Detection of negative ions in streamer discharge in air by transient cavity ringdown spectroscopy

Atmospheric-pressure discharges generated in air are expected to be electronegative, but experiments that examine negative ion densities are limited to date. In this work, we measured the temporal variation of the negative ion density in a streamer discharge generated in air. We adopted cavity ringdown spectroscopy, where negative ions were detected via weak optical absorption caused by laser photodetachment. The temporal variation of the absolute negative ion density was deduced by the transient analysis of the ringdown curve. Negative ions were detected after the disappearance of the discharge voltage and current. The negative ion density started the increase at 0.4 μs after the initiation of the discharge. The increase means the enhancement of the electron attachment frequency in the late phase of the secondary streamer with electron cooling. The survival of electrons until 0.4 μs is understood by the steep decrease in the cross section of dissociative recombination with the electron energy. The maximum negative ion density was observed at 1 μs, and it was around the noise level at 1.5 μs. The rapid decay is consistent with the destruction of negative ions by mutual neutralization with positive ions.