Low-energy-electron-diffraction ͑LEED͒ intensity measurements and multiple-scattering analysis for V͑100͒, supported by accurate characterization of surface impurity concentrations based on Auger-electron spectroscopy, are used to obtain a meaningful extrapolation of the first-layer relaxation to the clean surface value: d 12 = 1.36± 0.05 Å, corresponding to ⌬ 12 = −10% ± 3% relative to the bulk value d 0 = 1.514 Å. A highsensitivity probe for surface magnetism based on magneto-optic Kerr effect polarimetry using the cleanest surfaces achieved in the LEED experiment ͑ϳ5% C͒ yields a ͑sensitivity limited͒ null result with an estimated upper limit of 0.05 B /surface atom. These results are discussed within the framework of related experiments and in relation to the predictive accuracy of ab initio calculations that explore the surface structure and magnetism of V͑100͒ both of which are sensitive to different approximations for the exchange-correlation potential in density-functional theory.
Low-energy-electron-diffraction intensity measurements and multiple scattering analysis are used to determine the multilayer surface relaxation of clean and hydrogen-dosed Nb͑100͒ as a function of temperature. Accurate characterization of residual surface impurity concentration ͑oxygen͒ based on Auger electron spectroscopy is used to obtain a meaningful extrapolation of the first-layer relaxation to the clean surface value: d 12 = 1.481± .05 Å corresponding to ⌬ 12 = −10± 3%, a 10% relaxation relative to the bulk value d 0 = 1.645 Å. This experimental result for d 12 can be used to judge the accuracy of recent ab initio calculations for Nb͑100͒. Temperature-dependent changes in surface relaxation resulting from hydrogen dosing of Nb͑100͒ manifest an expansion of the near-surface lattice resulting from subsurface hydrogen atoms. The hydrogen-induced expansion of near-surface interplanar separation is determined to be 3 ± 1% at T =125 K, 4±1% at T = 300 K, and −1±1% at T = 400 K. The measured hydrogen-induced surface lattice expansion is consistent with the bulk lattice constant change ͑⌬ ϳ 4.5% ͒ that occurs when Nb is hydrated to form NbH. The observed relaxation of the hydrogen-dosed near-surface interplanar separation to the clean surface value for T Ͼ 400 K is consistent with the subsurface "hydrogen valve" model that has been used to account for unusual hydrogen uptake kinetics associated with Nb͑100͒.
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