Question

Provide a detailed description of the theory of Raman spectroscopy and explain (in general terms) how it is used in the field of physical chemistry.

Answer 1

Raman spectroscopy is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a system. Raman spectroscopy is commonly used in chemistry to provide a fingerprint by which molecules can be identified.

It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range.

If the final vibrational state of the molecule is more energetic than the initial state, the inelastically scattered photon will be shifted to a lower frequency for the total energy of the system to remain balanced. This shift in frequency is designated as a Stokes shift. If the final vibrational state is less energetic than the initial state, then the inelastically scattered photon will be shifted to a higher frequency, and this is designated as an anti-Stokes shift. Raman scattering is an example of inelastic scattering because of the energy and momentum transfer between the photons and the molecules during the interaction. Rayleigh scattering is an example of elastic scattering, the energy of the scattered Rayleigh scattering is of the same frequency (wavelength) as the incoming electromagnetic radiation.