Gas flow dependence of ground state atomic oxygen in plasma needle discharge at atmospheric pressure

We present clear evidence that ground state atomic oxygen shows two patterns near a surface in the helium plasma needle discharge. Two-photon absorption laser-induced fluorescence spectroscopy, combined with gas flow simulation, was employed to obtain spatially-resolved ground state atomic oxygen densities. When the feed gas flow rate is low, the radial density peaks along the axis of the needle. At high flow rate, a ring-shaped density distribution appears. The peak density is on the order of 1021 m-3 in both cases. The results are consistent with a previous report of the flow-dependent bacterial killing pattern observed under similar conditions.