Temporally and spatially resolved continuum radiation between 600 and 1000 nm from nanosecond discharge in water: implications for understanding the initiation mystery

Emission spectra of a nanosecond discharge initiated in liquid water provide an important tool for investigating the driving mechanisms and fundamental properties of in-liquid plasmas. In this work, we report for the first time on optical emission characteristics of the expanding discharge in deionized water resolved both in space and time in the near-infrared spectral range up to 1050 nm. We systematically examined ICCD images with associated emission spectra at fixed discharge conditions. The images reveal the morphology and dynamics of expanding discharge, while ICCD spectra obtained as a function of distance from the anode apex prove the initial emission characteristics based on structureless continua originating from bulk water without any distinguishable contribution coming from the tungsten anode surface. Furthermore, based on the comparison of 2D maps of emission spectra with images registered using a four-channel ICCD imager, we managed to connect the morphology of the luminous discharge phase with the specific characteristics of the plasma-induced emission in the vis-NIR region. We reveal that the initial diffuse morphology is associated with weak broadband emission continua, while the subsequent filamentary morphology shows much more intense spectra on time scales of tens of nanoseconds, consisting of significantly broadened Hα and several OI atomic lines superimposed on the broadband continuum. Preliminary analysis of line profiles yields corresponding electron densities between 10¹⁸ and 10²⁰ cm⁻³. All these findings provide important insight for understanding the mechanisms of direct nanosecond high-voltage discharge in liquid water.