The hydrogen-induced reconstruction on a high step density W(001) crystal, (√2 x √2)R45°-H, with steps oriented parallel to the [110] and ≈28Å average terrace width was investigated using low-energy electron diffraction symmetry, beam shape analyses, and EELS. The symmetry of the low-energy electron diffraction pattern was observed to change from p2mg for the (√2 x √2)R45° clean surface reconstruction to c2mm for the commensurate phase (√2 x √2)R45°-H reconstruction. Correspondingly, the shapes of the half-order beams indicate that the hydrogen-induced reconstruction domains were much less elongated than the clean surface domains. A splitting of each half-order beam into four beams at higher exposures indicated the existence of two domains of the incommensurate phase. A commensurate phase v1 vibrational loss peak centered at 160meV in the EELS spectrum broadens on the low-energy side during the incommensurate phase and then shifts toward 130meV and narrows as the (1 x 1)-H saturation structure develops. These observations imply that there was no long-range inhibition (≈20Å) to the formation of either commensurate or incommensurate phase; hydrogen induced 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 was 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. J.F.Wendelken, G.C.Wang: Surface Science, 1984, 140[2], 425-36