QUANTITATIVE MEASUREMENT OF INTEGRATED BAND INTENSITIES OF BENZENE (C 6 H 6 ) VAPOR IN THE MID-INFRARED AT 278, 298 AND 323 K Curtis P. Rinsland NASA Langley Research Center Mail Stop 401A Hampton, VA U.S.A. V. Malathy Devi, Department of Physics, The College of William and Mary, Box 8795, Williamsburg, VA , U.S.A. Thomas A. Blake, Robert L. Sams and Steven W. Sharpe Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88, Richland, WA 99352, U.S.A. Linda S. Chiou, Science Systems and Applications, Inc., 1 Enterprise Parkway, Suite 200, Hampton, VA U.S.A.

Infrared Spectrum of Benzene JQSRT-D R1 (in press) Benzene is a planar oblate symmetric top molecule with D 6h point group symmetry. The molecule has ten nondegenerate and ten doubly degenerate vibrational modes Although C 6 H 6 has twenty fundamental modes covering 410 to 3063 cm -1, only four fundamentals are infrared active because of the molecule’s high degree of symmetry. Based on Herzberg’s notation to label the infrared active bands the bands are – 4 parallel band centered at 674 cm -1 –three perpendicular bands 12 at 3048 cm -1, 13 at 1484 cm -1, and 14 at 1038 cm -1 –The molecule has a large number of infrared active combination, difference, and hot bands throughout the mid-infrared –The 4 is the most intense infrared band and the one that has been used for infrared remote sensing of the atmospheres of Titan, Jupiter, and in the interstellar medium and we focus on that region here

Atmospheric Importance of Benzene (C 6 H 6 ) Benzene is an aromatic hydrocarbon produced in the Earth’s atmosphere and is found in air due to emissions from the burning of coal and oil and also from gas stations, and from motor vehicle exhaust It is used in the manufacture of plastics, detergents, pesticides, and other chemicals and is a carcinogen with exposures that have led to the development of and death by leukemia in humans occupationally exposed The U.S. environmental protection agency (EPA) has classified it as a group A human carcinogen Few Earth atmosphere remote sensing measurements have been reported, likely due to its short atmospheric lifetime. It is destroyed in the Earth’s atmosphere primarily by reaction with OH radicals with an important influence on air quality and ozone production at elevated levels

Importance of Benzene in Titan’s Atmosphere The high abundances of N 2 and CH 4 in the atmosphere of Titan, Saturn’s largest moon, lead to high abundances of nitrogen and carbon compounds, and its atmosphere and smog-like haze are of particular interest because of its similarity to the atmosphere that may have existed on Earth before life began Thermal remote sensing measurements of the composition of Titan’s stratosphere and a search for benzene have been reported during fly bys with higher spectral resolution than obtained by Voyager 1 with an effort to detect benzene during both missions –The Infrared Space Observatory (ISO) measurements in 1997 with a grating spectrometer at 4.3 cm -1 resolution reported a tentative C 6 H 6 detection [[Coustenis et al. Icarus et al. 161, , 2003] –Cassini/CIRS with a Fourier transform spectrometer (July 2004-January 2006) with 2.54 or 0.53 cm -1 resolution reported a firm detection of C 6 H 6 near 70°N latitude [Coustenis et al. Icarus 189, 35-62, 2007] –The analysis of both sets of measurements were based on a prediction of the 674-cm -1 4 band Q branch assuming spectroscopic parameters of Dang-Nhu et al. [J. Mol Spectrosc. 134, , 1989]. –The measurements of Dang-Nhu were obtained with a tunable diode laser spectrometer with 30 absolute line intensities measured at room temperature in the P branch between and cm -1 with an absorption path length of 41 m The detection of C 6 H 6 in Titan’s stratosphere is consistent with known chemical reactions and model predictions and may serve as a precursor to more complex hydrocarbons, potentially leading to amino acids

Benzene in Jupiter’s atmosphere and the interstellar medium Measurements of the ν 4 C 6 H 6 band have also been reported from ISO upper atmospheric measurements of Jupiter and disk average spectra of Saturn Additionally, the 4 band Q branch of benzene has been measured in a proto-planetary nebula, and benzene is likely to survive in dense parts of envelopes of carbon-rich evolved stars surrounding interstellar molecular clouds in regions with attenuation of ultraviolet photons

Laboratory Measurements Integrated band intensities have been measured at temperatures of 278, 298, and 323 K from laboratory spectra covering cm -1 The spectra were recorded at the Pacific Northwest National Laboratory (PNNL) in Richland, Washington, U.S.A. The absorption spectra of benzene vapor were recorded with a Bruker-66V Fourier transform spectrometer The optics bench of the spectrometer was evacuated for these measurements to minimize water and carbon dioxide absorptions The instrument resolution was set to cm -1 (instrument resolution = 0.9/maximum optical path difference) The pressure of each benzene vapor sample was measured using high precision capacitance manometers and a minimum of nine sample pressures were recorded at each temperature Samples were introduced into a temperature-stabilized static cell (19.94(1) cm pathlength) that was hard-mounted into the spectrometer Two-hundred fifty-six interferograms were averaged for each sample spectrum Sample pressures ranged from approximately 0.1 to 22 torr A composite spectrum was calculated for each cell temperature from the individual absorbance spectra recorded at that temperature For the 5°C spectra the average type-A uncertainty is 0.40%, for the 25 °C spectra the average type-A uncertainty is 0.38%, and for the 50°C spectra the average type-A uncertainty is 0.54%

Overview of Composite Spectra

Composite Spectrum of the 4 Band

Comparison of PNNL 4 cross sections with previous measurements Our ν 4 integrated band intensities are (427(13) cm -2 atm -1 at 278 K, 428(13) at 298 K, and 426(13) cm -2 atm -1 at 323 K No dependence of the ν 4 integrated band intensity with temperature outside the 3% experimental error was found Our result is inconsistent with the ~21% variation inferred by Khlifi et al. (J. Mol Spectrosc. 1992;154: from a best fit of their ν4 integrated band intensities measured from 328 K to K using an FTIR with 4 cm -1 resolution Raulin et al. [Spectrochim Acta 1990;46A: ] reported a 4 integrated band intensity of neat benzene vapor at 300 K was measured to be 250(16) cm -2 atm -1 using an FTS with 1 cm -1 resolution When at least 500 Torr of nitrogen was used for broadening the integrated band intensity increased to 350 cm -2 atm -1, more consistent with our measurements Di Lorando et al. [Spectrochim Acta A 1999;55: ] reported integrated band intensities for the ν 4 band region from spectral measurements of neat benzene vapor at temperatures of 273, 298, and 323 K using a Bomem DA8 Fourier transform spectrometer at spectral resolutions of 0.03 and 1.0 cm-1. Their higher resolution measurements integrated from cm -1 (close to those we used). Their integrated band intensities are slightly lower, but in good agreement with the results obtained in our analysis

17 – 20 Difference Band

14 Band

13 Band

12 Band

Integrated Band Intensities A table of measured integrated band intensities at the 3 measurement temperatures for bands between 615 and 6080 cm -1 with the identification of the primary bands in each region is reported (Herzberg notation) Corresponding integration limits (cm -1 ) and integrated band intensity in cm molecule -1  and cm -2 atm -1 units are provided Measurements for each region have been compared with previously reported results

Summary and Prospects for Improvements Temperatures in Titan’s atmosphere range from 170 K in the high stratosphere and 70 K at the tropopause, much colder than the lowest temperature in our experiment Based on the benzene vapor pressure curve, we estimate it may be feasible to measure benzene vapor in the PNNL cell cooled to ~210 K The complexity and high density of lines in the Q branch region and the need for partition function calculations covering the same temperature range will make it difficult to create a line list for line-by-line analysis

PNNL C 6 H 6 composite spectra

Spectral Cross Sections Permission to distribute the PNNL cross section measurements has been received Files of integrated cross sections at the three temperatures are available for use by both HITRAN and GEISA