Ground- and excited-state scattering potentials for the stopping of protons in an electron gas

The self-consistent electron–ion potential V(r) is calculated for H+ ions in an electron gas system as a function of the projectile energy to model the electronic stopping power for conduction-band electrons. The results show different self-consistent potentials at low projectile-energies, related to different degrees of excitation of the electron cloud surrounding the intruder ion. This behavior can explain the abrupt change of velocity dependent screening-length of the potential found by the use of the extended Friedel sum rule and the possible breakdown of the standard free electron gas model for the electronic stopping at low projectile energies. A dynamical interpolation of V(r) is proposed and used to calculate the stopping power for H+ interacting with the valence electrons of Al. The results are in good agreement with the TDDFT benchmark calculations as well as with experimental dat

Impact-parameter dependence of electronic energy loss and straggling of incident bare ions on H and He atoms by using the coupled-channel method

A first-principles calculation based on an expansion of the time-dependent electronic wave function in terms of atomic orbitals (coupled-channel method) has been applied to evaluate the impact-parameter dependence of the electronic energy loss and the fluctuation in energy loss of swift ions colliding on H and He atoms at energies of 10 to 500 keV/amu. The results have been compared with experimental data as well as with other existing models, e.g., the local-density approximation in an electron-gas target, the harmonic-oscillator target treatment, and the first-order plane-wave-Born approximation. Our results show a nearly exponential shape of the mean electronic energy loss for small impact parameters, in contrast to the Cxaussian shapes obtained by Mikkelsen and Sigmun

Improved calculations of the electronic and nuclear energy loss for light ions penetrating H and he targets at intermediate velocities

A review is given on the use of the coupled-channel method to calculate the electronic and niuclear energy loss of ions penetrating the matter. This first principie calculation based on an expansion of the time dependent electronic wavefunction in terms of atomic orbitals lias been applied to evaluate the impact parameter dependence of the electronic energy loss, the stopping cross-section and the fluctuation in energy loss of ions colliding with H and He atoms at energies of 10keV/amu to 500keV/amu. The results have been compared to experimenta1 data as well as to others existing models e.g., local density approximation in an electron gas target, harmonic oscillator target treatment and first order plane-wave-Born approximatio

Impact-parameter dependence of the electronic energy loss of fast ions

In this work we describe a model for the electronic energy loss of bare ions at high velocities. Starting from first-order perturbation theory we propose a simple formula to calculate the impact-parameter dependence of the electronic energy loss for all impact parameters. The physical inputs are the electron density and oscillators strengths of the atoms. A very good agreement is obtained with full first-order calculations

Nonperturbative stopping-power calculation for bare and neutral hydrogen incident on he

The electronic stopping power of hydrogen beams penetrating He gas has been calculated by solving the time-dependent Schrõdinger equation. Special attention has been given to low incident energies where capture and loss of the projectile electron are the most important energy-loss mechanisms. These mechanisms are included in a consistent theoretical treatment and good agreement with experimental data has been obtained for high- as well as for low-energy projectiles in a gas target

Impact-parameter dependence of the energy loss of fast molecular clusters in hydrogen

The electronic energy loss of molecular clusters as a function of impact parameter is far less understood than atomic energy losses. For instance, there are no analytical expressions for the energy loss as a function of impact parameter for cluster ions. In this work, we describe two procedures to evaluate the combined energy loss of molecules: Ab initio calculations within the semiclassical approximation and the coupled-channels method using atomic orbitals; and simplified models for the electronic cluster energy loss as a function of the impact parameter, namely the molecular perturbative convolution approximation (MPCA, an extension of the corresponding atomic model PCA) and the molecular unitary convolution approximation (MUCA, a molecular extension of the previous unitary convolution approximation UCA). In this work, an improved ansatz for MPCA is proposed, extending its validity for very compact clusters. For the simplified models, the physical inputs are the oscillators strengths of the target atoms and the target-electron density. The results from these models applied to an atomic hydrogen target yield remarkable agreement with their corresponding ab initio counterparts for different angles between cluster axis and velocity direction at specific energies of 150 and 300 keV/u

Angular dependence of energy loss in proton-helium collisions

The energy loss of 50 to 250 keV protons scattered under single-collision conditions from He atoms is investigated in terms of its dependence on the angle of scattering. At the higher projectile energies we observe an enhanced energy loss at scattering angles around 0.5 mrad. Such a behavior cannot be understood on the basis of two-body scattering models. Based on our theoretical studies we show that the combined effects of the screened target potential and of electronic transitions have to be considered for the energy loss of proton scattering in light gases

Nonperturbative treatment of the screened-Coulomb contribution of projectile-electron loss

The electron-loss cross section of He+ ions impinging upon noble-gas targets (2≤Z2≤36) is calculated by using the coupled-channel method for the active projectile electron in the static screened field of the target atom. The calculations show a saturation of the projectile-electron-loss cross section with increasing target atomic number. This saturation effect due to neutral target systems is much more pronounced than for ionization or excitation by charged particles. Comparison with experimental data indicates a small electron-loss contribution from electron-electron interaction processes for heavy targets at intermediate velocities. Remaining discrepancies in the data are discussed in the light of the approximations involved in our theoretical treatment

Giant Barkas effect observed for light ions channeling in Si

Measurements of the electronic energy loss are presented for ⁴He and ⁷Li ions channeling along the Si main axial directions at intermediate to high projectile energies. The Barkas effect, an energy-loss enhancement proportional to the third power of the projectile charge at high energies, is clearly separated from other processes. It reaches about 50% for Li ions channeling along the Si direction. The observed Barkas contribution from the valence-electron gas is in fair agreement with the Lindhard model