Mainz 2022 – scientific programme

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P: Fachverband Plasmaphysik

P 2: Low Pressure Plasmas I

P 2.2: Talk

Monday, March 28, 2022, 14:15–14:30, P-H11

High-resolution terahertz spectroscopy with quantum-cascade lasers for atomic oxygen density measurements — •Jente Wubs, Uwe Macherius, Klaus-Dieter Weltmann, and Jean-Pierre van Helden — Leibniz Institute for Plasma Science and Technology (INP), Felix-Hausdorff-Straße 2, 17489 Greifswald, Germany

Absorption spectroscopy in the terahertz (THz) spectral region between 0.3 and 6 THz provides access to energies that correspond to molecular and atomic transitions involving rotational mode changes and fine structure splitting, respectively. In addition, THz frequencies lie well above typical values for the plasma frequency, allowing the electron density to be determined from a phase shift of transmitted THz radiation. Knowledge about the electron density as well as atomic and molecular densities in plasmas is highly relevant for technological and scientific applications. A promising diagnostic for measuring these densities is THz time-domain spectroscopy. It is based on the ultrafast generation and detection of broadband THz pulses; however, the spectral resolution is limited to approximately 1 GHz. THz quantum-cascade lasers (QCLs) operating in continuous-wave mode are therefore better suited for the detection of sharp absorption lines. Although these lasers have a relatively small tuning range, their narrow linewidth (below 10 MHz) makes them an excellent THz source for high-resolution spectroscopy. In this contribution, first results are presented on the absolute density of ground state oxygen atoms, measured in a low-pressure RF plasma using a QCL operating at 4.75 THz.