  Electronic energy loss of hydrogen ions in HfO2 was investigated in a wide
energy range in the medium and low energy ion scattering regime. Experiments by
Time-Of-Flight Medium-Energy Ion Scattering (TOF-MEIS) with proton and deuteron
projectiles were performed in backscattering geometry for nm-films of HfO2 on
Si with an ultrathin SiO2 interface layer prepared by ALD. At energies around
the stopping maximum excellent agreement is found with earlier results from
Behar et al. (Phys. Rev. A 80 (2009) 062901) and theoretical predictions.
Towards lower energies discrepancies between experiment and calculations
increase slightly. The low energy data exhibits excellent velocity
proportionality and indicates the absence of clear effects due to distinct
electronic states. Thus, also no apparent velocity threshold can be predicted
within the experiments uncertainty from the present data. The magnitude of the
energy loss is discussed in terms of a free-electron model and compared with
the expected electron densities from plasmon frequencies.
