A. L'Hoir

Surface relaxation and near-surface atomic displacements in the N/Cu(100) self-ordered system

The atomic displacements of Cu atoms induced by nitrogen adsorption on Cu(1 0 0) have been studied by channelling–blocking of swift 4He ions. This study has been performed at two adsorption stages. The first one corresponds to the formation of a dense, two-dimensional, self-ordered array of square-shaped islands covered by nitrogen. The second one corresponds to uniform coverage at saturation. We have determined by nuclear reaction analysis the absolute quantity of nitrogen adsorbed at these two stages. The values obtained, when confronted to previous observations of these stages by low energy electron diffraction and by scanning tunnelling microscopy, demonstrate that nitrogen remains mostly at the sample surface and that the N concentration in bulk Cu could not exceed 1%. However, channelling measurements show that this surface adsorption generates atomic displacements of Cu atoms down to depths of a few ten (1 0 0) interplanar distances. In the mean time, blocking measurements reveal that nitrogen adsorption induces a strong surface expansion: the interplanar distance between the first two (1 0 0) planes increases of about 0.2 A, in contrast with the weak contraction observed on bare Cu(1 0 0) surfaces. This observation supports the hypothesis that, when nitrogen is adsorbed, the surface is submitted to stress variations, from tensile to compressive stress for, respectively, bare and nitrogen-covered surface regions. The surface forces corresponding to such variations have been introduced in molecular dynamics simulations. For coverage leading to self-ordering, these simulations do indeed predict displacements of subsurface Cu atoms. The adjustment of these displacements to those measured by channelling gives the amplitude of the stress variation.

Monte-Carlo simulation of ion track structure in water: ionization clusters and biological effectiveness

Abstract The biological action of swift ions decreases for increasing stopping power (LET), and the loss of efficiency is particularly high for heavy ions when the LET reaches its maximum value. In one of the models developed in the literature to account for this behavior, the biological action of light ions (and X rays) is attributed to clusters of energy deposition by low energy electrons. We have performed Monte-Carlo simulations to test this model for heavy and light ions (here uranium and helium ions). For helium ions, the experimental inactivation cross sections variations with LET are well reproduced when clusters with E⩾340 eV in spheres of radius R≈ 4.54 nm are considered, with an efficiency ≈4%, in very good agreement with the literature. However, these parameters cannot be applied to uranium ions (here 1.4 MeV/u U32+): the calculated number of clusters is ≈5 times larger than expected, a result which sets a limit to the cluster model.

Influence of intrashell excitation (n=2) on the population of mestastable states of H- and He-like krypton ions in channeling conditions

Abstract We have observed delayed K α photons emitted by 60 A MeV H-like and He-like krypton ions leaving a 37 μm silicon crystal, both for random and 〈110〉 axial alignment of the target. We could extract intrashell (2s→ 2p) excitation probabilities, which are compared with values deduced from PWBA calculations.

Recent questions about the excitation mechanism at intermediate velocity

Abstract For both fundamental reasons and potential applications, the process of K-excitation by heavy ions at intermediate velocity has been re-investigated experimentally and theoretically and the main results are described here. The nature and the origin of the so-called “saturation effect” at high exciting charge is not yet elucidated. The importance of K-excitation is outlined in connection with problems of radiation damage.

Channelling experiment with a cooled and decelerated H-like gold ion beam extracted from a storage ring

We have performed a channelling experiment in which a ion beam, extracted by radiative recombination from the storage ring ESR at GSI after deceleration and cooling, was directed onto a thin silicon crystal. The directional properties of channelling have been used to test the quality of the extracted beam. An upper limit of the beam emittance has been measured to be via x-ray measurements. Coincidence measurements between photons and ions transmitted after capture of one electron in the crystal have allowed us to study L-REC and the associated emission.

Impact parameter dependent charge exchange studies with channeled heavy ions

We use decelerated (below 20 MeV/u) H-like heavy ions extracted from the GSI-ESR storage ring to study electron capture processes such as Radiative Electron Capture (REC) and Mechanical Electron Capture (MEC) in channeling conditions. With the help of simulations, we show that MEC occurs at relatively large impact parameters into highly excited states. REC studies provide information about the electron gas polarization.

Structural investigations on optical PLT thin films grown on (100) SrTiO3 by room and high-temperature channeling

Abstract It is well-known that cooling films through the paraelectric/cubic to the ferroelectric/tetragonal phase transition introduces twin structures, because of the coexistence of a- and c-oriented grains. In order to determine relative importance of these defects, we have performed room and high-temperature (450°C) channeling on a lanthanum-doped lead titanate (PLT) sample grown on (100) SrTiO3 (STO). The determined normalized yield minimum based on the Pb signal along the growth direction on this sample is 30% at room temperature. Moreover, the percentage of c-oriented grains, calculated from channeling along [101], is 70%. High-temperaturechanneling shows that the film defects mainly consist of dislocations, and that the defects introduced during growth are more important than those due to the phase transformation.