Photoionization capable, extreme and vacuum ultraviolet emission in developing low temperature plasmas in air

Experimental observation of photoionization capable extreme ultraviolet and vacuum ultraviolet emission from nanosecond timescale, developing low temperature plasmas (i.e. streamer discharges) in atmospheric air is presented. Applying short high voltage pulses enabled the observation of the onset of plasma formation exclusively by removing the external excitation before spark development was achieved. Contrary to the common assumption that radiative transitions from the b1∏u (Birge-Hopfield I) and b′1∑+ u (Birge-Hopfield II) singlet states of N2 are the primary contributors to photoionization events, these results indicate that radiative transitions from the c′4 1∑+ u (Carroll-Yoshino) singlet state of N2 are dominant in developing low temperature plasmas in air. In addition to c′4 transitions, photoionization capable transitions from atomic and singly ionized atomic oxygen were also observed. The inclusion of c′4 1∑+ u transitions into a statistical photoionization model coupled with a fluid model enabled streamer growth in the simulation of positive streamers.