Laser magnetic resonance rotational spectroscopy of 2Σ radicals: Ethynyl (CCH)

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Abstract
The first terrestrial measurement of the free ethynyl radical (CCH), made by far‐infrared laser magnetic resonance, is described. The N=6→7 rotational transition was observed for the lowest vibrational level of the 2Σ+ ground state. Because of the very weak spin coupling in this state, the LMR spectrum is complex and badly overlapped. A theoretical formalism for the prediction and analysis of such weakly coupled 2Σ states is presented, in which frequencies, linewidths, and intensities of all transitions are computed as a function of magnetic flux density, and the total absorption coefficient is computed at each field point in order to simulate the magnetic resonance spectrum. This formalism is used to analyze the LMR spectra of CCH. A combined least squares analysis of existing microwave, astronomical, and LMR data was carried out to determine an improved set of molecular parameters for this important interstellar molecule.