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Frequently Asked Questions

What is Raman scattering ? -- an inelastic light scattering phenomenon, discovered by Dr. C. V. Raman in 1928.

 

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Classical concept of Raman scattering: When a beam of light interacts with a material, part of it is transmitted, part it is reflected, and part of it is scattered. Over 99% of the scattered radiation has the same frequency as the incident beam: Mie and Rayleigh scattering. A small portion of the scattered radiation has frequencies different from that of the incident beam: Raman and Brillouin scattering, forms of inelastic scattering. The frequency differences between the incident and inelastically scattered radiation are determined by the properties of the molecules of which the material under study is made.

What is Raman spectroscopy ? -- A form of molecular spectroscopy based on the Raman effect, described above. A laser beam is used to irradiate a spot on the sample under investigation. The scattered radiation produced by the Raman effect contains information about the energies of molecular vibrations and rotations, and these depend on the particular atoms or ions that comprise the molecule, the chemical bonds connect them, the symmetry of their molecule structure, and the physico-chemical environment where they reside.

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Energy levels and transitions related to the Raman effect: Laser- produced, monochromatic light of ultra- violet, visible, or infrared frequency can be used as the excitation source. In conventional Raman spectroscopy, visible lasers are used (e.g., Ar+, Kr+, Nd:YAG, He-Ne, diode) to stimulate the molecules to high-energy " virtual" states of excitation. A Raman photon is emitted if a molecule then undergoes a transition to a higher vibrational energy state than its original state (Stokes-Raman), or to a lower energy vibrational state (Anti-Stokes Raman). Normally, Stokes Raman radiation has the higher intensity. Sometimes, the laser stimulates fluorescent radiations, but these are not part of the Raman scattering.

 

What does a Raman spectrum look like? -- Sharp and narrow peaks, located on the both sides of the excitation laser line, called Stokes and anti-Stokes lines.

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Stokes and anti-Stokes Raman spectral lines of CCl4, excited by the 514.5 nm line of an argon ion laser. The frequency differences between the excitation radiation and the Raman scattered radiation are called the Raman Shift. Raman shifts are reported in units of wavenumber (cm-1) and are defined by: D (cm-1) = (1/l o - 1/l R), where D is the Raman Shift, l o is the laser wavelength, and l R is the Raman radiation wavelength.

 

What information can we get from Raman spectrocopic studies ?-- identification of minerals and organic substances. From the identities of minerals, we know the chemical formulas and the arrangements of the atoms within them. Thus, we know whether the mineral was a carbonate, sulfate, phosphate, silicate, oxide, sulfide, hydroxide, etc. In some cases for which chemical compositions can vary, e.g., in the ratio of iron to magnesium ions, we can determine the cation ratio.

What is in situ planetary Raman spectroscopy?

This means analysis of the sample in its original location, in this case on the surface of a planet. Usually, the analysis is done without any preparation of the sample such as cleaning. For rocks and soils, that means characterizing their properties as they are found in nature. Through in-situ Raman spectroscopy, we can obtain the identities and characteristics of individual mineral grains.

Successful in situ planetary Raman spectroscopy is best done by using a Raman system that can be deployed by a rover (one similar to the Sojourner rover of the 1997 Pathfinder mission). It can also be done from a lander (such as the Viking landers of 1972). The Raman spectrometer needs to be placed directly against the surface of the target material.

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Imagine a Raman system deployed by a robotic rover during a planetary exploration mission – perhaps as in this excellent cartoon from the Buffalo News.

 

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