Femtosecond Two-Photon Absorption Laser Induced Fluorescence (fs-TALIF) Imaging of Atomic Nitrogen in Nanosecond Repetitive Discharges

Abstract

Presented herein is a feasibility study for absolute measurements of ground state atomic nitrogen inside of a nanosecond repetitive pulse (NRP) discharge using a femtosecond two-photon absorption laser induced fluorescence (TALIF) technique. A two-photon absorption scheme (λ = ×206.6 nm) is used to probe the ground state population of atomic nitrogen (2p 3 4 S) inside the discharge. The scheme involves the excitation of the 3p 4 S3/2 level with the subsequent fluorescence collection taking place from the radiative decay to the 3s 4 P triplet state around 745 nm. The main challenge is to demonstrate the ability to perform the measurements at above atmospheric conditions where the typical ns-TALIF diagnostics is unable to provide information due to the high rate of collisional quenching. As such, the current investigation focuses on high-pressure discharges in pure nitrogen. We demonstrate here the ability to measure fs-TALIF signals from atomic nitrogen in NRP discharges operated at pressures between 0.1-5 bar. Additionally, we perform fs-TALIF measurements in Krypton and we identify the main fluorescence channels using the λ = ×204.13 nm excitation scheme. We report that significant saturation effects was observed in both mixtures due to the presence of photoionization which resulted in a departure from the I 2 dependency of the TALIF intensity.