Time-resolved mass spectroscopic studies of an atmospheric-pressure helium microplasma jet

Using molecular beam mass spectroscopy, time-resolved measurements of the ionic species in the plasma plume of an atmospheric-pressure helium microplasma jet are made for a range of excitation frequencies (5, 10 and 25 kHz) and source-instrument orifice distances (1, 7 and 11 mm). Ionic species can only be observed in the visible plasma plume, with the main positive species being N-2(+) (65.26%) and O-2(+) (21.11%), and a few percentages of N+, O+, NO+ and He+. For the negative ions, the majority species are O-3(-) (22.68%), O-2(-) (H2O) (10.49%) and a large range of minority species observed, namely O-n(-), OHn-, (H2O)(n)(-), COn- and clusters, O-m(-) (H2O)(n), (OH)(m)(-) (H2O)(n), O-n(-) (CO3) and CO3- (H2O)(n). The flux of ions created from air species, such as NO+ and O-3(-), is seen to be maximized at a distance of several millimeters from the nozzle, whereas the He+ concentration continually decreases with distance from the exit orifice. The time-resolved measurements (time resolution down to 2 mu s) show that positive ions appear twice in one full period of the voltage waveform, correlated directly with positive and negative current peaks. The rise and fall times of the positive ions are typically tens of mu s. In contrast, the appearance of negative ions is correlated only with the negative part of discharge current, with one main peak in the detected ionic flux seen per cycle. The rise time of the negative ions is about 10 mu s, independent of mass; however, we observed longer decay times, from 100 to 150 mu s, increasing with mass. With increased driving frequency, the time modulation in the ionic fluxes is reduced, particularly for the negative species that show almost constant fluxes at 25 kHz throughout the cycle. The observations can be understood through a simple picture of the interaction of the He jet and the moist ambient air. The results indicate that the discrete plasma 'bullets' and their afterglow tail, that forms the jet, carry an associated positive or negative current depending on the time of their creation in the voltage cycle.