207 research outputs found
Electronic excitation of transition metal nitrides by light ions with keV energies
We investigated the specific electronic energy deposition by protons and He
ions with keV energies in different transition metal nitrides of technological
interest. Data were obtained from two different time-of-flight ion scattering
setups and show excellent agreement. For protons interacting with light
nitrides, i.e. TiN, VN and CrN, very similar stopping cross sections per atom
were found, which coincide with literature data of N2 gas for primary energies
<= 25 keV. In case of the chemically rather similar nitrides with metal
constituents from the 5th and 6th period, i.e. ZrN and HfN, the electronic
stopping cross sections were measured to exceed what has been observed for
molecular N2 gas. For He ions, electronic energy loss in all nitrides was found
to be significantly higher compared to the equivalent data of N2 gas.
Additionally, deviations from velocity proportionality of the observed specific
electronic energy loss are observed. A comparison with predictions from density
functional theory for protons and He ions yields a high apparent efficiency of
electronic excitations of the target for the latter projectile. These findings
are considered to indicate the contributions of additional mechanisms besides
electron hole pair excitations, such as electron capture and loss processes of
the projectile or promotion of target electrons in atomic collisions
On the correlation of angular distributions of keV ions and trajectory-dependent electronic excitations in transmission channelling geometry
We use energy discrimination of keV ions transmitted through a thin,
single-crystalline silicon membrane to correlate specific angular distribution
patterns formed in channelling geometry with trajectory-dependent electronic
energy loss. The integral energy and intensity distribution of transmitted ions
can thus be dissected into on one side axially channelled projectiles
travelling along rather straight trajectories and on the other side
dechannelled projectiles predominantly experiencing blocking. Angular
distributions of transmitted ions are further simulated with two different
Monte-Carlo codes.Comment: 9 pages, 5 figures, conferenc
Trajectory dependence of electronic energy-loss straggling at keV ion energies
We have measured the electronic energy-loss straggling of protons, helium,
boron and silicon ions in silicon using a transmission time-of-flight approach.
Ions with velocities between 0.25 and 1.6 times the Bohr velocity were
transmitted through single-crystalline Si(100) nanomembranes in either
channelling or random geometry to study the impact parameter dependence of
energy-loss straggling. Nuclear and path length contributions to the straggling
were determined with the help of Monte Carlo simulations. Our results exhibit
an increase in straggling with increasing ion velocity for channelled
trajectories for all projectiles as well as for protons and helium in random
geometry. In contrast for heavier ions, electronic straggling at low velocities
does not decrease further but plateaus and even seems to increase again. We
compare our experimental results with transport cross section calculations. The
satisfying agreement for helium shows that electronic stopping for light ions
is dominated by electron-hole pair excitations, and that the previously
observed trajectory dependence can indeed be attributed to a higher mean charge
state for random trajectories. No agreement is found for boron and silicon
indicating that local electron-promotion and charge-exchange events
significantly contribute to energy loss at low velocities
- …