HELIUM-ATOM DIFFRACTION STUDY OF THE SUBMONOLAYER STRUCTURES OF SODIUM ON CU(001)

The structure and low-frequency vibrations of submonolayer coverages o f sodium adsorbed on Cu(001) have been investigated using high-resolut ion helium-atom diffraction and time-of-flight measurements at T-s = 5 0 and 100 K. At low coverages, Theta(Na) less than or similar to 0.2, the overlayer structures are determined by a strong repulsive interact ion between neighboring sodium atoms, while the low-energy vibrations are characterized by a parallel frustrated translation mode (T-mode) f requency of <(h)over bar omega> similar to meV. At higher coverages, T heta(Na) greater than or similar to 0.2, a series of quasi-one-dimensi onal structures are observed, and are accompanied by a substantial low ering 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 r ange.