Analysis of Auger spectra from aHe+ion near a metal surface

Auger rates are computed and analyzed for a ${\mathrm{He}}^{+}$ ion in a wide range of ion–metal-surface separations. For neutralization-relevant distances close to the surface, the rate scales approximately quadratically with the density from the electron tails. (W∝${\mathit{n}}^{1.80}$ for ${\mathit{r}}_{\mathit{s}}$=2.07${\mathit{a}}_0$ and W∝${\mathit{n}}^{1.93}$ for ${\mathit{r}}_{\mathit{s}}$=3.99${\mathit{a}}_0$). Further away from the surface there is a gradual transition to a more linear scaling of the rate with the electron density (W∝${\mathit{n}}^{1.16}$). Computed Auger spectra are analyzed as well. For all ion-surface distances there is excellent agreement between the spectra and a convolution of the two hole energy distributions, left behind by the metal electrons neutralizing the ion and exciting to the Auger level, respectively. Closer to the surface the two hole spectra are almost identical. This is argued to indicate the localness of the Auger transition around the ion. For larger distances the hole spectra start to differ, indicating that the neutralization becomes increasingly nonlocal. The changeover in the scaling of the rate for larger ion-surface separations is consistent with the spectra analysis. The self-convolutions of the local density of states at the position of the ion do not agree quantitatively with the spectra. Therefore, despite the local character of the Auger process, the variations of the metal orbitals over the region of the ion cannot be neglected.