Two-photon absorption laser induced fluorescence (TALIF) detection of atomic iodine in low-temperature plasmas and a revision of the energy levels of I I

Abstract

In order to perfect the use of iodine, instead of xenon, in plasma thrusters, an experiment has been set up to investigate the feasibility of two-photon absorption laser induced fluorescence (TALIF) diagnostics in iodine plasmas. Levels (3P2)6p 2[1]o3⁄2 and (3P2)6p 2[3]o7⁄2 of atomic iodine were found qualified for that purpose, with a radiative lifetime of the former found equal to 35.5(9) ns, which appears consistent with the 32.5(1.2) ns predicted by the most recent calculations. Doppler-free two-photon spectroscopy has confirmed that the relative intensities of the hyperfine components follow well-known general formulas, which makes it possible to disentangle the hyperfine structure of 127I from Doppler broadening. Accurate temperature measurements are shown to be possible, even though the relatively heavy mass of the iodine atom does not make it the most favorable gauge for measurements based on Doppler e ect. Using an injection seeded pulsed laser as the primary laser source, the frequency-tripling of which has provided the ca 300 nm radiation used for excitation, absolute energy measurements of the two-photon excited levels revealed that all upper energy levels of I I have to be revised down, by -0.169(11)cm-1, which pulls every energy level off its presently tabulated value by more than thirty times the currently admitted uncertainty bar.

Measurement of the main neutral species densities and temperatures in iodine plasmas using optical absorption techniques

“…The main heating process is due to the energy gained by the atoms in the dissociation process. This has already been observed for Cl 2 (Cunge et al 2009) and on the same experimental set-up, in a recent article where the atomic temperature was measured by TALIF diagnostics (Esteves et al 2023). Remarkably, the temperatures measured by TALIF are always higher than the ones measured by absorption.…”

“…This suggests that quenching (Chichinin 2006) of I( 2 P • 1/2 ) states by molecular and/or atomic collisions is a dominant process in the experimental conditions of the present work. This observation is supported by the fact that no I( 2 P • 1/2 ) level population was ever detected using the TALIF method applied to atomic iodine, with the same plasma setup (Esteves et al 2023).…”

“…More recently, the ratio of negative ion, I − , to electron density was measured using laser-induced detachment in a low-pressure inductivelycoupled plasma chamber (Esteves et al 2022). Even more recently, a high-spectral resolution TALIF method was proposed to measure the temperature of the atoms and their relative ground-level density in a plasma generated by capacitive coupling in an iodine cell (Esteves et al 2023).…”