Double electron excitation in He ions interacting with an aluminum surface

Observation of Auger electron emission from Al under the impact of He ions, at energies below the excitation threshold of Al-Al collisions, gives evidence for charge transfer in He-Al collisions. Excitation of $2p$-shell electrons in Al is produced by vacancy transfer, following double excitation of He by promotion of its $1s$ level. Studies of ion scattering from surfaces currently neglect the double excitation of He. We discuss implications on the debate about electronic excitations and energy deposition, which is relevant to many fields, since helium ions are applied in the characterization and imaging of materials.

Figure 1

Auger spectra of Al under the impact of $\mathrm{N}{\mathrm{a}}^{+}$ and $\mathrm{H}{\mathrm{e}}^{+}$ ions at impact energies close to the observation threshold. The incidence angles of the $\mathrm{N}{\mathrm{a}}^{+}$ and $\mathrm{H}{\mathrm{e}}^{+}$ ions are 45° and 78°, respectively, with respect to the surface normal.

Figure 2

Al Auger emission intensities for several projectiles vs $\gamma E_0$, the maximum energy transfer in a projectile-Al collision. Data for noble-gas ions are taken from Ref. [18] and are shown after proper rescaling.

Figure 3

Schematic energy levels for the He-Al quasimolecule, indicating the diabatic promotion path for the $3d\sigma$ molecular orbital correlated to the He $1s$ level in the separated atom limits. The arrows indicate the two-electron transition that produces a hole in the $2p$ level of Al.