Raman spectra of ammonia (NH3), chlorine (Cl2), hydrogen sulfide (H2S), phosgene (COCl2), and sulfur dioxide (SO2) 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 NH3. Three Raman modes are observed in Cl2 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 H2S, COCl2, and SO2, respectively. Values of 3.68 ± 0.26x10−32 cm2/sr (3.68 ± 0.26x10−36 m2/sr), 1.37 ± 0.10x10−30 cm2/sr (1.37 ± 0.10x10−34 m2/sr), 3.25 ± 0.23x10−31 cm2/sr (3.25 ± 0.23x10−35 m2/sr), 1.63 ± 0.14x10−30 cm2/sr (1.63 ± 0.14x10−34 m2/sr), and 3.08 ± 0.22x10−30 cm2/sr (and 3.08 ± 0.22x10−34 m2/sr) were determined for the differential Raman cross section of the 967 cm−1 mode of NH3, sum of the 554, 547, and 539 cm−1 modes of Cl2, 870 cm−1 mode of H2S, 570 cm−1 mode of COCl2, and 1151 cm-1 mode of SO2, respectively, using the differential Raman cross section of 3.56 ± 0.14x10−31 cm2/sr (3.56 ± 0.14x10−35 m2/sr) for the 1285 cm−1 mode of CO2 as the reference.

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 (CH4, C2H6, C3H8, C6H6, CO, CO2, F2, HBr, HCl, HF, H2, H2O, H2S, N2, NH3, ND3, NO, N2O, O2, O3, and SO2) have been reported previously.3–8 In this paper, we report the measurement of the Raman spectra and cross sections of ammonia (NH3), chlorine (Cl2), hydrogen sulfide (H2S), phosgene (CCl2O), and sulfur dioxide (SO2) toxic gases in the fingerprint region 400-1400 cm−1 using the Raman cross section of the 1285 cm−1 mode of carbon dioxide (CO2) as the reference.9 Raman cross sections of NH3, Cl2, H2S, and CClO2 have been measured for the first time in the fingerprint region. Our value of the Raman cross section of SO2 is 1.5x larger than that reported previously.3,4

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 CO2, Cl2, COCl2, and SiO2 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.

Figure 1 shows the Raman spectrum for the 1265 and 1285 cm−1 modes of the 4.97-ppm CO2 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 1.73x103 counts. The differential Raman cross section for the 1285 cm−1 mode of CO2 is 35.6x10−32 cm2/sr (35.6x10−36 m2/sr) deduced from the measured value of 51.6x10−36 m2/sr for 488 nm excitation in Ref. 9.

FIG. 1.

Raman spectrum (average of 9 scans) for the 1265 and 1285 cm−1 modes of 5 ppm CO2 obtained with 10 W laser power and 15 s signal integration time for each scan.

FIG. 1.

Raman spectrum (average of 9 scans) for the 1265 and 1285 cm−1 modes of 5 ppm CO2 obtained with 10 W laser power and 15 s signal integration time for each scan.

Close modal

Figure 2 shows the Raman spectrum (average of 9 scans) of 200 ppm NH3, 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 7.17x103 counts. Using values of 7.17x103 and 1.73x103 counts for the Raman signals of NH3 and the 1285 cm−1 mode of CO2, values of 200 and 4.97 ppm for the concentration of NH3 and CO2, respectively, and a value of 3.56x10−31 cm2/sr (3.56x10−35 m2/sr) for the differential Raman cross section of the 1285 cm−1 mode of CO2, we obtain a value of 3.68 ± 0.26x10−32 cm2/sr (3.68 ± 0.26x10−36 m2/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 6.93x103 counts, which yields a value of 3.57 ± 0.25x10−32 cm2/sr (3.57 ± 0.25x10−36 m2/sr) for the differential Raman cross section of the 934 cm−1 mode. The 0.26x10−32 cm2/sr (7%) accuracy of the differential Raman cross section of the 967 cm−1 mode of NH3 is due to the 5% accuracy of the NH3 concentration and 4% accuracy of the Raman cross section of CO2, and 3% accuracy of the Raman signal because of the wavelength dependence of the Raman spectrometer.

FIG. 2.

Raman spectrum (average of 9 scans) for the 934 and 967 cm−1 modes of 200 ppm NH3 obtained with 10 W laser power and 15 s signal integration time for each scan.

FIG. 2.

Raman spectrum (average of 9 scans) for the 934 and 967 cm−1 modes of 200 ppm NH3 obtained with 10 W laser power and 15 s signal integration time for each scan.

Close modal

Chlorine is a diatomic molecule, which has a single vibrational mode that is Raman active. But Cl2 has two stable isotopes 35Cl (78.76%) and 37Cl (24.24%). Therefore, we expect three Raman modes, which are due to the 35Cl35Cl, 35Cl37Cl, and 37Cl37Cl isotopic molecules. Figure 3 is the Raman spectrum (average of 9 scans) of 4.60 ppm Cl2, 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 35Cl35Cl. The 547 cm−1 mode is due to the 35Cl37Cl isotopic molecules. The 539 cm−1 mode is due to the 37Cl37Cl 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 35Cl and 37Cl isotopes. The relative strengths of the observed Raman modes is consistent with those expected.

FIG. 3.

Raman spectrum (average of 9 scans) for the 539, 547, and 554 cm−1 modes of 4.6 ppm Cl2 obtained with 10 W laser power and 15 s signal integration time for each scan.

FIG. 3.

Raman spectrum (average of 9 scans) for the 539, 547, and 554 cm−1 modes of 4.6 ppm Cl2 obtained with 10 W laser power and 15 s signal integration time for each scan.

Close modal

The Raman signal of Cl2 integrated over the 530-561 cm−1 spectral range and corrected for the underlying background is 6.11x103, which yields a value of 1.37 ± 0.10x10−30 cm2/sr (1.37 ± 0.10x10−34 m2/sr) for the differential Raman cross section of Cl2.

Figure 4 is the Raman spectrum (average of 9 scans) of 50 ppm H2S, 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

FIG. 4.

Raman spectrum (average of 9 scans) for the 870 cm−1 modes of 50 ppm H2S obtained with 10 W laser power and 15 s signal integration time for each scan.

FIG. 4.

Raman spectrum (average of 9 scans) for the 870 cm−1 modes of 50 ppm H2S obtained with 10 W laser power and 15 s signal integration time for each scan.

Close modal

Value of the Raman signal of H2S integrated over the spectral range 850-880 cm−1 range and corrected for the underlying background is 1.58x104, which yields a value of 3.25 ± 0.23x10−31 cm2/sr (3.25 ± 0.23x10−35 m2/sr) for the differential Raman cross section of the 870 cm−1 mode of H2S.

Figure 5 is the Raman spectrum of 0.90 ppm COCl2, 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 COCl2 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 1.43x103, which yields a value of 1.63 ± 0.11x10−30 cm2/sr (1.63 ± 0.11x10−34 m2/sr) for the differential Raman cross section of COCl2.

FIG. 5.

Raman spectrum (average of 9 scans) for the 570 cm−1 mode of 0.90 ppm COCl2 obtained with 10 W laser power and 15 s signal integration time for each scan.

FIG. 5.

Raman spectrum (average of 9 scans) for the 570 cm−1 mode of 0.90 ppm COCl2 obtained with 10 W laser power and 15 s signal integration time for each scan.

Close modal

Figure 6 is the Raman spectrum of 1.00 ppm SO2, 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 SO2 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 2.99x103, which yields a value of 3.08 ± 0.22x10−30 cm2/sr (3.08 ± 0.22x10−34 m2/sr) for the differential Raman cross section of SO2. Our value of 3.08 ± 0.22x10−30 cm2/sr for differential Raman cross section of SO2 is larger by 1.5x than the value of 2.05x10−30 cm2/sr deduced from the value of 2.38x10−30 cm2/sr for 514.5 nm excitation reported by Fouche and Chang.3,4 The reason for this discrepancy is not known.

FIG. 6.

Raman spectrum (average of 9 scans) for the 1151 cm−1 mode of 1.0 ppm SO2 obtained with 10 W laser power and 15 s signal integration time for each scan.

FIG. 6.

Raman spectrum (average of 9 scans) for the 1151 cm−1 mode of 1.0 ppm SO2 obtained with 10 W laser power and 15 s signal integration time for each scan.

Close modal

The Raman spectra of toxic gases ammonia (NH3), chlorine (Cl2), hydrogen sulfide (H2S), phosgene (COCl2), and sulfur dioxide (SO2) 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 CO2 as the reference.

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.

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