Primary processes: from atoms to diatomic molecules and clusters

International audienceThis article presents a short review of the main progresses achieved at the GANIL facilities during the last thirty years in the field of ion-atom and ion-diatomic molecule collisions. Thanks to the wide range of projectile energies and species available on the different beam lines of the facility, elementary processes such as electron capture, ionization and excitation have been extensively studied. Beside primary collision mechanisms, the relaxation processes of the collision partners after the collision have been another specific source of interest. Progresses on other fundamental processes such as Young type interferences induced by ion-molecule collisions or shake off ionization resulting from nuclear beta decay are also presented. 1. Introduction For the electronic structures of atoms and molecules, precise theoretical knowledge and high-resolution experimental data are available. But the complete understanding of dynamic processes in atomic collisions remains a challenge, due to large theoretical problems in describing time-dependent many-particle reactions, and to experimental difficulties in performing complete experiments in which all relevant quantities are accessible. Elementary collisions involving ions, atoms and molecules play an important role in many gaseous and plasma environments, where they provide both the heating and cooling mechanisms. The study of such collisions is thus not only of fundamental importance, it is also essential for the understanding of large-scale systems such as astrophysical plasmas, planetary atmospheres, gas discharge lasers, semiconductor processing plasmas, and fusion plasmas. Collisions between ions and atoms (or simple molecules) give also access to the elementary processes responsible for energy transfer in ion-matter and ion-biological molecule collisions. Complete knowledge of these elementary processes is thus of primordial importance for ion induced modification of materials as well as for radiolysis, radiotherapy and biological damages due to radiation exposure

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Depth profiling of the chemical composition of free-standing carbon dots using X-ray photoelectron spectroscopy

Abstract The chemical and geometrical structure of free-standing carbon dots (Cdots) prepared from the pyrolysis of N-hydroxysuccinimide (NHS) have been characterized using X-ray photoelectron spectroscopy (XPS). An aerodynamic lens system was used to generate a sufficient particle density of monodispersed Cdots for XPS studies at the PLEIADES beamline at the SOLEIL synchrotron facility. Varying the X-ray excitation energy between 315 and 755 eV allows probing of the Cdots from the surface toward their core, owing to the kinetic energy dependence of the photoelectron inelastic mean free path. The C 1s, O 1s, and N 1s core-levels were recorded with high-spectral resolution to identify their main chemical components and branching ratios. While high-resolution transmission electron microscopy (HRTEM) reveals a defective graphitic core, the C 1s spectrum evidence two main peaks similar to those measured from the solid NHS. Their relative abundance as a function of the probing depth is strongly related to the chemical composition of the ligand shell that does not vary substantially over the first 3.4 nm. Combining the depth-resolved XPS and HRTEM studies, it was concluded that the Cdots possess a graphitic core surrounded by a relatively homogeneous shell of at least 3.4 nm thickness with a composition similar to that of the solid NHS

Mitigating space charge in time-resolved photoelectron spectroscopy to study laser-heated copper dynamics in the high fluence regime

International audienceThe performance of time-resolved photoelectron spectroscopy for the study of subpicosecond dynamics of laser-heated solids is often limited by space charge effects. The consequent shift and distortion of the photoelectron spectrum induced by electrons emitted by the ultrashort pump pulse is studied here using a fully coherent approach based on experimental measurements and space charge calculations. The temporal dynamics of the valence band of a copper sample is recorded before and after an 800 nm laser pump excitation at a fluence of 750mJ/cm$^2$. The probe pulse is produced using a laboratory-based high-harmonics source delivering 25 fs pulses up to 100 eV photon energy. We extract the laser-heating contribution by comparing these measurements with space charge calculations based on particle-in-cell simulations of the pump and probe electron clouds mutual interaction on their way to the detector. The deduced picosecond dynamics associated to the electronic density of states shift is attributed to lattice changes with the help of hydrodynamic simulations including the two-temperature model

Assessing the surface oxidation state of free-standing gold nanoparticles produced by laser ablation

Abstract The surface chemistry of gold nanoparticles produced by the pulsed laser ablation in liquids method is investigated by X-ray photoelectron spectroscopy (XPS). The presence of surface oxide expected on these systems is investigated using synchrotron radiation in conditions close to their original state in solvent but free from substrate or solvent effects which could affect the interpretation of spectroscopic observations. For that purpose we performed the experiment on a controlled free-standing nanoparticle beam produced by combination of an atomizer and an aerodynamic lens system. These results are compared with those obtained by the standard situation of deposited nanoparticles on silicon substrate. An accurate analysis based on Bayesian statistics concludes that the existence of oxide in the free-standing conditions cannot be solely confirmed by the recorded core-level 4f spectra. If present, our data indicate an upper limit of 2.15 ± 0.68% of oxide. However, a higher credence to the hypothesis of its existence is brought by the structureless valence profile of the free-standing beam. Moreover, the cross-comparison with the deposited nanoparticles case clearly evidences an important misleading substrate effect. Experiment with free-standing nanoparticles is then demonstrated to be the right way to further investigate oxidation states on Au nanoparticles

Surface chemistry of gold nanoparticles produced by laser ablation in pure and saline water

Abstract Pulsed laser ablation in liquid (PLAL) is a powerful method for producing nanoparticle colloids with a long-term stability despite the absence of stabilizing organic agents. The colloid stability involves different reactivities and chemical equilibria with complex ionic-specific effects at the nanoparticle/solvent interface which must be strongly influenced by their chemical composition. In this work, the surface composition of PLAL-produced gold nanoparticles in alkaline and saline (NaBr) water is investigated by X-ray photoelectron spectroscopy on free-flying nanoparticles, exempt from any substrate or radiation damage artifact. The Au 4f photoelectron spectra with a depth profiling investigation are used to evaluate the degree of nanoparticle surface oxidation. In alkaline water, the results preclude any surface oxidation contrary to the case of nanoparticles produced in NaBr solution. In addition, the analysis of Br 3d core-level photoelectron spectra agrees with a clear signature of Br on the nanoparticle surface, which is confirmed by a specific valence band feature. This experimental study is supported by DFT calculations, evaluating the energy balance of halide adsorption on different configurations of gold surfaces including oxidation or adsorbed salts

High intensity targets stations for S3

International audienceIn the framework of the SPIRAL2 project atGANIL, the Super-Separator-Spectrometer (S3) is dedicatedto experiments using the very high intensity stablebeams ([10 plA) delivered by the superconducting linearaccelerator up to 14.5 A.MeV. Specific target stations arerequired to sustain these unprecedented beam intensities. Aprototype target station for actinides was designed andmanufactured, which main characteristics are described inthis paper. In order to commission this system and checkthe behavior of target materials, preliminary tests with 7.7A.MeV 129Xe beams were performed at 100 pnA. Theresults of these tests are presented here

The Scientific Objectives of the SPIRAL2 project

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