By time-of-flight spectrometry we have measured the energy loss of hydrogen projectiles in helium gas. We show that for projectile velocities v\ below the Bohr velocity the presence of an appreciable minimum excitation energy leads to a significant deviation of the electronic stopping cross section from the v\ proportionality generally assumed; at 4 keV a dependence on roughly the third power of v\ is found.PACS numbers: 34.50.Bw, 61.80.MkIt is commonly assumed, e.g., in published tabulations, 1 " 4 that at low projectile velocities v\ the electronic stopping cross section for ions sW^-TTT-^ fr Tdo(T 9 vi)-Ta (vi) (l) TV as ** l min is proportional \o v\. Here, E\ is the kinetic energy of the projectile, TV is the atomic target density, and s is the path length. T is the energy transferred to the target and da denotes the cross section corresponding to T\ integration runs over all possible values of T, starting from a minimum excitation energy r m i n , the threshold energy; T is the mean transferred energy. However, it is well established 5 ' 6 that in the case of excitation or ionization of target electrons via Coulomb interaction the cross section cr is not proportional to v \. For nonzero 7" m in this should apply to e as well when this energy-loss channel dominates over losses due to charge exchange, since for small values of v\ 9 T is close to T mm and almost constant. 7 For the special case of protons (at fixed charge state) in H2 a similar behavior of e has been recently derived by Phelps 8 from individual collision processes (see also the discussion in Ref. 9). The influence of a nonzero r m j n was first considered by Fermi and Teller 10 and worked out in Refs. 11-13. We have found by theoretical arguments ! l that e(v 1) should be modified for projectile energies up to 500r m i n .In the following we provide experimental evidence that the stopping cross section of helium gas for hydrogen isotopes strongly deviates from v\ proportionality. Helium has been chosen because of the largest possible T m \ n (19.8 eV) and because of the large mismatch in the ground-state levels of projectile and target (H, -13.6 eV; He, -24.6 eV); this will reduce energy-loss processes made possible by charge exchange (the Massey criterion, see, e.g., Ref. 14).To measure e of gases for light ions we have developed a time-of-flight (TOF) facility. Here, only a short description of the apparatus and of the evaluation procedure is given; details will be published in a forthcoming paper.The beam of hydrogen ions is produced in a duoplasmatron ion source and accelerated to energies from 6 to 22 keV. After collimation, the beam is swept over the chopper baffle in a rectangular pattern, using two electrical deflectors working in orthogonal transverse directions. So, the beam passes the 0.25-cm-diam hole in the chopper baffle only once per cycle (this defines the start signal for TOF) and therefore allows us to optimize the deflecting-voltage change for speed without compromise for the return phase. 15 Neutral projectiles are remove...
The energy loss of slow ions during grazing scattering from a LiF(100) surface as a function of the projectile atomic number Z1 is observed to show oscillations similar to those occurring in metals. A model of stopping of ions in an electron gas where screening is calculated from density functional theory reproduces well the experimental data. The same model gives good agreement with the energy loss obtained in transmission experiments performed with H and He projectiles. Analysis of these results allows us to gain new insights in the stopping of slow ions in ionic crystals.
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