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The basic idea in CARS/CSRS is to monitor this enhanced third frequen cy for the information it provides about the chemical system under study. Of the two techniques, CARS is by far the most popular for analytical applications, since va in CARS is in the anti-Stokes region, where fluores cence is not a problem. CARS offers the analyst a number of advantages over spontaneous Raman scattering (3,4): • CARS is a more efficient process by orders of magnitude than spontaneous Raman. Spontaneous Raman yields only one scattered photon in ~ 1 0 8 in cident photons, while CARS may yield one scattered photon in 10 2 -10 3 inci dent photons. • The radiation produced in sponta neous Raman is scattered into 4π steradians, and only a small amount of this radiation can be collected. In CARS, on the other hand, the output is coherent and laserlike. This signal can be separated from the incident ra diation and collected with an efficien cy of approximately 90%. • As mentioned above, the CARS sig nal is generated in the anti-Stokes re gion, where it is removed in frequency from fluorescence emission, whereas most of the intensity in spontaneous Raman appears at Stokes-shifted frequencies (the anti-Stokes lines are relatively weak). • In spontaneous Raman, a wide range of frequencies is generated, ne cessitating the use of a monochromator. Resolution in spontaneous Raman is then limited by the performance of this monochromator. In CARS only one frequency (va) is monitored. No monochromator is needed for detec tion, and the spectral resolution in CARS depends only on the spectral width of the exciting laser. Thus, high resolution in conventional Raman spectroscopy is ~0.1—0.01 cm - 1 , with routine scanning at 5 c m - 1 . In CARS, routine spectra are recorded at ~0.4 c m - 1 , with high resolution at 10 -3 — 10~ 4 cm" 1 (4). A serious drawback of CARS is the presence of a nonresonant background signal from the matrix that can ob scure the resonant CARS signal at low sample concentrations. In convention al CARS, this background signal has limited the sensitivity to ~ 1 % in solu tion. Two methods, polarization suppres sion and resonance enhancement, have been widely used to eliminate or mitigate the nonresonant background signals in CARS. Polarization sup pression involves appropriate adjust ment of the pump, probe, and CARS
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A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
detection polarization orientations. Eckbreth predicts that "polarization CARS approaches will improve detec tion sensitivities by perhaps two to three orders of magnitude" (5). Detec tion limits can also be improved con siderably by utilizing the electronic resonances of the molecule (reso nance-enhanced CARS), since greatly enhanced intensity is obtained when the pump frequency (i>/) approaches the frequency of an allowed electronic dipole transition (ve). With reso nance-enhanced CARS, detection lim its as low as about 10~ 7 M have been reported. Relative to other analytical techniques, however, this is still a fair ly high number, and CARS is thus not widely applicable for trace analysis. CARS has proven useful in a num ber of analytical applications. Eck breth spoke in detail at the conference on the use of CARS in the diagnostic probing of hostile combustion envi ronments, including flames and plas mas. Recently, attention has also been directed to the other side of the tem perature spectrum, high-resolution CARS studies of cold supersonic jets (6). Due to its coherence and upward frequency shift, CARS discriminates very effectively against fluorescence, which is incoherent. Thus, CARS is attractive for the analysis of com pounds with intrinsically high fluores cence quantum yields, including many of the biological compounds under in tense study today. CARS finds other applications in high-resolution Raman spectroscopy and as a tool for photo chemical analysis and chemical kinet ics measurements, especially on the nanosecond and picosecond time scale. Recent CARS work has included the development of liquid chromato graphic CARS detection (LC/CARS) for the identification of environmental pollutants in water (3). Stuart A. Borman
References (1) Barrett, J. J. In "Chemical Applica tions of Nonlinear Raman Spectrosco py"; Harvey, Albert B., Ed.; Academic Press: New York, 1981; Chapter 3. (2) Wright, John C. In "Lasers in Chemi cal Analysis"; Hieftje, Gary M. et al., Eds.; Humana Press: Clifton, N.J., 1981; Chapter 4. (3) Carreira, L. A. et al. In "Chemical Ap plications of Nonlinear Raman Spectros copy"; Harvey, Albert B., Ed.; Academic Press: New York, 1981; Chapter 8. (4) Harvey, A. B. Anal. Chem. 1978,50, 905-12 A. (5) Eckbreth, Alan C; Schreiber, Paul W. In "Chemical Applications of Nonlinear Raman Spectroscopy"; Harvey, Albert B., Ed.; Academic Press: New York, 1981; Chapter 2. (6) Levy, Donald H. Science 1981,214, 263-69.
