Alkali-metal adsorption on the Si(100) surface studied by low-energyD+scattering

Low-energy ${\mathrm{D}}^{+}$ scattering is employed to explore the nature of the bonding of Na, K, Rb, and Cs adatoms on the Si(100) surface in a low-coverage regime (<0.25 monolayers). In contradiction to the commonly accepted picture, the Na-Si bond is found to differ sharply from the others. The ${\mathrm{D}}^{+}$ ions, although neutralized almost completely in scattering from Na, survive neutralization considerably in scattering from K, Rb, and Cs. These results indicate that Na has significant covalency with Si, while the others are adsorbed almost completely ionized.