Search

We have measured the kinetic energy distributions for the mutual neutralization of a large ensemble of atomic anions and cations. Ions are accelerated to equal velocities in a merged beam setup, enabling measurements at collision energies as low as 5 meV. Three-dimensional momentum imaging is performed with two position sensitive detectors located several meters downstream from the few centimeter long region where the beams overlap. An unprecedented resolution in the kinetic energy release (KER) spectra allows us to identify the states of both reactants and products down to their fine structure. Knowing the angular distribution of the products in the laboratory and center-of-mass frame, allows for total, partial, and differential cross sections to be retrieved., SCOPUS: cp.p, info:eu-repo/semantics/published

Synopsis The near-UV photodissociation of H3+ has been studied in a fast beam experiment. The kinetic energy release and branching ratio among the fragmentation channels were obtained by three-dimensional imaging of the dissociation products. It is found that the photodissociation of vibrationally excited H+3 mostly produces vibrationally cold H2+ and hot H2. The non-adiabatic repopulation of the ground potential energy surface after excitation is accounted for by modelling the wavepacket dynamics on the intersecting potential energy surfaces., SCOPUS: cp.p, DecretOANoAutActif, info:eu-repo/semantics/published

Experiments are conducted to investigate the role of the avoided crossing seam in the photodissociation of H+3 . Three-dimensional imaging of dissociation products is used to determine the kinetic energy release and branching ratio among the fragmentation channels. Vibrational distributions are measured by dissociative charge transfer of H+2 products. It is found that the photodissociation of hot H+3 in the near ultraviolet produces cold H+2 , but hot H2. Modelling the wavepacket dynamics along the repulsive potential energy surface accounts for the repopulation of the ground potential energy surface. The role of the avoided crossing seam is emphasized and its importance for the astrophysically relevant charge transfer eactions is underlined.

We present a combined experimental and theoretical study of the mutual neutralization (MN) process in collisions of lithium ions (Li+) with deuterium anions (D−) at collision energies below 1 eV. We employ a merged-beam apparatus to determine total and state-to-state MN cross sections. We perform nuclear dynamics calculations using the multichannel Landau–Zener model based on accurate ab initio molecular data. We obtain an excellent agreement between the experimental and theoretical results over the energy range covered in this work. We show that the basis sets used in the ab initio calculations have a limited influence on the total cross section, but strongly impacts the results obtained for the partial cross sections or the reaction branching ratios. This demonstrates the important role of high-precision measurements to validate the theoretical approaches used to study gas-phase reactive processes. Finally, we compute MN rate coefficients for Li+ + H− and Li+ + D−, and discuss their significance for astrochemistry models.

Experiments are conducted to investigate the role of the avoided crossing seam in the photodissociation of H3+. Three-dimensional imaging of dissociation products is used to determine the kinetic energy release and branching ratio among the fragmentation channels. Vibrational distributions are measured by dissociative charge transfer of H2+ products. It is found that the photodissociation of hot H3+ in the near-ultraviolet produces cold H2+, but hot H2. Modelling the wavepacket dynamics along the repulsive potential energy surface accounts for the repopulation of the ground potential energy surface. The role of the avoided crossing seam is emphasized and its importance for the astrophysically relevant charge transfer reactions underlined. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'., SCOPUS: ar.j, DecretOANoAutActif, info:eu-repo/semantics/published

We present measurements of absolute cross sections for projectile ionization and electron capture for C+, N+ and Cn N+ (n=1-3) projectiles impinging on He atoms with velocity v=2.2 a.u. Single and multiple electron processes are considered, as well as projectile dissociative excitation in the case of incident molecular cations. The measurements are compared to Classical Trajectory Monte Carlo (CTMC) calculations for C+, N++He collisions in the framework of the independent electron approximation. Cn N++He systems are described by means of the independent atom and electron (IAE) model which represents the molecule as a set of independent atoms. The impact parameter probabilities for excitation, ionization and electron transfer in C, C+, N-He collisions, underlying the IAE calculations, are also obtained by means of CTMC computations. A good agreement is generally found between measured and calculated cross sections, except for anionic production of C- and C n N-. The internal energy deposit due to electron excitation in C n N+ is also calculated with the IAE/CTMC model and compared to semi-empirical estimates based on either measured dissociation branching ratios for C2N+ and C3N+ (IdBarkach et al 2018 Mol. Astrophys. 12 25) or measured fragments kinetic energy release for CN+. Finally, measured dissociation branching ratios of excited C n N- and C n NQ+ species, with 1≤n ≤3 and 0≤Q≤4, are reported., SCOPUS: ar.j, DecretOANoAutActif, info:eu-repo/semantics/published

We present a combined experimental and theoretical study of the mutual neutralization (MN) process in collisions of lithium ions (Li+) with deuterium anions (D-) at collision energies below 1 eV. We employ a merged-beam apparatus to determine total and state-to-state MN cross sections. We perform nuclear dynamics calculations using the multichannel Landau-Zener model based on accurate ab initio molecular data. We obtain an excellent agreement between the experimental and theoretical results over the energy range covered in this work. We show that the basis sets used in the ab initio calculations have a limited influence on the total cross section, but strongly impacts the results obtained for the partial cross sections or the reaction branching ratios. This demonstrates the important role of high-precision measurements to validate the theoretical approaches used to study gas-phase reactive processes. Finally, we compute MN rate coefficients for Li+ + H- and Li+ + D-, and discuss their significance for astrochemistry models., SCOPUS: ar.j, info:eu-repo/semantics/published

We constructed semiempirical breakdown curves (BDC) for C2N, C3N, C2N+ and C3N+ molecules. These BDC, which are energy dependent dissociation branching ratios (BR) curves, were used to predict products branching ratios for various processes leading to the formation of C2N(+) and C3N(+) excited adducts. These processes, of astrochemical interest, are neutral-neutral and ion-molecule reactions, dissociative recombination and charge transfer reactions with He+. Model predictions of BR are compared to the literature data and to reported values in the kinetic database for astrochemistry KIDA. With the new BR values, the CnN abundances in cold cores were simulated., SCOPUS: ar.j, info:eu-repo/semantics/published

Ce travail avait pour ambition de caractériser deux processus collisionnels différents impliquant des paires d'ions. Ces derniers, que ce soit sous forme de cations ou sous forme d'anions, ont une importance majeure pour l'astrochimie. Ainsi, il est vital de pouvoir fournir des données théoriques ou expérimentales sur les processus conduisant tant à la formation qu'à la consommation de ces ions. Le premier processus que nous avons étudié, nommé la dissociation en paires d'ions, conduit à la formation d'ions secondaires. Nous avons caractérisé ce processus tant théoriquement qu'expérimentalement. Ainsi, au moyen de collisions à haute vitesse entre un jet d'atomes d'hélium et un faisceau d'agrégats carbonés neutres ou ionisés, nous avons obtenu les différents rapports de branchements et sections efficaces concernant la dissociation en paires d'ions de ces composés de tailles n et de charges q variées. Grâce à ce travail, qui a permis d'effectuer une étude expérimentale sur l'influence de ces deux paramètres, nous avons ainsi montré que la dissociation en paires d'ions est toujours présente, quelle que soit la taille ou la charge du composé étudié. Dans le but de décrire la dissociation en paires d'ions au moyen de méthodes ab initio, nous avons également calculé les courbes d'énergies potentielles relatives aux états très excités de l'ion moléculaire C_2+. Sur base de l'analyse de population de Mulliken des fonctions d'onde relatives à ces états moléculaires, nous avons ainsi pu mettre en évidence le canal ionique impliqué dans la dissociation en paires d'ions de C_2+, via une méthodologie purement ab initio. Nous avons également calculé les taux théoriques de dissociation en paires d'ions pour les molécules C_2+ et C_3+, au moyen d'un modèle statistique basé sur le nombre de limites de dissociation et d'états moléculaires s'y corrélant. Nous avons ainsi pu comparer les données expérimentales et théoriques pour ces deux systèmes et avons montré que ce modèle, malgré d, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

We constructed semiempirical breakdown curves (BDC) for C2N, C3N, C2N+ and C3N+ molecules. These BDC, which are energy dependent dissociation branching ratios (BR) curves, were used to predict products branching ratios for various processes leading to the formation of C2N(+) and C3N(+) excited adducts. These processes, of astrochemical interest, are neutral-neutral and ion-molecule reactions, dissociative recombination and charge transfer reactions with He+. Model predictions of BR are compared to the literature data and to reported values in the kinetic database for astrochemistry KIDA. With the new BR values, the CnN abundances in cold cores were simulated., info:eu-repo/semantics/inPress

We have measured total absolute cross sections for the mutual neutralization (MN) of O- with O+ and N+. A fine resolution (of about 50 meV) in the kinetic energy spectra of the product neutral atoms allows unique identification of the atomic states participating in the mutual neutralization process. Cross sections and branching ratios have also been calculated down to 1 meV center-of-mass collision energy for these two systems, with a multichannel Landau-Zener model and an asymptotic method for the ionic-covalent coupling matrix elements. The importance of two-electron processes in one-electron transfer is demonstrated by the dominant contribution of a core-excited configuration of the nitrogen atom in N++O- collisions. This effect is partially accounted for by introducing configuration mixing in the evaluation of coupling matrix elements. © 2018 American Physical Society., info:eu-repo/semantics/published

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

This paper compiles our merged-beam experimental findings for the associative ionization (AI) process from charged reactants, with the aim of guiding future investigations with e.g. the double electrostatic ion storage ring DESIREE in Stockholm. A reinvestigation of the isotopic effect in H-(D-) + He+ collisions is presented, along with a review of and NO+ production by AI involving ion pairs or excited neutrals, and put in perspective with the mutual neutralization and radiative association reactions. Critical parameters are identified and evaluated for their systematic role in controlling the magnitude of the cross section: isotopic substitution, exothermicity, electronic state density, and spin statistics., SCOPUS: ar.j, info:eu-repo/semantics/published

We will present measurements and modeling for two aspects of the CnN+ - He collisions (n=1-3, v=2.25 a.u) :cross sections for electronic excitation processes and fragmentation branching ratios for the excited and ionized CnNq+ molecules produced in the collision (q=-1,0,1,2-5)., SCOPUS: cp.j, info:eu-repo/semantics/published

We present measurements of ion-pair dissociation (IPD) of highly excited neutral and ionized carbon clusters Cn=2-5(q=0-3)+. The tool for producing these species was a high-velocity collision between Cn+ projectiles (v=2.25 a.u.) and helium atoms. The setup allowed us to detect in coincidence anionic and cationic fragments, event by event, leading to a direct and unambiguous identification of the IPD process. Compared with dissociation without anion emission, we found typical 10-4 IPD rates, not depending much on the size and charge of the (n,q) species. Exceptions were observed for C2+ and, to a lesser extent, C43+ whose IPDs were notably lower. We tentatively interpret IPDs of C2+ and C3+ by using a statistical approach based on the counting of final states allowed by energetic criteria. The model is able to furnish the right order of magnitude for the experimental IPD rates and to provide a qualitative explanation of the lower IPD rate observed in C2+., SCOPUS: ar.j, info:eu-repo/semantics/published

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/published

Ion pair collisions are an important process in many astrophysical environments. We present ab initio calculations of highly excited states of C2+ and identify the ion pair channel C2+/C-. We use these results to interpret recent experiments on carbon cluster dissociation., info:eu-repo/semantics/published

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

L’équipe « Astrophysique et édifices moléculaires » de l’Institut des Sciences Moléculaires d’Orsay (Université Paris – Sud) étudie différents clusters de carbones depuis plusieurs années. Afin d’améliorer la compréhension des phénomènes liés à ces clusters, de nombreuses réactions ont été étudiées dont notamment la relaxation de ces composés carbonés en paire d’ions suite à des collisions à hautes vitesses.Ce mémoire avait pour but de fournir des réponses théoriques à des interrogations expérimentales concernant la section efficace de relaxation de l’agrégat moléculaire C2+ en la paire d’ions C++/C-.Au moyen de méthodes de calcul ab initio multiconfigurationnelles, CASSCF et MRCI, nous avons pu modéliser une partie du paysage énergétique de C2+. Ainsi, nous avons pu obtenir les courbes d’énergie potentielle associées aux cinquante premiers états électroniques découlant de différentes dissociations moléculaires du système. En modifiant la base de calcul utilisée dans nos méthodes ab initio, nous avons pu faire apparaitre différents états Rydberg de C2+ et ainsi traiter leurs réactivités suivant la distance de liaison carbone – carbone. Nous avons également expérimenté différents traitements afin de représenter la dissociation moléculaire des états Rydberg de C2+ de la manière la plus adéquate possible.Au moyen de l’analyse de population de Mulliken des fonctions d’onde des états excités calculés, nous avons mis en évidence les canaux ioniques fondamentaux de la paire C++/C- qui interagissent et croisent les états électroniques de valence en formant une cascade de croisements évités qui vont conditionner la dynamique réactionnelle.Cette étude théorique, qui pour la première fois caractérise les états hautement excités de C2+, devrait permettre d’estimer prochainement la section efficace théorique de la relaxation de C2+ en la paire d’ions C++/C- afin de la comparer aux données expérimentales obtenues à l’Université de Paris – Sud., info:eu-repo/semantics/nonPublished