The Raman effect arises when the incident light excites molecules in the sample which subsequently scatter the light. While most of this scattered light is at the same wavelength as the incident light, some is scattered at a different wavelength. This inelastically scattered light is called Raman scatter. It results from the molecule changing it's molecular motions.
The energy difference between the incident light (Ei) and the Raman scattered light (Es) is equal to the energy involved in changing the molecule's vibrational state (i.e. getting the molecule to vibrate, Ev). This energy difference is called the Raman shift.
Ev = Ei - Es
Several different Raman shifted signals will often be observed; each being associated with different vibrational or rotational motions of molecules in the sample. The particular molecule and its environment will determine what Raman signals will be observed (if any).
A plot of Raman intensity versus Raman shift is a Raman spectrum.
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