A Laboratory Study of C3h + and the C3h Radical in Three New Vibrationally Excited Σ States Using a Pin-hole Nozzle Discharge Source

Rotational lines of the positive molecular ion C3H + and of the neutral C3H radical in three new vibrationally excited states with Σ 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 , i.e., toward the lower end of the ratio (10– 10) found for protonated ions using the same discharge nozzle. For each new Σ state of the C3H radical, spectroscopic constants, including those describing hydrogen hyperfine structure, have been determined to high precision. Lines of one Σ state (B= 11271MHz) are particularly intense in our molecular beam; for this state and a second one (B= 11306MHz), 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= 11271MHz is tentatively assigned to the ν5 bending mode, predicted to lie ∼300 cm above ground.