Nicolas Suas-David

Hydrocarbons are partly responsible for the opacity of warm Jupiters’ atmospheres in the infrared. Laboratory high-resolution spectroscopic data, including hot band rovibrational transitions, are crucial to model and interpret telescope observations. In this work, a set of six hot bands and 11 cold bands of ethylene (12C2H4) is observed using cavity ringdown spectroscopy between 5880 and 6200 cm−1. The ethylene sample is preheated to 650 and 850 K before being expanded through a Laval nozzle to produce a high Mach number expansion. The rotational temperature drops to ∼12–13 K in the jet, while the vibrational population accumulates in the first excited vibrational state ν10, from which all the observed hot bands originate. The observed transitions are assigned using the lower state combination difference approach; a set of A, B, and C rotational constants, along with the energy of the upper state, is determined using PGOPHER software for the 17 observed vibrational bands. The TheoReTS (Theoretical Reims-Tomsk Spectral data) model, employed to identify the upper vibrational states, will benefit from these newly identified transitions.

Les télescopes terrestres et spatiaux recueillent une énorme quantité d'informations dans le domaine infrarouge en provenance d'objets astrophysiques ''chauds'' (500-3000 K) tels que les atmosphères d'exoplanètes (Jupiters chauds), de naines brunes et les enveloppes circumstellaires d'étoiles AGBs. Cette thèse s'inscrit dans une approche d'astrophysique de laboratoire s'attachant à reproduire in situ certains aspects des conditions extrêmes rencontrées au sein de ce type d'environnements afin notamment de produire des données haute température de molécules clefs. Le nouveau dispositif mis en place à Rennes couple une Source Haute Enthalpie à un spectromètre par temps de déclin d'une cavité optique (CRDS). Le gaz étudié, chauffé dans le réservoir à une température avoisinant 2000 K, est expulsé dans une chambre basse pression au travers d'un injecteur circulaire. Le jet libre hypersonique ainsi formé est sondé en tout point et avec une très haute sensibilité. La simulation de nos écoulements stationnaires (CFD) associée à la modélisation du spectromètre a abouti à des spectres synthétiques en très bon accord avec les spectres expérimentaux. Ces données numériques ont été utilisées pour expliquer l'origine des profils de raie atypiques et plus généralement pour comprendre la structure des jets hypersoniques axisymétriques. Le cœur isentropique de ces écoulements est caractérisé par de fortes conditions hors équilibre. Une température de vibration très élevée (1350 K) et une température de rotation très basse (10 K) ont été obtenues à partir d’un jet de CO et d’argon. Ce découplage des degrés de liberté internes permet de simplifier la structure rotationnelle des spectres enregistrés et facilite l’étude des états vibrationnels excités des molécules en révélant la structure des bandes chaudes, absentes des bases de données spectroscopiques pour la plupart des molécules polyatomiques. Une approche complémentaire consiste à sonder la couche de choc produite par l'ajout d'un obstacle sur le trajet de l'écoulement. La température rotationnelle est brutalement élevée donnant ainsi accès aux transitions rotationnelles de hautes valeurs du nombre quantique J. Ces deux méthodes ont été appliquées avec succès au méthane qui joue un rôle majeur dans de nombreux environnements astrophysiques chauds. Enfin, outre la production de données spectroscopiques, ce dispositif expérimental a permis de mettre en évidence la relaxation des degrés de liberté internes du CO dans différents gaz porteurs (He et Ar) en suivant l’évolution des températures de rotation et vibration le long de l'écoulement hypersonique, aussi bien dans le cœur isentropique qu'au sein des couches limites. Ces températures sont comparées aux températures d'excitation obtenues par des méthodes ab initio afin de valider des calculs de taux de collision. Les données obtenues alimenteront à terme des bases de données, matière première au développement de codes de transfert radiatif permettant d'interpréter les observations en provenance des milieux astrophysiques ''chauds''.

We report high-resolution, cavity-enhanced direct frequency comb Fourier transform spectroscopy of cold acetylene (C₂H₂) molecules in a planar supersonic jet expansion. The experiment is based on a near-infrared frequency comb with a 300 MHz effective repetition rate, matched to a high-finesse enhancement cavity traversing the jet. The rotational and translational cooling of acetylene was achieved via expansion in argon carrier gas through a slit nozzle. By interleaving successive mode-resolved spectra measured at different comb repetition rates, we retrieved full absorption line profiles. Spectroscopic analysis reveals sharp, Doppler-limited transitions corresponding to a jet core rotational temperature below 7 K. Frequency comb and cavity stabilization were achieved through active Pound–Drever–Hall locking and mechanical vibration damping, enabling a spectral precision better than 2 MHz, limited by the vibrations induced by the pumping system. The demonstrated sensitivity reaches a minimum detectable absorption of 7.8 × 10⁻⁷ cm⁻¹ over an 18 m effective path length in the jet core. This work illustrates the potential of cavity-enhanced direct frequency comb spectroscopy for precise spectroscopic characterization of cold supersonic expansions, with implications for studies in molecular dynamics, reaction kinetics, and laboratory astrophysics.

Photoacoustic spectroscopy (PAS) is a sensitive broadband absorption technique relying on simple and inexpensive components. Despite this, PAS has never been applied in a cold supersonic environment. The reason for this is clear: how can one detect an acoustic signal under supersonic conditions? The flow provided by a Laval nozzle expansion offers a solution: such flows are thermal collisional environments propagating much faster than the local speed of sound, but inside which one may produce a photoacoustic signal by suitably chopped laser excitation of molecules in the flow. Downstream, a microphone can respond to the pressure oscillations induced by the laser excitation as the flow sweeps by after the absorbed radiation has been converted to translational energy of the molecules. Such a strategy offers a near-universal approach to spectroscopy and low-temperature kinetics in uniform flows. Here we show preliminary results demonstrating the feasibility of this technique and discuss future prospects.

Pulsed discharge nozzles (PDNs) have been successfully used for decades to produce rotationally cold (Trot ∼ 20 K) radicals and ions of astrophysical interest and to characterize these species spectroscopically. In this work, an evolution of the PDN, the piezostack pulsed discharge nozzle (P2DN), is used for the first time to investigate the characteristics of the still poorly understood supersonic plasma expansion. The P2DN allows for a better control of the reservoir pressure of which an accurate measurement is required to characterize the plasma expansion. This new source, thus, gives the opportunity to further optimize the plasma conditions and extend its use to new target species. The spatial distribution of an argon plasma and the effect of the supersonic flow for different pressures are studied by combining a two-dimensional extended fluid model (extFM) and a direct simulation Monte Carlo (DSMC) method. The combined simulation is validated with experimental results obtained through emission spectroscopy associated with a group-code collisional-radiative model to retrieve the plasma parameters. The validated numerical approach (DSMC-extFM) allows for an accurate characterization of the plasma structure in our typical experimental conditions (a reservoir pressure ranging from 90 to 905 mbar). Thus, this simulation will be used in future studies to improve the plasma conditions to favor the synthesis of (transient) hydrocarbon species as found in space, by seeding the argon gas with a suitable precursor, such as acetylene.

Chirped-Pulse Fourier-Transform millimeter wave (CP-FTmmW) spectroscopy is a powerful method that enables detection of quantum state specific reactants and products in mixtures. We have successfully coupled this technique with a pulsed uniform Laval flow system to study photodissociation and reactions at low temperature, which we refer to as CPUF (“Chirped-Pulse/Uniform flow”). Detection by CPUF requires monitoring the free induction decay (FID) of the rotational coherence. However, the high collision frequency in high-density uniform supersonic flows can interfere with the FID and attenuate the signal. One way to overcome this is to sample the flow, but this can cause interference from shocks in the sampling region. This led us to develop an extended Laval nozzle that creates a uniform flow within the nozzle itself, after which the gas undergoes a shock-free secondary expansion to cold, low pressure conditions ideal for CP-FTmmW detection. Impact pressure measurements, commonly used to characterize Laval flows, cannot be used to monitor the flow within the nozzle. Therefore, we implemented a REMPI (resonance-enhanced multiphoton ionization) detection scheme that allows the interrogation of the conditions of the flow directly inside the extended nozzle, confirming the fluid dynamics simulations of the flow environment. We describe the development of the new 20 K extended flow, along with its characterization using REMPI and computational fluid dynamics. Finally, we demonstrate its application to the first low temperature measurement of the reaction kinetics of HCO with O2 and obtain a rate coefficient at 20 K of 6.66 ± 0.47 × 10−11 cm3 molec−1 s−1.

Context. Interstellar carbonaceous particles and molecules are subject to intense shocks in astrophysical environments. Shocks induce a rapid raise in temperature and density which strongly affects the chemical and physical properties of both the gas and solid phases of the interstellar matter. Aims. The shock-induced thermal processing of C₆₀ particles in hydrogen has been investigated in the laboratory under controlled conditions up to 3900 K with the help of a material shock-tube. Methods. The solid residues generated by the exposure of a C₆₀/H₂ mixture to a millisecond shock wave were collected and analyzed using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman micro-spectroscopy, and infrared micro-spectroscopy. The gaseous products were analyzed by Gas Chromatography and Cavity Ring Down Spectroscopy. Results. Volatile end-products appear above reflected shock gas temperatures of ~2540 K and reveal the substantial presence of small molecules with one or two C atoms. These observations confirm the role played by the C₂ radical as a major product of C₆₀ fragmentation and less expectedly highlight the existence of a single C atom loss channel. Molecules with more than two carbon atoms are not observed in the post-shock gas. The analysis of the solid component shows that C₆₀ particles are rapidly converted into amorphous carbon with a number of aliphatic bridges. Conclusions. The absence of aromatic CH stretches on the IR spectra indicates that H atoms do not link directly to aromatic cycles. The fast thermal processing of C₆₀ in H₂ over the 800–3400 K temperature range leads to amorphous carbon. The analysis hints at a collapse of the cage with the formation of a few aliphatic connections. A low amount of hydrogen is incorporated into the carbon material. This work extends the range of applications of shock tubes to studies of astrophysical interest.

A new experimental setup coupling a High Enthalpy Source (HES) reaching 2000 K to a cw-cavity ring-down spectrometer has been developed to investigate rotationally cold hot bands of polyatomic molecules in the [1.5, 1.7] μm region. The rotational and vibrational molecular degrees of freedom are strongly decoupled in the hypersonic expansion produced by the HES and probed by cavity ring-down spectroscopy. Carbon monoxide has been used as a first test molecule to validate the experimental approach. Its expansion in argon led to rotational and vibrational temperatures of 6.7 ± 0.8 K and 2006 ± 476 K, respectively. The tetradecad polyad of methane (1.67 μm) was investigated under similar conditions leading to rotational and vibrational temperatures of 13 ± 5 K and 750 ± 100 K, respectively. The rotationally cold structure of the spectra reveals many hot bands involving highly excited vibrational states of methane.