A LABORATORY STUDY OF C 3 H + AND THE C 3 H RADICAL IN THREE NEW VIBRATIONALLY EXCITED 2 Σ STATES USING A PIN-HOLE NOZZLE DISCHARGE SOURCE

Abstract

Rotational lines of the positive molecular ion C3H+ and of the neutral C3H 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−1 above ground.