Helium-atom diffraction study of the submonolayer structures of sodium on Cu(001)

The structure and low-frequency vibrations of submonolayer coverages of sodium adsorbed on Cu(001) have been investigated using high-resolution helium-atom diffraction and time-of-flight measurements at T(s) = 50 and 100 K. At low coverages, Theta(Na) less than or similar to 0.2, the overlayer structures are determined by a strong repulsive interaction between neighboring sodium atoms, while the low-energy vibrations are characterized by a parallel frustrated translation mode (T-mode) frequency of <(h)over bar omega> similar to meV. At higher coverages, Theta(Na) greater than or similar to 0.2, a series of quasi-one-dimensional structures are observed, and are accompanied by a substantial lowering of the T-mode frequency to <(h)over bar omega> similar to 2 meV. The diffraction patterns and low T-mode frequencies strongly indicate that the sodium atoms form one-dimensional islands in this coverage range.