Ab initio modeling of molecular IR spectra of astrophysical interest: application to CH₄

Abstract

Aims. We describe an ab initio-based numerical method of obtaining infrared spectroscopic data (line list) of polyatomic molecules that allows calculation of complete sets of lines for temperatures up to several thousand Kelvin. While the main focus is on completeness and consistency, not spectroscopic accuracy, the approach is in principle "exact" for line positions and, although not exact for line strengths, of sufficient accuracy to be of value, especially in wavelength regions where there are gaps in reliable experimental data. Methods. Global potential energy and dipole moment hypersurfaces are fitted to the results of ab initio electronic structure calculations. The MULTIMODE software is then used to obtain rovibrational energy levels and dipole transition matrix elements. This information is used to calculate a complete set of Einstein coefficients of spontaneous emission A i j . Results. The method is applied to obtain a spectroscopic database for methane containing over 1.4 million lines up to an upper state energy of 6200 cm −1 (∼9000 K). The emission spectrum of CH 4 at 1000 K is calculated with the complete set of Einstein coefficients and compared with the one obtained from the HITRAN database. Gaps in the database are realistically filled in by the calculated spectrum. Conclusions. Consistent and complete databases are important for astrophysical applications. Databases obtained by the method described here fulfill this requirement and are sufficiently accurate for astrophysical applications such as model atmosphere calculations and the corresponding synthetic spectra.

First theoretical global line lists of ethylene (12C2H4) spectra for the temperature range 50−700 K in the far-infrared for quantification of absorption and emission in planetary atmospheres

“…Accurate first principles of rovibrational spectra of fiveatomic molecules such as methane and its isotopologs have been reported in Rey et al (2013aRey et al ( ,b, 2014aRey et al ( ,b, 2015a. Recently, high-temperature theoretical line lists for astrophysical applications have been constructed for methane in Warmbier et al (2009), Tennyson et al (2013), , Rey et al (2014b).…”

Perspective: Accurate ro-vibrational calculations on small molecules

“…49 This has led to the computation of extensive line list of transitions for hot molecules by a number of groups. 19,[50][51][52][53][54][55] Recent work has particularly focussed on providing line lists for hot methane, 51,[56][57][58] the use of which has also been shown to have a dramatic effect on models of astronomical objects, 43 see Fig. 1.…”

Spectroscopy of planetary atmospheres in our Galaxy

“…The strong transition of methane at 3 μm completely dominates the spectrum of BDs at 1600 K and is still identifiable at 1800 K (Borysow et al 2002). Statistical narrowband models (Perrin et al 2007), ab initio models (Warmbier et al 2009;Yurchenko et al, in preparation) or extrapolations from global effective models (Wenger et al 2008) have been proposed to reproduce the low resolution spectrum of CH 4 up or above 2000 K. From an experimental point of view, Nassar and Bernath (2003) measured line absorption cross sections up to 1273 K. Measurements of line absorption cross sections up to 1850 K in the 3 μm region were published by Thiévin et al (2008). Karkoschka and Tomasko (2010) have collected low temperature laboratory and planetary measurements to estimate methane transmission in the 0.4-5.5 μm wavelength interval.…”