MOLECULAR SPECTROSCOPY AND APPLICATIONS (Department of Optics and Matter-Radiation Interaction)

Institut Carnot de Bourgogne
UMR 5209 CNRS-Université de Bourgogne, 9 av. A. Savary, BP 47 870, F-21078 DIJON Cedex, FRANCE

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 dioxyde) in the Earth's atmosphere.
• It is an important constituent of some planetary atmospheres like the giant planets (Jupiter, Saturn, Uranus, Neptune) an Titan, as well as of the atmospheres of brown dwarfs, of some "cold" stars, etc.
• It plays an important role in combustion processes.

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

ν1	ν2	ν3	ν4
A1	E	F2	F2
Stretching	Bending	Stretching	Bending
Raman	Raman	IR	IR
2916 cm-1	1533 cm-1	3019 cm-1	1311 cm-1

We have approximately:  ν₁ ≈ν₃ ≈2ν₂ ≈2ν₄
Click here to view the vibrations!

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.

Figure 1: The polyads of methane

Our group is the world leader concerning the analysis of high-resolution spectra (infrared absorption and Raman scattering) of methane. All CH₄ spectra are analyzed in Dijon ! Needless to say, however, that the whole wavenumber range displayed in Figure 1 is far from being understood. At the present time (2007), 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. Figure 2 shows a recent exemple of a simulation of the infrared absorption spectrum of methane.

Figure 2: Calculated infrared absorption of methane

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.

Figure 3: Level mixings in the octad of ¹²CH₄

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.

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 cane be found in Vincent Boudon's home page.

 Calculate the methane partition function : click here.

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