Structure of Al(111)-(2x2)-Rb

The atomic geometry of the Al(111)-(2 x 2)-Rb phase formed by adsorption of one-quarter monolayer Rb on Al(111) at 100 K has been determined by analysis of low-energy electron diffraction measurements. The adsorbed Rb atoms are found to occupy on-top sites an a rumpled first layer of Al atoms, as in the corresponding (root 3 x root 3)R30 degrees phases formed by adsorption of one-third monolayer K, Rb, and Cs on Al(111) at low temperature. However, the adsorbed Rb atoms in the (2 x 2) phase have anomoiously large thermal vibrations.