Anti-Stokes Raman scattering, basic

Fig.8.41a-c. Basic diagram and phase matching schemes for Coherent Anti-Stokes Raman Scattering (CARS), (a) collinear, (b) BOXCARS, and (c) folded BOXCARS phase matching [8.91]... 

The introduction of lasers therefore has indeed revolutionized this classical field of spectroscopy. Lasers have not only greatly enhanced the sensitivity of spontaneous Raman spectroscopy but they have furthermore initiated new spectroscopic techniques, based on the stimulated Raman effect, such as coherent anti-Stokes Raman scattering (CARS) or hyper-Raman spectroscopy. The research activities in laser Raman spectroscopy have recently shown an impressive expansion and a vast literature on this field is available. In this chapter we summarize only briefly the basic background of the Raman effect and. present some experimental techniques which have been developed. For more thorough studies of this interesting field the textbooks and reviews given in [9.1-4] are recommended. 

A photon with frequency v0 is absorbed by a Raman-active molecule that is at the time of interaction already in an excited vibrational state. If the excess energy of the excited Raman-active mode is released, the molecule returns to the basic vibrational state, and the resulting frequency of scattered light is v0 + vm. This Raman frequency is called Anti-Stokes frequency. 

It is not in the scope of the present chapter to review all possible experimental setups for the various types of Raman scattering classical, microprobe, Fourier transform (FT), coherent anti-Stokes (CARS), surface enhanced (SERS), hyper (HRS), photo-acoustic (PARS), and so forth (see, e.g.. Refs. 29-31). A basic Raman-scattering instrument requires a laser-light source, an appropriate sample holder, a sample illumination optical unit, a scattered-light collection optical unit (these two may be combined in one system), a disperser (spectrometer) or an interferometer, a light detection unit, a recorder, and an appropriate microcomputer able to drive, control, and record all of the experimental parameters as well as the results and their processing. 

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