What's Their Difference?
The main difference between the two techniques lies in the nature of the molecular transitions taking place. For a transition to be Raman active there must be a change in the polarizability of the molecule during the vibration. This means that the electron cloud of the molecule must undergo positional change. On the other hand, for an IR detectable transition, the molecule must undergo dipole moment change during vibration. So, when a molecule is symmetrical, e.g. O2, we cannot observe any IR absorption lines, since the molecule cannot change its dipole moment. It has been observed that molecules with a strong dipole moment are typically hard to polarize.
Regarding the excitation wavelength, the Raman technique uses a monochromatic beam or laser, in the visible, near-infrared, or near ultraviolet range of the electromagnetic spectrum. In IR spectroscopy, a monochromatic beam is used in the infrared region of the electromagnetic spectrum. The wavelength here increases or decreases over time, in order to observe all the absorption lines within a specific range of the infrared region.
Another difference can be observed in the resulting spectra. The IR technique shows irregular absorbance (or transmittance) lines, depending on the material investigated. The Raman spectrum mainly comprises the elastic scattered light line (Rayleigh) and two equally distanced lines Stokes and anti-Stokes, with the second being rather weak and difficult to detect.
Finally, the Raman technique requires high-stability laser sources and sensitive amplification equipment to detect the weak signal. This makes Raman instruments more expensive compared to the IR equipment. However, the new advances in technology have altered this to some degree.