2015ApJS..217...10M

Rotational lines of the positive molecular ion C₃H⁺ and of the neutral C₃ H radical in three new vibrationally excited states with ²Σ symmetry have been detected in a supersonic molecular beam in the centimeter-wave band. The fundamental rotational line of the ion is quite weak, but is observed with similar intensity in a dc discharge through several different hydrocarbon gases when helium is the buffer gas. Under these conditions, the fractional abundance of C₃H⁺ relative to C₃H is estimated to be of order 10^–4, i.e., toward the lower end of the ratio (10^–3-10^–4) found for protonated ions using the same discharge nozzle. For each new ²Σ state of the C₃H radical, spectroscopic constants, including those describing hydrogen hyperfine structure, have been determined to high precision. Lines of one ²Σ state (B = 11271 MHz) are particularly intense in our molecular beam; for this state and a second one (B = 11306 MHz), millimeter-wave transitions have also been observed between 180 and 340 GHz using a long path dc glow absorption spectrometer. On the basis of intensity measurements with this spectrometer, the inferred rotation-vibration constant α, and theoretical calculations, the state with B = 11271 MHz is tentatively assigned to the ν₅ bending mode, predicted to lie ∼300/cm above ground.