Raman Spectroscopy

Vibrational Spectroscopy


     At the core of  Raman Spectroscopy theory is the idea that molecules can vibrate.  The vibrational modes of a molecule can best  be modeled by assuming that the compound is composed of atoms (balls) joined by chemical bonds composed of springs (rather than the traditional sticks).  When viewed in this way, it becomes apparent that an input of energy into the molecule will cause the molecule to vibrate. (3 and 4)
     Vibrational modes for the molecule are a function of the orientation of its atoms and bonds, the atomic mass of the atoms, bond order and hydrogen bonding, among other factors.  The vibrational modes for carbon dioxide an water are illustrated in the diagram to the right.Click on this link to view an animation of the vibrational modes of Water.  Click on this link to view an animation of the vibrational modes of Carbon Dioxide.
       Each Vibrational mode is initiated by a specific frequency, usually in the infrared region of the electromagnetic spectrum.  These modes are quantized much like atomic energy levels. The lowest vibrational energy level for a molecule is denoted as vo which is also called zero point energy.  The first excited state is indicated by v1, then v2, etc. (3and 4)

Figure 1 from: Smith, Ewin, Modern Raman Spectroscopy- A Practical Approach, Wiley, 2005, p. 8.

What is Raman Spectroscopy?


raman scatter

Figure 2 adapted from http://www.inphotonics.com/technote11.pdf

     In this type of spectroscopy, the species of interest (including molecules or polyatomic ions) is irradiated by photons of known energy.  In Stokes scatter (ceneter diagram ), the energy from the photon is absorbed and the target molecule is promoted to a higher (virtual) energy state.  Some of the energy from the incident photon is used by the molecule to excite it to higher level vibrational and rotational states, the rest is emitted as a photon of reduced energy.  This photon is commonly called the Raman photon. Stokes scatter results when the molecule is excited from ground state (v0) and results in a molecule at a higher energy state (v1).  The energy of this photon is equal to the difference between the incident light and the energy absorbed by the molecule.  The Raman photon energy is equivalent to a transition from v0 to v1 vibrational state for the molecule being studied.
     Anti-Stokes scatter (diagram on right) results when a molecule in an excited state (v1) is gains  energy from the incident photon.  It then decays back to a lower energy level, ground state (v0), with the emission of a higher energy photon than the incident radiation.  Since very few molecules reside in the excited state, Anti-Stokes scatter does not predominate in a Raman Spectra.   In both Stokes and Anti-Stokes scatter, the difference between the incident photon and the emitted photon is equal to the transition energy (v0 to v1) for the molecule. Rayleigh scatter simply releases a photon of equal energy.  (5)

How does Raman Spectroscopy Work?

     Raman spectroscopy occurs as a result of a molecular vibration causing a "change in polarizability" of the molecule.  In contrast, for a molecule to be infrared active, the vibration must cause a change in the permanent dipole moment.  A simple case of a Raman Active molecule would be a species such as CS2.  The symmetrical stretch out and then in (pictured to the right) will be detected by Raman spectroscopy.  Since the molecule has no permanent dipole, this stretch would be invisible in an infrared spectra. 
     If a molecule has a center of symmetry, Raman active vibrations would not be visible in the infrared.  For example, the symmetric stretch of CS2 is Raman active.  The asymmetric stretches, which induce a dipole, are infrared active.  As a result of this fundamental difference, it is often said that Raman and Infrared Spectra are complementary, meaning that, between the two, the analyst should be able to get a fairly complete picture of the vibrational modes of a molecule.  (5,6)

Figure 3 from: Smith, Ewin, Modern Raman Spectroscopy- A Practical Approach, Wiley, 2005, p. 10.

What kinds of compounds can be detected by Raman Spectroscopy?



Figure 4:
A Raman Sctrophotometer employing SERS Technology, http://epsc.wustl.edu/haskin-group/Raman/instrument.htm

     One of the reasons Raman Spectroscopy has been proposed for interplanetary exploration is that this method of analysis can detect a wide range of compounds from inorganic to organic.  Geologists on earth have successfully used this technique to identify minerals in rocks and other geologic formations.
        The detection of organic molecules has been more difficult.  Generally, organic molecules have a small photon absorption cross section.  As a result, until recently, only a very weak signal could be produced when analyzing organic molecules. Additionally, on the Martian surface, the presence of organic molecules is expected to be in a very low concentration (if any).  An adaptation designed to increase the amount of photon scattering is called Surface Active Raman Scattering (SERS).
         It has been observed that, when placed on or near a metal surface, compounds or polyatomic ions can increase the number of Raman photons scattered by a factor of 103 to 106.  Although this effect appears to be strongest on a silver surface, other metals such as gold or copper also demonstrate this ability to increase the Raman scatter.  This process enhances the electromagnetic field on the metal surface which, in turn, enhances the vibrational modes of the sample on its surface.  Additionally, the SERS method causes a "charge-transfer complex" to be formed between the metal and the sample.  This then causes resonance enhancement of the Raman signal to occur.  (5)
        The SERS method is particularly suited for electron rich molecules that contain lone electron pairs or pi electrons.  Compounds that respond well to SERS include aromatic amines, phenols, compounds containing oxygen and carboxylic acids.  Use of this technique would greatly improve the sensitivity of the Raman analysis on the Martian surface.  Even if amino acids are present on or near the surface, it is hypothesized that their population would be extremely low.  The diagram to the right shows how the SERS design can be incorporated into a Raman Spectrometer. (5,6,7)



   What compunds could be  an indicator of past or pesent life on Mars?  What kind of Raman signal is expected from those compounds?  Click on the "Searching for Signs of Life" link to find out more.


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