How is Raman spectroscopy different from infrared spectroscopy?

Raman spectroscopy differs from infrared spectroscopy as it measures scattered light, not absorbed light, to identify molecular vibrations.

Raman spectroscopy and infrared (IR) spectroscopy are both analytical techniques used to study vibrational, rotational, and other low-frequency modes in a system. They are based on the interaction of light with matter, but the fundamental difference lies in the type of light-matter interaction they measure.

Infrared spectroscopy is based on the absorption of light. When infrared light interacts with a molecule, it can be absorbed if the frequency of the light matches the frequency of the vibrations of the molecule. This absorption causes a transition from a lower energy level to a higher energy level, and the difference in energy is equal to the energy of the absorbed light. The result is an absorption spectrum, which can be used to identify the types of bonds present in the molecule, and therefore the molecule's structure.

On the other hand, Raman spectroscopy is based on the scattering of light. When light interacts with a molecule, most of it is scattered at the same frequency as the incident light, a phenomenon known as Rayleigh scattering. However, a small fraction of the light is scattered at different frequencies, due to the interaction with the vibrational and rotational modes of the molecule. This is known as Raman scattering. By measuring the frequencies of the scattered light, we can obtain a Raman spectrum, which provides information about the vibrational modes of the molecule.

Another key difference is that while infrared spectroscopy is most effective for studying molecules with a permanent dipole moment, Raman spectroscopy can be used to study any molecule, as it does not rely on the presence of a dipole moment. This makes Raman spectroscopy a more versatile technique, capable of studying a wider range of molecules.

In summary, while both techniques provide valuable information about the structure and composition of molecules, they do so through different mechanisms: infrared spectroscopy through the absorption of light, and Raman spectroscopy through the scattering of light.