Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation

A recent study of soft X-ray absorption in native and hydrogenated coronene cations, C$_{24}$H$_{12+m}^+$ $m=0-7$, led to the conclusion that additional hydrogen atoms protect (interstellar) Polycyclic Aromatic Hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30-200 eV) He atoms and pyrene (C$_{16}$H$_{10+m}^+$, $m=0$, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.Comment: 10 pages, 5 figure

Unusual hydrogen and hydroxyl migration in the fragmentation of excited doubly-positively-charged amino acids in the gas phase

We present a combined experimental and theoretical study of the fragmentation of doubly-positively- charged amino acids in the gas phase. The combination of ab initio molecular dynamics simulations with ion- molecule collisions followed by multiple-coincidence mass spectrometric techniques, allows us to obtain a complete picture of the fragmentation dynamics. In addition to the expected Coulomb explosion, we have found evidence of hydrogen and hydroxyl-group migration processes, which leads to unusual fragmentation product

Slow ion interaction with N-methylglycine and N-acetylglycine

N-acetyl glycine and N-methyl glycine molecules in the gas phase are ionized by electron exchange with slow O6+ ions at an energy of 48 keV. After ionization, the methyl and acetyl substituted glycines dissociate into fragments analogous to that resulting from ionization and fragmentation of amino acids and peptides, respectively. N-acetylglycine which contains a peptide bond also effectively tautomerizes to the diol form. Such tautomerization is typical for amino acids, however, we show that the tautomerization mechanism of the N-acetylglycine is differen

Charge and energy flows in ionised thymidine

We present a combined experimental and theoretical study of the ionisation and fragmentation of the nucleoside thymidine in the gas phase. Two sources of ionisation/excitation are used, namely UV photons and low-energy multiply charged ions, associated with coincidences measurements, respectively photoelec- tron/photofragment (PEPICO) and fragment/fragment. Coupling these experiments with quantum chemistry calculations, we obtain a complete picture of the fragmentation dynamics, in particular the charge and energy transfers within the molecular edific

Croissance moléculaire dans des agrégats d’hydrocarbures aromatiques polycycliques induite par des collisions avec des ions

This thesis concerns the experimental study of the interaction between low energy ions (keV range) andneutral isolated molecules or clusters of polycyclic aromatic hydrocarbons (PAH) in the gas phase. Theuse of ionising radiations on these complex molecular systems of astrophysical interest allowed to highlightprocesses of statistical fragmentation, corresponding to the redistribution of the energy through the degreesof freedom of the target, and non-statistical fragmentation, linked to binary collisions of the ions on thenuclei of the target.A mechanism of intermolecular growth in clusters of PAH is observed. It is associated to the ultrafast(< ps) formation of fragments inside the clusters following binary collisions. The presence of a molecularenvironment around the fragments formed during the interaction may initiate a process of reactivity betweenthe fragments and the molecules of the clusters. More precisely, the study focusses on the importance ofthe electronic stopping power SE and the nuclear stopping power SN of the projectile ion. It shows that themolecular growth is enhanced when SN is higher than SE. This can be explained by the fact that the depositof energy is mainly due to the interaction with the nuclei of the target.The process of growth has been observed for all the molecules of PAH studied during this thesis and alsofor nitrogenated analogues of the molecule of anthracene. This demonstrates that molecular growth may beefficiently induced by collisions of low energy ions with clusters of PAH.Cette thèse porte sur l’étude expérimentale de l’interaction entre des ions de basses énergies (de l’ordredu keV) et des molécules isolées ou des agrégats d’hydrocarbures aromatiques polycycliques (HAP) neutresen phase gazeuse. L’utilisation de rayonnements ionisants sur ces systèmes moléculaires complexes d’intérêtastrophysique a permis de mettre en évidence des processus de fragmentation statistique, correspondant àune redistribution de l’énergie à travers l’ensemble des degrés de liberté du système cible, et de fragmentationnon-statistique, associée aux collisions binaires des ions projectiles sur les atomes de la cible.Un mécanisme de croissance intermoléculaire a été observé dans les agrégats de HAP. Il est lié à la formationde fragments à l’intérieur des agrégats à une échelle de temps très courte (< ps) à la suite de collisions binaires.La présence d’un environnement moléculaire autour des fragments créés lors de l’interaction permet d’initierun processus de réactivité entre les fragments et les molécules constituant les agrégats. Plus particulièrement,l’étude se concentre sur l’importance du pouvoir d’arrêt électronique SE et nucléaire SN de l’ion projectile.Elle montre que lorsque SN est grand devant SE la croissance moléculaire devient plus importante car ledépôt d’énergie se fait principalement avec les noyaux de la cible.Les expériences réalisées au cours de cette thèse ont permis de généraliser ce processus de croissance pourl’ensemble des HAP étudiés ainsi qu’aux analogues azotés de la molécule d’anthracène. Elles démontrent queles ions représentent un outil efficace pour initier un mécanisme de croissance moléculaire dans les agrégatsde HAP

Ion-collision induced molecular growth in polycyclic aromatic hydrocarbon clusters: comparison of C16H10 structural isomers

International audienc

Ion collision-induced chemistry in pure and mixed loosely bound clusters of coronene and C 60 molecules

International audienc

Formation and Fragmentation of Protonated Molecules after Ionization of Amino Acid and Lactic Acid Clusters by Collision with Ions in the Gas Phase

International audienceCollisions between O3+ ions and neutral clusters of amino acids (alanine, valine and glycine) as well as lactic acid are performed in the gas phase, in order to investigate the effect of ionizing radiation on these biologically relevant molecular systems. All monomers and dimers are found to be predominantly protonated, and ab initio quantum–chemical calculations on model systems indicate that for amino acids, this is due to proton transfer within the clusters after ionization. For lactic acid, which has a lower proton affinity than amino acids, a significant non-negligible amount of the radical cation monomer is observed. New fragment-ion channels observed from clusters, as opposed to isolated molecules, are assigned to the statistical dissociation of protonated molecules formed upon ionization of the clusters. These new dissociation channels exhibit strong delayed fragmentation on the microsecond time scale, especially after multiple ionization

Unusual hydroxyl migration in the fragmentation of β-alanine dication in the gas phase

International audienceWe present a combined experimental and theoretical study of the fragmentation of doubly positively charged β-alanine molecules in the gas phase. The dissociation of the produced dicationic molecules, induced by low-energy ion collisions, is analysed by coincidence mass spectrometric techniques; the coupling with ab initio molecular dynamics simulations allows rationalisation of the experimental observations. The present strategy gives deeper insights into the chemical mechanisms of multiply charged amino acids in the gas phase. In the case of the β-alanine dication, in addition to the expected Coulomb explosion and hydrogen migration processes, we have found evidence of hydroxyl-group migration, which leads to unusual fragmentation products, such as hydroxymethyl cation, and is necessary to explain some of the observed dominant channels