Robert Georges

This study presents a high-resolution rovibrational investigation of 1,4-dioxane in the mid-infrared region, motivated by the need for accurate spectroscopic data in the context of atmospheric monitoring of this emerging pollutant. Jet-cooled spectra were recorded using two complementary setups: a continuous supersonic jet coupled to the Fourier transform spectrometer of the AILES beamline at the SOLEIL synchrotron facility (Jet-AILES) and a quantum cascade laser spectrometer coupled to a pulsed jet (SPIRALES). These measurements were combined with long-path cell spectra at room temperature and supported by anharmonic quantum chemical calculations. Four fundamental vibrational bands (ν34, ν33, ν16, and ν15) were analyzed at high resolution, leading to the assignment of more than 6100 rovibrational transitions. A global fit yielded accurate ground- and excited-state rotational and centrifugal distortion parameters. The results significantly improve the spectroscopic characterization of 1,4-dioxane and provide reliable line lists for future atmospheric detection in the mid-infrared range.

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.

Context . Polycyclic aromatic hydrocarbons (PAHs) are considered ubiquitous in the interstellar medium. There is now solid evidence for their presence. The mechanisms that lead to their formation and their destruction, remain debated, however. Of the processes that drive their evolution, the shock-induced alteration of PAHs has received little attention. Aims . Our objective is to explore the gaseous volatiles and solid residues generated by shock-processing of coronene C ₂₄ H ₁₂ , which is a prototypical compact PAH with seven aromatic rings. Methods . A pressure-driven shock tube was employed to sublimate and heat coronene up to 4000 K. The time evolution of the shock products was probed in situ by optical emission spectroscopy on a microsecond timescale. Solid residues were collected and analyzed ex situ by a variety of methods, including infrared microspectroscopy, Raman spectroscopy, X-ray diffraction, transmission electron microscopy, and laser desorption laser ionization mass spectrometry. The experiments were supported by molecular dynamics (MD) simulations. Results . The experiments revealed a dominant dehydrogenation pathway for coronene under shock conditions. In situ spectroscopy confirmed the presence of C ₂ radicals and a broad continuum emission attributed to large carbon clusters (C _n ) and small, weakly hydro-genated hydrocarbons (C _n H _x ). The ex situ analysis of solid residues indicates the formation of graphitic and graphenic nanostructures, including carbon nano-onions, nanotubes, and nanoribbons in the cooling phase. Laser desorption laser ionization mass spectrometry analysis validates the carbonization of the shock products, while MD simulations support the dehydrogenation and fragmentation processes of the rapid-heating phase. Conclusions . Shock waves drive the transformation of PAHs into small hydrocarbons and carbon clusters that recombine in the cooling phase into graphene-like structures. This affects the carbon life cycle of the interstellar medium. The identification of C _n H _x species as potential carriers of the broad green emission seen in the laboratory suggests a possible link to the λ 5450 diffuse interstellar band. This study underscores the value of shock tubes as a tool for simulating astrophysical environments and investigating the chemical evolution of large molecules and small particles in space.

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.

In the continuity of a previous jet-cooled rovibrational study of trans and cis conformers of 2-furfural in the mid-infrared region (700–1750 cm−1) [Chawananon et al., Molecules 28 (10), 4165 (2023)], the present work investigates the far-infrared spectroscopy of 2-furfural using a long path absorption cell coupled to a high-resolution Fourier transform spectrometer and synchrotron radiation at the AILES beamline of the SOLEIL synchrotron. Guided by anharmonic calculations, vibrational energy levels and excited-state rotational constants are sufficiently predictive for a complete assignment of all fundamental and combination bands up to 700 cm−1, as well as the rovibrational analysis of 4 (1) low-frequency modes of trans-(cis-)2-furfural. A global rovibrational simulation, including far-infrared rovibrational lines and microwave and millimeter-wave rotational lines assigned in a previous study [Motiyenko et al., J. Mol. Spectrosc., 244, 9 (2007)] provides a reliable set of ground- and excited-state rotational parameters involving ring torsion, bending, and ring puckering modes of 2-furfural. In a second step, a rovibrational analysis of several hot band sequences, mainly involving the lowest frequency ring CHO torsion mode, is carried out. Reliable values of some anharmonic coefficients are obtained experimentally and could serve as a benchmark for validating advanced anharmonic calculations related to these large amplitude motions of flexible molecules.

Cork popping represents a rich and complex fluid dynamics process, involving up to three phases (liquid, gas, and solid), three main chemical compounds (ethanol, water, CO2), and a moving cork gradually opening the bottle and blocking the fast progression of the expanding gas. In this work, we used high-speed, high-sensitivity schlieren imaging setups to provide a fresh perspective on this dynamical phenomenon. Our experimental results are systematically compared and interpreted on the basis of new computational fluid dynamics simulations. Our combined experimental and numerical works confirm the establishment of two supersonic expansions during cork popping from a champagne bottle.

Fullerene C₆₀ is one of the most iconic forms of carbon found in the interstellar medium (ISM). The interstellar chemistry of carbon-rich components, including fullerenes, is driven by a variety of energetic processes including UV and X-ray irradiation, cosmic-ray (CR) bombardment, electron impact, and shock waves. These violent events strongly alter the particle phase and lead to the release of new molecular species in the gas phase. Only a few experimental studies on the shock processing of cosmic analogs have been conducted so far. We explored in the laboratory the destruction of buckminsterfullerene C₆₀ using a pressure-driven shock tube coupled with optical diagnostics. Our efforts were first devoted to probing in situ the shock-induced processing of C₆₀ at high temperatures (≤ 4500 K) by optical emission spectroscopy. The analysis of the spectra points to the massive production of C₂ units. A broad underlying continuum was observed as well and was attributed to the collective visible emission of carbon clusters, generated similarly in large amounts. This proposed assignment was performed with the help of calculated emission spectra of various carbon clusters. The competition between dissociation and radiative relaxation, determined by statistical analysis, alludes to a predominance of clusters with less than 40 carbon atoms. Our laboratory experiments, supported by molecular dynamics simulations performed in the canonical ensemble, suggest that C₆₀ is very stable, and that high-energy input is required to process it under interstellar low-density conditions and to produce C₂ units and an abundance of intermediate-sized carbon clusters. These results provide some insights into the life cycle of carbon in space. Our findings hint that only J-type shocks with velocities above ~100 km s⁻¹ or C-type shocks with velocities above 9 km s⁻¹ can lead to the destruction of fullerenes. Observational tracers of this process remain elusive, however. Our work confirms the potential of shock tubes for laboratory astrophysics.