The interactive effects of light scattering at molecules can be separated into two main groups: inelastic and elastic scattering. Rayleigh scattering is elastic because the energy of the scattered photons remains unchanged, whereas the direction of travel is modified. In inelastic (Raman) scattering processes, the photon energy/frequency/wavelength of the light after interaction with the sample is different, and the distribution is spherical. Thus, it is possible to measure only the intensity and 3D angular distribution of the elastic scattering at the original wavelength (Rayleigh, excitation). The Raman photons, on the other hand, will carry a slightly different energy/frequency/wavelength after interaction. Raman spectra appear on both sides of the excitation energy. The side with lower energies (longer wavelengths) is called the Stokes side; the range of higher energies is called the anti-Stokes side. This behavior is due to the fact that the interacting molecule may absorb and transfer a certain amount into its rotational structure and vary the vibrational state (stretch), which is measured on the Raman Stokes side of the spectrum. Conversely, the molecule will add some energy from its own rotation, resulting in spectral features on the Raman anti-Stokes side.
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