19.3: Comparison with Other Types of Spectroscopy
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It is instructive to compare the process of Raman scattering with some other spectroscopic techniques. In the commonly used infrared absorption spectroscopy, infrared light excites certain vibrational frequencies of molecules and is absorbed by them, not re-emitted. This gives an absorption spectrum, with bands at characteristic wavenumbers. Other absorption techniques use higher energy radiation (e.g. ultraviolet) and raise electrons to an excited state.
Fluorescence occurs when light (often UV) is incident on a molecule and promotes an electron to an excited state. The molecule is also vibrating. Firstly it relaxes from its vibrational state, dissipating this energy (normally as heat). Then when it drops back down to the ground state, the photon released has less energy than the incident photon. The increased wavelength often means that the light is now in the visible region. This is how fluorescent lighting works, by ionisation of mercury to produce UV light, which is then absorbed by a fluorescent coating and re-radiated as visible light. Fluorescence can also be used for spectroscopy.
Below is an overview of some different interactions of light with a molecule.