raman spectroscopy

history C. V. Raman who discovered the scattering of photon got the Nobel prize in 1930. This scattering was named by Raman scattering. C. V. Raman http://science.scu.edu.tw/phy/M301/doc/Nb-1930%27.doc

principles Raman spectroscopy is commonly used in chemistry to provide a fingerprint by which molecules can be identified. It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range.

inelastic scattering Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more paths due to localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of reflected radiation from the angle predicted by the law of reflection. Reflections that undergo scattering are often called diffuse reflections and unscattered reflections are called specular (mirror-like) reflections.

Inelastic scattering inelastic scattering is a fundamental scattering process in which the kinetic energy of an incident particle is not conserved (in contrast to elastic scattering(Rayleigh scattering )).

Raman spectroscopy The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system. Infrared spectroscopy yields similar, but complementary, information.

Molecular Spectrum Vibrational energy spacings are 0.1 ~ 1 eV. Rotational energy spacings are 0.01 ~ 0.1 eV.

Applications Raman spectroscopy is commonly used in chemistry, since vibrational information is specific to the chemical bonds and symmetry of molecules. Therefore, it provides a fingerprint by which the molecule can be identified. Water does not generally interfere with Raman spectral analysis.

Raman spectroscopy of graphene G’ , 2D G 1583 2635 D 1320 monolayer bilayer HOPG

Surface-enhanced Raman spectroscopy (SERS) Normally done in a silver or gold colloid or a substrate containing silver or gold. Surface plasmons of silver and gold are excited by the laser, resulting in an increase in the electric fields surrounding the metal. Given that Raman intensities are proportional to the electric field, there is large increase in the measured signal (by up to 1011).

Resonance Raman spectroscopy The excitation wavelength is matched to an electronic transition of the molecule or crystal, so that vibrational modes associated with the excited electronic state are greatly enhanced. This is useful for studying large molecules such as polypeptides, which might show hundreds of bands in "conventional" Raman spectra. It is also useful for associating normal modes with their observed frequency shifts.

Surface-enhanced resonance Raman spectroscopy (SERRS) A combination of SERS and resonance Raman spectroscopy that uses proximity to a surface to increase Raman intensity, and excitation wavelength matched to the maximum absorbance of the molecule being analysed.

When electromagnetic radiation of energy content hυ irradiates a molecules, the energy may be transmitted, absorbed, or scattered.

Infrared absorption: Reyleigh scattering: Raman effect: An infrared photon whose frequency is the same as the molecular frequency υm. Reyleigh scattering: An elastic collision between the incident photon and the molecule. Raman effect: An inelastic collision between the incident photon and the molecule where as a result of the collision the vibrational or rotational energy of the molecule is changed by an amount ΔEm.