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Excitation and fragmentation of CnN+ (n=1-3) molecules in collisions with He atoms at intermediate velocity; fundamental aspects and application to astrochemistry

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Description

This thesis studies the aftermath of collision between singly positively charged Nitrogenated carbon species CnN+ (n = 0,1,2,3) and neutral Helium atom at a velocity of 2.25 au. At this velocity, close to the velocity of outer electrons in atoms and molecules, several electronic processes take place and are near their maximum of probability such as ionisation (single, double, triple... ), electronic excitation and electron capture (single and double). We looked at their cross sections and how their evolution with the molecule size. Following the collision the molecule can fragment, which leads to another interesting aspect, the fragmentation branching ratios. Collision experiments were done using a Tandem accelerator at Orsay that produced the CnN+ projectiles and a dedicated set-up, AGAT, to capture the flying fragments/intact molecule after collision according to their charge to mass ratio. Knowing the number of particles that are shot and the fact that our set-up allows no loss of fragments/intact molecule, we could get the probabilities of various fragments formed. Using these probabilities and a knowledge of the Helium jet profile used, we could measure their cross sections. The probabilities alone are sufficient to obtain the fragmentation branching ratios. The next step was to use a theoretical model to simulate the collision. We used Independent Atom and Electron (IAE) model coupled with Classical Trajectory Monte Carlo (CTMC) method to calculate the desired cross sections. A general good agreement was obtained, with the exception of double electron capture. The model could also predict, through the calculation of the species internal energy, the fragmentation branching ratios of cations CnN+ after electronic excitation. Also, the branching ratios were used to construct semi-empirical Breakdown Curves (BDCs), which are internal energy dependent dissociation branching ratios specific to each molecule, type, size and charge. With those, we could recommend products branching ratios to be used for various processes of astrochemical interest. The products branching ratios will be made available for a wider network of researchers under the international Kinetic Database for Astro- chemistry (KIDA). This thesis was realized under the doctoral programme of the Ecole Doctorale Ondes et Matiere (EDOM) with Institut des Sciences Moleculaires d'Orsay (ISMO) where the author was given an office and Universite Paris-Sud (Universite Paris-Saclay) where the author is formally enrolled. (author)

Abstract (French)

Dans cette these nous avons etudie des collisions entre des projectiles CnN+ (n=0,1,2,3) et des atomes d'Helium a vitesse intermediaire (2.25 u.a). A cette vitesse, proche de la vitesse des electrons sur les couches de valence externe des atomes et molecules, de nombreux processus electroniques prennent place avec une forte probabilite: ionisation (simple et multiple), excitation electronique, capture d'electron (simple et double). Nous avons mesure les sections efficaces absolues de tous ces processus. Un autre aspect interessant de la collision concerne la fragmentation des molecules excitees, que nous avons egalement mesuree precisement grace a un dispositif dedie. Les experiences ont ete effectuees aupres de l'accelerateur Tandem d'Orsay avec des faisceaux de quelques MeV d'energie cinetique. Le dispositif AGAT a permis de realiser les collisions (en condition de collision unique) et de mesurer tout a la fois les sections efficaces des processus et la fragmentation associee. Parallelement nous avons simule ces collisions d'un point de vue theorique en utilisant le modele a Atomes et Electrons Independants (IAE) couple a des calculs CTMC (Classical trajectory Monte Carlo). Sur cette base, nous avons predit les sections efficaces qui se sont trouvees etre en bon accord avec les mesures, a l'exception de la double capture d'electrons. Par ailleurs les rapports de branchement de dissociation des CnN+ apres excitation electronique sont bien reproduits en utilisant la distribution d'energie interne des especes calculees avec le meme modele IAE/CTMC. Ces experiences nous ont permis de construire des 'Breakdown Curves' (BDC), veritables cartes d'identite des molecules qui permettent de prevoir, dans le cadre d'une fragmentation statistique comment va fragmenter un systeme dont on connait l'energie interne. Avec ces BDC nous avons pu predire et recommander des rapports de branchement pour des voies de sortie de processus physiques et chimiques d'interet astrochimique. Ces donnees seront inserees dans la base internationale d'astrochimie the Kinetic Data Base for Astrochemistry KIDA. Cette these a ete realisee dans le cadre de l'ecole Doctorale Ondes et Matiere (EDOM) a l'Institut des Sciences Moleculaires d'Orsay (ISMO), a l'Universite Paris-Sud (Universite Paris Saclay). (auteur)

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Additional titles

Original title (English)
Excitation et fragmentation des molecules CnN+ (n = 1-3) en collision avec des atomes de He a vitesse intermediaire; aspects fondamentaux et application a l'astrochimie

Publishing Information

Imprint Pagination
211 p.
Report number
FRNC-TH--13822

INIS

Country of Publication
France
Country of Input or Organization
France
INIS RN
54020336
Subject category
S74: ATOMIC AND MOLECULAR PHYSICS;
Resource subtype / Literary indicator
Thesis
Descriptors DEI
ATOM-MOLECULE COLLISIONS; BRANCHING RATIO; CARBON NITRIDES; CROSS SECTIONS; DISSOCIATION; ELECTRON CAPTURE; EXCITATION; HELIUM; IONIZATION
Descriptors DEC
ATOM COLLISIONS; CAPTURE; CARBON COMPOUNDS; COLLISIONS; DIMENSIONLESS NUMBERS; ELEMENTS; ENERGY-LEVEL TRANSITIONS; FLUIDS; GASES; MOLECULE COLLISIONS; NITRIDES; NITROGEN COMPOUNDS; NONMETALS; PNICTIDES; RARE GASES

Optional Information

Notes
69 refs.; Available from the INIS Liaison Officer for France, see the INIS website for current contact and E-mail addresses