INFRARED-SPECTROSCOPY OF CARBOIONS .8. HOLLOW-CATHODE SPECTROSCOPY OF PROTONATED ACETYLENE, C2H3+

The vibration-rotation energy level pattern of protonated acetylene, C2H3+, in the nu(6)(C-H antisymmetric stretching) vibrational state is anomalous and irregular because of the coupling between the rotational motion and the tunneling of the three protons among their equilibrium positions. The resultant spectral anomaly and the coexistence of C-H bands of other carbocations such as CH3+, C2H2+, CH2+, etc., in our positive column discharge using He-dominated gas mixtures had made the analysis of the C2H3+ spectrum difficult. In the present paper we use a hollow cathode discharge to simplify plasma chemistry and to make a more definitive and extensive analysis of the spectrum. A 3 m hollow cathode discharge cell has been constructed with a multiple reflection optical system giving an effective path length of 30 m. A gas mixture of C2H2 and H-2 with pressures of 0.03 and 1.1 Torr, respectively, has produced spectral lines of C2H3+ from 3192 to 3083 cm(-1) which are almost completely free of those from other carbocations. The purity of the spectrum, together cm with the accurate ground state rotational constants recently reported by the Lille millimeter wave group, has allowed us to assign spectral lines up to J = 25 and K-a = 4 and to determine extensive sets of the A-E splittings due to the proton tunneling in the excited state. An attempt has been made to analyze the plasma chemistry in the hollow cathode on the basis of earlier plasma diagnostic studies of the negative glow region. A model was used in which the primary molecular ions H-2(+) and C2H2+ are generated due to ionization by ''hot'' primary and secondary electrons. In the plasma these ions undergo ion-neutral reactions to produce H-3(+) and C2H3+, which are dissociated by reactions with C2H2 and recombination with ''ultimate'' electrons. By assuming proper number densities of primary, secondary, and ultimate electrons, semiquantitative agreement with the experimentally estimated ion densities has been obtained.