Time-Selective TALIF Spectroscopy of Atomic Oxygen Applied to an Atmospheric Pressure Argon Plasma Jet

Abstract

Two-photon laser excitation diagnostics is applied to characterize the atomic oxygen production in an atmospheric pressure Ar+O2% plasma jet. The plasma jet device is driven by a 13.56-MHz radio-frequency power supply. It is found that in addition to the fast decaying two-photon absorption laser-induced fluorescence (TALIF), unexpected emissions from excited atomic argon and oxygen species with a long decay time are existent in TALIF processes. This phenomenon is believed to be common in TALIF diagnostics of atomic oxygen, especially in high-pressure plasmas because of intense collisional energy transfer processes. In order to avoid the influence of extra emissions on TALIF performance, time-selective detection is utilized to capture the pure atomic oxygen fluorescence through nanosecond imaging for characterizing the spatial distribution of O atoms in the plasma effluent. An approximate method for the absolute density calibration based on the atomic oxygen fluorescence in the open air, due to the laser-induced photodissociation of O2 molecules, is applied to the TALIF signal from a plasma jet. The 2-D spatial distributions of the ground state density of atomic oxygen are built in the plasma effluent under different O2 contents. With the increase in O2 admixtures, the generation of atomic oxygen is constricted to the plasma core area close to the gas outlet, and a maximum O density is obtained with 0.3% O2 in the core area.