why is it that we can obtain a Raman spectrum

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 the laser Raman effect,  the wavelength of  a small portion of the exciting optical radiation is shifted to a slightly longer wavelength.    The longer wavelength is usually also in the optical part of the spectrum,  As an example, consider using a green  Argon ion laser to observe the Raman effect in diamond.   The laser wavelength is .5145 micron.   The Raman active vibration mode in diamond is at an energy of 1332 cm^-1. (in the IR)    The wavelength of the Raman shifted light is at a wavelength of .5524 micron.   This is still in the green portion of the spectrum. But, the intensity of this Raman shifted light is much much less than that of the exciting beam.  It takes an optical system characterized by a very high rejection ratio to observe the weak Raman signal. 

So it is possible to observe the Raman effect using visible light, but it requires a bright source and a quality optical system that can reject the unshifted light.