Role ofdelectrons in electronic stopping of slow light ions

Abstract

In recent energy loss measurements, band structure effects in electronic stopping have been observed for materials with finite excitation thresholds, for example, noble metals such as Cu and Au. To further investigate the influence of the position of the d band relative to the Fermi edge, electronic stopping of hydrogen and helium ions in Ag and Pt was determined. For Ag, the electronic stopping power exhibits a velocity dependence similar to Cu and Au. No particular effect due to the comparatively large d-band offset in Ag is found. In the case of Pt, the electronic stopping power is virtually velocity proportional for H + ions and exhibits a distinct velocity dependence for He + ions. For hydrogen the results are compatible with modeling the conduction band as a free electron gas with an energy-dependent effective number of electrons. For He + , however, the observed effects point towards an additional energy loss mechanism, e.g., by charge-exchange processes.

Energy loss of low-energy ions in transmission and backscattering experiments

“…Note that for He scattered from polycrystalline Cu it was possible to quantitatively reproduce the energy spectrum in a wide energy range including the surface peak by a Monte Carlo (MC) simulation, which includes only multiple elastic collisions and electronic stopping along the trajectory [27]. It is, however, still an open question whether impact parameterdependent inelastic losses are responsible for the observed major discrepancies between electronic stopping data deduced from TR and BS experiments [4,6,7,28]. In this context it is interesting that around the stopping maximum electronic stopping data are consistent within experimental uncertainties ( ± 3%) when obtained in transmission and backscattering geometries [29].…”

Electronic Stopping of Slow Protons in Oxides: Scaling Properties

“…To evaluate ox, experimental spectrum widths were compared to corresponding Monte Carlo simulations (TRBS, [34]), in order to disentangle electronic and nuclear stopping. In the simulations, a screened Coulomb potential (ZBL, [35]) was used to handle 5 scattering in close and distant collisions; ox was optimized to reproduce the width of the experimental spectrum ( [11]). …”

“…At higher ion velocities, such a behavior has been observed for Al2O3, SiO2 and H2O ice [44] as well as for HfO2 versus SiO2 [28] and traced back to an O 2p 6 configuration as if in oxides the ionic character of the local bonds would prevail. At low ion velocities, however, details of the density of states (DOS) might be highly relevant, since even the subtle differences between specific metals have clear impact on the observed Se, e.g., for Au and Pt [10,11]. In order to obtain quantitative information on the unperturbed electronic density of states (DOS) of all presented oxides, DFT calculations were performed with the VASP code [45,46].…”

“…For very slow protons with v << vF, deviations from velocity-proportionality of Se were reported for target materials featuring excitation thresholds in their electronic band structures; in noble metals, the d-bands exhibit an excitation threshold Ed of several eV with respect to the Fermi energy EF, so that at ion energies above  1 keV, excitation of the d-bands becomes more and more effective and the stopping cross section rises with steeper slope [8,9,10,11,12]. Time-dependent density-functional theory (TD-DFT) calculations of Se of protons in Au confirmed this interpretation [13].…”

Highly accurate nuclear and electronic stopping cross sections derived using Monte Carlo simulations to reproduce measured range data

“…Recent studies on the slowing-down of energetic ions in matter have addressed a wide variety of topics. Arranged roughly in the order of increasing projectile energy or velocity, noteworthy examples include the following: the correlation between electronic stopping and ion induced electron emission [1,2,3], the role of s and d electrons in electronic stopping of slow protons and deuterons [1,4,5,6,7], systematic differences in electronic stopping of low-energy H + and He + ions [6,8], the importance of exact knowledge of nuclear stopping (screening length) in low-energy ion scattering [9], the applicability of the reciprocity approach [10] for predicting ranges of slow heavy ions in compounds [11,12,13], modelling of range distributions in crystalline silicon [14], and measurements and interpretation of electronic stopping of low-and medium-mass ions in solids at energies around the Bragg peak and below [15,16,17]. However, reasonably accurate knowledge of electronic stopping cross sections S e is still available merely for a very limited number of projectile-target combinations, often within narrow ranges of energy.…”