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Infrared absorption by collisional CH4+X pairs, with X=He, H2, or N2
Michael Buser1, Lothar Frommhold
1Physics Department, University of Texas, Austin, Texas 78712, USA.
The Journal of Chemical Physics
|January 11, 2005
Summary
Collision-induced absorption spectra of methane (CH4) with helium, hydrogen, and nitrogen show measured absorption exceeding theoretical calculations. Excess spectra reveal common features but defy current models, suggesting new induction mechanisms in CH4-X complexes.
Area of Science:
- Molecular Spectroscopy
- Intermolecular Forces
- Quantum Chemistry
Background:
- Collision-induced absorption (CIA) provides insights into intermolecular interactions.
- Existing models for CH4-X systems (methane with helium, hydrogen, or nitrogen) based on multipole-induced and dispersion forces do not fully explain experimental data.
- Discrepancies between theoretical and measured absorption spectra highlight limitations in current understanding of CH4-X interactions.
Purpose of the Study:
- To compare existing measurements of collision-induced rototranslational absorption spectra of CH4-X gaseous mixtures with theoretical calculations.
- To analyze and present the excess absorption spectra (measured minus calculated) for CH4-He, CH4-H2, and CH4-N2 systems at various temperatures.
- To investigate the underlying mechanisms responsible for the observed excess absorption and explore potential new dipole induction models.
Main Methods:
- Comparison of experimental collision-induced absorption spectra with theoretical calculations.
- Analysis of refined multipole-induced and dispersion force-induced dipole moments for CH4-He, CH4-H2, and CH4-N2 pairs.
- Characterization of excess absorption spectra, focusing on spectral intensities and shape across different collision partners and temperatures.
Main Results:
- Measured absorption spectra consistently exceed theoretical calculations across various frequencies for all CH4-X pairs.
- Excess absorption spectra exhibit distinct features for each collision partner (X) but share common characteristics between 200-500 cm(-1).
- The observed excess absorption cannot be adequately modeled by previously proposed ad hoc exchange force-induced dipole components.
Conclusions:
- Current theoretical models based on polarization and dispersion forces are insufficient to explain the observed collision-induced absorption in CH4-X systems.
- The significant excess absorption suggests the presence of additional, uncharacterized dipole induction mechanisms in these collisional complexes.
- These mechanisms may be related to the collisional distortion of the methane (CH4) molecular frame, warranting further investigation.