Neutral Products Desorption from DNA Thin Films Induced by Low-Energy Electrons (0.5-20 eV)

International audienceLow-energy electrons (LEEs) are produced in great amount in the biological medium, when submitted to high-energy radiations. They have the ability to induce strand breaks in the DNA duplex, as proven by electrophoresis analysis of irradiated dry deposits. LEE interactions with target molecules induce the formation of different species such as anions, cations, radicals and neutrals. The desorption of anionic species from oligonucleotides and DNA under LEEs irradiation has been intensively explored. The involved mechanisms and sites were successfully identified, including the resonant formation of transient negative ions (TNI) below 15 eV. However, the desorption of neutral products was less explored [4], due to their difficult detection. Exploring this aspect will provide additional information and complete the picture of the dissociating pathways followed by TNIs

Interaction d'atomes et de molécules d'hydrogène<br />avec des glaces d'eau à très basse température :<br />formation de H2 dans le milieu interstellaire

The molecular hydrogen, a major component of the interstellar medium (ISM) isformed on dust grains through surface catalysis. This key reaction is a source of uncertaintyin the description of the dynamic of the ISM, as well for the reaction efficiencyand for its energy budget. This reaction and its sub-processes (sticking and diffusion onthe grain surface, desorption) are studied here experimentally in the case of molecularhydrogen formation on water ice surfaces similar to those covering dust in the cold denseISM.Ultra-high vacuum, cryogenics, atomic beams, mass spectrometry and UV spectroscopyare combined in the experimental setup named FORMOLISM. We have studiedthe effects of surface heterogeneity and porosity on the reaction. We have focused onmolecular hydrogen desorption to improve the interpretation of formation experimentsand because the molecular hydrogen adsorbed on grains modify the formation efficiency.Adsorption energies of H2, HD and D2 on these ices are obtained. A mechanism of isotopicsegregation is highlighted and the deuteration of molecular hydrogen on ice mantlesis quantified. The study of molecular formation reveals that on porous ices the reactionenergy is released to the surface because of the efficient recapture of the formedmolecules, and that the reaction is effective up to temperatures higher (20 K) than fornonporous ices (13 K). For these latter, the formed molecules are directly released intothe gas phase where they are detected in rovibrationally excited states.La physico-chimie et l'évolution des différents milieux qui constituent le milieu interstellairedépendent étroitement de H2, son principal constituant moléculaire. En particulier,la connaissance incomplète du bilan énergétique et de l'efficacité de la réaction deformation d'hydrogène moléculaire par catalyse hétérogène sur les grains de poussière estune source importante d'incertitude dans la description de la dynamique du milieu, notammentlors de la formation d'étoiles. L'´etude de cette réaction et de ses sous-processus(collage et diffusion sur les grains, désorption) est abordée théoriquement et expérimentalementdepuis plus de 40 ans.Cette thèse vise par une approche expérimentale à caractériser la réaction de formationd'hydrogène moléculaire à la surface des glaces d'eau. Elle s'articule autour dudispositif FORMOLISM. Ultravide, cryogénie, jets atomiques, spectrométrie de masseet spectroscopie UV sont réunis pour étudier en particulier les effets de l'hétérogénéitéet de la porosité de la surface. L'étude de la désorption de l'hydrogène moléculaire s'estrévélée indispensable à l'interprétation des expériences de formation. Nous avons mesuré les distributions d'énergies d'adsorption de H2, HD et D2. Ces mesures permettentd'estimer la quantité d'hydrogène moléculaire en surface des grains interstellaires. Laprésence d'hydrogène moléculaire modifie l'efficacité de la réaction. Un mécanisme deségrégation isotopique a été mis en évidence et son importance pour la deutération del'hydrogène moléculaire en surface des manteaux de glace a été étudiée. Les expériencessur la formation révèlent que sur les glaces poreuses l'énergie dégagée par la réaction esttransmise à la surface par la rétention des molécules formées. La réaction reste efficaceà des températures plus élevées (20 K) que sur les glaces non poreuses (13 K). Sur cesdernières, les molécules formées sont directement libérées en phase gazeuse où elles sontdétectées dans des états rovibrationnellement excités

Interaction d'atomes et de molécules d'hydrogène avec des glaces d'eau à très basse température (Formation de H2 dans le milieu interstellaire)

La physico-chimie et l'évolution des différents milieux qui constituent le milieu interstellaire dépendent étroitement de R2, son principal constituant moléculaire. En particulier, la connaissance incomplète du bilan énergétique et de l'efficacité de la réaction de formation d 'hydrogène moléculaire par catalyse hétérogène sur les grains de poussière est une source importante d'incertitude dans la description de la dynamique du milieu, notamment lors de la formation d'étoiles. L'étude de cette réaction et de ses sous-processus (collage et diffusion sur les grains, désorption) est abordée théoriquement et expérimentalement depuis plus de 40 ans. Cette thèse vise par une approche expérimentale à caractériser la réaction de formation d'hydrogène moléculaire à la surface des glaces d'eau. Elle s'articule autour du dispositif FORMOLISM. Ultravide, cryogénie, jets atomiques, spectrométrie de masse et spectroscopie UV sont réunis pour étudier en particulier les effets de l'hétérogénéité et de la porosité de la surface. L'étude de la désorption de l'hydrogène moléculaire s'est révélée indispensable à l'interprétation des expériences de formation. Nous avons mesuré les distributions d'énergies d'adsorption de H2, HD et D2. Ces mesures permettent d'estimer la quantité d'hydrogène moléculaire en surface des grains interstellaires. La présence d'hydrogène moléculaire modifie l'efficacité de la réaction. Un mécanisme de ségrégation isotopique a été mis en évidence et son importance pour la deutération de l'hydrogène moléculaire en surface des manteaux de glace a été étudiée. Les expériences sur la formation révèlent que sur les glaces poreuses l'énergie dégagée par la réaction est transmise à la surface par la rétention des molécules formées. La réaction reste efficace à des températures plus élevées (20 K) que sur les glaces non poreuses (13 K). Sur ces dernières, les molécules formées sont directement libérées en phase gazeuse où elles sont détectées dans des états rovibrationnellement excités.The molecular hydrogen, a major component of the interstellar medium (ISM) is formed on dust grains through surface catalysis. This key reaction is a source of uncertainty in the description of the dynamic of the ISM, as weIl for the reaction efficiency and for its energy budget. This reaction and its sub-processes (sticking and diffusion on the grain surface, desorption) are studied here experimentally in the case of molecular hydrogen formation on water ice surfaces similar to those covering dust in the cold dense ISM. Ultra-high vacuum, cryogenies, atomic beams, mass spectrometry and UV spectroscopy are combined in the experimental setup named FÜRMüLISM. We have studied the effects of surface heterogeneity and porosity on the reaction. We have focused on molecular hydrogen desorption to improve the interpretation of formation experiments and because the molecular hydrogen adsorbed on grains modify the formation efficiency. Adsorption energies of R2, RD and D2 on these iees are obtained. A mechanism of isotopie segregation is highlighted and the deuteration of molecular hydrogen on ice mantles is quantified. The study of molecular formation reveals that on porous ices the reaction energy is released to the surface because of the efficient recapture of the formed molecules, and that the reaction is effective up to temperatures higher (20 K) than for nonporous ices (13 K). For these latter, the formed molecules are directly released into the gas phase where they are detected in rovibrationally excited states.CERGY PONTOISE-BU Neuville (951272102) / SudocSudocFranceF

Strain relaxation and epitaxial relationship of perylene overlayer on Ag(110)

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Electron induced decomposition towards quantitative data

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Electron induced decomposition towards quantitative data

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Electron Processing at 50 eV of Terphenylthiol Self-Assembled Monolayers: Contributions of Primary and Secondary Electrons

International audienceAromatic self-assembled monolayers (SAMs) can serve as platforms for development of supramolecular assemblies driven by surface templates. For many applications, electron processing is used to locally reinforce the layer. To achieve better control of the irradiation step, chemical transformations induced by electron impact at 50 eV of terphenylthiol SAMs are studied, with these SAMs serving as model aromatic SAMs. High-resolution electron energy loss spectroscopy (HREELS) and electron-stimulated desorption (ESD) of neutral fragment measurements are combined to investigate electron-induced chemical transformation of the layer. The decrease of the CH stretching HREELS signature is mainly attributed to dehydrogenation, without a noticeable hybridization change of the hydrogenated carbon centers. Its evolution as a function of the irradiation dose gives an estimate of the effective hydrogen content loss cross-section, σ = 2.7−4.7 × 10 −17 cm 2. Electron impact ionization is the major primary mechanism involved, with the impact electronic excitation contributing only marginally. Therefore, special attention is given to the contribution of the low-energy secondary electrons to the induced chemistry. The effective cross-section related to dissociative secondary electron attachment at 6 eV is estimated to be 1 order of magnitude smaller. The 1 eV electrons do not induce significant chemical modification for a 2.5 mC cm −2 dose, excluding their contribution

Physisorption and desorption of H 2 , HD and D 2 on amorphous solid water ice. Effect on mixing isotopologue on statistical population of adsorption sites

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Tuning the properties of molecular organic layers at the micron-scale using low-energy electrons.

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D2 desorption kinetics on amorphous solid water: from compact to porous ice films

International audienceThe desorption kinetics of D 2 from amorphous solid water (ASW) films have been studied by the temperature-programmed desorption (TPD) technique in the 10-30 K temperature range. Compact (and nonporous) films were grown at 120 K over a copper substrate. Ultra-thin porous films were additionally grown at 10 K over the compact base. The TPD spectra from compact and from up to 20 monolayers (ML) porous films were compared. The simulation of the TPD experimental traces provides the corresponding D 2 binding-energy distributions. As compared to the compact case, the binding-energy distribution found for the 10 ML porous film clearly extends to higher energies. To study the transition from compact to porous ice, porous films of intermediate thicknesses (o10 ML), including ultra-thin films (o1 ML), were grown over the compact substrate. The thermal D 2 desorption peak was found to shift to higher temperatures as the porous ice network was progressively formed. This behavior can be explained by the formation of more energetic binding sites related to porous films. TPD spectra were also modelled by using a combination of the two energy distributions, one associated to a bare compact ice and the other associated to a 10 ML porous ice film. This analysis reveals a very fast evolution of the binding-energy distribution towards that of porous ice. Our results show that few ML of additional porous film are sufficient to produce a sample for which the D 2 adsorption can be described by the energy distribution found for the 10 ML porous film. These experiments then provide evidence that the binding energy of D 2 on ASW ice is primarily governed by the topological and morphological disorder of the surface at molecular scale