Signals of strong electronic correlation in ion scattering processes

Previous measurements of neutral atom fractions for $\mathrm{S}{\mathrm{r}}⁺$ scattered by gold polycrystalline surfaces show a singular dependence with the target temperature. There is still not a theoretical model that can properly describe the magnitude and the temperature dependence of the neutralization probabilities found. Here, we applied a first-principles quantum-mechanical theoretical formalism to describe the time-dependent scattering process. Three different electronic correlation approaches consistent with the system analyzed are used: (i) the spinless approach, where two charge channels are considered $(\mathrm{S}{\mathrm{r}}⁰$ and $\mathrm{S}{\mathrm{r}}⁺)$ and the spin degeneration is neglected; (ii) the infinite-U approach, with the same charge channels $(\mathrm{S}{\mathrm{r}}⁰$ and $\mathrm{S}{\mathrm{r}}⁺)$ but considering the spin degeneration; and (iii) the finite-U approach, where the first ionization and second ionization energy levels are considered very, but finitely, separated. Neutral fraction magnitudes and temperature dependence are better described by the finite-U approach, indicating that $e$-correlation plays a significant role in charge-transfer processes. However, none of them is able to explain the nonmonotonous temperature dependence experimentally obtained. Here, we suggest that small changes in the surface work function introduced by the target heating, and possibly not detected by experimental standard methods, could be responsible for that singular behavior. Additionally, we apply the same theoretical model using the infinite-U approximation for the Mg-Au system, obtaining an excellent description of the experimental neutral fractions measured.