Accurate spectroscopic models for methane polyads derived from a potential energy surface using high-order contact...

Area of Science:

• Molecular Spectroscopy
• Theoretical Chemistry
• Computational Physics

Background:

• Accurate spectroscopic data for methane (CH4) is vital for atmospheric science and planetary research.
• Existing models often require extensive experimental data or adjustable parameters for high accuracy.

Purpose of the Study:

• Develop a novel spectroscopic model for precise theoretical predictions of methane's vibration-rotation line positions and intensities.
• Validate the model against experimental data and established databases.

Main Methods:

• Utilized high-order contact transformations (CT) on ab initio potential energy surfaces to derive resonance coupling parameters.
• Employed effective polyad Hamiltonians to converge vibrational and rotational energy levels.
• Determined dipole transition moment parameters from ab initio computed line strengths.
• Fine-tuned diagonal parameters to achieve experimental accuracy for specific spectral regions.

Main Results:

• Achieved an average discrepancy of 0.74 cm⁻¹ for experimental bands and 0.001 cm⁻¹ for ground state rotational levels up to J=17 without adjustable parameters.
• Accurately predicted over 5600 Dyad and Pentad line positions with an accuracy of ~10⁻³ cm⁻¹.
• Obtained line intensity predictions with good agreement with the Hitran-2008 database (4.4% for Dyad, 1.8% for Pentad).

Conclusions:

• The new spectroscopic model provides highly accurate theoretical predictions for methane spectra.
• The model's reliance on ab initio data and CT methods reduces the need for empirical adjustments.
• This work offers a valuable tool for spectroscopic databases and atmospheric modeling.