Internal electronic structure of adatoms on Fe(110) and Fe(100) surfaces: A low-energy Li⁺ scattering study

The neutralization of 400–3000 eV ⁷Li⁺ ions scattered from clean and adsorbate-covered Fe(110) and Fe(100) surfaces was measured with time-of-flight spectroscopy. Li singly scattered from bromine, iodine, and cesium adatoms has a consistently larger neutral fraction than that for scattered from substrate sites. This suggests that the local electrostatic potential directly above these adatoms is reduced from that of the clean substrate. The neutral fraction of Li scattered from halogen adatoms is surprising in that it decreases as the emission angle moves off-normal, yet increases in the usual manner for cesium and silver adatoms. This indicates that the charge distribution associated with a halogen adsorbate is nonuniform, most likely due to internal polarization. A semiquantitative theoretical analysis shows that a nonuniform internal electron density would give rise to the observed behavior. The polarization of halogen adatoms is likely responsible for anomalous work function changes observed previously. Alkali-ion scattering is shown to be an effective tool for detecting the internal electronic structure of an adatom.