Apparent Velocity Threshold in the Electronic Stopping of Slow Hydrogen Ions in LiF

The electronic energy loss of hydrogen ions (protons and deuterons) in thin supported films of LiF has been studied in backscattering geometry for specific energies from $700\mathrm{eV}/\mathrm{u}$ to $700\mathrm{keV}/\mathrm{u}$, using Rutherford backscattering spectroscopy and time-of-flight low-energy ion scattering spectroscopy. For specific energies below $8\mathrm{keV}/\mathrm{u}$, our data confirm velocity proportionality for the stopping cross section $\epsilon$ (like in a metal) down to $3.8\mathrm{keV}/\mathrm{u}$, as observed previously for protons and antiprotons despite the large band gap (14 eV) of LiF. Below $3.8\mathrm{keV}/\mathrm{u}$, the present results indicate an apparent velocity threshold at about 0.1 a.u. for the onset of electronic stopping.

Figure 1

(a) Experimental RBS spectra for 150 keV protons scattered from $\mathrm{Au}/\mathrm{B}/\mathrm{Si}$ (open circles) and $\mathrm{LiF}/\mathrm{Au}/\mathrm{B}/\mathrm{Si}$ (filled circles). The corresponding simulated spectra (SIMNRA) are also shown as dashed lines and full lines, respectively. For details, see text. (b) As (a) for 4 keV $\mathrm{H}_2{}^{+}$ ions and TRBS simulations.

Figure 2

(a) Stopping cross section per molecule for hydrogen ions in LiF, obtained for protons, $\mathrm{H}_2{}^{+}$, $\mathrm{H}_3{}^{+}$, deuterons, $\mathrm{D}_2{}^{+}$ and $\mathrm{D}_3{}^{+}$, as a function of the specific energy ($\mathrm{keV}/\mathrm{u}$). Also shown are the corrected data from Ref. 8 (triangles), and data from tabulations: SRIM2003 [27] (dashed line), PSTAR [26] (full line). For details, see text. (b) As (a) for energies below $25\mathrm{keV}/\mathrm{u}$, as a function of the ion velocity in a.u. Also shown are the data from Ref. 10 (filled circles).