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

Basic theory

The Raman effect occurs when light impinges upon a molecule and interacts with the electron cloud of the bonds of that molecule. The amount of deformation of the electron cloud is the polarizability of the molecule. The amount of the polarizability of the bond will determine the intensity and frequency of the Raman shift. The molecule must be symmetric to observe the Raman shift. The photon (light quantum), excites one of the electrons into a virtual state. When the photon is released the molecule relaxes back into vibrational energy state. The molecule will typically relax into the first vibration energy state, and this generates Stokes Raman scattering. If the molecule was already in an elevated vibrational energy state, the Raman scattering is then called Anti-Stokes 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. 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.

Raman active fibers, such as aramid and carbon, have vibrational modes that show a shift in Raman frequency with applied stress.

Raman microspectroscopy

The advantages of Raman spectroscopy, which are described in previous topics, are that the pretreatment of samples is not necessary, samples are not destroyed, and the interference of water is weak. Therefore Raman spectroscopy is suitable for cell and proteins. However, biological samples had not been studied using Raman spectroscopy due to the strong interference of fluorescence.

By using Raman microspectroscopy, in vivo time- and space-resolved Raman spectra of micro regions of samples can be measured. As a result, the fluorescence of water, media, and buffers can be removed. As regards spatial resolutions, for example, the lateral and depth resolutions were 250 nm and 1.7 µm, respectively, using a confocal Raman microspectrometer with the 632.8 nm line from a He-Ne laser with a pinhole of 100 µm diameter. Consequently in vivo time- and space-resolved Raman spectroscopy is suitable to measure cells, proteins, organs, and erythrocytes.

One application of this method is near-infrared time- and space-resolved Raman spectroscopy. The fluorescence of samples is very weak when near-infrared light is used, but detectors for near-infrared light had not been developed until recently. Now that such detectors are available, near-infrared time- and space-resolved Raman spectroscopy is a useful method for biological samples.

History

Inelastic scattering of light is sometimes called the Raman effect, named after one of its discoverers, the Indian scientist Sir C. V. Raman (1928, together with K. S. Krishnan and independently by Grigory Landsberg and Leonid Mandelstam). 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.

Other Types

Some of the many types of Raman Spectroscopy are:

Hyper Raman - A very powerful laser is used to excite a sample. The resulting scattered light is very weak.
Bulk Raman - The typical Raman effect, normally tested under potassium bromide powder.
Surface Enhanced Raman Spectroscopy (SERS) - Normally done in a silver or gold colloid or a substrate containing silver or gold. Silver and gold are easily excited by the laser, and the resulting electric fields cause other nearby molecules to become Raman active. The result is amplification of the Raman signal (by up to 10⁹).
Resonance Raman Spectroscopy - Used to study large molecules such as polypeptides.
Spontaneous Raman Spectroscopy - Used to study the temperature dependence of the Raman spectra of molecules.
Optical Tweezers Raman Spectroscopy (OTRS) - Used to study single biochemical processes in cells trapped by optical tweezers.
Confocal Raman Microscopy - A confocal pinhole restricts the amount of light collected to come from a small region.

External links

Raman Spectroscopy Tutorial - A detailed explanation of Raman Spectroscopy including Resonance-Enhanced Raman Scattering and Surface-Enhanced Raman Scattering.
The Science of Spectroscopy - supported by NASA. Spectroscopy education wiki and films - introduction to light, its uses in NASA, space science, astronomy, medicine & health, environmental research, and consumer products.
Slashdot - Discussion on using Raman spectroscopy and confocal laser scanning microscopy for 3D images of fossils embedded in solid rock
The Science Show, ABC Radio National - Interview with Scientist on NASA funded project to build Raman Spectrometer for 2009 Mars mission: a cellular phone size device to detect almost any substance known, with commercial
Power Technology in depth look at Raman Spectroscopy - Applications of Raman Spectroscopy and Distributed Feedback Diodes

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