New Approach for Spectroscopic Data Reduction Using Ab Initio Calculations and Experimental Lines: Application to Methane

Our aim is to address the question: how to generate line lists for molecular spectra containing line positions determined within experimental accuracy and predicted ab initio intensities in a consistent way for atmospheric, planetological and astrophysical applications? In this work we have developed a new combined spectroscopic model¹ for methane vibrational polyads which involves all resonance terms very accurately derived from the molecular potential energy surface² via high-order contact transformation (CT) method¹. A subsequent “fine tuning” of the diagonal parameters allows achieving experimental accuracy for about 6000 Dyad and Pentad line positions whereas all resonance coupling parameters were held fixed to ab initio values. For determination of dipole transition moment parameters, we have applied two complementary methods. Initial values for the effective dipole moment were first computed from ab initio dipole moment surface (DMS)², then the final parameters were fitted to selected ab initio line strengths previously computed³ from the same DMS by variational method. The new polyad model allows generating a spectral line list for the Dyad and Pentad bands with the accuracy ∼10⁻³ cm⁻¹ for line positions combined with ab initio predictions for line intensities. Future prospectives for an application of this approach to other molecules will be discussed.