High-temperature Hypersonic Laval Nozzle For Non-lte Cavity Ringdown Spectroscopy

The SMAUG apparatus (Spectroscopy of Molecules Accelerated in Uniform Gas flows) was developed to produce hot spectra of various molecules of interest for hot astrophysical atmospheres, like the one surrounding hot Jupiters, reaching up to 2500 K. High-temperature IR spectroscopic data is needed to retrieve temperature and concentration profiles from astronomical spectra. A small dimension Laval nozzle connected to a compact high enthalpy source equipped with cavity ringdown spectroscopy (CRDS) is used to produce high-resolution infrared spectra of polyatomic molecules in the 1.67~$\mu$m region.$^{a}$. The experimental setup can operate according to two complementary working regimes: non-local thermodynamic equilibrium (non-LTE) (vibrationally hot and rotationally cold) and LTE conditions, to interpret the complex pattern of highly-excited vibrational states. Two different gases, carbon monoxide (CO) and methane (CH$_{4}$) were used as test molecules. Using non-LTE conditions vibrational (T$_{vib}$) and rotational (T$_{rot}$) temperatures were extracted from the recorded infrared spectrum leading to T$_{vib}$~=~1346(52)~K and T$_{rot}$~=~12(1)~K for CO. A rotational temperature of 39(3)~K was measured for CH4, while two vibrational temperatures were necessary to reproduce the observed intensities. The population distribution between vibrational polyads was correctly described with T$_{vib,I}$~=~825(50)~K" , while the population distribution within a given polyad was modelled correctly by T$_{vib,II}$~=~39(5)~K" , testifying to a more rapid vibrational relaxation between the vibrational energy levels constituting a polyad. Using a “post-shock CRDS” technique CO and CH$_{4}$ LTE spectra were recorded at 950 K and 1400 K.\\ $^{a}$\textit{E.Dud$\acute{a}$s \emph{et al.} High-temperature hypersonic Laval nozzle for non-LTE cavity ringdown spectroscopy. J Chem Phys 2020;152:134201. https://doi.org/10.1063/5.0003886.