Studying species distribution in laser-induced plasma by molecular and atomic fluorescence

Abstract

The spatial distribution profiles of particles in plasma sources push forward our understanding of plasma evolution and physicochemical processes occuring inside. Optical probe methods, such as laser-induced fluorescence, are advanced tools for spatially resolved plasma studies. In our work, we focused on investigation of distribution of neutral Ca and Fe atoms and CaO and FeO molecules in laser-induced plasma by means of laser-induced fluorescence. The development of excitation-emission schemes for Fe and FeO and the practical implementation of schemes for Ca and CaO allowed us to construct distribution maps of these species in laser plasma at 10 and 100 Torr pressures. Both atomic and molecular fluorescence was observed much further from the plasma formation point than the region of bright spontaneous atomic emission. Additionally, by comparing fluorescence intensity distributions with plasma imaging data, we explain the origin of some pecularities in observable plasma inhomogeneity.Distributions of Ca and CaO fluorescence intensity, as well the distribution of CaO/Ca intensity ratio, demostrate that the monoxide is distributed within the plume by the shock wave, but its concentration in the outer layers of plasma is influenced by recombination with atmospheric oxygen.