Knowledge Bank

Spontaneous Raman scattering is a two photon process in which perhaps one photon in roughly $107$ is scattered to low frequency, i.e., into the Stokes region. In contrast, coherent anti-Stokes Raman scattering (CARS) is a four photon process in which two intense laser beams with the frequencies $\omega p$ and $\omega s$ are mixed in a nonlinear medium to produce a collimated output beam at the frequency $2\omega p-\omega s.1$ Resonance enhancement of the output intensity occurs when, $\omega p-\omega s$ is equal to a molecular vibration frequency, and thus CARS may provide an alternative to spontaneous $Raman.2,3$ Possible advantages of CARS are: (1) large S/N ratio, (2) collimated output beam, and (3) no interference from fluorescence. On the other hand, complications can arise from the phase matching conditions, which depend on $\omega p-\omega s$, and from interference with nonresonant electronic contributions. We have used a $N2$ laser with peak output power of 400 kW as the pump for two dye heads to carry out CARS experiments on variety of organic liquids including several monosubstituted benzene derivatives and simple alcohols. We have been concerned with experimental and theoretical comparisons of CARS with spontaneous Raman. Our CARS spectra do not mirror the spontaneous Stokes spectra, but instead show severe distortions, intensity variations, and frequency shifts, Experimental difficulties and computer simulations of spectra will be discussed.