Mass Spectrometry Study of Reactive Species in a Microhollow Cathode Discharge in He+H2O Mixtures

Reactive species created by a microhollow cathode discharge (MHCD) in He+H2O mixtures were investigated at 160 Torr using the molecular beam mass spectrometry. Ion currents of He+, HeH+, O+, OH+, H+(H2O), and H+(H2O)2 were measured as functions of H2O relative concentration (0.24-14%) and discharge current (5-15 mA). When the concentration exceeds 6%, most of the ions were decreased, but H+(H2O)2 (cluster ion) significantly increased at discharge current of 5 mA, whose ion current was very low compared with those of other ions. With increasing the discharge current, the cluster ion showed a sharp decrease, while the other ions were almost constant. These features were reasonably explained by the increase of the gas temperature and the plasma rarefaction due to the Joule heating of the working gas.