Though water has 3 raman active vibrations ( Symm. stretch, asymm. stretch and bend) yet it is quite weak raman scatterer and never interfares with Raman signals of other constituents..why??
The first half sentence is true that water is a weak Raman scatterer. It can be explained from the basic principle of Raman scattering:
" In the classical wave interpretation, light is considered as electromagnetic radiation, which contains an oscillating electric field that interacts with a molecule through its polarizability. Polarizability is determined by the electron cloud’s ability to interact with an electric field. For example, soft molecules such as benzene tend to be strong Raman scatterers while harder molecules like water tend to be fairly weak Raman scatterers. " (Source: https://bwtek.com/raman-theory-of-raman-scattering/)
However, the second half of the sentence is not true. Raman spectrum of water may interfere with other components. For instance, the O-H stretching mode at high wavenumber range at around 3400~3600cm-1 and the bending mode at around 1600cm-1 can interfere with other components that share the same bonds.
In general, the impact of water can be ignored. This is due to not only its weak peak intensity but also the large peak FWHM, i.e. broad peak (probably the broadest among all of the O-H stretching mode of the materials). However, when water molecules were bonded with a compound to form hydrate forms, its Raman intensity is significantly increased and the peak becomes sharper. There is no other peaks detected in the same wavelength region of bonded water molecules. Hence, there is still no interference from water.
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