Localized Surface Phonons and Surface Structural Studies of NICKEL(977)

Helium atom scattering was used to study the vibrational properties and structural transitions of a stepped metallic surface, Ni(977). Vibrational properties were studied through phonon spectroscopy via time-of-flight measurements. The backfolding of the surface Rayleigh mode and its softening were observed in the direction perpendicular to the steps. Two new modes localized at the steps were observed in the direction parallel to the steps. An analytical 1D lattice model is proposed and local forces at the steps are determined. Diffraction measurements on this stepped surface show no roughening transition on the terraces. However, step roughening was observed. This roughening is localized to the step row and its nearest neighbor rows. The kink energetics were determined using a simple square packing model. Step doubling and singling on Ni(977) was also investigated through diffraction measurements. Kinetic studies of the reconstruction at very low oxygen coverages show that step doubling is second order in single step density and step singling is first order in double step density. Oxygen atoms at the steps facilitate both step doubling and step singling, i.e., the process is surfactant -mediated. The reconstruction was also found to be a continuous phase transition with a critical temperature of 470K and a critical exponent of 0.15.