Transcript QUANTITATIVE MEASUREMENT OF INTEGRATED BAND …

QUANTITATIVE MEASUREMENT OF INTEGRATED BAND INTENSITIES OF
BENZENE (C6H6) VAPOR IN THE MID-INFRARED AT 278, 298 AND 323 K
Curtis P. Rinsland
NASA Langley Research Center
Mail Stop 401A
Hampton, VA 23681-2199
U.S.A.
V. Malathy Devi, Department of Physics, The College of William and Mary,
Box 8795, Williamsburg, VA 23187-8795, 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 23666 U.S.A.
Infrared Spectrum of Benzene
JQSRT-D-08-00036R1 (in press)
• Benzene is a planar oblate symmetric top molecule with D6h point
group symmetry. The molecule has ten nondegenerate and ten
doubly degenerate vibrational modes
• Although C6H6 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 (C6H6)
• 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
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The high abundances of N2 and CH4 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 C6H6 detection [[Coustenis et al.
Icarus et al. 161, 383-403, 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 C6H6 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, 237-239, 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 657.5 and 664.5 cm-1 with an absorption path length of 41 m
The detection of C6H6 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 ν4 C6H6 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
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Integrated band intensities have been measured at temperatures of 278, 298, and
323 K from laboratory spectra covering 600-6500 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 0.112 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
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Our ν4 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 ν4 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;154:235-239 from a best fit of their ν4 integrated band intensities
measured from 328 K to 219.7 K using an FTIR with 4 cm-1 resolution
Raulin et al. [Spectrochim Acta 1990;46A:671-683] 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;55:1535-1544] reported integrated band
intensities for the ν4 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 640-705 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  10-19
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 C6H6 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