Fundamentals
Raman scattering intensity
Stokes/anti-Stokes Raman spectra
Raman bands are detected symmetrically but with different absolute intensities around the wavenumber of a monochromatic light source, preferably a laser. The bands shifted to higher wavenumbers (blue shifted) are called anti-Stokes Raman bands and bands shifted to lower wavenumbers (red shifted) are called Stokes Raman bands. Usually, the intensities of the Stokes shifted Raman bands are more intense and therefore used for qualitative and/or quantitative analysis. The intensity ratios of Stokes and anti-Stokes Raman bands are determined by Boltzmann’s law and can be used for the temperature measurement.
Comparison of Raman scatter, Rayleigh scatter and fluorescence intensities
Normal Raman scattering (inelastically scattered) is a very weak phenomenon. Compared to Rayleigh scattering (elastically scattered light) and the competing fluorescence and thermal emission the Raman scattering intensities are up to several orders of magnitudes lower. They can easily covered by these and this makes it difficult or impossible to detect the Raman scattering. Detecting such a weak phenomenon needs high sensitivity detection and a carefully designed experimental set up. Resonance Raman scattering (RR) or surface enhanced Raman scattering (SERS) can enhance the Raman scattering intensities significantly by several orders of magnitude.
Factors affecting Raman scatter intensities
For the normal Raman scattering it is assumed that the wavelength of the Raman scattering exciting light is far away from any electronic or vibrational absorption of the sample. The Raman scattering intensity is mainly affected by the:
- Light source intensity (directly proportional)
- Wavelength of the source (l-4dependency)
- Concentration of the sample or number of molecules (directly proportional)
- Scattering properties of the sample (sample specific)
