The structure of carbon dioxide aggregates is investigated by means of direct absorption IR specroscopy in the region of the antisymmetric stretching vibration v₃. The (CO₂)_N particles are generated under dynamic (supersonic cooling in Laval nozzles) and static (collisional cooling cells) conditions over a broad mean size range (20 < < 10⁵). The vibrational exciton approach is used to interpret the observed spectral features. The particles generated by supersonic cooling remain globular in shape even for the largest explored aggregate sizes ( ≈ 10⁵), thus highlighting the absence of agglomeration between primary clusters under our jet conditions. This is in contrast to collisional cooling where the primary particles strongly agglomerate after a few seconds. The spectra for the larger particles ( > 10⁴) are well reproduced by the simulations if cuboctahedral or octahedral rather than spherical aggregate shapes are assumed.

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.

Survey jet-cooled spectra of acetic acid have been recorded in the infrared region (200–4000 cm⁻¹) over a wide range of expansion conditions. From the variations of the relative intensities of the signals, vibrational transitions have been assigned unambiguously to the trans-monomer and cyclic-dimer. The IR-active fundamental frequencies have been determined at the instrumental accuracy of 0.5 cm⁻¹. This analysis of the jet-cooled spectra supported by electronic structure calculations permitted us to characterize the trans-monomer/cyclic-dimer equilibrium. From static cell spectra at 298 K, variations of the molar fractions ratio as a function of the total pressure were used to estimate the equilibrium constant and the Gibbs free energy of dimerization at 298 K. The very good agreement with the literature data shows that the present method is able to produce, from a single study, a free energy value as reliable as the one obtained from a large collection of data. In addition, the semi-empirical free energy value was used to estimate the accuracy of electronic structure calculations and in turn the accuracy of the derived useful information such as the dissociation energy of the complex (i.e. the strength of the hydrogen bonds) or the relative energies within the conformational landscape.

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.

A small dimension Laval nozzle connected to a compact high enthalpy source equipped with cavity ringdown spectroscopy (CRDS) is used to produce vibrationally hot and rotationally cold high-resolution infrared spectra of polyatomic molecules in the 1.67 µm region. The Laval nozzle was machined in isostatic graphite, which is capable of withstanding high stagnation temperatures. It is characterized by a throat diameter of 2 mm and an exit diameter of 24 mm. It was designed to operate with argon heated up to 2000 K and to produce a quasi-unidirectional flow to reduce the Doppler effect responsible for line broadening. The hypersonic flow was characterized using computational fluid dynamics simulations, Pitot measurements, and CRDS. A Mach number evolving from 10 at the nozzle exit up to 18.3 before the occurrence of a first oblique shock wave was measured. Two different gases, carbon monoxide (CO) and methane (CH₄), were used as test molecules. 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 30 ± 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 ${\text{T}}_{\text{vib}}^{\text{I}}=894\pm 47\text{\hspace{0.17em}K}$ , while the population distribution within a given polyad (namely, the dyad or the pentad) was modeled correctly by ${\text{T}}_{\text{vib}}^{\text{II}}=54\pm 4\text{\hspace{0.17em}K}$ , testifying to a more rapid vibrational relaxation between the vibrational energy levels constituting a polyad.