Raman spectroscopy

In Raman spectroscopy (named after the Indian physicist CV Raman ) refers to the spectroscopic study of the inelastic scattering of light by molecules or solids ( Raman scattering). It serves inter alia, the investigation of the material properties eg semiconductors or pigments (such as objects of art ).

Principle of operation

To apply the Raman spectroscopy for molecules, the polarizability in rotation or oscillation of the molecule must change. In Raman spectroscopy, the material to be examined is irradiated with monochromatic light, usually from a laser. In the spectrum of the scattered light in addition to the sample of the incident frequency ( Rayleigh scattering) are still observed more frequencies. The frequency differences to the incident light corresponding to the characteristic of the material energies of rotational, vibrational, phonon or spin- flip processes. From the obtained spectrum can be similar to the spectrum of infrared spectroscopy, draw conclusions on the substance investigated. The lines which occur in a Raman spectrum are also referred to as a Stokes lines.

This is due to an interaction of light with matter, the so-called Raman effect of mechanical transfer of energy from the light on the matter ( " Stokes " side of the spectrum), or energy from matter to light ( " anti- -Stokes side "of the spectrum). Since the wavelength of light, i.e., its color, depends on the energy of the light, this transfer of energy results in a shift in the wavelength of the scattered light relative to the incident light, the so-called Raman shift.

Testify

From the spectrum ( frequency and its intensity) and the polarization of the scattered light can be experienced include the following material properties: crystallinity, crystal orientation, composition, strain, temperature, doping and relaxation. Raman spectroscopy also allows statements on aqueous systems, which are difficult to access via infrared spectroscopy. So not only abiotic but also biotic systems of analysis are available. In principle it is even possible to differentiate individual species of bacteria by means of Raman spectroscopy.

Raman scattering of molecules usually has a very small scattering cross section (about 10 to 30 cm2), so that it requires a relatively high concentration of molecules or a high laser intensity in order to obtain a detectable signal. Raman spectra of individual molecules are not possible.

Variants and developments

In addition to the classical Raman spectroscopy still exist some variants and developments. These include

• On the non-linear Raman scattering based processes, such as coherent anti -Stokes Raman scattering (English coherent anti - Stokes Raman scattering CARS)
• On the surface-enhanced Raman scattering ( engl. surface enhanced Raman scattering, SERS )-based method
• The tip-enhanced Raman spectroscopy ( engl. tip- enhanced Raman spectroscopy, TERS ) as a combination of SERS and atomic force microscopy (german atomic force microscopy, AFM).

Using surface-enhanced Raman scattering is possible Raman spectroscopy also on single molecules. Here, the Raman signals on the surfaces of intelligent designter metal structures around 106 to 108 amplified (opposite signals without metallic surface ) by its locally very high electromagnetic field strengths occur, which lead to a strong intensity entry.