Large neutral fractions in collisions of ${\text{Li}}^{+}$ with a highly oriented pyrolytic graphite surface: Resonant and Auger mechanisms

We report measurements of neutral atom fractions for ${\text{Li}}^{+}$ scattered by a highly oriented pyrolytic graphite surface as a function of the incident ion energy and exit angle. We found an unexpected large neutralization probability that increases with both energy and exit angle. Based on a dynamical quantum calculation of the scattering process that accounts for the extended features of the surface and the localized nature of the atom-atom interactions, we propose that the resonant neutralization to the Li ground state is a relevant mechanism in this collision. However, to understand the whole process including the angular dependence, and taking into account the Li energy-level variations, we conclude that other mechanisms such as Auger neutralization to the ground state and resonant neutralization to excited states should also be considered.

Figure 1

(Color online) TOF-ISS spectra of total scattered ions plus neutral (open squares) and neutral (full circles) particles for $\alpha /\beta ={25}^0/{20}^0$ and for 3 keV ${\text{Li}}^{+}$ incoming energy. The striped area indicates the elastic peak width considered.

Figure 2

(Color online) Experimental neutral fractions as (a) a function of incident energies (exit angle as a parameter) and (b) a function of exit angles (${\text{Li}}^{+}$ incident energy as a parameter).

Figure 3

(Color online) ${\text{Li}}^{+}$ neutralization probability as a function of exit angle. Experimental data: 2 keV (full squares) and 5 keV (full down triangles). Theoretical neutral fractions: dash line corresponds to the spin-less calculation and solid line to the calculation including correlation effects.

Figure 4

Normal distance dependence of the calculated neutral fraction along the trajectory for the HOPG surface for $\alpha /\beta ={10}^0/{35}^0$. Both curves correspond to 4 keV energy of incoming ${\text{Li}}^{+}$ (open squares) and ${\text{Li}}^0$ (full squares). Striped region corresponds to the HOPG total local DOS. The full gray line indicates the projectile ionization level energy as a function of $z$, referred to the surface Fermi level $\left(E_{\text{Fermi}}=0\right)$.

Figure 5

(Color online) Evolution of the ionization level energy along the ion trajectory for $\alpha /\beta ={35}^0/{10}^0$ when the interaction with only one surface atom is considered (dashed line) and the interaction with many surface atoms is included (dot line). The striped area corresponds to the HOPG total local DOS.

Figure 6

Neutral fraction as a function of the exit angle for 2 keV ${\text{Li}}^{+}$ incoming energy. Full circles correspond to experimental results, open circles to theoretical results including only RN, and dashed line corresponds to the calculated neutral fraction when a rough estimation of the Auger neutralization contribution is added to RN (see text).

Figure 7

Evolution of the affinity (dashed line) and excited $\text{Li-}1s^{2}2p$ (solid line) level energies along the ion trajectory for $\alpha /\beta ={35}^0/{10}^0$ (the corresponding asymptotic values are also indicated, $\Phi =4.67\text{eV}$ is the HOPG work function). The striped area corresponds to the HOPG total local DOS. The following coupling terms are shown in the inset: $\text{Li}\left(2s\right)\text{-C}\left(2s\right)$ (full circles); $\text{Li}\left(2s\right)\text{-C}\left(2p_z\right)$ (full squares); $\text{Li}\left(2p_z\right)\text{-C}\left(2s\right)$ (empty circles); and $\text{Li}\left(2p_z\right)\text{-C}\left(2p_z\right)$ (empty squares).