Carbon tetrachloride (CCl<sub>4</sub>) is one of a number of volatile organic chemicals (VOCs) of interest as pollutants which are regulated at very low levels (ppm to ppb) by the EPA. A need exists to measure these chemicals at these regulatory levels rapidly and, in many cases, <i>in situ.</i> Raman spectroscopy is one method which has the potential to be a useful tool for remote monitoring and has already found a number of applications for process and field monitoring of chemicals. While carbon tetrachloride has been detected with these remote sampling systems, it is doubtful that the requisite sensitivity could be obtained with visible or near-IR lasers because of the relatively small cross section for the Raman effect. Large increases in the Raman cross section have been observed when the excitation frequency is at or near an electronic absorption of the molecule. Resonance enhancement of the Raman cross sections can be many orders of magnitude (10<sup>3</sup> to 10<sup>6</sup>). Hence, large gains in sensitivity might be expected. Such measurements are complicated by background florescence as well as the need to use lasers in the UV region of the spectrum to access the electronic transitions of simple VOCs.
Robert E. Barletta and James T. Veligdan, "Resonance Raman Spectrum of Carbon Tetrachloride," Appl. Spectrosc. 49, 532-534 (1995)