Question

If vibrational transitions correspond to energies in the IR region of the electromagnetic radiation, why is it that we can obtain a Raman spectrum by using a visible wavelength laser as a light source?

Answer 1

In I. R spectroscopy , the molecules are IR active only when there is a change in dipole moment when vibrational transition occurs . If we take any molecule and apply energy/frequency that is in the range of IR ( 3 X 10¹² - 3 X 10¹⁴Hz) then molecule undergoes change in it's vibrational level ( that is transition between ground vibrational state and excited state )  and if there is change in dipole moment is observed during transition , then it will be IR active and will give IR spectroscopy .  

Now in Raman spectroscopy we will obtain Raman spectra only if molecular rotation or vibration must cause some change in the component of molecular polarizibility and the change in polarizibility will be reflected by change in magnitude or direction of Polarisibility ellipsoid.

Now if suppose IR region energy is applied , it will cause change in its rotational and vibrational level that is a molecule will absorb radiation and goes to higher energy level and then it will emit radiation and come back to its ground state , In most of cases it was observed that it was same radiation with same frequency emitted that it absorbed but in very few case this frequency(emitted) was not same as compared to incident radiation . So this was observed by Raman and formed basis of Raman spectroscopy .

Raman Spectroscopy requires Laser as a source in visible or ultra voilet region and we do not want our radiation to probe innter details of bond but we want only to sense average polarizibility in various directions through molecule . So Lasers are used as source due to their highly monochromatic nature and high beam fluxes.

So answer of your question would be that We obtain a raman spetra by using visible wavelength as source but we will only obtain raman spectra if molecule is raman active ( that shows change in polarizibility of molecule ) and also Raman spectroscopy is considered complimentary to IR spectroscopy .