Raman spectroscopy

Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other low-frequency modes in a system. It relies on inelastic scattering, or Raman scattering of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. Phonons or other excitations in the system are absorbed or emitted by the laser light, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the phonon modes in the system. Infrared spectroscopy yields similar, but complementary information.

Typically, a sample is illuminated with a laser beam. Light from the illuminated spot is collected with a lens and sent through a monochromator. Wavelengths close to the laser line (due to elastic Rayleigh scattering) are filtered out and those in a certain spectral window away from the laser line are dispersed onto a detector.

Spontaneous Raman scattering is typically very weak, and as a result the main difficulty of Raman spectroscopy is separating the weak inelastically scattered light from the intense Rayleigh scattered laser light. Raman spectrometers typically use holographic diffraction gratings and multiple dispersion stages to achieve a high degree of laser rejection. A photon-counting photomultiplier tube (PMT) or, more commonly, a CCD camera is used to detect the Raman scattered light.

Raman scattering has a stimulated version, analogous to stimulated emission, called stimulated Raman scattering.

Applications

Raman spectroscopy is commonly used in chemistry, since vibrational information is very specific for the chemical bonds in molecules. It therefore provides a fingerprint by which the molecule can be identified. Another way that the technique is used is to study changes in chemical bonding, e.g. when a substrate is added to an enzyme.

In solid state physics, spontaneous Raman spectroscopy is used to, among other things, characterize materials, measure temperature, and find the orientation of a sample.

As with single molecules, a given solid material has characteristic phonon modes that can help an experimenter identify it. In addition, Raman spectroscopy can be used to observe other low frequency modes of the solid, such as plasmons, magnons, and superconducting gap excitations.

The spontaneous Raman signal gives information on the population of a given phonon mode in the ratio between the Stokes (downshifted) intensity and anti-Stokes (upshifted) intensity.

Raman scattering by a crystal gives information on the crystal orientation. The polarization direction of the Raman scattered light with respect to the crystal and the polarization direction of the laser light can be used to find the orientation of the crystal, if the crystal structure is known.

History

Inelastic scattering of light is sometimes called the Raman effect, named after its discoverer, the Indian scientist Sir C. V. Raman. Raman won the Nobel Prize in Physics in 1930 for his discovery, accomplished using filtered sunlight as a monochromatic source of photons, a colored filter as a monochromator, and a human eye as detector. The technique became widely used after the invention of the laser.