected or may travel straight through without any interaction at all. In Raman spectroscopy we are observing the scattering process of photons upon interrogation with a monochromatic (single wavelength) laser and unlike in IR spectroscopy, the photons are not required to have the same specific energy that matches the energy gap between the ground and vibrational excited states of the analyte, as no absorption process is occurring. The interaction of a scattered photon promotes the molecule to a virtual energy state, before it relaxes almost immediately due to the unstable nature of the virtual state and returns to its original ground state. The most common occurrence is that the molecule relaxes and returns to its original energy level and thus the emitted photon has the same energy as that of the incident photon. This is termed elastic scattering and is known as Rayleigh scattering. This process occurs in 99.99999% of all scattering processes. However, much less frequently the molecule relaxes to a different energy state to its original and thus emits a photon with differing energy to that of the incident photon. This results in an energetic gain or loss and a measurable shift in wavelength. This is termed inelastic scattering and is what we call Raman scattering. There are two types of Raman scattering …