Spectroscopy is the study of light through light-matter interactions.
There are three main types of interactions:
Quantized interactions in which involved molecules (Here: atoms, full or parts of molecules or crystals,..) jump from a stationary state to another stationary state. It is the most usual spectroscopy.
Coherent interactions in which the involved molecules are excited to a non-stationary state during the interaction and return to their initial state. A detectable effect requires the coherence, that is the relations between the molecular parameters allowing the interaction (oscillating dipoles...) and the local electromagnetic fields are the same for all molecules.
Mixed interactions: coherent and involving transitions of some molecules; example : He-Ne lasers.
Except refraction which is widely used, the coherent interactions are generally studied using lasers, so that, due to the intensity of these sources, the effects depend non-linearly on the intensity of the lasers. Some interactions require an intensity threshold.
See also
Coherent anti-Stokes Raman spectroscopy
Coherent Stokes Raman spectroscopy
Raman spectroscopy
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