Néstor R. Arista

Stopping of molecules and clusters

Abstract The interaction of swift cluster and molecular ions with solids has been studied with increasing interest in the past decades. Experiments have produced valuable information on cluster energy losses, screening effects, charge states, and dynamical interactions among correlated ions, which in many cases have not been obtained using single atomic ions. These studies show differences (vicinage effects) in the energy loss per nucleon and also in charge equilibrium conditions both for small and large clusters. Theoretical studies dealing with vicinage effects in ion cluster interactions, using the most basic models, have provided a semi-quantitative view of cluster energy losses which partially agree with experiments, but still leave several questions open. In particular, the dielectric models predict a diminished stopping power in the low-velocity range, as a result of negative interferences, whereas at high velocities a significant enhancement of the energy loss is expected. The behavior at high velocities has been confirmed experimentally using small molecular ions. In addition, there is clear experimental evidence of diminished stopping values at low velocities, but still the possible influence of various mechanisms should be clarified, like the role of charge state equilibrium, alignment, and non-linear quantum effects. Modifications in charge state equilibrium may be specially important in the case of large clusters. The purpose of this work is, on one side, to review the current knowledge and recent progress in this field, and, on the other, to reformulate the theory of cluster energy loss in order to incorporate the effects of charge equilibrium according to the most recent experimental evidences.

Wavenumber dependence of the energy loss function of graphite and aluminium

Abstract The energy loss function of a material is a key parameter in the dielectric formalism used to describe the optical spectra and the excitations produced by swift charges in solids. Modelling the experimental energy loss function as a sum of Mermin type functions in the optical limit (i.e., at zero momentum transfer), we have calculated its dependence on the momentum transfer. We compare the result with available experimental data for graphite and aluminium and with other theoretical models. We find a reasonably good agreement of our results with these data, and better agreement than is obtained with other models.

Energy loss of ions in solids: Non-linear calculations for slow and swift ions

Abstract The historical approach to describe the energy loss of swift ions in solids is based on the Bohr, Bethe and Bloch theories. As is well known, the central parameter in these theories is the ratio η=Z1e2/ℏv, whose value is generally used to delimit the ranges of applicability of the Bohr (η>1) and Bethe (η

Proton energy loss in allotropic forms of carbon

We have investigated theoretically the electronic stopping of protons in different solid forms of carbon: glassy, amorphous, graphite, diamond and C60-fullerite. The energy loss is described within the dielectric formalism and the target properties are modelled by a sum of Mennin-type energy loss functions. For each allotropic carbon form, we observe remarkable differences in the stopping cross section and in the energy loss straggling at proton velocities around and lower than that corresponding to the maximum in the energy loss. A comparison of our results with available experimental data shows a reasonably good agreement.

Stopping power of low-velocity ions in solids: inhomogeneous electron gas model

Abstract A self-consistent model for the calculation of electron scattering and stopping coefficients of slow ions in a non-homogeneous electron gas is developed. The model permits to account for realistic electron density profiles in the evaluation of the average energy loss of slow ions in solids. The screening parameter in the potential and the scattering phase shifts are calculated in a non-perturbative way, as a function of the local electron-gas density. We show that the inhomogeneity effect can modify the Z1 dependence of the low-velocity stopping powers. The differences between previous calculations and experimental stopping-power data, for channeled and random incidence trajectories of ions in solids, are well explained by this approach.

Electronic energy loss of swift protons in the oxides Al2O3, SiO2 and ZrO2

This work was supported by the Spanish Comision Interministerial de Ciencia y Tecnologia (projects 1FD97-1358-C02-01, BFM2000-1050-C02-01 and BFM2000-1050-C02-02) and the Argentinian FONCYT (project PICT 0303579).

Large hydrogen cluster stopping in carbon

Abstract We have investigated theoretically the stopping of clusters impinging on a target at high velocities. All energy losses are assumed to be due to electronic interaction which is described within the linear dielectric formalism. We focus on hydrogen clusters consisting of ten or more H2 molecules, moving in a carbon target. Two different models are employed to specify the target properties, namely the Drude and the Mermin dielectric functions. Interference effects, quantified as usual by the vicinage function, are identified as intra- and inter molecular contributions. The influence of different approximations for the atomic pair correlation function is also studied. Results are given for the stopping power per atom as a function of cluster size and velocity.

Interaction of swift ions with surface modes in microcapillaries and nanotubes

Abstract The interaction of atomic ions with surface modes in cylindrical channels in solids is described in terms of the dielectric formalism using approximations as well as realistic dielectric functions for alumina and graphitic structures. The formulation is applied to evaluate induced potentials, self-energies, energy losses and Stark splitting of atomic lines. The formulation is extended to ion clusters. The main features emerging from the scaling properties are described and applied.

Exit angle, energy loss and internuclear distance distributions of H2+ ions dissociated when traversing different materials

Abstract We have performed computer simulations of the trajectory followed by each proton resulting from the dissociation of H2+ molecules when traversing a thin solid target. We use the dielectric formalism to describe the forces due to electronic excitations in the medium, and we also consider the Coulomb repulsion between the pair of protons. Nuclear collisions with target nuclei are incorporated through a Monte Carlo code and the effect of the coherent scattering is taken into account by means of an effective force model. The distributions of exit angle, energy loss and internuclear separations of the protons fragments are discussed for the case of amorphous carbon and aluminum targets.

Differences in the energy loss of protons and positive muons in solids

We evaluate the magnitude of the difference in the electronic energy loss of positive muons and protons in solids in the low-energy range using both the dielectric formalism and the non-linear transport cross-section (TCS) method. The effects of mass increase at low energies, showing stopping differences between 5% and 1% for velocities in the range 0.2<v<1 a.u. We find that nuclear stopping effects must also be taken into account for a consistent analysis in the low-energy range.