The electronic stopping power, S, of HfO 2 films for proton and alpha particle beams has been measured and calculated. The experimental data have been obtained by the Rutherford backscattering technique and cover the range of 120-900 and 120-3000 keV for proton and alpha particle beams, respectively. Theoretical calculations of the energy loss for the same projectiles have been done by means of the dielectric formalism using the Mermin energy loss function-generalized oscillator strength ͑MELF-GOS͒ model for a proper description of the HfO 2 target on the whole momentum-energy excitation spectrum. At low projectile energies, a nonlinear theory based on the extended Friedel sum rule has been employed. The calculations and experimental measurements show good agreement for protons and a quite good one for alpha particles. In particular, the experimental maximums of both stopping curves ͑around 120 and 800 keV, respectively͒ are well reproduced. On the basis of this good agreement, we have also calculated the inelastic mean-free path ͑IMFP͒ and the stopping power for electrons in HfO 2 films. Our results predict a minimum value of the IMFP and a maximum value of the S for electrons with energies around 120 and 190 eV, respectively.

Si-K x-ray and backscattering yields have been measured as a function of the projectile entrance angle for atomic and molecular (H 2 + and H 3 + ) hydrogen ions channeling at kinetic energies of 150 keV per proton along the Si ͗100͘ crystal direction. A large enhancement of the x-ray production has been observed for well-aligned H 3 + molecule beams. It is shown that this effect results from the Coulomb explosion of the molecule fragments during the channeling motion. Moreover, the shape and intensity of the measured angular distribution allows a quantitative determination of the corresponding heating of the transversal ion motion (2.6± 0.6 eV for H 2 + and 5.1± 0.8 eV for H 3 + molecules) in the channel. These values are consistent with the stored potential energies per particle and they depend significantly on the collective wake forces and molecular alignment conditions. PACS number (s): 61.85.ϩp, 34.50.Bw, 36.40.Ϫc, 68.55.LnBeams of molecules and cluster ions are useful tools in fundamental research with promising applications in material science and plasma physics. In particular, significant coherence effects (vicinage effects) have been predicted theoretically and in some cases experimental stopping forces for these structured projectiles show clear deviations from simple additive rules concerning the projectile constituents. 1,2 Other effects related to the correlated motion of cluster atoms, such as Coulomb explosion, 3 enhanced electron emission, 4 desorption and sputtering 5,6 have also been reported and reviewed in recent publications. 7 In the case of crystalline materials, ions entering nearly parallel to a particular crystal axis or plane become channeled as their motion is guided by correlated collisions with target atoms. The average transversal momentum of channeled atomic particles increases due to the inelastic scattering with the target electrons and scattering at displaced target atoms. This effect enhances the number of close encounters with the atomic rows and is named transverse heating. Recently, it has been observed that charge changing processes of fast heavy ions may even lead to transversal cooling, namely, a reduction of the transversal energy. 8 In addition to the channeling motion, molecular ions undergo a breakup process, since they lose their bonding electrons due to ionization in the first monolayers of the material. The combination of these two correlated motions, namely, the ion channeling and the breakup of the cluster under quasi-Coulombic forces, leads to a transverse Coulomb heating. Pioneering investigations of this Coulomb heating have been performed in transmission 9 as well as using measured energy spectra of backscattered protons from H + , H 2 + , and H 3 + beams. 10 However, although the effect is visible in the dechanneling profile, it was not evaluated quantitatively and has been strongly overestimated by recent computer simulations. 11This paper reports on a quantitative determination of the transverse Coulomb-heating energy (abbreviated Coulomb heating) using th...

In the present work we report on the energy loss ratio R n of fast H 2 + clusters in thin films ͑30-50 Å͒ of LaScO 3 and HfO 2 . The medium energy ion scattering technique was employed covering a broad energy range ͑40-200 keV/amu͒. The energy loss ratio data showed no clear evidence of collective excitations in these materials. The experimental results were interpreted in terms of three different theoretical approaches: the dielectric formalism with the Brandt-Reinheimer theory for semiconductor materials; the detailed simulation of the molecular fragments dynamics through the target; and finally the unitary convolution approximation adapted for hydrogen molecules. Only the simulation agrees with the experimental results for both oxides. The unitary convolution approximation works quite well for HfO 2 but overestimates slightly the LaScO 3 data. The overall results indicate that the energy loss ratio depends critically on the description of the electronic properties of such oxides.

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 data.

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