Raman Spectroscopy

Raman spectroscopy is used to measure vibrational and rotational transitions of molecules, similar to IR spectroscopy (with different selection rules). The difference is that Raman spectroscopy does not use IR light, but visible light. This makes it possible to use detectors and optics that are standard in optics.

By Dr. Jakob Bierwagen, AHF analysentechnik AG

Raman effect

In contrast to elastic Rayleigh scattering, the Raman effect involves inelastic scattering of photon in matter. In this process, energy is removed from or added to the matter. The remaining energy is re-emitted in the form of a photon. This photon has now, according to the rotation and vibration states of the material more or less energy compared to before and thus differs in frequency to the excitation photons.

The bands that are formed are called Stokes lines (photon has given energy to material) or anti-Stokes lines (photon has received energy from material). The energy difference is usually expressed in wavenumbers (cm-1) and ranges from 10 – 4000 cm^-1.

Suppress excitation laser with optical filter

The Raman effect is very weak, which is why the Raman signal is many orders of magnitude smaller than the output power of the excitation laser. Therefore, a very powerful suppression of the excitation laser by optical filters is extremely important. Furthermore, one needs a very steep transmission edge of the filter, which is close to the excitation wavelength, in order to be able to measure signals with only a small energy difference. Furthermore, one has to guarantee that the laser does not carry unwanted wavelengths, which can be achieved with laser clean-up filters or even ultra-narrow bandpass filters.

Finally, one can use the Raman effect for markerless microscopy. To do this, one needs not only very steep filters, but also appropriate dichroic beam splitters and notch filters.