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

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Abstract
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.