Introduction to the Raman Effect

Raman scattering is similar to infrared absorption in that they are both vibrational spectroscopies that probe the discrete vibrations of molecules and molecular bonds. The Raman effect relies on the scattering of the incident photons, whereas infrared occurs through light absorption. These vibrational techniques have different selection rules for the types of molecular vibrations that can be observed, and so they are often considered complimentary. For a molecular vibration to be detected with infrared spectroscopy the vibration must alter the permanent dipole of the molecule. In contrast a Raman vibration is only active when the polarizability of the molecule changes during the vibration. This polarizability occurs when the electron cloud of the molecule becomes more readily deformed in one extreme of the vibration than in the other extreme. Essentially a Raman spectrum identifies information about the backbone structure of the molecule, whereas the stronger infrared features are indicative of the molecules polar segments.

The Raman effect can be observed by exciting a sample with a single monochromatic light source (i.e. a laser). The incident photons promote electrons to a virtual excited state and then the photons are scattered back from the molecule. Most of the scattered radiation has the same wavelength as the excitation photons. This process is known as Rayleigh scatter. However, a tiny portion of the scattered radiation (1 x 10^-7) is shifted to a longer wavelength and will produce a Stokes Raman scattering band. The wavelength-shifted Raman bands identify the different functional groups of the molecules (i.e. the various Raman bands are used to determine the type of molecules located in the sample mixture). A key advantage with the Raman effect is that it is a linear process. Therefore a sample’s Raman band for an individual molecule will be twice as strong when the sample contains twice as many of those molecules. Basically the locations of the different Raman bands are used to determine the type of molecule, and the Raman scattered intensity directly correlates to that molecule’s concentration.