Dear Chitra Sulkan, otherwise to say, there is a complementarity between both techniques, and each one has its powerfull and weaknesses. It stays in terms of flexibility, Rayman has six possible modes of analysis. Please have a look at the following documents. My Regards

The articles are informative. Thank you @Abdelkader BOUAZIZ

In order to properly interpret the Raman and FTIR spectra, you. need to be able to perform the symmetry analysis. If you are a chemistry major, this subject is typically taught. Otherwise, you will not be able to discuss quantitatively regarding the number of bands observed. When the molecule has a center of symmetry (the highest level of symmetry), any bands observed in IR spectrum do not coincide with the Raman spectrum. On the other hand, if the molecule has no symmetry at all, then all the IR bands observed coincide with the Raman spectrum in frequency. The total number of IR and Raman bands, if no symmetry exists, should be 3N-6 where N is the number of atoms in the molecule. 6 is the 3 translational and 3 rotational motion which are IR and Raman inactive.

Difference exists in intensity, however. Even if the bands with the same frequency is observed in both spectra, if it appears strong in IR, it typically appear weak in Raman, and vise versa. Therefore, for example, water appears very strongly in OH stretching around 3400 cm-1 and others and using aqueous solution is difficult to study using IR. This is opposite in Raman. Water is nearly transparent in Raman and thus aqueous solution is considered a very easy sample to study. Thus, if observing a band is the only purpose, change the technique that shows stronger bands.

We can predict the frequency of the band rather accurately using the technique called normal coordinate analysis. However, intensity of the vibrational spectrum cannot be calculated easily unless you can solve the Hamiltonian. Thus, only the simplest molecules, such as carbon dioxide or methane can be studied for their intensities. More complex molecules are too difficult and time consuming to do so. In a recent years, ab initial calculation program exists and approximation method allows intensity of the spectrum to be calculated. However, accuracy of the prediction is far from sufficient at the present time.

Thank you very much @ Hatsuo Ishida

The Raman domain is spherically related to molecular vibrations, whereas FTIR has an operation range that provides insight into molecular structure. The relationship between Raman scattering and molecular structure is not always straightforward, but there are two main reasons why it is utilized in polymer science. First, Raman scattering can yield a wealth of data about a material that would be inaccessible to FTIR readings because of the limited operating range of FTIR spectroscopy. While the physical process behind both types of spectroscopy is different, they share a common characteristic: both techniques allow us to gain insight into a sample's molecular structure via frequency analysis.

thank you@ Noureddine Boumdouha

They are complementary techniques.

FTIR is active when you have variation of the dipole moment of the molecule, which is not the case for example in homonuclear diatomics.

RAMAN is active when you have a change in the polarizability of the molecule.

Let me explain, for example symmetrical stretching is inactive in IR but active in RAMAN. Antisymmetrical stretching is active in IR but not in RAMAN ....

So, to use FTIR or RAMAN you should know what you are looking for first and you should also know if the functions are active or inactive.

Hope my answer helps you,

Best Regards,

Z@K

Thank you Abdelmoheiman Zakaria Benbouzid