Hydrogen-induced Reconstruction Research Articles

An eels study of hydrogen-induced reconstruction on stepped W(001) surfaces

Previous studies have shown that the size of hydrogen-induced reconstruction domains may be limited by steps on the surface (ref.1,2). It has also been shown that for steps oriented parallel to [100], domains with a single displacement direction are preferentially selected (ref.3,4), although the preferred displacement direction has been unclear. Angle-resolved EELS data presented here are consistent with a preferred tungsten atom displacement which is perpendicular to the step edges and with promotion of a second adsorption state by the steps.

Summary Abstract: Hydrogen-induced reconstruction of stepped W(001) surfaces studied with LEED and EELS

Summary Abstract: Hydrogen-induced reconstruction of stepped W(001) surfaces studied with LEED and EELS

Hydrogen-induced reconstruction: Analysis of the Ni[110](1X2)-H structure

Hydrogen-induced reconstruction: Analysis of the Ni[110](1X2)-H structure

Hydrogen-induced reconstruction: Analysis of the Ni{110}(1 × 2)-H structure

New experiments and dynamical theory calculations have been applied to a determination of the Ni{110}(1 × 2)-H structure by low energy electron diffraction. The analysis is based on 10 distinct diffracted beam intensity-energy spectra and includes assessment by the Pendry R-factor. In addition, however, we have sought to obtain an approximate fit of the relative diffracted beam intensities, as an important feature of the reconstruction experimentally is very intense fractional order beams. This added criterion, coupled with the effects of the adsorbed hydrogen atoms in electron scattering, causes us to reject the previously proposed row-pairing model of the metal surface reconstruction. Substantially improved agreement between experiment and theory is found for a missing row reconstruction model. Some of the implications of this conclusion are discussed.

Hydrogen-induced reconstruction on a high step density W(001) surface

The hydrogen-induced reconstruction on a high step density W(001) crystal, ( 2 × 2 )R45°-H, with steps oriented parallel to the [110] and ∼ 28 Å average terrace width has been investigated using LEED symmetry, beam shape analyses, and EELS. The symmetry of the LEED pattern is observed to change from p2mg for the ( 2 × 2 )R45° clean surface reconstruction to c2mm for the commensurate phase ( 2 × 2 )R45°-H reconstruction. Correspondingly, the shapes of the half-order beams indicate that the hydrogen-induced reconstruction domains are much less elongated than the clean surface domains. A splitting of each half-order beam into four beams at higher exposures indicates the existence of two domains of the incommensurate phase. A commensurate phase v 1 vibrational loss peak centered at 160 meV in the EELS spectrum broadens on the low-energy side during the incommensurate phase and then shifts toward 130 meV and narrows as the (1×1)-H saturation structure develops. These observations imply that there is no long-range inhibition ( ∼ 20 Å) to the formation of either commensurate or incommensurate phase; hydrogen induces a switching of the atomic displacements from 〈110〉 directions on a clean surface to 〈100〉 directions, even with steps oriented parallel to the [110]; and in the incommensurate phase there is a distribution of hydrogen site geometries with the most probable geometry more like the commensurate phase geometry than the saturation phase geometry.

Hydrogen-induced reconstruction on a high step density W(001) surface

Hydrogen-induced reconstruction on a high step density W(001) surface

Finite size effects on reconstruction domains of tungsten(001) surfaces

The size of reconstruction domains on low-temperature, clean W(001) and also hydrogen-covered W(001) have been studied for both planar and stepped surfaces through analysis of LEED intensities and beam shapes. The stepped surface with step edges parallel to the [110] has an average terrace width of ∼28 Å. For the low temperature, clean, planar surface the average equilibrium reconstruction domain size is ∼130 Å. On the stepped surface, ‘‘zig–zag’’ orientational domains with atomic displacements perpendicular to the steps are preferred and the domain size perpendicular to the steps is limited by the terrace width, while the size parallel to the steps is nearly the same as on the planar surface. Hydrogen-induced reconstruction domains for the commensurate phase are similar in size to the clean surface domains on the planar surface. On the stepped surface, the ‘‘dimer’’ domains are also limited by the terrace width perpendicular to the steps and are additionally limited parallel to the steps to approximately half their planar surface dimension. With increasing hydrogen exposure and the development of the incommensurate phase, the domain dimensions are observed to shrink to less than 12 Å before the transition to the (1×1)-H structure occurs on both planar and stepped surfaces.

Desorption of hydrogen from tungsten (100)

The desorption energy, E d, and the preexponential factor, ν, for the desorption of hydrogen from W(100) have been determined as a function of coverage, θ, from a series of isobars. The values of E d and ν depend strongly on coverage and weakly on temperature; they show a compensation effect, both quantities decreasing at about 1 4 of the hydrogen saturation coverage: E d drops from about 40 kcal per mole of H 2 to about 20 kcal per mole of H 2 and ν from 10 3 to 10 −5 s −1 cm 2. These values differ from those extracted from earlier thermal desorption studies, but they are compatible with the measurements reported. Correlation of E d( θ) and ν( θ) with structural data for the H/W(100) surface shows that the higher E d and ν values are associated with desorption from a reconstructed substrate. Effects of the reconstruction also appear in the coverage dependence of the partial molar entropy of the H/W surface layer, $ ̄ s s, which is zero or less at low coverages and rises to about 10 cal/K per mole of H at 1 4 of saturation. Analysis of $ ̄ s s(θ) indicates that the hydrogen-induced reconstruction reduces the entropy of the tungsten surface by 1 k per W atom.

Hydrogen-induced reconstruction of the W(100) surface, studied by surface core level spectroscopy

We report W(4ƒ) surface core level shifts which yield new information on the energetics of the W(100) (1 × 1) → C(2 × 2)H phase transition. At small hydrogen coverages we find two co-existing surface core levels from atoms on normal lattice sites and from atoms in reconstructed domains. These surface levels are shifted to smaller binding energy (toward E F ) by 0.35 eV and 0.13 eV relative to the bulk level, respectively. The most stable configuration is obtained at a fractional coverage θ H ≅ 0.2, at which all surface atoms are shown to be paired with neighboring atoms in the surface plane.