Nonlinear Raman Spectroscopy

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Under the non-linear Raman spectroscopy is a group of spectroscopic examination methods based on the non-linear Raman scattering based light of solids or gases. (Narrow-band) lasers are used for excitation , which is why these methods belong to laser spectroscopy .

The group of nonlinear Raman spectroscopy can be divided into three main methods according to the effect used:

  1. Induced Raman scattering, also called stimulated Raman scattering (engl. Stimulated Raman scattering , SRS)
  2. Coherent anti-Stokes Raman scattering (Engl. Coherent anti-Stokes Raman scattering , CARS)
  3. Hyper-Raman Effects (English: Hyper-Raman Effects )

functionality

The difference to (linear) Raman spectroscopy lies in the special type of excitation. Either two laser beams of different frequencies (pump and Stokes laser) with suitable frequencies are required (e.g. with CARS) or several photons of a single laser beam (e.g. with SRS). The photons are superimposed in the so-called Raman medium (depending on the application, a solid or atoms or molecules in a gaseous state). The interaction of the photons with the material in the medium creates a laser-like output beam. The emitted beam is then amplified resonantly when the superposition of the frequencies of the input photons corresponds to a Raman resonance. The signal appears shifted by the amount of the Raman resonance to lower or higher frequencies relative to the frequency of the input photons. These rays are called Stokes and Anti-Stokes rays, respectively.

Coherent anti-Stokes Raman scattering

CARS spectroscopy is used, among other things, to examine material properties, thermodynamic properties (e.g. temperature) or for species-selective microscopy . It is used, among other things, in molecular spectroscopy , plasma and combustion diagnostics , in microscopy and for quality assurance of diamonds . It may also in the study of biological systems for the visualization of nearly any selectable molecular species by excitation of characteristic vibration modes (CH 2 -oscillation for lipids , Amidschwingung for proteins , phosphate oscillation for DNA) can be used, which thus not only by fluorescent dyes must be selected.

Stimulated Anti-Stokes Raman Scattering

The stimulated Anti-Stokes Raman scattering (engl. Stimulated anti-Stokes Raman scattering , SARS) is also based on the principle of non-linear superposition of photons input in a Raman medium. In contrast to CARS, with SARS not photons of different frequencies are superimposed, but several photons of the same frequency. These photons are superimposed in a so-called four-photon process , with resonances in the Raman medium, for example due to the energy difference between two vibration states of a molecule. The resulting photons are either shifted to lower frequencies (Stokes) or to higher frequencies (Anti-Stokes).

As a rule, the process is used to shift the frequency of a laser to higher frequencies (anti-Stokes rays), which cannot be achieved with existing lasers. The shifted radiation extends into the UV and vacuum UV spectrum and is used in molecular spectroscopy.

Individual evidence

  1. ^ SAJ Druet, JPE Taran: CARS spectroscopy . In: Progress in Quantum Electronics . tape 7 , no. 1 , 1981, p. 1-72 , doi : 10.1016 / 0079-6727 (81) 90002-1 .
  2. ^ AP Hickman, JA Paisner, WK Bischel: Theory of multiwave propagation and frequency conversion in a Raman medium . In: Physical Review A . tape 33 , no. 3 , 1986, pp. 1788–1797 , doi : 10.1103 / PhysRevA.33.1788 .
  3. ^ EO Potma, XS Xie : Detection of single lipid bilayers with coherent anti-Stokes Raman scattering (CARS) microscopy . In: Journal of Raman spectroscopy . tape 34 , no. 9 , 2003, p. 642–650 , doi : 10.1002 / jrs.1045 .

literature

See also