2021 |
Basalgète, R., Dupuy, R., Féraud, G., Romanzin, C., Philippe, L., Michaut, X., Michoud, J., Amiaud, L., Lafosse, A., Fillion, J. - H., & Bertin, M. (2021). Complex organic molecules in protoplanetary disks: X-ray photodesorption from methanol-containing ices: I. Pure methanol ices. ASTRONOMY & ASTROPHYSICS, 647, A35.
Résumé: Context. Astrophysical observations show complex organic molecules (COMs) in the gas phase of protoplanetary disks. X-rays emitted from the central young stellar object that irradiate interstellar ices in the disk, followed by the ejection of molecules in the gas phase, are a possible route to explain the abundances observed in the cold regions. This process, known as X-ray photodesorption, needs to be quantified for methanol-containing ices. This Paper I focuses on the case of X-ray photodesorption from pure methanol ices.
Aims. We aim at experimentally measuring X-ray photodesorption yields (in molecule desorbed per incident photon, displayed as molecule/photon for more simplicity) of methanol and its photo-products from pure CH3OH ices, and to shed light on the mechanisms responsible for the desorption process.
Methods. We irradiated methanol ices at 15 K with X-rays in the 525–570 eV range from the SEXTANTS beam line of the SOLEIL synchrotron facility. The release of species in the gas phase was monitored by quadrupole mass spectrometry, and photodesorption yields were derived.
Results. Under our experimental conditions, the CH3OH X-ray photodesorption yield from pure methanol ice is ~10−2 molecule/photon at 564 eV. Photo-products such as CH4, H2CO, H2O, CO2, and CO also desorb at increasing efficiency. X-ray photodesorption of larger COMs, which can be attributed to either ethanol, dimethyl ether, and/or formic acid, is also detected. The physical mechanisms at play are discussed and must likely involve the thermalization of Auger electrons in the ice, thus indicating that its composition plays an important role. Finally, we provide desorption yields applicable to protoplanetary disk environments for astrochemical models.
Conclusions. The X-rays are shown to be a potential candidate to explain gas-phase abundances of methanol in disks. However, more relevant desorption yields derived from experiments on mixed ices are mandatory to properly support the role played by X-rays in nonthermal desorption of methanol (see Paper II).
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Basalgète, R., Dupuy, R., Féraud, G., Romanzin, C., Philippe, L., Michaut, X., Michoud, J., Amiaud, L., Lafosse, A., Fillion, J. - H., & Bertin, M. (2021). Complex organic molecules in protoplanetary disks: X-ray photodesorption from methanol-containing ices: II. Mixed methanol-CO and methanol-H<sub>2</sub>O ices. ASTRONOMY & ASTROPHYSICS, 647, A36.
Résumé: Context. Astrophysical observations show complex organic molecules (COMs) in the gas phase of protoplanetary disks. X-rays emitted from the central young stellar object (YSO) that irradiate interstellar ices in the disk, followed by the ejection of molecules in the gas phase, are a possible route to explain the abundances observed in the cold regions. This process, known as X-ray photodesorption, needs to be quantified for methanol-containing ices.
Aims. We aim at experimentally measuring X-ray photodesorption yields (in molecule desorbed per incident photon, displayed as molecule/photon for more simplicity) of methanol and its photo-products from binary mixed ices: 13CO:CH3OH ice and H2O:CH3OH ice.
Methods. We irradiated these ices at 15 K with X-rays in the 525–570 eV range from the SEXTANTS beam line of the SOLEIL synchrotron facility. The release of species in the gas phase was monitored by quadrupole mass spectrometry, and photodesorption yields were derived.
Results. For 13CO:CH3OH ice, CH3OH X-ray photodesorption yield is estimated to be ∼10−2 molecule/photon at 564 eV. X-ray photodesorption of larger COMs, which can be attributed to either ethanol, dimethyl ether, and/or formic acid, is detected with a yield of ∼10−3 molecule/photon. When methanol is mixed with water, X-ray photodesorption of methanol and of the previous COMs is not detected. X-ray induced chemistry, dominated by low-energy secondary electrons, is found to be the main mechanism that explains these results. We also provide desorption yields that are applicable to protoplanetary disk environments for astrochemical models.
Conclusions. The X-ray emission from YSOs should participate in the enrichment of the protoplanetary disk gas phase with COMs such as methanol in the cold and X-ray dominated regions because of X-ray photodesorption from methanol-containing ices.
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Rathore, A., Cipriani, M., Huang, C. - C., Amiaud, L., Dablemont, C., Lafosse, A., Ingolfsson, O., De Simone, D., & De Gendt, S. (2021). Electron-induced fragmentation mechanisms in organic monomers and their implications for photoresist optimization for EUV lithography. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 23, 9228–9234.
Résumé: Secondary electrons generated during the Extreme Ultraviolet Lithography (EUVL) process are predominantly responsible for inducing important patterning chemistry in photoresist films. Therefore, it is crucial to understand the electron-induced fragmentation mechanisms involved in EUV-resist systems to improve their patterning performance. To facilitate this understanding, mechanistic studies were carried out on simple organic EUV-resist monomers, methyl isobutyrate (MIB) and methacrylic acid (MAA), both in the condensed and gas phases. Electron-stimulated desorption (ESD) studies on MIB in the condensed phase showed desorption peaks at around 2 and 9 eV electron energies. The gas-phase study on MIB showed that the monomer followed the dissociative ionization (DI) fragmentation pathway, under single collision conditions, which opened up at electron energies above about 11 eV. No signs of dissociative electron attachment (DEA) were detected for MIB in the gas phase under single collision conditions. However, DEA was an active process in MAA in the gas phase under single collision conditions at around 2 eV, showing that slight modifications of the molecular structures of photoresists may serve to sensitize them to certain electron-induced processes.
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2018 |
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Sala, L., Szymańska, I. B., Dablemont, C., Lafosse, A., & Amiaud, L. (2018). Response under low-energy electron irradiation of a thin film of a potential copper precursor for focused electron beam induced deposition (FEBID). Beilstein J. Nanotechnol., 9, 57–65.
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2017 |
Houdoux, D., Houplin, J., Amiaud, L., Lafosse, A., & Dablemont, C. (2017). Interfacial water on organic substrates at cryogenic temperatures: hydrogen bonding and quantification in the submonolayer regime. Phys Chem Chem Phys, 19(3), 2304–2312.
Résumé: Water molecules were used to probe the physical and chemical properties of a model hydrophilic organic organized layer. To this end, H2O adsorption on mercaptoundecanoic acid self-assembled monolayers (SAMs) was investigated at the molecular level under ultra-high vacuum by high resolution electron energy loss spectroscopy (HREELS), through the sensitivity of the water OH stretching modes to the molecular environment. The water interfacial layer formation and structure were studied upon deposition at 28 K. A direct sensitive quantification in the submonolayer regime (10-80% of completion) was achieved by the sole measurement of the OH stretching mode frequencies, and the dominant basic (-COO(-))/acidic (-COOH) forms of the terminal functions could be probed. The surface densities of the water interfacial layer and the SAM terminal functions were measured independently, and demonstrated to be comparable. This means that the SAM terminal functions provided anchors for water adsorption through two hydrogen bonds and that the SAM acted as a template even at 28 K. Upon annealing at 110 K, the water molecules were observed to form clusters of higher molecular density, dewetting the supporting substrate. Finally, the vanishing of the supporting substrate vibrational signature, due to the masking effect by the deposited water layer, was used to estimate the depth probed by HREELS through water layers to be 11 +/- 2 A.
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2015 |
Amiaud, L., Fillion, J. - H., Dulieu, F., Momeni, A., & Lemaire, J. - L. (2015). Physisorption and desorption of H2, HD and D2 on amorphous solid water ice. Effect on mixing isotopologue on statistical population of adsorption sites. Phys Chem Chem Phys, 17(44), 30148–30157.
Résumé: We study the adsorption and desorption of three isotopologues of molecular hydrogen mixed on 10 ML of porous amorphous water ice (ASW) deposited at 10 K. Thermally programmed desorption (TPD) of H2, D2 and HD adsorbed at 10 K have been performed with different mixings. Various coverages of H2, HD and D2 have been explored and a model taking into account all species adsorbed on the surface is presented in detail. The model we propose allows to extract the parameters required to fully reproduce the desorption of H2, HD and D2 for various coverages and mixtures in the sub-monolayer regime. The model is based on a statistical description of the process in a grand-canonical ensemble where adsorbed molecules are described following a Fermi-Dirac distribution.
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Houplin, J., Amiaud, L., Dablemont, C., & Lafosse, A. (2015). DOS and electron attachment effects in the electron-induced vibrational excitation of terphenylthiol SAMs. Phys Chem Chem Phys, 17(45), 30721–30728.
Résumé: Low energy electron scattering on terphenylthiol (TPT, HS-(C6H4)2-C6H5) self-assembled monolayers (SAMs) deposited onto gold was investigated using high resolution electron energy loss spectroscopy (HREELS) by recording specular elastic and inelastic excitation functions. The electron elastic reflectivity could be directly compared to the sample density-of-states (DOS) above vacuum level. A high reflectivity region was observed in the range 7.2-8.6 eV. Inelastic excitation functions were studied to get insights into the mechanisms involved in the excitation of a selection of vibrational modes (dipolar and impact scattering). In particular, a resonant mechanism was observed in the excitation of the stretching mode nu(CC) at 196 meV. The purely resonant contribution to the electron-induced excitation of the stretching modes nu(CH) (379 meV) could be extracted from the overtone excitation. It is located at 7.2 eV above the vacuum level and is characterized by a width of 3.4 eV.
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Houplin, J., Amiaud, L., Sedzik, T., Dablemont, C., Teillet-Billy, D., Rougeau, N., & Lafosse, A. (2015). A combined DFT/HREELS study of the vibrational modes of terphenylthiol SAMs. Eur. Phys. J. D, 69(9), 9 pp.
Résumé: Self-assembled monolayers of p-terphenylthiol (TPT, HS-(C6H4)(2)-C6H5) deposited onto gold can serve as model systems for aromatic lithography resists. Such thin molecular films are suitably probed using high resolution electron energy loss spectroscopy, due to its high surface sensitivity. Extended energy loss spectra were measured at different probing energies. The TPT monolayer overlapping.(CH) stretching modes could be modelled by a single effective anharmonic oscillator sustained by a Morse potential energy curve, thanks to the resonant excitation of the associated overtone series at 6 eV. A remarkably good agreement was obtained between the TPT monolayer energy loss spectrum and the computer-simulated infrared vibrational spectrum of the isolated TPT molecule. Density Functional Theory calculations for TPT, fully deuterated TPT and benzenethiol isolated molecules were performed with the exchange correlation functional B3LYP and a dispersion correction, using a triple zeta+ polarisation basis set. By comparing the vibrational patterns obtained for these parent systems, (re-) assignments of all the features observed in the TPT self-assembled monolayer energy loss spectrum are discussed. The obtained vibrational assignments can be confidently transposed to other related systems, such as benzenethiol and biphenyl self-assembled monolayers.
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Houplin, J., Dablemont, C., Sala, L., Lafosse, A., & Amiaud, L. (2015). Electron Processing at 50 eV of Terphenylthiol Self-Assembled Monolayers: Contributions of Primary and Secondary Electrons. Langmuir, 31(50), 13528–13534.
Résumé: Aromatic 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, sigma = 2.7-4.7 x 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.
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2014 |
Amiaud, L., Houplin, J., Bourdier, M., Humblot, V., Azria, R., Pradier, C. - M., & Lafosse, A. (2014). Low-energy electron induced resonant loss of aromaticity: consequences on cross-linking in terphenylthiol SAMs. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 16(3), 1050–1059.
Résumé: Aromatic self-assembled monolayers (SAMs) can be used as negative tone electron resists in functional surface lithographic fabrication. A dense and resistant molecular network is obtained under electron irradiation through the formation of a cross-linked network. The elementary processes and possible mechanisms involved were investigated through the response of a model aromatic SAM, p-terphenylthiol SAM, to low-energy electron (0-10 eV) irradiation. Energy loss spectra as well as vibrational excitation functions were measured using High Resolution Electron Energy Loss Spectroscopy (HREELS). A resonant electron attachment process was identified around 6 eV through associated enhanced excitation probability of the CH stretching modes [small nu](CH)ph at 378 meV. Electron irradiation at 6 eV was observed to induce a peak around 367 meV in the energy loss spectra, attributed to the formation of sp3-hybridized CHx groups within the SAM. This partial loss of aromaticity is interpreted to be the result of resonance formation, which relaxes by reorganization and/or CH bond dissociation mechanisms followed by radical chain reactions. These processes may also account for cross-linking induced by electron irradiation of aromatic SAMs in general.
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Piekarczyk, A., Bald, I., Flosadóttir, H. D., Ómarsson, B., Lafosse, A., & Ingólfsson, O. (2014). Influence of metal ion complexation on the metastable fragmentation of DNA hexamers. Eur. Phys. J. D, 68(6), 146.
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2013 |
Houplin, J., Amiaud, L., Humblot, V., Martin, I., Matar, E., Azria, R., Pradier, C. M., & Lafosse, A. (2013). Selective terminal function modification of SAMs driven by low-energy electrons (0-15 eV). Phys. Chem. Chem. Phys., 15(19), 7220–7227.
Résumé: Low-energy electron induced degradation of a model self-assembled monolayer (SAM) of acid terminated alkanethiol was studied under ultra-high vacuum (UHV) conditions at room and low (similar to 40 K) temperatures. Low-energy electron induced chemical modifications of 11-mercaptoundecanoic acid (MUA, HS-(CH2)(10)-COOH) SAMs deposited on gold were probed in situ as a function of the irradiation energy (<11 eV) by combining two complementary techniques: High Resolution Electron Energy Loss Spectroscopy (HREELS), a surface sensitive vibrational spectroscopy technique, and Electron Stimulated Desorption (ESD) analysis of neutral fragments. The SAM's terminal functions were observed to be selectively damaged at around 1 eV by a resonant electron attachment mechanism, observed to decay by CO, CO2 and H2O formation and desorption. CO2 and H2O were also directly identified at low temperature by vibrational analysis of the irradiated SAMs. At higher irradiation energy, both terminal functions and spacer alkyl chains are damaged upon electron irradiation, by resonant and non-resonant processes.
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2011 |
Amiaud, L., Martin, I., Milosavljevic, A. R., Michaelson, S., Hoffman, A., Azria, R., & Lafosse, A. (2011). Low-energy electron scattering on deuterated nanocrystalline diamond films-a model system for understanding the interplay between density-of- states, excitation mechanisms and surface versus lattice contributions. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 13(24), 11495–11502.
Résumé: Electron energy loss spectrum, elastic reflectivity and selected vibrational excitation functions were measured by High Resolution Electron Energy Loss Spectroscopy (HREELS) for deuterated nanocrystalline dc GD CVD diamond films. The electron elastic reflectivity is strongly enhanced at about 13 eV, as a consequence of the second absolute band gap of diamond preserved up to the surface for D-nano-crystallites. The pure bending modes delta(CD(x)) at 88 meV and 107 meV are dominantly excited through the impact mechanism and their vibration excitation functions mimic the electron elastic reflectivity curve. Pure diamond phonon mode nu(CC) can be probed through the resolved fundamental loss located at 152 meV and through the multiple loss located at 300 meV. In addition to the well-known 8 eV resonance, two supplementary resonances located at 4.5 eV and 11.5 eV were identified and clearly resolved for the first time. A comprehensive set of data is now available on low-energy electron scattering at hydride terminated polycrystalline diamond films grown either by HF (microcrystalline) or dc GD (nanocrystalline) chemical vapour deposition. The careful comparison of the vibrational excitation functions for hydrogen/deuterium termination stretching modes nu(sp(3)-CH(x)) and nu(sp(3)-CDx), for hydrogen termination bending modes delta(CH(x)) mixed with diamond lattice modes nu(CC), for deuterium termination bending modes delta(CD(x)), and for multiple loss 2 nu(CC) demonstrates the close interplay between three characteristics: (i) the density-of-states of the substrate, (ii) the vibrational excitation mechanisms (dipolar and/or impact scattering including resonant scattering) and (iii) the surface versus lattice character of the excited vibrational modes. This work shows clearly that excitation function measurement provides a powerful and sensitive tool to clarify loss attributions, involved excitation mechanisms, and surface versus lattice characters of the excited vibrational modes.
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Azria, R., Lafosse, A., Amiaud, L., Michaelson, S., & Hoffman, A. (2011). Hydrogenated polycrystalline diamond films: Elastic and inelastic electron reflectivity. PROGRESS IN SURFACE SCIENCE, 86(5-8), 94–114.
Résumé: The microstructure and properties of carbon-based thin films depend on the deposition process and conditions used, including pressure, gas phase composition, and substrate temperature, as well as the energy of the reactive species (atoms or ions). For instance concerning diamond films, each method results in different type of films which may differ in terms of diamond grain size (from nano to micro), grain boundary nature, hydrogen content, defect density, amorphous or graphitic components, morphological properties and different chemical and physical properties. Among them, the well-known negative electron affinity, very attractive for the detection and emission of electrons, and high conductivity of diamond surfaces are properties of fully hydrogenated diamond surfaces. Similarly, diamond grain size may influence the electronic and optical properties of the films. More generally the chemical and physical characterization of the uppermost surface atomic layer of diamond films presents a great challenge. In this review we present results on hydrogen bonding configuration in hydrogenated polycrystalline diamond films of varying size (few nanometers up to micrometers) obtained by high resolution electron energy loss spectroscopy (HREELS). More precisely we will present energy loss spectra extended up to 800 meV, as well as elastic and inelastic reflectivity curves (associated to different vibrational modes of hydrogenated diamond surfaces), measured over the 3-18 eV electron energy range. We will show in particular that due to the specific features of diamond bulk electronic band structure, which is maintained up to the surface in the case of fully hydrogenated diamond, it is possible to extract from these data valuable information about the surface properties and composition such as diamond or graphitic like nature of the films, surface versus lattice nature of the vibrational modes. (C) 2011 Elsevier Ltd. All rights reserved.
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Kristensen, L. E., Amiaud, L., Fillion, J. - H., Dulieu, F., & Lemaire, J. - L. (2011). H-2, HD, and D-2 abundances on ice-covered dust grains in dark clouds. ASTRONOMY & ASTROPHYSICS, 527.
Résumé: Aims. We seek to study the abundances of H-2, HD, and D-2 adsorbed onto ice-covered dust grains in dark molecular clouds in the interstellar medium. Methods. We use our previously developed detailed model describing temperature-programmed desorption (TPD) experiments of H-2 and its isotopologues on water ice. We here extrapolate these model results from laboratory conditions to conditions similar to those found in dark molecular clouds. Results. By means of our model we are able to infer three important results. (i) The time scale for H-2 and isotopologues to accrete onto dust grains is less than 10(4) yrs. (ii) Due to the higher binding energy of D-2 with respect to HD, D-2 becomes the most abundant deuterated species on grains by similar to 50% with respect to HD (a few times 10(-5) with respect to H-2). (iii) The surface coverage of D-2 as a function of temperature shows that at very low temperatures (i. e., less than 10 K), D-2 may be two orders of magnitude more abundant than HD. Possible implications for deuteration of water on grain surfaces are discussed when it forms through reactions between OH and H-2.
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2009 |
Fillion, J. H., Amiaud, L., Congiu, E., Dulieu, F., Momeni, A., & Lemaire, J. L. (2009). D-2 desorption kinetics on amorphous solid water: from compact to porous ice films. Phys. Chem. Chem. Phys., 11(21), 4396–4402.
Résumé: The 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 (<10 ML), including ultra-thin films (<1 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.
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Lafosse, A., Bertin, M., & Azria, R. (2009). Electron driven processes in ices: Surface functionalization and synthesis reactions. PROGRESS IN SURFACE SCIENCE, 84(5-6), 177–198.
Résumé: The ability to control and orientate chemical reactivity in the condensed phase is a major challenge of modern research. Upon interaction with condensed molecules electrons drive bond cleavage thus generating a population of very reactive species in the condensed medium. These reactive species may interact either within the volume leading to the synthesis of new molecules or with the substrate surface by forming strong chemical bonds. The former reaction is known as electron induced synthesis and the latter one as electron induced surface functionalization. High-energy electrons achieve only a low chemical specificity due to the large number of dissociating open channels. in contrast, electrons with energies below ionization threshold of the irradiated matter are capable of high selectivity because of the dissociative electron attachment mechanism. In this review recent studies of electron interaction with condensed molecules on hydrogenated diamond substrates will be described. In particular electron induced functionalization of diamond surfaces by CH(2)CN groups, decarboxylation reactions in condensed films of pure organic acids RCOOH (R = H, CH(3), C(2)H(5), CFA carbamic acid formation in CO(2):NH(3), HCOOH:NH(3) and CF(3)COOH:NH(3) binary ice mixtures, and glycine formation in a CH(3)COOD:NH(3) Mixture are presented and discussed. (c) 2009 Elsevier Ltd. All rights reserved.
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Lafosse, A., Bertin, M., Hoffman, A., & Azria, R. (2009). Surface functionalization by low-energy electron processing of molecular ices. SURFACE SCIENCE, 603(10-12), 1873–1877.
Résumé: Low-energy electron processing of condensed molecular films (also called ices) is an efficient method to induce functionalization of a substrate, and thereby to modify and adjust its electronic and chemical properties. This method takes advantage of a resonant mechanism specific to low-energy electrons (by opposition to photons), the dissociative electron attachment (DEA), the key process allowing the chemical selectivity to be directly and easily controlled by the kinetic energy of the processing electrons. The functionalization procedure is described and illustrated by high resolution electron energy loss spectroscopy (HREELS) results on the induced anchoring of CH(2)CN organic chains on synthetic diamond by electron irradiation at 2 eV of condensed acetonitrile. The range of application of the proposed functionalization method is worth to be extended to other organic/inorganic interfaces, such as organic layers on metallic and semiconducting substrates. (C) 2009 Elsevier B.V. All rights reserved.
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2008 |
Abouaf, R., & Teillet-Billy, D. (2008). Low energy electron collisions in H2S and H2Se: Structure in dissociative attachment cross-sections. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY, 277(1-3), 79–83.
Résumé: Dissociative electron attachment between 0 and 4 eV has been investigated in hydrogen sulfide and hydrogen selenide with an improved electron resolution (0.040 eV). HS- and HSe- cross-sections versus electron energy present vertical onsets revealing that the potential surfaces of the resonances which are reached around 2 eV are bound. A well-developed and intriguing structure is observed in HS-, S-, HSe- and Se- cross-sections. It could reveal interferences due to an attractive resonance having a lifetime of the order of one vibrational period. The strong similarity between the anion behaviour in H2S and H2Se is in contrast with H2O where no dissociative attachment process occurs in this energy range. (C) 2008 Elsevier B.V. All rights reserved.
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Abouaf, R., Ptasinska, S., & Teillet-Billy, D. (2008). Low energy electron impact on gas phase 5-nitrouracil. CHEMICAL PHYSICS LETTERS, 455(4-6), 169–173.
Résumé: Excitation of the first electronic states, vibrational excitation between 1 and 3 eV, and negative ion formation (0-2 eV) in gas phase 5-nitrouracil are presented. Five singlet states are clearly identified at 4.76; 5.72; 6.82; 7.82 and 9.2 eV. A band at 3.7 eV could be assigned to a triplet state. The vibrational modes observed indicate resonant states having both pi* and sigma* character. The most intense anions (M-NO2) , (M-H) appear at zero energy with huge cross sections (>= 10 18 m(2)). Anion cross sections versus electron energy reveal structures interpreted using the anion energy thresholds found by DFT calculations. (C) 2008 Elsevier B.V. All rights reserved.
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Amiaud, L., Momeni, A., Dulieu, F., Fillion, J. H., Matar, E., & Lemaire, J. L. (2008). Measurement of the adsorption energy difference between ortho- and para-D(2) on an amorphous ice surface. Phys. Rev. Lett., 100(5), 056101.
Résumé: Molecular hydrogen interaction on water ice surfaces is a major process taking place in interstellar dense clouds. By coupling laser detection and classical thermal desorption spectroscopy, it is possible to study the effect of rotation of D(2) on adsorption on amorphous solid water ice surfaces. The desorption profiles of ortho- and para-D(2) are different. This difference is due to a shift in the adsorption energy distribution of the two lowest rotational states. Molecules in J('')=1 rotational state are on average more strongly bound to the ice surface than those in J('')=0 rotational state. This energy difference is estimated to be 1.4 +/- 0.3 meV. This value is in agreement with previous calculation and interpretation. The nonspherical wave function J('')=1 has an interaction with the asymmetric part of the adsorption potential and contributes positively in the binding energy.
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Burean, E., Abouaf, R., Lafosse, A., Azria, R., & Swiderek, P. (2008). High-resolution vibrational electron-energy-loss spectroscopy of gaseous and solid glycine. JOURNAL OF PHYSICAL CHEMISTRY C, 112(25), 9405–9411.
Résumé: The vibrational spectra of thin solid films of glycine deposited at temperatures between 12 and 130 K, as well as gaseous glycine, have been measured by high-resolution electron-energy-loss (HREEL) spectroscopy. The spectra are dominated by hydrogen-bonded glycine dimers when the films are deposited at low temperatures. The HREEL spectra of films deposited at higher temperature (130 K) have a different habitus that can be explained on the basis of a proton transfer reaction leading to the formation of zwitterionic glycine. Gas-phase HREEL spectra show bands characteristic of the neutral monomer of glycine. The results are discussed in relation to a previous study aiming at glycine synthesis in mixed ices under the effect of low-energy electrons.
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Hoffman, A., Lafosse, A., Michaelson, S., Bertin, M., & Azria, R. (2008). Nano size effects in the high resolution electron energy loss spectra and excitation function of hydrogenated diamond films. SURFACE SCIENCE, 602(18), 3026–3032.
Résumé: The present work studies the high resolution electron energy loss spectra (HR-EELS) and excitation function of hydrogenated diamond films composed of diamond grains of varying size of similar to 300, similar to 20 nm and similar to 5 nm. It is reported that HR-EELS modes associated with C-C lattice phonons are comparably more intense than modes associated with the C-H surface vibrational modes in the case of nano-crystal line compared to micro-crystalline diamond films. Excitation function measurements of the different modes show a pronounced maximum at similar to 8 eV for the nano-diamond films, whereas for micro-crystalline films similar modes showed maximum intensities at 13 eV with a smaller contribution at 8 eV. It is reported that the elastic electron reflectivity is substantially larger for the nano-diamond films, showing broad maxima at similar to 8 and similar to 13 eV in both cases. It is suggested that these effects are associated with the larger probability for inelastic scattering of electrons by lattice phonons mediated by scattering with internal grain boundaries and defects in the case of nano-crystalline diamond films. (C) 2008 Elsevier B.V. All rights reserved.
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Lafosse, A., Bertin, M., Michaelson, S., Azria, R., Akhvlediani, R., & Hoffman, A. (2008). Surface defects induced by in-situ annealing of hydrogenated polycrystalline diamond studied by high resolution electron energy loss spectroscopy. DIAMOND AND RELATED MATERIALS, 17(6), 949–953.
Résumé: Plasma hydrogenation of polycrystalline diamond films results in a fully hydrogenated well-ordered diamond surface and etching of the amorphous phase located at grain boundaries. Vacuum annealing to 1000 degrees C followed by in-situ hydrogenation by thermal activated hydrogen of the bared diamond surface results in the formation of sp(3)-CH(x) adsorbed groups located on the top surface. Annealing of the in-situ hydrogenated surface to 600 degrees C results in desorption of these species and partial reconstruction of the film surface. Some irreversible surface degradation was detected in the in-situ annealed and hydrogenated surface compared to the state of the surface obtained upon plasma hydrogenation. (C) 2008 Elsevier B.V. All rights reserved.
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Lemaire, J. L., Fillion, J. H., Dulieu, F., Momeni, A., Baouche, S., Amiaud, L., & Cobut, V. (2008). Formation of molecules in the interstellar medium. In search for formation mechanisms of molecular H2 on the interstellar dust grains: the FORMOLISM experiment. Actual Chim., (315), III-IX.
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2007 |
Amiaud, L., Dulieu, F., Fillion, J. H., Momeni, A., & Lemaire, J. L. (2007). Interaction of atomic and molecular deuterium with a nonporous amorphous water ice surface between 8 and 30 K. J. Chem. Phys., 127(14), 12 pp.
Résumé: Molecular and atomic interactions of hydrogen on dust grains covered with ice at low temperatures are key mechanisms for star formation and chemistry in dark interstellar clouds. We have experimentally studied the interaction of atomic and molecular deuterium on nonporous amorphous water ice surfaces between 8 and 30 K, in conditions compatible with an extrapolation to an astrophysical context. The adsorption energy of D(2) presents a wide distribution, as already observed on porous water ice surfaces. At low coverage, the sticking coefficient of D(2) increases linearly with the number of deuterium molecules already adsorbed on the surface. Recombination of atomic D occurs via a prompt reaction that releases molecules into the gas phase. Part of the newly formed molecules are in vibrationally excited states (v=1-7). The atomic recombination efficiency increases with the presence of D(2) molecules already adsorbed on the water ice, probably because these increase the sticking coefficient of the atoms, as in the case of incident D(2). We have measured the atomic recombination efficiency in the presence of already absorbed D(2), as it is expected to occur in the interstellar medium. The recombination efficiency decreases rapidly with increasing temperature and is zero at 13 K. This allows us to estimate an upper limit to the value of the atom adsorption energy E(a)similar to 29 meV, in agreement with previous calculations. (C) 2007 American Institute of Physics.
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2006 |
Amiaud, L., Fillion, J. H., Baouche, S., Dulieu, F., Momeni, A., & Lemaire, J. L. (2006). Interaction of D-2 with H2O amorphous ice studied by temperature-programed desorption experiments. J. Chem. Phys., 124(9), 9 pp.
Résumé: The gas-surface interaction of molecular hydrogen D-2 with a thin film of porous amorphous solid water (ASW) grown at 10 K by slow vapor deposition has been studied by temperature-programed-desorption (TPD) experiments. Molecular hydrogen diffuses rapidly into the porous network of the ice. The D-2 desorption occurring between 10 and 30 K is considered here as a good probe of the effective surface of ASW interacting with the gas. The desorption kinetics have been systematically measured at various coverages. A careful analysis based on the Arrhenius plot method has provided the D-2 binding energies as a function of the coverage. Asymmetric and broad distributions of binding energies were found, with a maximum population peaking at low energy. We propose a model for the desorption kinetics that assumes a complete thermal equilibrium of the molecules with the ice film. The sample is characterized by a distribution of adsorption sites that are filled according to a Fermi-Dirac statistic law. The TPD curves can be simulated and fitted to provide the parameters describing the distribution of the molecules as a function of their binding energy. This approach contributes to a correct description of the interaction of molecular hydrogen with the surface of possibly porous grain mantles in the interstellar medium. (c) 2006 American Institute of Physics.
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2005 |
Dulieu, F., Amiaud, L., Baouche, S., Momeni, A., Fillion, J. H., & Lemaire, J. L. (2005). Isotopic segregation of molecular hydrogen on water ice surface at low temperature. Chem. Phys. Lett., 404(1-3), 187–191.
Résumé: Experimental studies of adsorption and desorption of molecular hydrogen on an amorphous porous solid water ice surface between 10 and 35 K reveal a very efficient isotopic segregation process. A statistical model, which take into account thermodynamic aspects of adsorption sites and isotopic competition, is proposed to understand the enhancement of deuterium fractionation. This mechanism could play a key role in chemistry at the surface of interstellar dust grains. (C) 2005 Elsevier B.V. All rights reserved.
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