Analytical Applications of a New Spectroscopic Tool: Oherent Anti-Stokes Raman Spectroscopy (CARS)

Abstract

Conventional, spontaneous Raman spectroscopy has been an extremely useful technique for analytical applications (1–5). It is used for both qualitative and quantitative analysis in cases which complement infrared methods. For example, homonuclear diatomic molecules, H2 D2, O2 N2, Cl2, etc. cannot be detected by infrared spectroscopy. Moreover, certain “symmetrical” vibrational modes are often only weakly allowed in the infrared spectrum, but such vibrations are frequently very intense in the Raman effect. An example is the vibrational spectrum of Thiokol rubber (6). The ir spectrum of this material shows the presence of C-O-C linkage but no indication of C-S or S-S bonds. The Raman spectrum exhibits no suggestion of ether-like moieties but very strongly displays the intense C-S and S-S vibrations. Hence, both spectra are necessary to identify the material. Raman spectroscopy has certain advantages over infrared spectroscopy, especially with respect to sample handling. With Raman spectroscopy, one can obtain results from samples in sealed tubes, whereas careful sample preparation is often required for infrared spectroscopy. Raman spectra of aqueous solutions are much easier to obtain since Raman scattering in water is relatively weak, whereas infrared absorption in water is nearly ubiquitous. Thus, Raman spectroscopy is a potent tool for the bio-related sciences. Raman spectroscopy is also becoming a very useful technique as a diagnostic tool for combustion and aerodynamic applications (7–12).