Time-Resolved Imaging for Detecting Reactive Species at the Plasma-Liquid Interface*

When attempting to measure the density of atomic species near a liquid surface by utilizing femtosecond two-photon laser-induced fluorescence (fs-TALIF), a challenge arises when trying to measure the atomic emission lifetime due to the spatial variations in quenching and reflections caused by the gas-liquid interface. By collecting a series of images with varying times, and fitting the decay of each pixel intensity with an equation that is a sum of two distinct decay functions, we can separate the images stemming from each phase (gas or liquid), even when there is a spatial overlap of the two signals. The plasma source used in the experiment was the Cooperation on Science and Technology (COST) Reference Microplasma Jet (COST-jet) used with a helium and oxygen gas mixture. The images were collected using a gated intensifier with a gate width on the ns scale, which was then deconvoluted from the pixel intensities to measure sub ns lifetimes. By varying the fitting method and weighting the signal based on the uncertainty of the fit, distinct regions of different quenching regimes can be identified. Using such a method, we can estimate emission lifetimes around a plasma-liquid interface to allow imaging of the spatial distribution of reactive species solvated in the liquid.