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

The vibration-rotation energy level pattern of protonated acetylene, C 2H3+, 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 po sitive column discharge using He-dominated gas mixtures had made the a nalysis of the C2H3+ spectrum difficult. In the present paper we use a hollow cathode discharge to simplify plasma chemistry and to make a m ore definitive and extensive analysis of the spectrum. A 3 m hollow ca thode discharge cell has been constructed with a multiple reflection o ptical system giving an effective path length of 30 m. A gas mixture o f 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 al most completely free of those from other carbocations. The purity of t he spectrum, together cm with the accurate ground state rotational con stants recently reported by the Lille millimeter wave group, has allow ed us to assign spectral lines up to J = 25 and K-a = 4 and to determi ne 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 che mistry in the hollow cathode on the basis of earlier plasma diagnostic studies of the negative glow region. A model was used in which the pr imary molecular ions H-2(+) and C2H2+ are generated due to ionization by ''hot'' primary and secondary electrons. In the plasma these ions u ndergo ion-neutral reactions to produce H-3(+) and C2H3+, which are di ssociated by reactions with C2H2 and recombination with ''ultimate'' e lectrons. By assuming proper number densities of primary, secondary, a nd ultimate electrons, semiquantitative agreement with the experimenta lly estimated ion densities has been obtained.