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International audience; Peptide chain formation from amino acids such as glycine is a key step in the emergence of life. Unlike their synthesis by living systems, how peptide chains grow under abiotic conditions is an open question given the variety of organic compounds discovered in various astrophysical environments, comets and meteorites. We propose a new abiotic route in the presence of protonated molecular dimers of glycine in a cold gaseous atmosphere without further need for a solid catalytic substrate. The results provide evidence for the preferential formation of mixed protonated dimers of glycine consisting of a dipeptide and a glycine molecule instead of pure protonated glycine dimers. Additional measurements mimicking a cosmic-ray impact in terms of internal excitation show that a single gas-phase collision induces polymerization via dehydration in both the mixed and pure dimer ions. Peptide chain growth is thus demonstrated to occur via a unimolecular gas-phase reaction in an excited cluster ion.

International audience; Nitroimidazolic radiosensitizers are used in radiation therapy to selectively sensitize cancer cells deprived of oxygen, and the actual mechanism of radiosensitization is still not understood. Selecting five radiosensitizers (1-methyl-5-nitroimidazole, ronidazole, ornidazole, metronidazole, and nimorazole) with a common 5-nitroimidazolic ring with different substitutions at N1 and C2 positions of the imidazole moiety, we investigate here their binding to nucleobases (A, T, G, and C) and nucleosides (As, Td, Gs, and Cd) via the positive electrospray ionization mass spectrometry experiments. In addition, quantum chemical calculations at the M062x/6-311+G(d,p) level of theory and basis set were used to determine binding energies of the proton bound dimers of a radiosensitizer and a nucleobase. The positive electrospray ionization leads to the formation of proton bound dimers of all radiosensitizers except 1-methyl-5-nitroimidazole in high abundance with C and smaller abundance with G. Ronidazole and metronidazole formed less abundant dimers also with A, while no dimers were observed to be formed at all with T. In contrast to the case of the nucleoside Td, the dimer intensity is as high as that with Cd, while the abundance of the dimer with Gs is smaller than that of the former. The experimental results are consistent with the calculations of binding energies suggesting proton bound dimers with C and G to be the strongest bound ones. Finally, a barrier-free proton transfer is observed when protonated G or C approaches the nitroimidazole ring.

International audience; Nitroimidazoles are important compounds in medicine, biology, and the food industry. The growing need for their structural assignment, as well as the need for the development of the detection and screening methods, provides the motivation to understand their fundamental properties and reactivity. Here, we investigated the decomposition of protonated ronidazole [Roni+H]+ in low-energy and high-energy collision-induced dissociation (CID) experiments. Quantum chemical calculations showed that the main fragmentation channels involve intramolecular proton transfer from nitroimidazole to its side chain followed by a release of NH2CO2H, which can proceed via two pathways involving transfer of H+ from (1) the N3 position via a barrier of TS2 of 0.97 eV, followed by the rupture of the C–O bond with a thermodynamic threshold of 2.40 eV; and (2) the –CH3 group via a higher barrier of 2.77 eV, but with a slightly lower thermodynamic threshold of 2.24 eV. Electrospray ionization of ronidazole using deuterated solvents showed that in low-energy CID, only pathway (1) proceeds, and in high-energy CID, both channels proceed with contributions of 81% and 19%. While both of the pathways are associated with small kinetic energy release of 10–23 meV, further release of the NO• radical has a KER value of 339 meV.

Significance Gas-to-particle conversion, also known as atmospheric aerosol nucleation, is responsible for about half of the global cloud condensation nuclei. It has been further argued that homogenous ternary nucleation including water is the major pathway for atmospheric aerosol formation. In contrast to earlier results on pure protonated water clusters, the shape of the measured velocity distributions of water molecules evaporated from excited water clusters doped with pyridinium, an abundant anthropogenic ion in the troposphere, shows evidence for out-of-equilibrium evaporation prior to thermalization. Water molecule evaporation is found to be much slower for the doped clusters, indicating in turn how such contaminants facilitate the growth of water clusters and consequently the nucleation processes at the early stages of atmospheric aerosol formation., Atmospheric aerosols are one of the major factors affecting planetary climate, and the addition of anthropogenic molecules into the atmosphere is known to strongly affect cloud formation. The broad variety of compounds present in such dilute media and their specific underlying thermalization processes at the nanoscale make a complete quantitative description of atmospheric aerosol formation certainly challenging. In particular, it requires fundamental knowledge about the role of impurities in water cluster growth, a crucial step in the early stage of aerosol and cloud formation. Here, we show how a hydrophobic pyridinium ion within a water cluster drastically changes the thermalization properties, which will in turn change the corresponding propensity for water cluster growth. The combination of velocity map imaging with a recently developed mass spectrometry technique allows the direct measurement of the velocity distribution of the water molecules evaporated from excited clusters. In contrast to previous results on pure water clusters, the low-velocity part of the distributions for pyridinium-doped water clusters is composed of 2 distinct Maxwell–Boltzmann distributions, indicating out-of-equilibrium evaporation. More generally, the evaporation of water molecules from excited clusters is found to be much slower when the cluster is doped with a pyridinium ion. Therefore, the presence of a contaminant molecule in the nascent cluster changes the energy storage and disposal in the early stages of gas-to-particle conversion, thereby leading to an increased rate of formation of water clusters and consequently facilitating homogeneous nucleation at the early stages of atmospheric aerosol formation.

International audience; Velocity distributions of neutral water molecules evaporated after collision induced dissociation of protonated water clusters H+(H2O)n≤10 were measured using the combined correlated ion and neutral fragment time-of-flight (COINTOF) and velocity map imaging (VMI) techniques. As observed previously, all measured velocity distributions exhibit two contributions, with a low velocity part identified by statistical molecular dynamics (SMD) simulations as events obeying the Maxwell–Boltzmann statistics and a high velocity contribution corresponding to non-ergodic events in which energy redistribution is incomplete. In contrast to earlier studies, where the evaporation of a single molecule was probed, the present study is concerned with events involving the evaporation of up to five water molecules. In particular, we discuss here in detail the cases of two and three evaporated molecules. Evaporation of several water molecules after CID can be interpreted in general as a sequential evaporation process. In addition to the SMD calculations, a Monte Carlo (MC) based simulation was developed allowing the reconstruction of the velocity distribution produced by the evaporation of m molecules from H+(H2O)n≤10 cluster ions using the measured velocity distributions for singly evaporated molecules as the input. The observed broadening of the low-velocity part of the distributions for the evaporation of two and three molecules as compared to the width for the evaporation of a single molecule results from the cumulative recoil velocity of the successive ion residues as well as the intrinsically broader distributions for decreasingly smaller parent clusters. Further MC simulations were carried out assuming that a certain proportion of non-ergodic events is responsible for the first evaporation in such a sequential evaporation series, thereby allowing to model the entire velocity distribution.

Measurement of velocity distributions of evaporated water monomers from small mass- and energy-selected protonated water clusters allows probing the extent of thermalization after excitation of these ultimately small nanodroplets. Electronic excitation of a molecule in the cluster is here induced by a single collision with an argon atom in the keV energy range. The measured velocity distributions of the departing neutral molecules exhibit bimodal shapes with a lower-velocity part consistent with a complete redistribution of the deposited energy in the entire cluster and a higher-velocity contribution corresponding to evaporation before complete energy redistribution. Statistical molecular dynamics calculations reproduce the bimodal shape of the velocity distributions by assuming an initial spreading of the excitation energy among all modes, thereby reproducing the lower velocity contribution of the distribution. By contrast, assuming the deposited energy to be initially localized among the modes of a single molecule leads to calculated distributions with two components whose shape is in accordance with the experimental results. The characteristics and the relative abundance of these two contributions in the velocity distributions obtained are presented and discussed as a function of the number of molecules (n = 2-10) in the ionized nanodroplet H+(H2O)n.Measurement of velocity distributions of evaporated water monomers from small mass- and energy-selected protonated water clusters allows probing the extent of thermalization after excitation of these ultimately small nanodroplets. Electronic excitation of a molecule in the cluster is here induced by a single collision with an argon atom in the keV energy range. The measured velocity distributions of the departing neutral molecules exhibit bimodal shapes with a lower-velocity part consistent with a complete redistribution of the deposited energy in the entire cluster and a higher-velocity contribution corresponding to evaporation before complete energy redistribution. Statistical molecular dynamics calculations reproduce the bimodal shape of the velocity distributions by assuming an initial spreading of the excitation energy among all modes, thereby reproducing the lower velocity contribution of the distribution. By contrast, assuming the deposited energy to be initially localized among the modes of a sing...

The velocity of a molecule evaporated from a mass-selected protonated water nanodroplet is measured by velocity map imaging in combination with a recently developed mass spectrometry technique. The measured velocity distributions allow probing statistical energy redistribution in ultimately small water nanodroplets after ultrafast electronic excitation. As the droplet size increases, the velocity distribution rapidly approaches the behavior expected for macroscopic droplets. However, a distinct high-velocity contribution provides evidence of molecular evaporation before complete energy redistribution, corresponding to non-ergodic events.

International audience; How the information obtained from the gas phase experiments can reflect the processes in solution is a crucial question for analytical chemistry, and particularly the selective host–guest recognition mechanisms which are fundamental in biology. Here we combine ElectroSpray Ionization mass spectrometry (ESI-MS) and the Collision Induced Dissociation (CID) experiments to the density functional theory to investigate the interaction of acetylcholine and the choline cation with a triphosphonate cavitand. While the relative abundance of the cation complexes in the ESI mass spectrum reflects the preferential capture of the acetylcholine ion over the choline ion by the cavitand in the solution, the gas phase CID measurements indicate that after desolvation the choline cation is the most strongly bound to the host. The experimental results are interpreted by theory that underlines the role of the counterion in the stabilization of the complexes in solution and therefore in the selective recognition of substrates of biological interest.

International audience; The evaporation of water molecules from high-velocity argon atoms impinging on protonated water clusters has been computationally investigated using molecular dynamics simulations with the reactive OSS2 potential to model water clusters and the ZBL pair potential to represent their interaction with the projectile. Swarms of trajectories and an event-by-event analysis reveal the conditions under which a specific number of molecular evaporation events is found one nanosecond after impact, thereby excluding direct knockout events from the analysis. These simulations provide velocity distributions that exhibit two main features, with a major statistical component arising from a global redistribution of the collision energy into intermolecular degrees of freedom, and another minor but non-ergodic feature at high velocities. The latter feature is produced by direct impacts on the peripheral water molecules and reflects a more complete momentum transfer. These two components are consistent with recent experimental measurements and confirm that electronic processes are not explicitly needed to explain the observed non-ergodic behavior.

accepté dans International Journal of Mass Spectrometry; We report on collision induced fragmentation of isolated molecular nanosystems studied with an event by event detection technique including the correlated detection of both neutral and charged fragments. This work focuses on the dissociation induced by collisional excitation without ionisation and electron-capture. Two molecular cluster cations are investigated: the collision of protonated hydrogen clusters at 60keV/amu with helium targets and that of protonated water clusters at 8keV with an argon gas. In addition to the molecular evaporation process the dissociation channel leading to the production of the H3+ or H3O+ molecular cations (loss of all the molecules) is observed with an unexpected abundance. The cross section for the production of these cations is observed to increase with the number of molecules in the cluster. Such an increase cannot be associated with the direct collisional excitation of the cation core of the cluster.

To be published International Journal of Mass Spectrometry; The first fragmentation ratios are presented for the ionization and dissociative ionization of gas-phase DNA bases following 80 keV (1.8 v0) proton impact. Event-by-event determination of the projectile charge state after collision enables also to distinguish the relative contributions of electron capture (EC) by the projectile from direct ionization (DI) of the target molecule (without projectile neutralization) thus yielding branching rations for these two different ionization processes. Results have been compared with recent similar experiments on uracil [Tabet et al. unpublished] and water [Gobet et al. Phys. Rev. A 70 (2004) 062716]. Although in all cases both processes (EC and DI) produced the same fragment ion groups in the mass spectra, fragmentation is for EC larger than for DI. Moreover the fragmentation ratio for dissociative ionization was observed to be for thymine larger than for adenine, cytosine, and uracil.

This research has benefited from the support of the TMO Cancer Avesian from the Cancer Plan 2009/2013, Action 3.3. H.C gratefully acknowledges the 'Grand Equipement National de Calcul Intensif./Centre Informatique National de l'Enseignement Superieur' (GENCI/CINES) for HPC resources/computer time 507 (Project cpt2130).; International audience; The different fragmentation channels of cytosine, adenine and guanine have been studied through DFT calculations. The electronic structure of bases, their cations, and the fragments obtained by breaking bonds provides a good understanding of the fragmentation process that can complete the experimental approach. The calculations allow assigning various fragments to the given peaks. The comparison between the energy required for the formation of fragments and the peak intensity in the mass spectrum is used. For cytosine and guanine the elimination of the HNCO molecule is a major route of dissociation, while for adenine multiple loss of HCN or HNC can be followed up to small fragments. For cytosine, this corresponds to the initial bond cleavage of N3-C4/N1-C2, which represents the main dissociation route. For guanine the release of HNCO is obtained through the N1-C2/C5-C6 bond cleavage ( reverse order also possible) leading to the largest peak of the spectrum. The corresponding energies of 3.5 and 3.9 eV are typically in the range available in the experiments. The loss of NH3 or HCN is also possible but requires more energy. For adenine, fragmentation consists of multiple loss of the HCN molecule and the main route corresponding to HC8N9 loss is followed by the release of HC2N1.

International audience; Proton transfer (PT) from protonated pyridine to water molecules is observed after excitation of microhydrated protonated pyridine (Py) clusters PyH+(H2O)(n) (n=0-5) is induced by a single collision with an Ar atom at high incident velocity (95x10(3)ms(-1)). Besides the fragmentation channel associated with the evaporation of water molecules, the charged-fragment mass spectrum shows competition between the production of the PyH+ ion (or its corresponding charged fragments) and the production of H+(H2O) or H+(H2O)(2) ions. The increase in the production of protonated water fragments as a function of the number of H2O molecules in the parent cluster ion as well sd the observation of a stable H+(H2O)(2) fragment, even in the case of the dissociation of PyH+(H2O)(2), are evidence of the crucial role of PT in the relaxation process, even for a small number of solvating water molecules.

International audience; The velocity distribution of water molecules evaporated from a mass-selected protonated water nanodroplet after a collision-induced electronic excitation is reported. In their Communication (DOI: 10.1002/anie.201505890), M. Farizon et al. show that besides the Maxwell–Boltzmann statistics behavior, a distinct high-velocity contribution provides evidence for molecular evaporation before complete energy redistribution, corresponding to non-ergodic events.

Electron-capture in collisions of singly charged protonated water cluster H+(H2O)3, ions with Ar atoms is studied at the impact energy of 8 keV in the frame of the COrrelated Ion and Neutral fragments Time-Of-Flight, COINTOF, technique. In contrast to methods based only on the detection of the charged fragments, dissociation induced by collisional-excitation and electron-capture induced dissociation can be simultaneously recorded in the present set-up. The time of flight measurements of both the neutral and the corresponding charged species resulting from the charge-exchange process provide a direct observation of the dissociation of the neutralized protonated water cluster. Thus, the present COINTOF method provides new valuable insights into the collision process through the detection of produced neutral fragments. Moreover, it opens new possibilities to measure kinetic energy release also in the dissociation of the produced neutrals, which is our future endeavour in the development of the presented COINTOF set up.

We report on a cluster fragmentation study involving collisions of high-energy hydrogen cluster ions with atomic helium or fullerenes. The experimental characterization of the cluster fragmentation, not only by the average fragment size distribution but also by a statistical analysis of the fragmentation events, has become possible because of a recently developed multicoincidence technique in which all the fragments of all the collisions occurring in the experiment are mass analyzed on an event by event basis. From the breakup in two fragments to the complete disintegration of the cluster, the fragmentation phenomenon exhibits a transition with an increase of the fluctuations. The fragmentation events with more than one H 3 + fragment evidence the cluster multifragmentation process. An important aspect of these results is that high-energy cluster collision can induce a reaction in the cluster.

We report the first account of a cluster fragmentation study involving high energy cluster-cluster collisions in which all the fragments of each collision occurring in the experiment are mass analyzed on an event by event basis. This allows an unbiased look at the true nature of fragmentation including a statistical analysis in terms of fluctuations in the fragment size distribution.

The possible fragmentation mechanisms of the uracil cation have been investigated through DFT calculations. The comparison between the peak intensity obtained from a mass spectrum for given mass/charge ratios and the required energy to form given fragments, allows one to propose a fragmentation channel. Particular attention has been brought to the possible hydrogen release from formed and activated fragments that might occur during the fragmentation process. Although the H-loss has never been observed experimentally through direct mechanism impact of high energy particles, it is predicted from calculated free energy to be a highly possible process.

The production of H 3 + ions resulting from single collisions of mass-selected ionic hydrogen clusters, H + (n=9, 25, 31), with helium at high velocity (1.55 times the Bohr velocity) has been studied. A strong double H 3 + ion production resulting from one incident cluster is observed. Moreover, evidence for a triple H 3 + fragment production is presented forn=25 and 31. Thus, in this energy range, the collision gives rise to multifragmentation processes. The formation of H 3 + ions takes place in the fragmentation of the multicharged cluster resulting from the collision.

International audience; The fragmentation process of the uracil RNA base has been investigated via DFT calculations in order to assign fragments to the ionisation mass spectrum obtained after dissociation induced by collision experiments. The analysis of the electronic distribution and geometry parameters of the cation allows selection of several bonds that may be cleaved and lead to the formation of various fragments. Differences are observed in the electronic behaviour of the bond breaking as well as the energy required for the cleavage. It is reported that N(3)-C(4) and N(1)-C(2) bonds are more easily cleaved than the C(5)-C(6) bond, since the corresponding energy barriers amount to ΔG = +1.627, +1.710, +5.459 eV, respectively, which makes the C(5)-C(6) bond cleavage almost prohibited. Among all possible formed fragments, the formation of the OCN(+) fragment for the peak at m/z = 42 Da is excluded because of an intermediate that was not observed experimentally and too a large free energy barrier. Based on the required free energy, it is observed that two fragment derivatives: C(2)H(4)N(+) and C(2)H(2)O˙(+) may be formed, with a small preference for C(2)H(4)N(+). This latter product is not formed through a retro Diels Alder reaction in contrast to C(2)H(2)O˙(+). The following sequence is proposed for the peak at 42 Da: C(2)H(4)N(+) (from N(1)-C(2), C(4)-C(5) cleavages) > C(2)H(2)O˙(+) (from N(3)-C(4), N(1)-C(2) and C(5)-C(6) cleavages) > C(2)H(4)N(+) (from N(1)-C(2), N(3)-C(4) and C(4)-C(5)) > C(2)H(2)O˙(+) (from C(5)-C(6), N(1)-C(2) and N(3)-C(4) cleavages) > NCO(+) (from N(1)-C(2), C(4)-C(5) and N(3)-C(4) cleavages). Finally the peak at 28 Da is assigned to CNH(2)(+) derivatives that can be formed through two different paths, the easiest one requiring 5.4 eV.

Triphosphonate cavitand, 3 i PO, has been designed for the recognition of ammonium in solution. This Letter presents a study of the selective guest–host recognition between Acetylcholine and Choline ions, ACh + and Ch + , and the host. We are able to observe that the ACh + ion replaces the Ch + ion in the 3 i PO-Ch + complexes with a concomitant exchange/substitution constant of 7 × 10 4 M −1 . Conversely a replacement of the ACh + ions by the Ch + ions in the 3 i PO-ACh + complexes has been also observed with a concomitant exchange constant of 5.8 × 10 3 M −1 . The structure of the 3 i PO-ACh + complex is determined by single crystal X-ray crystallography.

Ab initio self-consistent-field (SCF) Hartree–Fock and configuration interaction (CI) calculations have been carried out for H + 2n + 1 (n=1–6) clusters using a triple-zeta plus polarization basis set. Fully optimized structures and energies of H + 11 and H + 13 are presented. These structures can be thought as the addition of H2 molecules to a deformed H + 9. Dissociation energies as a function of cluster size follow the pattern established experimentally by Hiraoka and Mori. Nevertheless, our energy results on the biggest clusters suffer from the lack of size consistency of CI with single and double substitutions (CISD) calculations. Analytic gradient techniques have been used to locate stationary point geometries and to predict harmonic vibrational frequencies and infrared intensities at the two levels of theory SCF (n=1–6) and CISD (n=1–4) both with triple-zeta polarization basis sets. Of special interest are the new vibrational modes of H + 11 and H + 13, which have no counterpart in the H + 9 cluster. Our predicted frequencies compare fairly well with the experimental results of Okumura, Yeh, and Lee.

9 pages, accepted for publication in International Journal of Mass Spectrometry; International audience; The influences of water molecules surrounding biological molecules during irradiation with heavy particles (atoms, ions) are currently a major subject in radiation science on a molecular level. In order to elucidate the underlying complex reaction mechanisms, we have initiated a joint experimental and theoretical investigation with the aim to make direct comparisons between experimental and theoretical results. As a first step, studies of collisions of a water molecule with a neutral projectile (C atom) at high velocities (>= 0.1 a.u.), and with a charged projectile (proton) at low velocities (

The first partial cross-section measurements are presented for the various channels of H3+ and H2 dissociation in collisions with helium atoms at 50 and 60 keV/amu. The experimental technique includes the use of a removable grid in front of a surface barrier detector to enable neutrals of mass 2 amu to be separated into H2 and 2H channels.

A complete set of cross sections is reported for the ionization of water molecules by neutral atomic hydrogen impact at velocities of the order of the Bragg peak. The measured relative cross sections are normalized by comparison with proton impact results for the same target conditions and previous absolute data. Event by event coincidence analysis of the product ions and the projectile enables partial cross sections for target ionization and target plus projectile ionization to be determined, as well as total cross sections for electron loss reactions., To be published in Chemical Physics Letters

Absolute dissociation cross sections are reported for Hn+ clusters of varied mass (n = 3, 5, ..., 35) following collisions with He atoms at 60 keV / amu. Initial results have been published in a previous brief report for a smaller range of cluster sizes [Ouaskit et al., Phys. Rev. A 49, 1484 (1994)]. The present extended study includes further experimental results, reducing the statistical errors associated with the absolute cross sections. The previously suggested quasi-linear dependence of the Hn+ dissociation cross sections upon n is developed with reference to expected series of geometrical shells of H2 molecules surrounding an H3+ core. Recent calculations identify n = 9 as corresponding to the first closed H2 shell [e.g. Stich et al., J. Chem. Phys. 107, 9482 (1997)]. Recurrence of the distinct characteristics observed in the dissociation cross section dependence upon cluster size around n = 9 provides the basis for the presently proposed subsequent closed shells at n = 15, 21, 27, and 33, in agreement with the calculations of Nagashima et al. [J. Phys. Chem. 96, 4294 (1992)]., Comment: To be published in Physical Review A

Mass analyzed product ions have been detected in coincidence with the projectile following the ionization of water by proton impact. Measurement of the projectile charge state postcollision enables the different ionization processes to be identified: direct ionization, single electron capture, and double electron capture. A complete set of partial and total absolute cross sections is reported for the direct ionization and electron capture processes initiated by proton collisions at 20--150 keV. The cross sections for the direct ionization of ${\mathrm{H}}_{2}\mathrm{O}$ by proton impact are compared with previous electron impact results [Straub et al., J. Chem. Phys. 108, 109 (1998)].

Electron attachment (EA) to uracil (U), thymine (T) and cytosine (C) was studied in the electron energy range from about 0 to 12 eV using a high resolution crossed electron/molecule beams technique. The dominant negative ions formed via EA reactions to U, T and C are (U-H) - , (T-H) - and (C-H) - . The respective partial EA cross sections could be determined yielding peak values of σ (1.0 eV)=3x10 -20 m 2 , σ (1.05 eV)=1.2x10 -19 m 2 , σ (1.54 eV)=2.3x10 -20 m 2 , respectively. Based on (i) a comparison of the resonance positions for the different bases and on (ii) high level ab initio calculations we can assign certain resonances to the site specific loss of hydrogen during the EA reaction. At higher electron energies, in the range between about 3 and 12 eV we observe further product anions (e.g., for U the ions CN - , OCN - and C3OH2N - ), however, with significantly lower cross section values.

By selecting specific decay reactions in high-energy collisions (60 keV/amu) of hydrogen cluster ions with a helium target (utilizing event-by-event data of a recently developed multicoincidence experiment) and by deriving corresponding temperatures for these microcanonical cluster ensembles (analyzing respective fragment distributions), we are able to construct caloric curves for H+3(H2)(m) cluster ions (6

Ionization and fragmentation of water and uracil molecules was studied both by electron and proton impact. A special coincidence technique allows on an event by event basis the investigation of product ions formed upon the collision of protons with neutral molecules including the identification of the charge state of the projectile. This enables the characterization of the ionization processes occurring, i.e. direct ionization, single electron capture or double electron capture for 0, 1 or 2 electrons that are transferred from the target to the projectile, respectively. For uracil the fragmentation patterns obtained by electron and proton impact ionization reveal close similarities and indicate a comparable amount of excitation for the two different ionization mechanisms at high enough projectile energies.

High-energy collisions ( 60 keV/amu) of hydrogen cluster ions with a helium target have been completely analyzed on an event-by-event basis. By selecting specific decay reactions we can start after the energizing collision with a microcanonical cluster ion ensemble of fixed excitation energy and we derive corresponding temperatures of the decaying cluster ions. The relation between the temperature and the excitation energy (caloric curve) exhibits the typical prerequisites of a first-order phase transition in a finite system, in the present case signaling the transition from a bound cluster to the gas phase.

Using a recently developed multicoincidence technique, fragmentation resulting from collisions between high energy $(60 \mathrm{k}\mathrm{e}\mathrm{V}/\mathrm{u})$ ${\mathrm{H}}_{27}^{+}$ ions with an He target are investigated on an event by event basis. The data obtained are analyzed in terms of fluctuations in the fragment size distribution. A comparison with results from two different three-dimensional (3D) lattice bond percolation models of the same size (27 constituents) indicates the presence of a critical behavior in the decaying finite system and the importance of accounting in the percolation model for the specific binding situation in the hydrogen cluster ion.

The most abundant decay channels have been studied quantitatively for high-energy (60 keV/amu) cluster ions H (3) (+)(H (2))(m = 1-14) colliding with He atoms employing a recently developed multicoincidence technique for the simultaneous detection of the correlated fragments on an event-by-event basis. This allows us to identify decay reactions and their underlying decay mechanisms responsible for the occurrence of the U-shaped fragmentation pattern.

IPM

IPM; International audience; Electron capture cross sections of hydrogen cluster ions Hn+ colliding with atomic helium have been measured in a large range of cluster size (5⩽n⩽35) for the same velocity (1.5v0, 60 keV/u). While the electron capture cross section decreases from the H+ ion to the H3+ one, the cluster electron capture cross section is found to be independent of the cluster size and nearly equal to the capture cross section of the H3+ ion. Electron capture by hydrogen clusters on a helium atom is a process involving only the H3+ core of the cluster where the positive charge is localized. It appears that this very localized electron capture is not disturbed by the presence of molecules, up to sixteen, around the H3+ core.

Using a recently developed multicoincidence technique, the hydrogen cluster fragmentation resulting from a collision between a high energy H+25 ion with a C60 target is investigated on an event-by-event basis. For a sample of 6000 collisions, statistical methods (conditional moments, scaled factorial moments) are applied to these data sets. The results obtained provide strong experimental evidence for the presence of a critical behavior in these finite systems.

IPM; We report the direct observation of the multi-ionization of the $H_{21}^+$ hydrogen cluster in a single collision with a helium atom at 60 keV/u. Up to quadruple ionization of the cluster was observed and new multi-fragmentation channel were detected. Moreover, the results show two different fragmentation processes of doubly charged $H-{21}^{2+}$ clusters: emission of an $H_2^+$ dimer, or emission of an $H-3^+$ trimer after rearrangement in the cluster prior to fragmentation.

The structure of Hn + hydrogen clusters which are a nucleation of H2 molecules around a H3 + core has been theoretically studied for n=5−13 (odd) using ab initio methods. For n larger than 9, the predicted structures can be thought as an addition of H2 molecules around a weakly deformed H9 + core. Besides, mass-selected ionic hydrogen cluster beams of high energy are now available at the cluster acceleration facility at the Institut de Physique Nucleaire de Lyon. Fragmentation of 60-keV/amu — Hn + clusters induced by single collision on helium atoms is investigated for n = 9, 25 and 31. The deduced inclusive mass distributions of the charged fragments are measured from mass 3 to n−2. These distributions appear strongly different from those obtained at lower energy. An important production of intermediate-mass fragments is observed.

The interest of physicists in working with high energy cluster beams is increasing, in particular in France where the hydrogen cluster accelerator at Lyon has been recently upgraded to deliver Hn+ beams in the energy range 60−100 keV u , n As a matter of fact, the use of mass-selected high-energy clusters as projectiles appears to be an important source of information on the interaction of particles and clusters with matter (solid, gas, electron, plasma) and related phenomena, and on the clusters themselves. Results (experiments and Monte Carlo simulation) on the interaction of Hn+ clusters with thin foils and some perspectives of investigation with fast clusters are presented.

Dissociative electron attachment (DEA) to water in the gaseous phase has been studied using two different crossed electron–molecule beam apparatus. Ion yields for the formation of the three fragments H$−$, O− and OH$^−$ wer measured as a function of the incident electron energy. The kinetic energies of the fragment ions were measured and compared with the values derived from ab initio calculations to provide information on the energy partitioning in the fragmentation process. Isotope and temperature effects on the attachment process are discussed and the production of OH$^−$ via DEA is confirmed.

Fragment size distributions including multiple fragmentation events have been measured for high energy H + 25 cluster ions (60 keV amu −1 ) colliding with a neutral C 60 target. In contrast to earlier collision experiments with a helium target the present studies do not show a U-shaped fragment mass distribution, but a single power-law falloff with increasing fragment mass. This behaviour is similar to what is known for the intermediate regime in nuclear collision physics and thus confirms a recently predicted scaling from nuclear to molecular collisions.

The interaction of high energy radiation (� , � , � rays or heavy ions) with living cells does not in general directly lead to DNA strand breaks. The primary interaction essentially removes electrons from the components of the complex molecular network, i.e., electrons from valence states of the chemical bonds but also electrons from localized inner shells of the individual atoms. As a result of the subsequent charge transfer and energy dissipating processes, chemical bonds can be ruptured generating neutral or ionic radicals as additional secondary species. Electrons as the most abundant secondary species are created with an estimated quantity of � 4 � 10 4 electrons per MeV primary quantum deposited [1]. The larger majority possesses initial kinetic energies up to about 20 eV [2]. In the course of successive inelastic collisions within the medium they are thermalized within 10 � 12 s before they reach some stage of solvation, then as chemical rather inactive species. Moreover, damage of the genom in a living cell by ionizing radiation is about one-third direct and two-thirds indirect [3]. Direct damage concerns reactions directly in the DNA and its closely bound water molecules and indirect damage results from energy deposition in water molecules and other biomolecules in the surrounding of the DNA. It is believed that almost all the indirect damage is due to the attack of the highly reactive hydroxyl radical [4,5]. The importance of reactions of presolvated electrons with amino acids and nucleotides has already been pointed out more than two decades ago by time resolved pulse radiolysis experiments [6]. More recently, the ability of free ballistic electrons (3‐20 eV) to efficiently induce single and double strand breaks in supercoiled DNA has clearly been shown by Sanche and co-workers [7]. In these studies it was demonstrated that the DNA strand breaks were initiated by the formation and decay of transient negative ion (TNI) states, localized on the various DNA components (base, phosphate, deoxyribose, or hydration water). Resonances in DNA strand break curves were observed in the energy range around 10 eV ,s imilar to those TNI states formed by these components in the gas phase or in homogeneous films as exemplified in Ref. [7]