A Measurement of the Rotational Spectrum of the CH Radical in the Far‐Infrared

Rotational and fine-structure transitions between the lower rotational levels of the CH radical in its X 2 Π state have been observed in absorption in the laboratory with a tunable far-infrared (TuFIR) spectrometer. The molecules were generated in an electric discharge through a mixture of methane and carbon monoxide in helium. The experimental line widths were limited by Doppler broadening and the measurements have a 1 σ experimental uncertainty of 100 kHz. The frequencies have been used together with all previous measurements of CH in the v = 0 level of the X 2 Π electronic state to determine its molecular parameters and to predict an accurate set of rotational transition frequencies.