Raman transitions

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. In quantum computation, it is not only used for characterising physical systems, but to control their evolution by changing the population of atomic energy levels in a coherent manner. It relies on inelastic scattering, or Raman scattering of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. When used for spectroscopy, 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 spectroscopy has a stimulated version, analogous to stimulated emission, called stimulated Raman scattering.

Basic theory

Raman scattering red-shifted
Raman scattering red-shifted

The Raman effect occurs when light falls upon a molecule and interacts with the electric dipole of that molecule. The photon (light quantum), excites the one of the electrons into a virtual state, that is to say, the energy is not yet enough to excite into a full quantum state. Then almost immediately another photon is released and the molecule is left in a lower state. During this proces energy can be deposited into the molecule, which leaves the molecule in a higher vibrational state. The exitation photon then has had a higher energy than the photon coming out. (This is called red shift of Stokes shift.) An Anti-Stokes shift is also possible but this is rather rare, because normally there are very few molecules already in a vibrational state. An exception to this is coherent anti-Stokes Raman spectroscopy or CARS, where molecules are artificially put into a vibrational state first.

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. The fingerprint region of organic molecules is in the range 500-2000 cm-1. Another way that the technique is used is to study changes in chemical bonding, e.g. when a substrate is added to an enzyme.

Raman gas analyzers have many practical applications, for instance they are used in medicine for real-time monitoring of anaesthetic and respiratory gas mixtures during surgery.

In solid state physics, spontaneous Raman spectroscopy is used to, among other things, characterize materials, measure temperature, and find the crystallographic 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 excitations 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 of the Raman scattered light with respect to the crystal and the polarization of the laser light can be used to find the orientation of the crystal, if the crystal structure (specifically, its point group) is known.

History

Inelastic scattering of light is sometimes called the Raman effect, named after one of its discoverers, the Indian scientist Chandrasekhara Venkata Raman (1928, together with K. S. Krishnan and independently by Grigory Landsberg and Leonid Isaakovich Mandelshtam). Raman won the Nobel Prize in Physics in 1930 for this 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.

Category:Miscellaneous

Last modified: 
Monday, October 26, 2015 - 17:56