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High-temperature hypersonic Laval nozzle for non-LTE cavity ringdown spectroscopy.

Eszter Dudás1, Nicolas Suas-David1, Shuvayan Brahmachary2

  • 1Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) - UMR 6251, F-35000 Rennes, France.

The Journal of Chemical Physics
|April 10, 2020
PubMed
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This study demonstrates a novel method for generating high-resolution infrared spectra of molecules using a Laval nozzle and cavity ringdown spectroscopy (CRDS). The technique achieves vibrationally hot and rotationally cold molecular states for detailed spectral analysis.

Area of Science:

  • Physical Chemistry
  • Spectroscopy
  • Fluid Dynamics

Background:

  • High-resolution molecular spectroscopy requires precise control over molecular internal states.
  • Cavity ringdown spectroscopy (CRDS) offers high sensitivity for detecting molecular species.
  • Generating vibrationally hot and rotationally cold molecules is challenging but crucial for detailed spectral analysis.

Purpose of the Study:

  • To develop and characterize a system for producing vibrationally hot and rotationally cold molecular gases.
  • To record high-resolution infrared spectra of polyatomic molecules (CO and CH4) in the 1.67 µm region.
  • To investigate the thermodynamic conditions (vibrational and rotational temperatures) within the generated flow.

Main Methods:

  • Utilized a small-dimension Laval nozzle machined from isostatic graphite, capable of withstanding high stagnation temperatures (up to 2000 K).

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  • Integrated the Laval nozzle with a compact high enthalpy source and cavity ringdown spectroscopy (CRDS) for spectral acquisition.
  • Employed computational fluid dynamics (CFD) simulations, Pitot measurements, and CRDS to characterize the hypersonic flow (Mach 10-18.3).

    • Main Results:

    • Successfully produced vibrationally hot (Tvib ≈ 1346 K for CO) and rotationally cold (Trot ≈ 12 K for CO) molecular gases.
    • Recorded high-resolution infrared spectra for carbon monoxide (CO) and methane (CH4) in the 1.67 µm region.
    • Determined distinct vibrational temperatures for methane, indicating rapid vibrational relaxation within specific polyads (TvibII ≈ 54 K).

    Conclusions:

    • The developed Laval nozzle and CRDS system effectively generates molecular gases with controlled vibrational and rotational states.
    • The technique enables high-resolution infrared spectral analysis of molecules under unique thermodynamic conditions.
    • Observed multi-temperature vibrational distributions highlight complex relaxation dynamics in hypersonic flows.