Paul Pirlot Jankowiak

This work presents calculations of state-to-state collisional rate coefficients for the rotational excitation of the deuterated formaldehyde isotopologs, HDCO and D ₂ CO, induced by collisions with molecular hydrogen, coupled with experimental verification by pressurebroadening measurements. Time-independent scattering calculations were performed using the close-coupling method to compute inelastic cross sections for collisions with both para- H ₂ ( j ₂ = 0) and ortho- H ₂ ( j ₂ = 1). These calculations are based on the H ₂ CO–H ₂ potential energy surface previously determined at the CCSD(T) level of theory and transformed to account for the isotopic substitution. The resulting rate coefficients cover the lowest 100 rotational states of HDCO and 60 states of ortho- and para- D ₂ CO with internal energies less than 200 cm ⁻¹ . Collisional data were obtained for temperatures up to 100 K for HDCO and D ₂ CO and extended up to 300 K using the coupled-states approximation. The computed rate coefficients show systematic differences between ortho- H ₂ and para- H ₂ collisions, with the former generally yielding larger values by a factor of 2–3. Rate coefficients for HDCO–H ₂ are found to be lower by a factor of 2 to 3 compared to those of H ₂ CO–H ₂ and D ₂ CO–H ₂ . Pressure-broadening coefficients were also calculated using the random-phase approximation and compared with new laboratory measurements, yielding agreement by about 5–30%.

Excitation collisionnelle de radicaux d'intérêt astrophysique: La modélisation des spectres moléculaires interstellaires nécessite la compréhension des processus radiatifs et collisionnels de transferts d'énergie entre espèces chimiques. Des taux de collisions d’état à état précis pour les molécules interstellaires, en collision avec H2, H et He, sont alors essentiels. Ces données peuvent être obtenues via la résolution des équations close-coupling indépendantes du temps, étant la méthode la plus précise pour les calculs à basse températures. Cependant, certaines molecules comme les radicaux présentent des structures énergétiques complexes en raison de leurs spins électroniques et nucléaires, rendant la description précise des transitions fines et hyperfines théoriquement coûteuse en ressources numériques. D'importants systèmes collisionnels souffrent donc d'un manque de données pour les interprétations astrophysiques.Ce travail de thèse vise à quantifier l'excitation collisionnelle de radicaux d’intérêt astrophysique en développant des outils méthodologiques et numériques pour surmonter ces défis. De nouveaux taux de collision fins et hyperfins ont été calculés pour les systèmes suivants: NH-H2, ND-H2, C2H-H2, C2D-H2, 13CCH-H2 et C13CH-H2.

Understanding the interplay between inelastic and reactive processes in low temperature molecular collisions is a true theoretical challenge. This study addresses this challenge by employing the statistical adiabatic channel model (SACM) to quantify the rotational excitation processes in the OH^{+}+H_{2} reactive system, such process being key in astrochemistry. The SACM approach demonstrates very good agreement with reduced dimensional close-coupling calculations in describing pure inelastic collisions in the low energy regime and satisfactory agreement with experimental measurement for treating reactive processes. Hence, the SACM approach can be considered as a good alternative to consider molecular collisions in reactive systems characterized by strongly bounded intermediate complexes, in absence of exact calculation in the low energy regime. Our findings reveal that reactive processes dominate the pure collisional excitation at all temperatures studied (5-300 K) by at least about one order of magnitude. This suggests a strong revision of the predicted abundance of OH^{+} in astrophysical environments since astrochemical models are presently significantly overestimating the impact of OH^{+} excitation induced by H_{2} collisions.

The NH and ND radicals are of key importance in the comprehension of nitrogen chemistry and the enhancement of deuterated molecules in the interstellar medium. Observations by space telescopes yield spectra that can resolve the fine and hyperfine structure of these radicals, a consequence of the electronic and magnetic interactions of nitrogen, hydrogen, and deuterium nuclei. Accurate rate coefficients, induced by collisions with H₂, are required to interpret spectra of these radicals. We report the first rate coefficients for fine and hyperfine transitions of NH and ND in collision with both ortho- and para-H₂. Based on a recent four-dimensional potential energy surface, fine-structure resolved cross sections and rate coefficients are computed with the time-independent close-coupling method over a temperature range of 5–300 K. Our calculations include the first 25 energy levels of NH and ND. Hyperfine resolved cross sections and rate coefficients are determined using the infinite-order sudden (IOS) approximation between 5 and 200 K for NH and 100 K for ND. We consider the first 71 and 105 energy levels of NH and ND, respectively. General propensity rules are discussed. We found a significant isotopic substitution effect in the rate coefficients. In addition, the rate coefficients for collisions with H₂ are larger than those with He by a factor of up to 5, leading to lower critical densities for collisional excitation with H₂ than He. The impact of the new set of collisional data has been investigated in simple radiative transfer models of the NH emission seen toward the Orion Bar and the ejecta of the η Carinae binary star. We observed significant differences by a factor of 5 between the presently determined column densities for NH compared to those from the literature using He as a collider.

Context. The global context of making numerous data produced by researchers available requires collecting and organising the data, assigning meaningful metadata, and presenting the data in a meaningful and homogeneous way. The BASECOL database, which collects inelastic rate coefficients for application to the interstellar medium and to circumstellar and cometary atmospheres, meets those requirements. Aims. We aim to present the scientific content of the BASECOL2023 edition. Methods. While the previous versions relied on finding rate coefficients in the literature, the current version is populated with published results sent by the producers of data. The paper presents the database, the type of data that can be found, the type of metadata that are used, and the Virtual Atomic and Molecular Data Centre (VAMDC) standards that are used for the metadata. Finally, we present the different datasets species by species. Results. As the BASECOL database, interconnected with the VAMDC e-infrastructure, uses the VAMDC standards, the collisional data can be extracted with tools using VAMDC standards and can be associated with spectroscopic data extracted from other VAMDC connected databases such as the Cologne database for molecular spectroscopy (CDMS), the jet propulsion laboratory molecular spectroscopy database (JPL), and the high-resolution transmission molecular absorption database (HITRAN).

The determination of physical conditions in interstellar clouds requires reliable estimation of radiative and collisional data for molecules detected in space. In this work, rate coefficients for (de-)excitation of C₂H and C₂D induced by collisions with both ortho- and para-H₂ are presented. Calculations have been carried out using a recently published four dimensional C₂H–H₂ potential energy surface. Fine structure resolved cross sections were computed with the time-independent close-coupling approach. We report cross sections for transitions between the first 41 levels of C₂H and corresponding rate coefficients up to 500 K. We also computed cross sections for transitions between the first 31 levels of C₂D and corresponding rate coefficients up to 200 K. Then, hyperfine structure resolved cross sections and rate coefficients were computed using the recoupling technique. The hyperfine structure resolved rate coefficients for C₂H–H₂ and C₂D–H₂ are calculated for the first 38 hyperfine C₂H energy levels and first 55 hyperfine C₂D energy levels both for temperatures up to 100 K. These collisional data were used in a simple radiative transfer modeling.

The computation of hyperfine resolved cross sections and rate coefficients for open-shell molecules in collision with H₂ is a true methodological and numerical challenge. Such collisional data are however required to interpret astrophysical observations. We report the first hyperfine resolved rate coefficients for (de-)excitation of ¹³CCH and C¹³CH isotopologues induced by collisions with para-H₂. These calculations have been performed using a recently published C₂H–H₂ potential energy surface. Hyperfine resolved cross sections and rate coefficients between the first 98 energy levels of the two isotopologues were determined using a recoupling technique for temperatures ranging from 5 to 100 K. Significant isotopic substitution effects were found, showing the necessity of computing isotopologue specific collisional data. These rate coefficents have then been used in a simple radiative transfer modeling for typical molecular cloud conditions.

Collisional data for the excitation of NH by H₂ are key to accurately derive the NH abundance in astrophysical media. We present a new four-dimensional potential energy surface (PES) for the NH–H₂ van der Waals complex. The ab initio calculations of the PES were carried out using the explicitly correlated partially spin-restricted coupled cluster method with single, double, and perturbative triple excitations [RCCSD(T)-F12a] with the augmented correlation-consistent polarized valence triple zeta basis set. The PES was represented by an angular expansion in terms of coupled spherical harmonics. The global minimum corresponds to the linear structure with a well depth D_e = 149.10 cm⁻¹. The calculated dissociation energy D₀ is found to be 30.55 and 22.11 cm⁻¹ for ortho-H₂ and para-H₂ complexes, respectively. These results are in agreement with the experimental values. Then, we perform quantum close-coupling calculations of the fine structure resolved excitation cross sections of NH induced by collisions with ortho-H₂ and para-H₂ for collisional energies up to 500 cm⁻¹. We find strong differences between collisions induced by ortho-H₂ and para-H₂. Propensity rules are discussed. The cross sections are larger for fine structure conserving transitions than for fine structure changing ones, as predicted by theory. These new results should help in interpreting NH interstellar spectra and better constrain the abundance of NH in interstellar molecular clouds.