Raman spectra of ammonia (NH_{3}), chlorine (Cl_{2}), hydrogen sulfide (H_{2}S), phosgene (COCl2), and sulfur dioxide (SO_{2}) toxic gases have been measured in the fingerprint region 400-1400 cm^{−1}. A relatively compact (<2′x2′x2′), sensitive, 532 nm 10 W CW Raman system with double-pass laser and double-sided collection was used for these measurements. Two Raman modes are observed at 934 and 967 cm^{−1} in NH_{3}. Three Raman modes are observed in Cl_{2} at 554, 547, and 539 cm^{−1}, which are due to the 35/35 35/37, and 37/37 Cl isotopes, respectively. Raman modes are observed at 870, 570, and 1151 cm^{−1} in H_{2}S, COCl_{2}, and SO_{2}, respectively. Values of 3.68 ± 0.26x10^{−32} cm^{2}/sr (3.68 ± 0.26x10^{−36} m^{2}/sr), 1.37 ± 0.10x10^{−30} cm^{2}/sr (1.37 ± 0.10x10^{−34} m^{2}/sr), 3.25 ± 0.23x10^{−31} cm^{2}/sr (3.25 ± 0.23x10^{−35} m^{2}/sr), 1.63 ± 0.14x10^{−30} cm^{2}/sr (1.63 ± 0.14x10^{−34} m^{2}/sr), and 3.08 ± 0.22x10^{−30} cm^{2}/sr (and 3.08 ± 0.22x10^{−34} m^{2}/sr) were determined for the differential Raman cross section of the 967 cm^{−1} mode of NH_{3}, sum of the 554, 547, and 539 cm^{−1} modes of Cl_{2}, 870 cm^{−1} mode of H_{2}S, 570 cm^{−1} mode of COCl_{2}, and 1151 cm-1 mode of SO_{2}, respectively, using the differential Raman cross section of 3.56 ± 0.14x10^{−31} cm^{2}/sr (3.56 ± 0.14x10^{−35} m^{2}/sr) for the 1285 cm^{−1} mode of CO_{2} as the reference.

## I. INTRODUCTION

The Raman spectra and cross sections of toxic gases are important for the detection of these gases using Raman spectroscopy in the fingerprint region 400-1400 cm^{−1}. Raman spectra and cross sections are important for homeland security and other applications.^{1,2} Raman cross sections of a number of gases (CH_{4}, C_{2}H_{6}, C_{3}H_{8}, C_{6}H_{6}, CO, CO_{2}, F_{2}, HBr, HCl, HF, H_{2}, H_{2}O, H_{2}S, N_{2}, NH_{3}, ND_{3}, NO, N_{2}O, O_{2}, O_{3}, and SO_{2}) have been reported previously.^{3–8} In this paper, we report the measurement of the Raman spectra and cross sections of ammonia (NH_{3}), chlorine (Cl_{2}), hydrogen sulfide (H_{2}S), phosgene (CCl_{2}O), and sulfur dioxide (SO_{2}) toxic gases in the fingerprint region 400-1400 cm^{−1} using the Raman cross section of the 1285 cm^{−1} mode of carbon dioxide (CO_{2}) as the reference.^{9} Raman cross sections of NH_{3}, Cl_{2}, H_{2}S, and CClO_{2} have been measured for the first time in the fingerprint region. Our value of the Raman cross section of SO_{2} is 1.5x larger than that reported previously.^{3,4}

## II. EXPERIMENTAL

Schematic of the optical setup of the Raman system is the same as reported previously.^{10,11} The 532 nm pump laser propagates in the horizontal direction (z-axis) through the flow cell. The laser was polarized perpendicular to the direction of propagation (z-axis). The Raman signal is collected along the x-axis. The magnification *M* of the image of the laser focal spot on the spectrometer slit is 1.6, which is the ratio of the 40 mm focal length focusing lens and the 25 mm focal length collection lens. The diameter *D* of the laser focal spot, located in the center of the flow cell, was 50 μm. There is small (<3%) wavelength dependence of the Raman spectrometer between 545 nm and 575 nm.

Ammonia (34 L of 200 ppm balance nitrogen) was purchased from Icon Safety. Hydrogen sulfide (34 L of 50 ppm balance nitrogen) was purchased from Cross Company. Carbon dioxide (103 L of 4.97 ppm balance air), chlorine (103 L of 4.60 ppm balance air), phosgene (103 L of 0.90 balance nitrogen), and sulfur dioxide (103 L of 1.00 ppm balance air) were purchased from Airgas. The actual gas concentrations of CO_{2}, Cl_{2}, COCl_{2}, and SiO_{2} were determined to ±5% accuracy by Airgas by direct comparison to calibration standards traceable to N.I.S.T. weights and/or N.I.S.T. Gas Mixture reference materials. The accuracy of the other gases is also assumed to be ±5%. The flow rate of the gases through the flow cell was ∼0.5 LPM.

## III. RESULTS AND DISCUSSION

Figure 1 shows the Raman spectrum for the 1265 and 1285 cm^{−1} modes of the 4.97-ppm CO_{2} obtained with 10 W laser power and 15 second signal integration time. The spectrum of Fig. 1 is the average of 9 scans for a total signal integration time of 135 s. The Raman signal for the 1285 cm^{−1} mode integrated over the 1280-1290 cm^{−1} spectral range and corrected for the underlying background is 1.73x10^{3} counts. The differential Raman cross section for the 1285 cm^{−1} mode of CO_{2} is 35.6x10^{−32} cm^{2}/sr (35.6x10^{−36} m^{2}/sr) deduced from the measured value of 51.6x10^{−36} m^{2}/sr for 488 nm excitation in Ref. 9.

Figure 2 shows the Raman spectrum (average of 9 scans) of 200 ppm NH_{3}, which shows two Raman modes at 934 and 967 cm^{−1}, in agreement with the infrared (IR) spectrum.^{12} The Raman signal for the 967 cm^{−1} mode integrated over the 950-981 cm^{−1} spectral range and corrected for the underlying background is 7.17x10^{3} counts. Using values of 7.17x10^{3} and 1.73x10^{3} counts for the Raman signals of NH_{3} and the 1285 cm^{−1} mode of CO_{2}, values of 200 and 4.97 ppm for the concentration of NH_{3} and CO_{2}, respectively, and a value of 3.56x10^{−31} cm^{2}/sr (3.56x10^{−35} m^{2}/sr) for the differential Raman cross section of the 1285 cm^{−1} mode of CO_{2}, we obtain a value of 3.68 ± 0.26x10^{−32} cm^{2}/sr (3.68 ± 0.26x10^{−36} m^{2}/sr) for the differential Raman cross section of the 967 cm^{−1} mode. The Raman signal for the 934 cm^{−1} mode integrated over the 921-950 cm^{−1} spectral range and corrected for the underlying background is 6.93x10^{3} counts, which yields a value of 3.57 ± 0.25x10^{−32} cm^{2}/sr (3.57 ± 0.25x10^{−36} m^{2}/sr) for the differential Raman cross section of the 934 cm^{−1} mode. The 0.26x10^{−32} cm^{2}/sr (7%) accuracy of the differential Raman cross section of the 967 cm^{−1} mode of NH_{3} is due to the 5% accuracy of the NH_{3} concentration and 4% accuracy of the Raman cross section of CO_{2}, and 3% accuracy of the Raman signal because of the wavelength dependence of the Raman spectrometer.

Chlorine is a diatomic molecule, which has a single vibrational mode that is Raman active. But Cl_{2} has two stable isotopes ^{35}Cl (78.76%) and ^{37}Cl (24.24%). Therefore, we expect three Raman modes, which are due to the ^{35}Cl^{35}Cl, ^{35}Cl^{37}Cl, and ^{37}Cl^{37}Cl isotopic molecules. Figure 3 is the Raman spectrum (average of 9 scans) of 4.60 ppm Cl_{2}, which shows three Raman modes at 554, 547, and 539 cm^{−1} in agreement with those reported by Hochenbleicher and Schrotter.^{13} The 554 cm^{−1} mode is due to the more abundant isotopic molecules ^{35}Cl^{35}Cl. The 547 cm^{−1} mode is due to the ^{35}Cl^{37}Cl isotopic molecules. The 539 cm^{−1} mode is due to the ^{37}Cl^{37}Cl isotopic molecules. Frequency separation between these modes is expected to be 7.4 cm^{−1} based on the masses of the two isotopes. The relative Raman signals of the 554, 547, and 539 cm^{−1} modes should be 1.00, 0.31, and 0.10, respectively, based on the relative abundance of ^{35}Cl and ^{37}Cl isotopes. The relative strengths of the observed Raman modes is consistent with those expected.

The Raman signal of Cl_{2} integrated over the 530-561 cm^{−1} spectral range and corrected for the underlying background is 6.11x10^{3}, which yields a value of 1.37 ± 0.10x10^{−30} cm^{2}/sr (1.37 ± 0.10x10^{−34} m^{2}/sr) for the differential Raman cross section of Cl_{2}.

Figure 4 is the Raman spectrum (average of 9 scans) of 50 ppm H_{2}S, which shows the Raman mode at 870 cm^{−1}. This mode has not been reported previously. The Raman cross section of the mode at 2611 cm^{−1} was reported previously.^{1,6}

Value of the Raman signal of H_{2}S integrated over the spectral range 850-880 cm^{−1} range and corrected for the underlying background is 1.58x10^{4}, which yields a value of 3.25 ± 0.23x10^{−31} cm^{2}/sr (3.25 ± 0.23x10^{−35} m^{2}/sr) for the differential Raman cross section of the 870 cm^{−1} mode of H_{2}S.

Figure 5 is the Raman spectrum of 0.90 ppm COCl_{2}, which shows the 570 cm^{−1} Raman mode; there is appreciable fluorescence background of unidentified origin. This mode is due to the symmetric the Cl-C-Cl bend. Our value of 570 cm^{−1} is consistent with the value of 573 cm^{−1} for liquid COCl_{2} reported by Ananthakrishnan.^{14} The value of the Raman signal integrated over the 559-580 cm^{−1} range and corrected for the underlying background is 1.43x10^{3}, which yields a value of 1.63 ± 0.11x10^{−30} cm^{2}/sr (1.63 ± 0.11x10^{−34} m^{2}/sr) for the differential Raman cross section of COCl_{2}.

Figure 6 is the Raman spectrum of 1.00 ppm SO_{2}, which shows the Raman mode at 1151 cm^{−1}. This value of 1151 cm^{−1} is consistent with the value of 1145 cm^{−1} for the liquid SO_{2} reported by Dickinson and West.^{15} The value of the Raman signal integrated over the 1140-1161 cm^{−1} range and corrected for the underlying background is 2.99x10^{3}, which yields a value of 3.08 ± 0.22x10^{−30} cm^{2}/sr (3.08 ± 0.22x10^{−34} m^{2}/sr) for the differential Raman cross section of SO_{2}. Our value of 3.08 ± 0.22x10^{−30} cm^{2}/sr for differential Raman cross section of SO_{2} is larger by 1.5x than the value of 2.05x10^{−30} cm^{2}/sr deduced from the value of 2.38x10^{−30} cm^{2}/sr for 514.5 nm excitation reported by Fouche and Chang.^{3,4} The reason for this discrepancy is not known.

## SUMMARY

The Raman spectra of toxic gases ammonia (NH_{3}), chlorine (Cl_{2}), hydrogen sulfide (H_{2}S), phosgene (COCl_{2}), and sulfur dioxide (SO_{2}) have been measured in the fingerprint region 400-1400 cm^{−1}. The Raman cross sections of these gases have been determined using the Raman cross section of the 1285 cm^{−1} mode of CO_{2} as the reference.

## ACKNOWLEDGMENTS

We thank William Herzog, Steven Christesen (ECBC), and David Sickenberger (ECBC) for several discussions regarding this work. We thank Richard Kreis (ECBC) for the fabrication of the flow cell. This work was sponsored by the Defense Threat Reduction Agency through the Edgewood Chemical and Biological Center under the Air Force Contract FA8721-05-C-0002. Opinions, interpretation, and recommendations are those of the authors, and do not necessarily represent the view of the United States Government.