Coherent anti-Stokes Raman spectroscopy (CARS) is a relatively new kind of Raman spectroscopy which is based on a nonlinear conversion of two laser beams into a coherent, laser-like Raman beam of high intensity in the anti-Stokes region. The emission is often many orders of magnitude greater than normal Raman scattering and, because of the coherent and anti-Stokes character of radiation, the method is very useful for obtaining Raman spectra of fluorescing samples, gases in discharges, plasmas, combustion, atmospheric chemistry. In this paper we outline the basic theory behind CARS and describe its unusual effects and drawbacks. We review the research to date on various materials, and indicate the possible future direction, utility and applications of CARS such as surface studies, fluctuation phenomena, reaction dynamics, photochemistry, kinetics, relaxation, and energy transfer.

Coherent anti-Stokes Raman spectroscopy offers significant advantages over standard incoherent Raman spectroscopy. The advantages include 105 improvement in conversion efficiency, spectral and spatial discrimination against fluorescence, low incident average power, and no requirement for a monochromator. The potential disadvantages of signal scaling as power cubed and requirement for a high peak power tunable laser source are largely overcome by presently available tunable laser sources. The method appears particularly useful for investigating biological compounds where background fluorescence is a problem for conventional spontaneous Raman studies. We show that for low concentrations, the density-squared signal dependence becomes linear due to coherent mixing with an intense solvent Raman mode or the third-order background signal.

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The direct measurement of nonequilibrium vibrational level populations of N2 at the center of an electric discharge has been demonstrated by a new diagnostic technique: coherent anti-Stokes Raman spectroscopy (CARS). On the assumption of a Boltzmann equilibrium among only the lowest vibrational levels, a method has been developed and utilized for the direct determination of vibrational populations of all other levels, even in the case of extreme deviations from a Boltzmann equilibrium. Limitations of the method are discussed and illustrated.