Résumé: Five rovibrational bands of the Kr–H2O complex have been recorded in the ν2 bending region of H2O using a quantum cascade laser coupled to a pulsed slit supersonic jet. Four of them have been unambiguously assigned to the Σ and Π states in v2=1,111,110, and 212 rotational levels of the monomer based on ground state combination differences and similarities with the vibration–rotation tunneling states of Ar–H2O. One of the bands is tentatively assigned to a combination band with two quanta of intermolecular van der Waals stretching mode (νs). Due to the efficient rovibrational cooling in our pulsed supersonic expansion, all observed bands originate from the lowest lying states. Four of them are from the lowest ortho ground state Σe(101) and one of them is from the lowest para ground state Σe(000). The jet-cooled spectra have been analyzed in terms of a nearly free internal rotor model taking into account Coriolis couplings between close lying Σ and Π levels. Molecular parameters for the five upper vibrational states, band origin, rotational and centrifugal distortion constants have been accurately determined. The β parameter describing the Coriolis coupling between the Σ and Π states originating from the (212) state has also been obtained.

Résumé: Catechol is an oxygenated aromatic volatile organic compound and a biogenic precursor of secondary organic aerosols. Monitoring this compound in the gas phase is desirable due to its appreciable reactivity with tropospheric ozone. From a molecular point of view, this molecule is attractive since the two adjacent hydroxy groups can interchangeably act as donor and acceptor in an intramolecular hydrogen bonding due to the tunnelling between two symmetrically equivalent structures. Using synchrotron radiation, we recorded a rotationally-resolved Fourier Transform far-infrared (IR) spectrum of the torsional modes of the free and bonded -OH groups forming the intramolecular hydrogen bond. Additionally, the room temperature, pure rotational spectrum was measured in the 70–220 GHz
frequency range using a millimeter-wave spectrometer. The assignment of these molecular transitions was assisted by anharmonic high-level quantum-chemical calculations. In particular, pure rotational lines belonging to the ground and the four lowest energy, vibrationally excited states were assigned. Splitting due to the tunnelling was resolved for the free -OH torsional state. A global fit combining the far-IR and millimeter-wave data provided the spectroscopic parameters of the low-energy far-IR modes, in particular those characterizing the intramolecular hydrogen bond dynamics.

Résumé: Electronic spectra are measured for mass-selected C^+{2????}(n = 6–14) clusters over the visible and near-infrared spectral range through resonance enhanced photodissociation of clusters tagged with N2 molecules in a cryogenic ion trap. The carbon cluster cations are generated through laser ablation of a graphite disk and can be selected according to their collision cross section with He buffer gas and their mass prior to being trapped and spectroscopically probed. The data suggest that the C^+{2????}(n = 6–14) clusters have monocyclic structures with bicyclic structures becoming more prevalent for C^+{22} and larger clusters. The C^+{2????} electronic spectra are dominated by an origin transition that shifts linearly to a longer wavelength with the number of carbon atoms and associated progressions involving excitation of ring deformation vibrational modes. Bands for C^+{12}, C^+{16}, C^+{20}, C^+{24}, and C^+{28} are relatively broad, possibly due to rapid non-radiative decay from the excited state, whereas bands for C^+{14}, C^+{18}, C^+{22}, and C^+_{26} are narrower, consistent with slower non-radiative deactivation.

Résumé: New transitions are reported in the submillimeter wave, terahertz, and far-infrared rotation-torsion spectrum of doubly-deuterated methanol CD2HOH. The newly assigned transitions allow us to spectroscopically characterize torsional states with 0⩽K⩽12 and 0⩽vt⩽2. Three line position analyses are carried out. In the first one, restricted to rotation-torsion lines involving torsional states with 3⩽K⩽12 and vt⩽2, rotational energies are evaluated with a J(J+1) Taylor-type expansion for each torsional state. 4853transitions were accounted for with a unitless standard deviation of 11.4. In the second analysis, 126torsional subband centers are fitted to obtain refined torsional parameters including the hindering potential. In the third analysis, 5911rotation-torsion transitions involving torsional states with vt⩽2, and J⩽26 are reproduced with a unitless standard deviation of 3.2 using a four-dimensional rotation-torsion fitting Hamiltonian.

Résumé: Hydrocarbons such as methane and ethane are present in many solar system objects, including comets, moons and planets. The interaction of these hydrocarbons with water ice at low temperatures could lead to the formation of inclusion compounds, such as clathrate hydrates (water based host cages trapping guest molecules), modifying their retention, stability and therefore evolution. The occurrence of clathrate hydrates on solar system surfaces could be established by remote sensing of their spectroscopic signatures. In this study, we measure and analyse ethane clathrate hydrate spectra recorded in the temperature range from 5.3 to 160 K, covering most of the temperature range of interest for solar system objects. Specific infrared band signatures are identified for the ethane encaged guest. We provide evidence that ethane clathrate hydrate outcrops can be detected by remote sensing on the surface of planetary bodies.

Résumé: The high resolution vibrational spectrum of ethyl cyanide (C2H5CN) has been investigated in the far-IR using synchrotron-based Fourier transform spectroscopy. The assignment was performed using the Automated Spectral Assignment Procedure (ASAP) allowing accurate rotational energy levels of the four lowest fundamental vibrations of the species, namely the v13=1 @ 205.934099(8)cm−1, v21=1 @ 212.141101(8)cm−1, v20=1 @ 372.635293(15)cm−1, and v12=1 @ 532.699617(16)cm−1 states, to be determined. The analysis not only confirms the applicability of the ASAP in the treatment of (dense) high-resolution infrared spectra but also reveals some of its limitations. Complementary to the infrared study, the pure rotational spectrum of C2H5CN was also studied in selected frequency ranges from 75 to 255GHz. New observations of a prototypical high-mass star-forming region, G327.3–0.6, performed with the Atacama Large Millimeter Array show that vibrational satellites of C2H5CN can be very intense, of order several tens of Kelvin in units of brightness temperature.

Résumé: The NH2 radical is a key component in many astrophysical environments, both in its neutral and cationic forms, being involved in the formation of complex N-bearing species. To gain insight into the photochemical processes into which it operates and to model accurately the ensuing chemical networks, the knowledge of its photoionization efficiency is required, but no quantitative determination has been carried out so far. Combining a flow-tube H-abstraction radical source, a double imaging photoelectron-photoion spectrometer, and a vacuum-ultraviolet synchrotron excitation, the absolute photoionization cross section of the amino radical has been measured in the present work for the first time at two photon energies: σionNH2(12.7 eV) = 7.8 ± 2.2 Mb and σionNH2(13.2 eV) = 7.8 ± 2.0 Mb. These values have been employed to scale the total ion yield previously recorded by Gibson et al. ( J. Chem. Phys. 1985, 83, 4319−4328). The resulting cross section curve spanning the 11.1–15.7 eV energy range will help in refining the current astrophysical models.

Résumé: VUV photoionization dynamics of the S2 molecule were re-investigated from threshold up to 15.0 eV, using synchrotron radiation coupled with double imaging photoelectron/photoion coincidence featuring high resolution capabilities. We measured the first threshold photoelectron spectrum of S2 achieving higher resolution than previous literature to derive accurate spectroscopic constants for a few electronic states of the cation including the X2ΠΩg ground state and the a4Πu, b4Σg–, and B2Σg– states. We also recorded the total ion yield for S2 up to a photon energy of 15.0 eV which, combined with the threshold photoelectron spectrum, led to the assignment of various autoionizing Rydberg series.

Résumé: Context. Carbon is one of the most abundant components in the Universe. While silicates have been the main focus of solid phase studies in protoplanetary discs (PPDs), little is known about the solid carbon content especially in the planet-forming regions (~0.1–10 au). Fortunately, several refractory carbonaceous species present C-H bonds (such as hydrogenated nano-diamond and amorphous carbon as well as polycyclic aromatic hydrocarbons), which generate infrared (IR) features that can be used to trace the solid carbon reservoirs. The new mid-IR instrument MATISSE, installed at the Very Large Telescope Interferometer (VLTI), can spatially resolve the inner regions (~1–10 au) of PPDs and locate, down to the au-scale, the emission coming from carbon grains.
Aims. Our aim is to provide a consistent view on the radial structure, down to the au-scale, as well as basic physical properties and the nature of the material responsible for the IR continuum emission in the inner disk region around HD 179218.
Methods. We implemented a temperature-gradient model to interpret the disk IR continuum emission, based on a multiwavelength dataset comprising a broadband spectral energy distribution and VLTI H-, L-, and N-bands interferometric data obtained in low spectral resolution. Then, we added a ring-like component, representing the carbonaceous L-band features-emitting region, to assess its detectability in future higher spectral resolution observations employing mid-IR interferometry.
Results. Our temperature-gradient model can consistently reproduce our dataset. We confirmed a spatially extended inner 10 au emission in H- and L-bands, with a homogeneously high temperature (~1700 K), which we associate with the presence of stochastically heated nano-grains. On the other hand, the N-band emitting region presents a ring-like geometry that starts at about 10 au with a temperature of 400 K. Moreover, the existing low resolution MATISSE data exclude the presence of aromatic carbon grains (i.e., producing the 3.3 μm feature) in close proximity tothe star (≲1 au). Future medium spectral resolution MATISSE data will confirm their presence at larger distances.
Conclusions. Our best-fit model demonstrates the presence of two separated dust populations: nano-grains that dominate the near- to mid-IR emission in the inner 10 au region and larger grains that dominate the emission outward. The presence of such nano-grains in the highly irradiated inner 10 au region of HD 179218 requires a replenishment process. Considering the expected lifetime of carbon nano-grains from The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS model), the estimated disk accretion inflow of HD 179218 could significantly contribute to feed the inner 10 au region in nano-grains.Moreover, we also expect a local regeneration of those nano-grains by the photo-fragmentation of larger aggregates.

Résumé: ABSTRACT
Line coupling and line mixing effects of CH3Cl lines in the ν1 band perturbed by N2 have been investigated. We have taken into account the non-diagonality of the exp(−S2) operator within specified line spaces as well as the k-degeneracy of the transitions (due to the double degeneracy of the j,k levels with k ≠ 0). These transitions should be considered as doublets coupled by the line mixing process. A new problem appears in the calculation when the atom-atom potential model is introduced. In order to overcome this difficulty, a pragmatic approach is proposed. Comparisons of theoretically calculated matrix elements of W with measurements of QR(j,k) doublets as well as some QQk sub-branches, which are strongly affected by line mixing, have been made. The results show that the formalism improved in this way leads to rather accurate predictions.

Résumé: Anharmonicities provide a wealth of information about the vibrational dynamics, mode coupling and energy transfer within a polyatomic system. In this contribution, we show how driven molecular dynamics trajectories can be used to extract anharmonicity properties under very short times of a few hundreds of vibrational periods, using two exciting fields at- and slightly off-resonance. Detailed analyses on generic quartic potential energy surfaces and applications to various model systems are presented, giving good agreement with perturbation theory. Application to a realistic molecule, cubane (C$$8$$H$$8$$), modelled with a tight-binding quantum force field, further indicates how the method can be applied in practical cases.

Résumé: In this work, we report quantitative absorbance measurements of nitric oxide (NO) diluted in nitrogen between 1700 to 2000 cm−1 and present three line mixing modeling approaches for the measured spectra. Static cell measurements were taken using a narrow-linewidth, external-cavity quantum-cascade laser at temperatures of 293 K and 802 K and pressures of 20–34 atm. The measured results exhibit considerable deviations from the spectra simulated by a superposition of Lorentzian line profiles due to significant line mixing coupling effects at high-number-density conditions. Our previous work demonstrated a line mixing model based on relaxation matrix theory and the Modified Exponential Gap (MEG) law for the NO R-branch. With expanded access to the P- and Q-branches, the measured data indicated significant line mixing effects between lines of different branches in addition to those within the same branch. An empirical two-scaling-factor inter-branch MEG model is presented that delivers strong agreement across the measured spectra, with residuals less than 2% for the spectrum at 293 K and 34 atm. In addition, the Energy Corrected Sudden (ECS) scaling law is shown to produce reasonable agreement across the measured spectra, excluding the Q-branch. In the Q-branch peak, the ECS model overpredicts the measured data by about 7%. The different line mixing models presented and discussed in this work will improve NO absorption predictions vital for laser absorption applications in high-number-density gas conditions. Future studies may seek to account for inter-spin-split coupling to further improve the ECS application to NO absorption.

Résumé: Context. Under cold conditions in dense cores, gas-phase molecules and atoms are depleted from the gas-phase to the surface of interstellar grains. Considering the time scales and physical conditions within these cores, a portion of these molecules has to be brought back into the gas-phase to explain their observation by milimeter telescopes.
Aims. We tested the respective efficiencies of the different mechanisms commonly included in the models (photo-desorption, chemical desorption, and cosmic-ray-induced whole-grain heating). We also tested the addition of sputtering of ice grain mantles via a collision with cosmic rays in the electronic stopping power regime, leading to a localized thermal spike desorption that was measured in the laboratory.
Methods. The ice sputtering induced by cosmic rays has been added to the Nautilus gas-grain model while the other processes were already present. Each of these processes were tested on a 1D physical structure determined by observations in TMC1 cold cores. We focused the discussion on the main ice components, simple molecules usually observed in cold cores (CO, CN, CS, SO, HCN, HC3N, and HCO+), and complex organic molecules (COMs such as CH3OH, CH3CHO, CH3OCH3, and HCOOCH3). The resulting 1D chemical structure was also compared to methanol gas-phase abundances observed in these cores.
Results. We found that all species are not sensitive in the same way to the non-thermal desorption mechanisms, and the sensitivity also depends on the physical conditions. Thus, it is mandatory to include all of them. Chemical desorption seems to be essential in reproducing the observations for H densities smaller than 4 × 104 cm−3, whereas sputtering is essential above this density. The models are, however, systematically below the observed methanol abundances. A more efficient chemical desorption and a more efficient sputtering could better reproduce the observations.
Conclusions. In conclusion, the sputtering of ices by cosmic-rays collisions may be the most efficient desorption mechanism at high density (a few 104 cm−3 under the conditions studied here) in cold cores, whereas chemical desorption is still required at smaller densities. Additional works are needed on both mechanisms to assess their efficiency with respect to the main ice composition.