The quadrupole moment of cyanogen: a comparative study of collision-induced absorption in gaseous C₂N₂, CO₂, and mixtures with argon

Abstract

The collision-induced spectra of C 2 N 2 gas and a gaseous mixture of C 2 N 2 and Ar at 298 K have been obtained in the spectral region below 120 cm −1 using far-infrared laser and microwave techniques as well as a Fourier-transform spectrometer. In addition, the collision-induced spectra of a gaseous mixture of CO 2 and Ar are reported at temperatures of 233 and 298 K in the spectral region below 230 cm −1 . The theoretical values for the spectral moments α 1 and γ 1 for CO 2 are much smaller than the experimental values, as expected for a molecule with a relatively large quadrupole moment. However, for CO 2 –Ar mixtures, the agreement between the theoretically and experimentally determined spectral moments is relatively good, resulting in a value of 4.6 B for the quadrupole moment of CO 2 instead of the generally accepted value of 4.3 B. The quadrupole moment of C 2 N 2 is estimated to be 6.2 ± 0.4 B from our data and the theory for the spectral moments, if a correction is made for an overestimate of the quadrupole moment similar to that obtained for the CO 2 –Ar mixture. This value is considerably smaller than a previously reported calculated result of 9.0 B. Line-shape expressions based on information theory (IT6) do not yield good agreement with experiment, a result that is attributed to the large anisotropy of the molecules.