Relaxation of Ni(111) Surface by Oxygen Adsorption

We have shown by displacement-sensitive high-energy ${\mathrm{He}}^{+}$-ion scattering that the clean Ni (111) surface has a bulklike structure with ∼20% enhancement of surface thermal vibration, and that the $p(2\mathrm{}\times 2)$ and $(\sqrt{3}\times \sqrt{3})R30°$ superstructures on the oxygen-adsorbed surface are accompanied by expansion of the first nickel layer by ∼0.15 Å. In addition, the absolute oxygen coverages for these two superstructures and at saturation were measured to be ∼¼, $\sim \frac{1}{3}$, and ∼3 monolayers, respectively.