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Laboratoire Interdisciplinaire Carnot de Bourgogne

Méthane et planétologie

MODELING THE ABSORPTION SPECTRUM OF METHANE


For applications to Earth's, planetary and exo-planetary atmospheres.

Methane (CH4), the simplest stable hydrocarbon molecule, is a species of primary importance for many domains:

 
  • It is the third major greenhouse gas (after water and carbon dioxide) in the Earth's atmosphere.
  • It is an important constituent of some planetary atmospheres like the giant planets (Jupiter, Saturn, Uranus, Neptune) and Titan, Pluto, … , as well as of the atmospheres of brown dwarfs, of some "cold" stars, etc.
  • It has been recently discovered in the atmosphere of exoplanets of "hot-jupiter" type like HD189733b or HD209458b.
  • It plays an important role in combustion processes.

IllustrationHotCH4

Illustration : M. Louviot.

Although it is a small and simple molecule, its rovibrational spectroscopy is very complicated. This is mainly due to the high symmetry of this tetrahedral system (which leads to the existence of many degeneracies) and to its intricated vibrational structure.
As a matter of fact, the four fundamental vibration frequencies of CH4 (three of then being degenerate oscillators) obey a simple approximate relation (see Table 1), which leads to a specific vibrational structure with vibrational levels grouped into "packets" called polyads.

 

Table 1: The normal modes of methane

CH4modes

 

Figure 1 shows the first ten polyads of methane. It is clear that the complexity of the spectrum increases rapidly with energy, due to the increasing number of levels and sublevels.

 

Polyads

Figure 1: The polyads of methane, illustrating the complexity of this spectrum. Horizontal lines represent vibration energy levels. The black curve gives the number of vibrational sublevels for each polyad. The names correspond to the different absorption bands. Different spectral regions are illustrated by images and spectra: in pink, a simulated spectrum for lower polyads and, in red, an example of the spectra recorded on Titan by the Huygens probe (JPL images PIA05381 and PIA06220 – Credit: NASA/JPL/Space Science Institute).

 

Our group is the world leader concerning the analysis of high-resolution spectra (infrared absorption and Raman scattering) of methane. Needless to say, however, that the whole wavenumber range displayed in Figure 1 is far from being understood. At the present time, the octad is the highest polyad which is fully understood (a global fit of the 0 to 4800 cm-1 region for both positions and intensities has been performed), the analysis of the tetradecad being still partial, but in good progress and already convincing (see Figure 4 below)). Figure 2 shows an exemple of a simulation of the infrared absorption spectrum of methane.

 

CH4 220K calc LR

Figure 2: Calculated infrared absorption of methane. GS is the ground vibrational state. Some Earth-observing satellite/instrument wavenumber ranges are indicated at the top.

 

Figure 3 illustrates the complexity of the level mixings in the excited polyads of methane. The example shown is that of the reduced energy levels in the octad, as a function of the rotational quantum number J. The colors show, for each rovibrational level, the contribution of the different normal mode vibrations after diagonalization of the effective Hamiltonian.

 

Fig Niv P4mP0

Figure 3: Level mixings in the Tetradecad of methane. Rovibrational levels are plotted a a function of the rotational quantum number J. The colors corresponds to the 14 different interacting vibrational levels.

 

Figure 4 shows a recent comparison between an experimental and an simulated spectrum, in the so-called Tetradecad region. See the following paper doe more details:

Fig tetra global 296K

Figure 4: The Tetradecad region of methane at room remperature, compared to the simulation.

 

All our simulations and analyses are performed thanks to the group theoretical and tensorial tools developed in our group since many years for the spectroscopy of spherical tops, which is implemented in the STDS software.

We have started in 2013 the study of hot methane emission spectra, in collaboration with the Institut de Physique de Rennes (Prof. Robert Georges) and the AILES BEAMLINE of the Soleil Synchotron (Dr. Olivier Pirali). Figure 5 shows an example of such an emission spectrum in the bending Dyad region.

 

Hot CH4

Figure 5: Emission spectrum of methane at ca. 1173 K, compared to the simulation.

 

The 13CH4, 12CD4 and 12CH3D isotopologues are also studied. In the case of 12CH3D, the MIRS software is used.

Our results are widely used for atmospheric and planetary applications. Some references illustrating recent applications to the case of Titan's atmosphere can be found on the CH4@Titan ANR (2009-2012) page.

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See also the following reference:

 

To calculate the methane partition function: click here.

 

Our calculated line lists can be downloaded for a database build up for the VAMDC (Virtual Atomic and Molecular Data Centre) consortium :

vamdc portal logo

 

These linelists are also used to parly update the HITRAN database. See the following reference:

 

A special issue of the Journal of Molecular Spectroscopy dedicated to "Methane spectroscopy and its applications to planetary atmosphere including Earth's" has been published in 2013, with Athena Coustenis and Vincent Boudon as guest editors.

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