Rotational lines of the positive molecular ion C_3H^+ and of the neutral C_3H 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 C_3H^+ relative to C_3H 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 C_3H 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 ν_5 bending mode, predicted to lie ~300 cm^(−1) above ground
