Knowledge Bank

We have constructed a 3m hollow cathode tube to conduct infrared spectroscopy of protonated carbocations. This system has been applied to $C2H2/H2$ discharges. As shown by Amano, we obtained an ion spectrum of protonated acetylene almost exclusively, in contrast to our earlier scans of positive column discharges which contained spectra of many other carbocations such as $CH3.CH2+$ and $C2H2+$ as well as of $C2H3+$ Our discharge assumes a length of 1.6m when the cathode is filled with a flowing mixture of $C2H2/H2=3/112$ mTorr and powered with 0.5 amp RMS at 11 kHz. Single-mode IR radiation from our difference frequency spectrometer was reflected 20 times through the discharge off a vacuum enclosed White cell, detected with noise subtraction, and demodulated at the discharge frequency to selectively display the concentration modulated ion absorption lines. We obtained a signal to noise of 70 for the strongest spectral lines. The purity and strength of the observed $C2H3v6=3142.165cm-1$ band together with the recent millimeter wave results by the Lille group2 have allowed us to considerably extend and strengthen our previous assignment; transitions up to $J=25$ and $Ka=4$ have been assigned using ground state combination differences. The observed splitting in the excited state due to proton tunneling will be presented. From the intensity pattern we estimate a rotational temperature of 250 K for $C2H3+$ in this water-cooled plasma.