Experimental investigation of the energy dependence and evidence for the softness of the He/Ni(110) potential

Helium-diffraction experiments on Ni(110) were performed with incident energies Ei between 17 and 270 meV for different angles of incidence θi different azimuthai orientations α of the sample, and surface temperatures of 100 and 200 K. with the aim of obtaining information on the dependence of the corrugation function on the effective incoming energy Eiz = Ei cos2θi. Hard-wall fits of both in-plane and out-of-plane intensities yielded the coefficients ζ(10) and ζ(01) which describe the corrugation amplitudes parallel and perpendicular to the close-packed Ni rows, respectively. Too small values of ζ(10) and ζ(01) were systematically observed whenever the coefficients were determined by in-plane beams, the deviations becoming larger with increasing θ1,. This can be understood qualitatively on the basis of calculations using a soft exponential potential performed recently by Armand. The present results indicate that the corrugation parameters can be determined reliably at small θi; effects due to the finite steepness of the potential become pronounced at medium and large θi and should permit determination of softness parameters. The results allow the conclusion that ζ(01) = 0.075 A ̊ is practically independent of energy, whereas ζ(10) increases from < 0.02 A ̊ to 0.055 A ̊ with increasing Eiz. These findings are discussed in connection with recent theoretical calculations of surface electron densities performed by Hamann and Manninen et al. © 1982.