Electronic interaction of very slow light ions in Au: Electronic stopping and electron emission

Electronic stopping and ion-induced electron emission during the interaction of slow $\left(v<0.1\text{a}\text{.u}\text{.}\right)$ ${\text{H}}^{+}$ and ${\text{D}}^{+}$ ions with polycrystalline Au targets have been investigated. Electronic stopping is deduced from time-of-flight low-energy ion scattering measurements in backscattering geometry using polycrystalline films of several nanometers thick. Current measurements are performed to determine the ion-induced electron emission yield. Both in electronic stopping and in ion-induced electron emission we find a rather sharp, distinctive threshold velocity of $\sim 0.19\text{a}\text{.u}\text{.}$ for specific excitation and emission of $5d$ electrons. Below this threshold the projectiles interact exclusively with the $6s$ electrons.

Figure 1

(Color online) Schematic representation of the experimental setup ACOLISSA. Shown are the TOF beam lines before and after scattering off the surface of the target. Additionally, dotted lines indicate directions of incoming and outgoing high energy beam trajectories.

Figure 2

(Color online) Schematic illustration of the voltage switching states and the corresponding time-dependent electric current at the collector electrode and the target, respectively.

Figure 3

(Color online) Experimental TOF-LEIS spectra for 700 eV ${\mathrm{H}}^{+}$ (1.4 keV ${\mathrm{H}}_2^{+}$) ions scattered off the in situ deposited Au/B/Si film. The corresponding simulated TRBS spectrum corrected for the stopping cross section is also shown (solid line). Simulations obtained when using an $\epsilon$ value that are larger or smaller than the optimum value ${\epsilon }_0$ by 18% are also shown as dashed and dotted lines, respectively.

Figure 4

(Color online) Stopping cross section for hydrogen ions obtained for ${\mathrm{H}}_n^{+}$ and ${\mathrm{D}}_n^{+}$ with $n=1$, 2, and 3 (circles and squares, respectively) as a function of the ion velocity in a.u. Also shown are data from Refs. 9, 26, 27, 28 and SRIM.

Figure 5

(Color online) Total electron emission from Au excited by impinging protons. The total number of emitted electrons per incident ion is shown. The dashed lines labeled (a) and (b) indicate linear fits of two distinct regimes in the data set. A horizontal dashed line represents the potential emission contribution ${\gamma }_{\text{PE}}$ to the total yield. The expected emission thresholds for $6s$ electrons (at around 0.10 a.u.) and $5d$ electrons (at around $0.17\text{a}\text{.u}\text{.}$) are indicated by arrows (cf. text). The statistical error is estimated to $\pm 5\%$ but never smaller than 0.02 and indicated at selected points. For comparison, previous results are included (Refs. 15, 37).