Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on the reversibility of hydrogen loading in thin single-crystalline Y films grown by vapor deposition on W͑110͒. Under a H 2 partial pressure of 1ϫ10 Ϫ5 mbar the hexagonal-closedpacked Y films convert to the face-centered-cubic Y dihydride. Unloading is accomplished by annealing the dihydride to 1000 K. No loss of crystallinity is observed during these martensitic transformations of the Y lattice. Moreover, we demonstrate a model to determine the H concentration in Y in situ.The interaction of hydrogen with Y, La, and the rare-earth ͑RE͒ metals has been the subject of numerous investigations due to the interesting temperature-and concentrationdependent structures and properties observed in the solid solution ͑␣ phase͒ as well as in the stable dihydride ͑␤͒ and trihydride ͑␥͒ phases.1 An example is the recent observation of switchable optical properties of Y and La hydride films where shiny, metallic dihydride films become transparent semiconductors in the trihydride phase.2 It is obvious that the geometrical and electronic structure of RE hydrides are key quantities for the understanding of all these properties.For Y, the geometrical structure of the host-lattice and the hydrogen atom positions are well known over the entire range of H concentrations: Y crystallizes in the hexagonalclosed-packed ͑hcp͒ structure, while its dihydride transforms to a face-centered-cubic ͑fcc͒ CaF 2 -type structure.1 The insulating trihydride, finally, possesses a hcp unit cell ͑HoD 3 type͒. 3 The electronic structure of Y hydrides, however, is still a matter of dispute. 1Up until recently most work on hydrogen in Y has been done using polycrystalline bulk or powder samples. Existing photoemission spectroscopy data therefore only yields information on the occupied density of states or on charge transfer from Y to H via core-level shifts. [4][5][6][7] The reason for the absence of experimental band-structure data based on angleresolved ultraviolet photoemission spectroscopy ͑ARUPS͒ is due to the fact that loss of single crystallinity during the first transition ͑␣ to ␤͒ is difficult to avoid and that most bulk samples decompose into powder while going from ␤ to ␥. 1Recently an x-ray diffraction study demonstrated that in thin, monocrystalline Y films, the structural coherence is maintained during cycling ex situ between the dihydride and the trihydride phases. In this study we demonstrate that it is possible to transform thin, single-crystalline Y films into the dihydride phase and to unload them again without loss of order. We used x-ray photoelectron diffraction ͑XPD͒ to observe, in real space and near the surface, the changes occurring due to the H-induced structural transitions. In contrast to the previous study, 8 experiment, loading, and unloading are done in situ, and the films are not capped by a protective Pd layer, a most crucial prerequisite for future ARUPS studies on the electron...

Pd clusters formed by physical vapor deposition at room temperature (RT) on highly oriented pyrolytic graphite were investigated by a combination of X-ray photoelectron spectroscopy and diraction, angle-resolved ultraviolet photoelectron spectroscopy, scanning tunneling microscopy and calculations based on the local density functional theory. Dierent coverages with nominally 3, 5 and 10 A were studied after deposition at RT and after heat treatment at 600°C. Local ordering exhibiting growth with an fcc(1 1 1) orientation is already observed at the lowest coverage, but with no preferred azimuthal orientation in accordance with the substrate itself. However electronic structure features characteristic for a Pd(1 1 1) single crystal appear only after heat treatment.

A laboratory set-up for ultraviolet (UV) photoelectron spectroscopy combining a high intensity UV source and a toroidal UV monochromator, both being commercially available, is described. The source is modified in order to maximize the solid angle collected by the monochromator. Scanning its exit arm length allows for the optimization of the photon intensity as well as the energy resolution by minimizing the defocus term only. By using several different gases such as He, Ne or H 2 one obtains a powerful tool to sample extended regions in k-space. Fermi energy maps are presented for Cu(110) in the photon energy range from 10.2 to 48.3 eV. As a result of the variable photon energy an actual zooming into the well-known Shockley surface state is possible.

The electronic structure of hydrogen adsorbed on Ni͑110͒ at 150 K has been studied using angle-resolved photoelectron spectroscopy with monochromatized He I␣ radiation. At a saturation coverage of 1.5 monolayers, we observe a general shift of the spectral weight away from the Fermi level to higher binding energies indicating the hybridization of hydrogen levels with metal d states. Along selected Ni 3d bands the intensity shift is obvious whereas other bands remain unchanged. In addition, contrary to previous observations, a hydrogen induced state is visible at a binding energy of roughly 1. To our knowledge there are no recent photoemission experiments of hydrogen adsorption on Ni͑110͒. Previous investigations on Ni͑110͒ ͑Refs. 2 and 3͒ and on Ni͑111͒ ͑Refs. 4-6͒ reported in great detail on the hydrogen-induced bonding states mentioned above. However, they could not unequivocally decide the debate which metal states are involved in the hydrogen bonding and what symmetry they have. Another uncertainty existed due to a lack of information on the geometrical positions of hydrogen adsorbed on the different Ni single crystal surfaces. This issue has been resolved in the last decade using more powerful diffraction methods, scanning tunneling microscopy, and electron energy loss spectroscopy. [7][8][9][10][11][12][13][14][15][16]

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