Journal of Quantitative Spectroscopy and Radiative Transfer
10μm ethylene: spectroscopy, intensities and a planetary modeler's atlas
Introduction
Ethylene, C2H4 is a simple alkene which has a planar asymmetric top conformation. Ethylene is observed in the terrestrial atmosphere as a tropospheric pollutant. Earth atmospheric sources include automobiles, forest fires and plant life [1]. In the atmosphere, photochemical reactions with molecular oxygen, the hydroxyl radical, and ozone produce formaldehyde, nitrous oxide, and formic acid [2]. C2H4 is also detected in the atmospheres of the outer solar system bodies including Jupiter [3], Saturn [4], and Neptune and Titan [5], [6], [7]. In these atmospheres it is a by-product of methane photochemistry. As a minor atmospheric constituent with a rich spectrum in the window, ethylene is well positioned as an atmospheric-physics probe. For example, ethylene has been used as a probe of atmospheric convection on Titan [8] and as a probe of short time variations (time scales on the order of days) in thermal infrared auroral emission on Jupiter [9].
Spectroscopically, ethylene is a planar asymmetric top with a very intense c-type band centered at approximately , the ν7 band system. It is this band that is seen in the spectra of the outer planets and was used for the detection of ethylene in the circumstellar shell around IRC+10216 [10]. The return to the Saturn/Titan system by CASSINI has prompted us to undertake a careful analysis of this system of bands with emphasis on absolute frequency and line intensity calibration.
It is expected that the planetary modeler's atlas, which is an end product of this investigation, will be useful to investigators making high resolution spectroscopic observations of a number of outer solar system objects. An example of the quality of the combined ethylene planetary modeler's atlas is shown in Fig. 1. The figure compares a spectrum calculated using the modeler's atlas compared to a Titan spectrum recorded by the ISO/SWS combination [11]. Some of the discrepancies between the observations and the model may be attributable to instrumental corrections [12].
Experimental details concerning the laser spectra used for intensity calibration of the broadband FTS spectrum have been published [13]. The FTS spectrum was obtained at the McMath–Pierce FTS instrument at Kitt Peak National Observatory [14]. Single-sided interferograms produced spectra with a resolution. The data set contains a spectrum from 800 to obtained from 30 co-added interferograms. The 30-cm-long sample-cell contained Torr (actual) of research grade ethylene at (nominal).
Section snippets
Frequency calibration
For frequency calibration, a low pressure () of CO2 was added to a tandem 6-m White cell operated at 97.1-m path length. We have previously demonstrated the quality of frequency calibration possible for data recorded on the McMath–Pierce FTS using these calibration techniques [15]. The CO2 laser lines in the region provided the frequency standards. A total of 52 candidate calibration lines were identified. Forty of these survived visual inspection for overlap with other lines and
Intensities
The intensity of an individual absorption transition is given aswhere SAB is the line strength , h is Plank's constant , c is the speed of light in a vacuum (cm/s), k is Boltzman's constant (erg/K), T is temperature (erg) is the energy of the lower rotation–vibration state, Q=QVQR is the total partition function equal to the product of rotation and vibration partition functions, νAB(cm−1) is the transition
Production of a planetary modeler's atlas
The principal goal of this effort is to produce a planetary modeler's spectral atlas for ethylene in the region. The ingredients needed to produce such an atlas include an experimental database containing a large number of well-resolved ethylene transitions with the observed frequencies and measured absolute intensities. The experimental database ideally contains the assignments of the included ethylene lines for validation purposes.
In addition, a theoretical data base containing quantum
Band Intensities
Retrieved values for the vibrational band intensities, S0iv, at , are: ; ; . The strength of ν4 was held fixed to . None of the transitions in the merged intensity database involved a significant ν4 mixing component C4. The vibrational band intensities above are presented as the best available intensity values. They are the best current fitting values from the non-linear least-squares regression
Modeler's atlas
The modeler's atlas is found at http://aurora.phys.utk.edublass/ethyatlas/.
The full atlas contains over 11,000 transitions from 700 to . The atlas is available in downloadable form. A form is available on the website which allows one to request a section of the atlas to be prepared for download. Table 5 displays a short section of the atlas near . Also available at the same URL are segments of synthetic spectra compared with the actual observed data. Fig. 2 displays
Conclusions
The modeler's atlas has been successfully used in several investigations. The comparison in Fig. 1 with ISO/SWS observations is quite good and a vast improvement over prior model calculations [11]. A recent heterodyne spectroscopy study of ethylene frequencies and intensities presents a set of observations which compare favorably with data in the atlas [24]. It should be noted that the transition frequencies in the atlas are, in fact, calculated frequencies based on the model of Cauuet et al.
Acknowledgements
We thank A. Coustenis of Observatoire de Paris-Meudon for providing Fig. 1. We acknowledge support from the NASA Planetary Atmospheres program in the form of grants to the Planetary Systems Branch of the Laboratory for Extraterrestrial Physics (RTOP 693-154-50-80-01) over the several year period that the entire ethylene project has been in progress. We also acknowledge the assistance of the University of Tennessee in the form of internal Research Incentive grants which have partially supported
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2019, Journal of Quantitative Spectroscopy and Radiative TransferFirst high-resolution analysis of the ν<inf>2</inf>+ν<inf>6</inf> band of the C<inf>2</inf>H<inf>2</inf>D<inf>2</inf>-cis isotopologue of ethylene
2019, Journal of Quantitative Spectroscopy and Radiative TransferExtended analysis of the ν<inf>12</inf> band of <sup>12</sup>C<inf>2</inf>H<inf>4</inf> for astrophysical applications: Line strengths, widths, and shifts
2019, Journal of Quantitative Spectroscopy and Radiative TransferCitation Excerpt :For these reasons, for many years the ethylene molecule and its different isotopologues have been extensively studied. We quote here only a few examples out of a rather long list of references on this topic, which means only spectroscopic studies of the “parent” molecule from 2000 up to now (see Refs. [23–48] and references cited therein). The ethylene sample, manufactured by Linde AG with a purity of 99.99%, was purified with three freeze–pump–thaw cycles and recorded at pressures between 10 to 15000 Pa with absorption path lengths of 4.0524 ± 0.002 m (White–cell, 4 pathes), 8.0524 ± 0.004 m (White–cell, 8 pathes) and 13.28 ± 0.02 mm (single–path cell).
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Present address: Department of Astronomy, University of Maryland, College Park, MD, USA.