18th, Laser Aided Plasma Diagnostics

High resolution Two-photon Absorption Laser-Induced Fluorescence (TALIF) with a single-mode nanosecond Ti:Sapphire laser

Sep 28, 2017, 10:55 AM

15m

ORAL Low-temperature plasmas Oral

Speaker

Dr Cyril Drag (Laboratoire Aimé-Cotton)

Description

Parameters of laboratory discharges in molecular gases, as the gas translational temperature, the collisions rates and the density, are difficult to determine. We measure, with excellent spatial and temporal resolution, the velocity distribution of oxygen ground-state atoms in plasmas from the Doppler broadening of their laser excitation spectra. The method is based on the well-known Two-Photon Absorption Laser-induced Fluorescence (TALIF) technique. We have developed a pulsed tunable ultraviolet laser with very narrow bandwidth which allows the Doppler profiles to be measured with high precision. This laser consists of a pulsed Nd:YAG-pumped Ti:Sapphire ring cavity that is injection-seeded by a singlemode CW Diode laser. The single-mode infrared output pulses are frequency quadrupled by two non-linear crystals to reach the necessary UV wavelength (226 nm, 0.2 mJ) for TALIF excitation. Measuring the Doppler profile in different experimental conditions, we have determined the temperature of oxygen atoms with an uncertainty of only +/- 10K in a simple DC glow discharge [1]. By reflecting the laser beam upon itself with a flat mirror, we could make Doppler-free TALIF spectra, allowing observation of the pressure-broadening effects [2]. Accurate measurement of the injection-seeding wavelength provides new data on the excitation energy of the fine structure of the O 3p 3P level. We have also observed the isotope shift of the 2p4 3P2 → 3p 3P2 transition making the experiment on 16O and 18O isotopes [3]. We will discuss the possibility of the determination of the quenching rates by the analysis of the TALIF signal. Nevertheless, we will show how the Amplified Spontaneous Emission (ASE) can modify the decay rate of the fluorescence. Finally, we will discuss different possibilities to measure the density of Oxygen atoms in the plasma. [1] J-P. Booth, D. Marinov, M. Foucher, O. Guaitella, D. Bresteau, L. Cabaret and C. Drag, “Gas temperature measurements in oxygen plasmas by high-resolution Two-Photon Absorption Laser-induced Fluorescence”, *J. of Instrum.* **10**, C11003 (2015) [2] D. Marinov, C. Drag, C. Blondel, O. Guaitella, J. Golda, B. Klarenaar, R. Engeln, V. Schulz-von der Gathen and J.-P. Booth, “Pressure broadening of atomic oxygen two-photon absorption laser induced fluorescence”, *Plasma Sources Sci. Technol.* **25**, 06LT03 (2016) [3] D. Marinov, J.-P. Booth, C. Drag and C. Blondel, “Measurement of the isotope shift of the 2p4 3P2 -> 2p33p 3P2 two-photon transition of O I and a revision of the triplet energy levels of atomic Oxygen”, *J. Phys. B: At. Mol. Opt. Phys.* **50**, 065003 (2017)