[ Members ] [ What's New ] [ Photo Album ] [ Publications ] [ Raman Project ] [ Lunar Project ]
Frequently Asked Questions
What is Raman scattering ? -- an inelastic light scattering phenomenon,
discovered by Dr. C. V. Raman in 1928.
|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.
|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.
|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
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
|Imagine a Raman system deployed by a robotic rover during
a planetary exploration mission perhaps as in this excellent cartoon from the
[ Up ] [ FAQs ] [ Overview ] [ Missions ] [ Current brassboard ] [ Instrument ] [ Applications ]