OH Density Distribution Measurements in Transient Surface Discharges by High Resolution Laser-Induced Fluorescence

A high spatial resolution is crucial to investigate species gradients near substrates and in a plasma sheath at atmospheric pressure. This is particularly the case for ubiquitous surface discharges. In this contribution, we report on the measurements of OH radical density distributions generated by a pulsed surface discharge with a spatial resolution of 10 μm in the direction perpendicular to the substrate. The surface discharge is generated upon the impingement of a plasma jet onto a quartz substrate operating in humid He in an ambient air environment. The OH density was obtained by Rayleigh calibration in air utilizing a 4-level LIF model which considered both radiative and collisional transport processes. The spatial distribution of the air and water vapor fraction was obtained by spatially resolved measurements of the OH(A) lifetime. These measurements enable the visualization of strong gradients in the OH radical density on lengths scales of 10s of micrometer. From these data, both the diffusive and kinetic flux to the substrate can be deduced, enabling the determination of the surface loss probability of the quartz substrate equal to 0.01 under atmospheric pressure plasma conditions for the first time.