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Description
Two-photon induced polarization spectroscopy (TIPS) is a laser diagnostic technique that relies on the simultaneous absorption of two photons, one being contributed by a strong, circularly polarized pump beam and the other by a weaker, linearly polarized probe beam. The two beams are crossed at a slight angle thus defining the measurement volume. Through consideration of the two-photon selection rules while choosing a suitable two-photon absorption transition to probe, it can be assured that the absorption of one photon from each beam induces a polarization rotation within the probe beam. The induced polarization rotation can be detected behind an analyzing polarizer which is a linear polarizer in crossed alignment with the linear polarization of the probe beam.
TIPS has been used for detection of various molecules as well as of atomic hydrogen and xenon in the past. The study presented in this paper will feature some of the very first measurements of atomic oxygen ever performed using TIPS. The laser diagnostic setup has been developed in the High Enthalpy Flow Diagnostics Group (HEFDiG) at the Institute of Space Systems (IRS). The microwave-powered plasma source has been developed by the Institute of Interfacial Process Engineering and Plasma Technology (IGVP).
The TIPS signal is created in the moment of absorption thus rendering the technique immune to quenching effects and allowing for measurements at high pressures where fluorescence-based techniques would struggle or not work at all. This paper will feature measurements of atomic oxygen performed in an O$_2$-plasma created at atmospheric pressure in a microwave-powered burner.
The absolute number density calibration for measurements of atomic oxygen in the O$_2$ plasma is realized using xenon as a transfer species. This calibration approach originates with TIPS with atomic hydrogen and has since become a popular tool for calibration of laser-induced fluorescence measurements of atomic oxygen. A xenon cold gas cell is positioned at the measurement location and allows for TIPS measurements with controllable number densities of xenon. Since the ratio of the two-photon absorption cross-sections for the oxygen and the xenon transition is known, absolute number densities can be determined.
In the final paper, TIPS measurements of atomic oxygen in ambient pressure O$_2$ plasma will be presented along with TIPS measurements of xenon for absolute number density calibration. A detailed discussion of the absolute number density calibration method for TIPS with atomic oxygen will be presented along with different approaches through accurate lineshape modelling.
Summary
This paper will present the current advancements in measurements of absolute atomic oxygen number densities. So far, two-photon induced polarization spectroscopy has been applied to atmospheric pressure plasma conditions. This is the first time that this technique has been used successfully for quantitative measurements of atomic oxygen. The paper will contain a detailed discussion on the calibration approach through lineshape modelling while using xenon as a reference species.
