Raman Spectroscopy

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Under Raman spectroscopy [ rɑːmən ] (named after the Indian physicist CV Raman ) is the spectroscopic analysis of the inelastic scattering of light on molecules or solids ( Raman scattering ). It serves u. a. the investigation of the material properties z. B. of semiconductors or pigments (such as works of art ).

Working principle

In order to be able to use Raman spectroscopy with molecules, the polarizability must change when the molecule rotates or vibrates. In Raman spectroscopy, the material to be examined is irradiated with monochromatic light, usually from a laser . In addition to the irradiated frequency ( Rayleigh scattering ), other frequencies are also observed in the spectrum of the light scattered on the sample . The frequency differences to the radiated light correspond to the energies of rotation , oscillation , phonon or spin-flip processes that are characteristic of the material . Similar to the spectrum of infrared spectroscopy , conclusions can be drawn about the examined substance from the spectrum obtained . The lines appearing in a Raman spectrum are also referred to as Stokes lines.

The reason lies in the interaction of light with matter, the so-called Raman effect, in which energy is transferred from light to 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. H. its color , which depends on the energy of the light, this energy transfer causes a shift in the wavelength of the scattered light compared to the incident light, the so-called Raman shift.

The achievable spatial resolution varies depending on the measuring equipment used. A resolution of a few micrometers can be achieved if the laser light used for spectroscopy is focused with the aid of a microscope. The resolution should not be confused with the value given below for the scattering cross section, as this in turn corresponds to a reaction probability.

statement

From the spectrum (frequency and the associated intensity ) and the polarization of the scattered light one can u. a. experience the following material properties: crystallinity , crystal orientation , composition , strain , temperature , doping and relaxation . Raman spectroscopy also allows statements to be made about aqueous systems that are difficult to access using infrared spectroscopy. Not only abiotic , but also biotic systems are accessible for analysis. In principle, it is even possible to differentiate between individual species of bacteria using Raman spectroscopy.

The Raman scattering of molecules usually has a very small scattering cross-section (approx. 10 −30 cm 2 ), so that a relatively high concentration of molecules or a high laser intensity is required to obtain a detectable signal. Raman spectra of individual molecules are not possible in this way.

If the Raman spectrum of a thermally or mechanically treated sample is compared with that of an equivalent untreated sample, statements can be made about the internal stresses that have arisen. The emergence of compressive stress leads to a shift to higher frequencies, while a shift to lower frequencies is caused by tensile stress.

Variants and further developments

In addition to the classic Raman spectroscopy, there are still a few variants and further developments. This includes

With the help of surface-enhanced Raman scattering, Raman spectroscopy is also possible on individual molecules. The Raman signals on the surfaces of intelligently designed metal structures are amplified by 10 6 to 10 8 (compared to signals without a metallic surface), as very high electromagnetic field strengths occur locally , which lead to a strong input of intensity.

With the help of special spectrometers for fast data acquisition, these methods can also be used for " real-time " applications. So-called real-time Raman spectroscopy is used in the field of medical in-vivo diagnostics.

See also

literature

  • Josef Brandmüller, Heribert Moser: Introduction to Raman Spectroscopy. Steinkopff, Darmstadt 1962 ( Scientific Research Reports . Natural Science Series 70, ISSN  0084-0920 ).
  • DB Chase, JF Rabolt (Ed.): Fourier Transform Raman Spectroscopy. From Concept To Experiment. Academic Press, San Diego CA et al. 1994, ISBN 0-12-169430-5 .
  • Jeanette G. Grasselli (Ed.): Analytical Raman Spectroscopy. Wiley, New York NY et al. 1991, ISBN 0-471-51955-3 ( Chemical Analysis 114 A Wiley Interscience Publication ).
  • Michael J. Pelletier: Analytical Applications Of Raman Spectroscopy. Reprinted. Blackwell Science, Malden MA et al. 2001, ISBN 0-632-05305-4 .
  • Bernhard Schrader (Ed.): Infrared And Raman Spectroscopy. Methods and Applications. VCH, Weinheim et al. 1995, ISBN 3-527-26446-9 .

Web links

Individual evidence

  1. ↑ a b Ingrid De Wolf: Micro-Raman spectroscopy to study local mechanical stress in silicon integrated circuits . In: Semiconductor Science and Technology . tape 11 , no. 2 , 1996, p. 139-154 , doi : 10.1088 / 0268-1242 / 11/2/001 .
  2. ↑ M. Krause, P. Rösch, B. Radt, J. Popp: Localizing and identifying living bacteria in an abiotic environment by a combination of Raman and fluorescence microscopy . In: Analytical Chemistry . tape 80 , no. 22 , 2008, p. 8568-8575 , doi : 10.1021 / ac8014559 .
  3. ↑ a b Thomas Hellerer: CARS microscopy: development and application . Munich, 2004 ( Abstract & PDF - Ludwig Maximilians University Munich, Faculty of Chemistry and Pharmacy).
  4. ↑ PG Etchegoin, EC Le Ru: A perspective on single molecule SERS: current status and future challenges . In: Physical Chemistry Chemical Physics . tape 10 , no. 40 , 2008, p. 6079-6089 , doi : 10.1039 / b809196j ( PDF ). PDF ( Memento of the original from March 11, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  @1@ 2Template: Webachiv / IABot / www.victoria.ac.nz
  5. ↑ M. Schmitt, C. Krafft, J. Popp: Molecular Imaging: Raman, CARS and TERS . In: BIOspectrum . tape 14 , no. 6 , 2008, p. 605-607 ( PDF ).
  6. ↑ Jianhua Zhao, Harvey Lui, David I., Haishan Zeng: Real-Time Raman Spectroscopy for Noninvasive in vivo Skin Analysis and Diagnosis . In: Domenico Campolo (Ed.): New Developments in Biomedical Engineering . InTech, Vienna 2010, ISBN 978-953-7619-57-2 , p. 455-474 , doi : 10.5772 / 7603 ( PDF ).