Measured angular distributions for ${\mathrm{O}}^{-}$ and ${\mathrm{Cu}}^{+}$ ejected by 1500-eV ${\mathrm{Ar}}^{+}$-ion bombardment of chemisorbed oxygen on Cu(001) exhibit anisotropies which vary by greater than a factor of 2. Molecular-dynamics calculations of the ion-impact event agree well with experiment if the oxygen atom is placed in a fourfold coordination site 1.2-1.5 Å above the Cu(001) plane. Angle-resolved secondary-ion mass spectrometry is proposed as a powerful new method for the determination of adsorbate positions.

• Received 29 March 1979

DOI:https://doi.org/10.1103/PhysRevLett.43.220