Collision Broadening of Rotational Absorption Lines. V. Pressure Broadening of Microwave Absorption Spectra Involving Asymmetric-Top Molecules

• Published in: The Journal of chemical physics 1969-01, Vol.51 (9), p.3891-3901
• Main Authors: Murphy, Joseph S., Boggs, James E.
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.1672608

Pressure broadening of microwave absorption spectra by collisions involving asymmetric-top molecules either as absorbing or as perturbing molecules has been investigated within the framework of a pressure-broadening theory presented earlier. Modifications of this theory are required because of the increased mathematical complexity involved in asymmetric-top calculations and because asymmetric-top molecules may have two energy levels spaced relatively close together with all other levels widely spaced. This latter condition may result in saturation of certain energy-level pairs and must be considered for accurate linewidth calculations. The linewidth calculations are found to be in very good over-all agreement with experimental values in most cases, although a few discrepancies occur. Comparison of calculated and experimental linewidth data suggest that H2O and SO2 have quadrupole contributions of about 10%–15% of the observed values while linewidths involving CH2Cl2, CH2CF2, and C2H4O can be explained solely on the basis of dipole–dipole interaction. A value of (2.87 ± 0.10) D·Å was determined for the molecular quadrupole moment of N2 from linewidth data for broadening of the 22,0→31,3 transition of H2O by N2 and appears to be in excellent agreement with the value obtained previously from data for NH3–H2 broadening.