What is Raman spectroscopy?
Raman spectroscopy is a chemical identification technique that detects vibrational, rotational, and other low-frequency modes of molecules. When a sample is put into a Raman spectrophotometer, it is bombarded with photons. One out of every 10 million of these photons will bounce off the molecule with a different energy level than before. This is known as Raman scattering. The amount of photons that scatter this way and the change in energy that they experience gives a specific 'structural fingerprint' that is unique to every molecule.
Raman scattering was first observed by C. V. Raman and his research assistant K. S. Krishnan in February of 1928. In Raman's experiments, he focused sunlight onto liquids and dust-free gasses with his eye acting as a detector. He noticed that the light would scatter inelastically when it struck the surface. In 1930, Raman published his findings in the journal Nature and won a Nobel Prize in Physics for his work.
To understand Raman requires a basic knowledge of the main types of spectroscopy. There are two general categories:
Other motions such as the umbrella-like opening and closing in NH3 can be used to describe vibrational modes as well. Each vibration mode is characterized by a unique frequency. This is what allows each molecule to be distinguished.
such as Raman and IR, detect each of these modes based on their respective energies. Varying portions of the electromagnetic spectrum from ultraviolet to infrared and beyond interact with matter in various ways. Vibrational spectroscopies measure the energy changes in molecules that result in modifying the way their bonds vibrate.
This means that bonds can expand, contract, and flex. Vibrational modes can be described by a variety of different motions such as symmetric stretching, asymmetric stretching, bending, twisting, wagging, rocking, and breathing.
Because each atom has a different mass, and each type of bond has a different strength. One can imagine that a molecule is made up of various balls of different masses connected by springs of different strengths. Bonds between atoms behave like springs.
Infrared, or IR,
spectroscopy is widely used for the detection and analysis of vibrational
modes. This method
involves passing an
electromagnetic (light) wave
through a sample and measuring the amount of light that gets absorbed.
For IR, the wavelengths of light that are absorbed relate to vibrational modes that change the dipole moment of the molecule. In FT-IR spectroscopy a beam containing a spectrum of IR light is split using optics and passes through a sample and a reference cell. The beam that passes through the sample is absorbed at the appropriate excitation wavelengths.
The light that passes through the reference is directed toward the sample beam in such a way that light of the same wavelengths destroys each other. This leaves a light equivalent to that absorbed by the sample which is then detected.
Raman vs. Infared
In order to understand the difference between detection by of vibrations by infrared and Raman one must understand the effects of vibrations molecular dipole. Vibrations that change the net dipole will be detected in infrared while those that do not are likely detected in Raman. This can be easily shown in carbon dioxide.
Carbon dioxide has four vibrational modes: a symmetric stretch, and asymmetric stretch, and two bends. Since oxygen is more electronegative than carbon, electron density will be brought toward the oxygen. This causes a dipole shift during the asymmetric stretching and bending modes as the electrons are being pulled unevenly in the molecule. This causes these modes to be infrared active while the symmetric stretch, which does not undergo this effect, is Raman active.