S. Steydli

Investigation of slow collisions for (quasi) symmetric heavy systems: what can be extracted from high resolution x-ray spectra

We present a new experiment on (quasi) symmetric collision systems at low velocity, namely Ar17 + ions (v = 0.53 au) on gaseous Ar and N2 targets, using low- and high-resolution x-ray spectroscopy. Thanks to an accurate efficiency calibration of the spectrometers, we extract absolute x-ray emission cross sections combining low-resolution x-ray spectroscopy and a complete determination of the ion beam–gas jet target overlap. Values with improved uncertainty are found in agreement with previous results (Tawara et al 2001 Phys. Rev. A 64 042712). Resolving the whole He-like Ar16 + Lyman series from n = 2–10 with our crystal spectrometer enables us to determine precisely the distribution of the electron capture probability and the preferential npref level of the selective single-electron capture. Evaluation of cross sections for this process as well as for the contribution of multiple-capture is carried out. Their sensitivity to the l-distribution of n levels populated by single-electron capture is clearly demonstrated, providing a stringent benchmark for theories. In addition, the hardness ratio is extracted and the influence of the decay of the metastable 1s2s 3S1 state on this ratio is discussed.

Suppression of the thermal hysteresis in magnetocaloric MnAs thin film by highly charged ion bombardment

We present the investigation on the modifications of structural and magnetic properties of MnAs thin film epitaxially grown on GaAs induced by slow highly charged ions bombardment under well-controlled conditions. The ion-induced defects facilitate the nucleation of one phase with respect to the other in the first-order magneto-structural MnAs transition, with a consequent suppression of thermal hysteresis without any significant perturbation on the other structural and magnetic properties. In particular, the irradiated film keeps the giant magnetocaloric effect at room temperature opening new perspective on magnetic refrigeration technology for everyday use.

Primary processes: from atoms to diatomic molecules and clusters

This 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.

Modulating the phase transition temperature of giant magnetocaloric thin films by ion irradiation

Magnetic refrigeration based on the magnetocaloric effect at room temperature is one of the most attractive alternative to the current gas compression/expansion method routinely employed. Nevertheless, in giant magnetocaloric materials, optimal refrigeration is restricted to the narrow temperature window of the phase transition (Tc). In this work, we present the possibility of varying this transition temperature into a same giant magnetocaloric material by ion irradiation. We demonstrate that the transition temperature of iron rhodium thin films can be tuned by the bombardment of ions of Ne 5+ with varying fluences up to 10 14 ions cm --2 , leading to optimal refrigeration over a large 270--380 K temperature window. The Tc modification is found to be due to the ion-induced disorder and to the density of new point-like defects. The variation of the phase transition temperature with the number of incident ions opens new perspectives in the conception of devices using giant magnetocaloric materials.

Kossel Effect in Periodic Multilayers

The Kossel effect is the diffraction by a periodically structured medium, of the characteristic X-ray radiation emitted by the atoms of the medium. We show that multilayers designed for X-ray optics applications are convenient periodic systems to use in order to produce the Kossel effect, modulating the intensity emitted by the sample in a narrow angular range defined by the Bragg angle. We also show that excitation can be done by using photons (X-rays), electrons or protons (or charged particles), under near normal or grazing incident geometries, which makes the method relatively easy to implement. The main constraint comes from the angular resolution necessary for the detection of the emitted radiation. This leads to small solid angles of detection and long acquisition times to collect data with sufficient statistical significance. Provided this difficulty is overcome, the comparison or fit of the experimental Kossel curves, i.e., the angular distributions of the intensity of an emitted radiation of one of the element of the periodic stack, with the simulated curves enables getting information on the depth distribution of the elements throughout the multilayer. Thus the same kind of information obtained from the more widespread method of X-ray standing wave induced fluorescence used to characterize stacks of nanometer period, can be obtained using the Kossel effect.

Note: Observation of the angular distribution of an x-ray characteristic emission through a periodic multilayer

We present the observation of the angular distribution of a characteristic x-ray emission through a periodic multilayer. The emission coming from the substrate on which the multilayer is deposited is used for this purpose. It is generated upon proton irradiation through the multilayer and detected with an energy sensitive CCD camera. The observed distribution in the low detection angle range presents a clear dip at a position characteristic of the emitting element. Thus, such a device can be envisaged as a spectrometer without mechanical displacement and using various ionizing sources (electrons, x-rays, and ions), their incident direction being irrelevant.

Numerical simulations of purification and final charge state analysis of the slow ion beam for the FISIC project

Synopsis The Fast Ion –Slow Ion Collisions (FISIC) project consists of a crossed-beam arrangement to study ion-ion collisions in the intermediate velocity regime. For the low energy channel, ion trajectory simulations has been used to develop a complete beam line that includes a new Omega type purification system located just before the collision point and a charge state analyzer after interaction.

Low energy Ne ion beam induced-modifications of magnetic properties in MnAs thin films

Investigations of the complex behavior of the magnetization of manganese arsenide thin films due to defects induced by irradiation of slow heavy ions are presented. In addition to the thermal hysteresis suppression already highlighted in Trassinelli et al (2014 Appl. Phys. Lett. 104 081906), we report here on new local magnetic features recorded by a magnetic force microscope at different temperatures close to the characteristic sample phase transition. Complementary measurements of the global magnetization in different conditions (applied magnetic field and temperatures) enable the film characterization to be completed. The obtained results suggest that the ion bombardment produces regions where the local mechanical constraints are significantly different from the average, promoting the local presence of magneto-structural phases far from the equilibrium. These regions could be responsible for the thermal hysteresis suppression previously reported, irradiation-induced defects acting as seeds in the phase transition.

Hints on the origin of the thermal hysteresis suppression in giant magnetocaloric thin films irradiatied with highly charged ions

In a recent experiment we demonstrated the possibility to suppress the thermal hysteresis of the phase transition in giant magnetocaloric MnAs thin film by interaction with slow highly charged ions (Ne 9+ at 90 keV) [1]. This phenomenon has a major impact for possible applications in magnetic refrigeration and thus its reproducibility and robustness are of prime importance. Here we present some new investigations about the origin and the nature of the irradiation-induced defects responsible for the thermal hysteresis suppression. Considering in particular two samples that receive different ion fluences (two order of magnitude of difference), we investigate the reliability of this process. The stability of the irradiation-induced defects with respect to a soft annealing is studied by X-ray diffraction and magnetometry measurements, which provide some new insights on the mechanisms involved.

X-ray spectroscopy as a tool to enlighten the growth of Van der Waals nanoparticles in a supersonic jet

The clustering thermodynamics in a supersonic jet of argon is studied using x-ray emission induced by keV electrons and slow highly charged ions (HCIs). The high sensitivity of the HCI interaction dynamics allows to probe the very first steps of the argon bunch in a supersonic jet.