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

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).
• 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.