RAMAN Spectroscopy
RAMAN spectroscopy is a part of vibrational spectroscopy, often IR inactive oscillations can be observed in RAMAN. In contrast to IR spectroscopy, those oscillations are RAMAN-allowed which lead to a change in the polarizability of the molecule. In the technique, also called RAMAN scattering, a molecule is excited by monochromatic light of the visible or near infrared spectral range. The energy introduced by the exciting radiation is usually re-emitted (by electron oscillations) while maintaining the frequency, this elastic scattering is called Rayleigh scattering. If the electron oscillations induced by the excitation lead to a change in the polarizability of the entire molecule, then a small part of the input energy is used for the excitation of the molecular oscillation and the inelastically scattered (Stokes scattering), frequency-reduced radiation, is reemitted again. The scattering takes place diffusely in all spatial directions. The highest RAMAN intensities are observed in oscillations in which the entire molecule is contracted or stretched. A vivid example is the symmetrical stretching vibration of CO2. The description of the molecular vibrations excited by RAMAN spectroscopy is analogous to IR spectroscopy. In addition, RAMAN spectroscopy is not sensitive to water or aqueous media compared to IR spectroscopy.
However, the observed spectrum clearly depends on the wavelength and intensity of the excitation radiation:
I ~ ν4I0N(∂α/∂q)²
(ν Frequency of excitation radiation, I0 Intensity of excitation radiation, N Number of scattering molecules, (∂α/∂q) Change in polarizability)
Therefore only lasers are used for excitation, common laser wavelengths are 514.5 nm (argon ion or diode) and 785 nm (NIR diode), but practically all available laser wavelengths are possible. The excitation radiation must be suppressed for the detector, since it is orders of magnitude more intense than the RAMAN scattering. So-called notch filters or Bragg gratings are used for this, but more robust and durable are hard-coated long-pass filters. The latter, however, do not permit the observation of anti-Stokes bands[1], although their observation can often be dispensed with. In use, a high quality (OD value) of the filters must be ensured. In the case of inexpensive long-pass color filters, it may also be necessary to use several filters.
Fluorescence is a competing process for RAMAN scattering. Due to the intensive excitation of the laser it often occurs in RAMAN spectra and prevents the observation of the RAMAN bands by superposition. The simultaneous fluorescence excitation can be avoided by changing the excitation wavelength (excitation at larger wavelengths, but the RAMAN bands can then also be significantly smaller, see above equation) or by observing the anti-Stokes bands in the RAMAN spectrum.
[1] Anti-Stokes scattering occurs when molecules are already in the excited state and this energy is added to the excitation radiation during re-emission, this means the re-emission occurs at a higher energy/higher frequency. However, the intensity of the scattering is 90 % lower than that of the Stokes scattering.