Energy loss of hydrogen projectiles in gases

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 N2gas for primary energies ⩽25 keV. In case of the chemically rather similar nitrides with metal constituents from the 5thand 6thperiod, i.e. ZrN and HfN, the electronic stopping cross sections were measured to exceed what has been observed for molecular N2gas. For He ions, electronic energy loss in all nitrides was found to be significantly higher compared to the equivalent data of N2gas. 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.

Stopping power in insulators and metals without charge exchange

The slowing-down process of pointlike charged particles in matter has been investigated by measuring the stopping power for antiprotons in materials of qualitatively very different nature. Whereas the velocity-proportional stopping power observed for metal-like targets such as aluminum over a wide energy range of 1-50 keV is in agreement with expectations, it is surprising that the same velocity dependence is seen for a large band-gap insulator such as LiF. The validity of these observations is supported by several measurements with protons and several checks of the target properties. The observations call for both a qualitative explanation and a quantitative theoretical model.

Antiproton stopping at low energies: confirmation of velocity-proportional stopping power

The stopping power for antiprotons in various solid targets has been measured in the low-energy range of 1-100 keV. In agreement with most models, in particular free-electron gas models, the stopping power is found to be proportional to the projectile velocity below the stopping-power maximum. Although a stopping power proportional to velocity has also been observed for protons, the interpretation of such measurements is difficult due to the presence of charge exchange processes. Hence, the present measurements constitute the first unambiguous support for a velocity-proportional stopping power due to target excitations by a pointlike projectile.

Charge exchange and threshold effect in the energy loss of slow projectiles

In this Letter, we study charge exchange and energy loss of protons, taking into account the dynamics of both nuclei and electrons during the collision with atomic hydrogen, helium, and neon targets. We obtain the nuclear and electronic contributions to the energy loss as well as the charge exchange probability, and the total cross section for charge exchange. We find a low-energy threshold in the electronic energy loss due to the quantization of excited states. We find that the electronic stopping cross section is not proportional to the velocity of the projectile at very low velocities (energies), as is predicted by electron gas theory. This confirms recent experimental results.