The comparative scarcity of Raman scattering means that it is more difficult to detect than Rayleigh scattering. Notwithstanding, the advent of technologies such as lasers and sensitive charge coupled device (CCD) detectors has allowed Raman scattering to become the basis of a measurement technique known as Raman spectroscopy. This technique is based on the fact that light that is Raman scattered by a given molecule in a sample will have various different frequency shifts which correspond to the various different vibrational modes of the molecule. Because these different types of vibrations depend on the chemical bonds and symmetry of the molecule, the frequency shifts translate into a specific molecular structure. Thus, Raman spectroscopy has come to be seen as a powerful investigative tool capable of providing “optical fingerprints” by which molecules can be identified.
More technically speaking, a Raman scattering event proceeds as follows. An incoming photon interacts with a molecule, polarising the cloud of electrons around the nuclei and inducing a dipole moment. If the molecule was already vibrating at one of its characteristic vibrational frequencies, the oscillations of the induced dipole moment will be amplitude modulated at the vibrational frequency of the molecule.* As a result of this interaction, the molecule is excited to a “virtual” energy state created by the photon-induced polarisation and referred to as “virtual” to distinguish it from the molecule’s own excited vibrational/electronic energy states. This state, whose energy is determined by the frequency of the incident photon, is not stable, and the photon is quickly re-radiated, or scattered. During the scattering process, energy is transferred either from the incident photon to the molecule or from the molecule to the scattered photon. The end result is that the scattered photon is left with an energy that differs from that of the incident photon by one vibrational unit. Because the vibrational states of the molecule are dictated by its chemical structure, the shift in energy of the scattered photon will likewise then contain information about that chemical structure.
*A rotating molecule also leads to a periodic modulation of the dipole moment induced by the incoming photon and hence to Raman scattering, but the vibrational Raman effect dominates over the rotational Raman effect.
Put very simply, different molecules are made up of different atoms in different configurations, so each molecule bends, stretches, and vibrates in a slightly different way. Some of the photons scattered by a molecule will change the way the molecule is vibrating, and in turn, the energy of those photons will be changed by a very small amount. This change in energy is directly proportional to the vibration of the molecule, and hence to it’s chemical configuration, so Raman scattered light can be thought of as an “optical fingerprint” that can be used to identify a molecule by its chemical structure.
It is interesting to note that in Raman scattering, energy is transferred from photon to molecule and vice versa because nuclear motion is induced during the scattering process. This change in energy from incoming to scattered photon means that the process is inelastic. If nuclear motion is not induced during scattering and only electron cloud distortion is involved in the process, then the photon is scattered with only a negligible change in energy (electrons are light compare to nuclei), and the process is elastic. This process is none other than Rayleigh scattering, which is discussed here.
- Ewen Smith & Geoffrey Dent, Modern Raman Spectroscopy: A Practical Approach, John Wiley & Sons Ltd, 2005.
- Hermann Haken & Hans Christoph Wolf, Molecular Physics and Elements of Quantum Chemistry, Springer-Verlag Berlin Heidelberg, 2004.