The millimeter-wave spectrum of CCN (X ∼ ^2Πr): A combined Fourier transform and direct absorption study

New pure rotational measurements have been conducted for the CCN radical (X∼^2Πr) in the Ω = 1/2 spin-orbit ladder using a combination of Fourier transform millimeter-wave (FTmmW) and millimeter direct absorption techniques. For the FTmmW work, the species was created in a supersonic jet in a pulsed DC discharge from a dilute mixture of CH₄ and (CN)₂ in argon, while an AC discharge of pure (CN)₂ in argon was used for the direct absorption study. Six hyperfine components arising from the nitrogen nuclear spin in the lambda-doublets of the J = 5/2 → 3/2 transition of CCN near 59 GHz were recorded, partly as a test of the new FTmmW system. Four transitions were also measured in the 224-296 GHz range using direct absorption, as well; at the higher frequencies, lambda-doubling was resolved but the hyperfine structure was effectively collapsed. All measurements required significant signal-averaging. The spectra were analyzed in a combined fit with the previous J = 3/2 → 1/2, FTMW data, using a case (a) Hamiltonian. Rotational and lambda-doubling constants for CCN were significantly improved over previous analyses. Based on the hyperfine and quadrupole constants, the most dominant resonance structure for this radical is the CCN arrangement. Unlike CN and C₃N, however, there is still substantial electron density on the nitrogen nucleus such that the CCN structure is also important, as found for CCP and CCAs.