Microwave Linewidths of Symmetric-Top Molecules

Abstract

The Anderson—Tsao and Curnutte impact theory of pressure broadening is applied to microwave rotational lines of symmetric‐top molecules. The effect of elastic and inelastic collisions on the optical cross section is examined. Simple but accurate expressions are obtained for microwave linewidths due to a variety of intermolecular interactions. The theory is applied to existing microwave linewidths, which happen to involve interactions between polar molecules. The theoretical width of the J = 1→2 line of PF₃ is in good agreement with experiment, although the agreement is less satisfactory for the J = 0→1 line of CH₃F and CHF₃, and poor for the J = 1→2 line of CHF₃. However, the theory satisfactorily accounts for widths of the NH₃ 3–3 inversion line broadened by CHF₃ as well as other symmetric‐top molecules.