C. Nyberg

On the structure of dense CO overlayers

Abstract A series of dense CO structures formed on the Ni(100), Cu(100) and Pd(100) surfaces have been investigated using high resolution electron energy loss spectroscopy (EELS). The structures were monitored by low energy electron diffraction (LEED). The simple half-monolayer structures i.e. the c(2×2) CO on Ni and Cu and the p (2 2 × 2 ) R 45° CO on Pd are fully compatible with on-top site (Ni, Cu) and bridge site (Pd) adsorption, respectively. The denser structures form in a somewhat different manner on the three surfaces, with the common factor that the CO molecules bend in order to accommodate the increasing CO-CO repulsion while still retaining simple coordinations with respect to the substrate surface atoms.

Vibrational excitations of p(2×2) oxygen and c(2×2) hydrogen on Pd(100)

Abstract The p(2×2) oxygen and c(2×2) hydrogen structures on Pd(100) have been investigated by angle-resolved high-resolution electron energy loss spectroscopy. Dipole excited vibrational modes are observed at 44 and 64 meV for the oxygen and hydrogen structures respectively and are interpreted to correspond to atomic adsorption in the hollow site.

Vibrational excitations of nitric oxide adsorbed on Pd(100)

Abstract High resolution electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED) have been used to study the adsorption of nitric oxide on Pd(100). We have observed the vibrational excitations of adsorbed NO for several ordered structures. At low coverage a loss at 38 meV with a shoulder on the high energy side and a loss at 185 meV are observed. At high coverages the EEL spectrum exhibits a loss at 25 meV, a loss at 41 meV with a shoulder on the high energy side and a loss at 208 meV. The observation of two different losses in the high energy region, corresponding to the internal NO stretch, is characteristic for the presence of two different NO species on the surface. These may be monomeric and dimerized NO or NO adsorbed in bridge and on top sites. We propose that the NO molecules form monomers in the low coverage region and dimers in the high coverage region on the Pd(100) surface.

Adsorption of hydrogen on a stepped nickel surface

Abstract The adsorption of hydrogen and deuterium on a stepped nickel surface, Ni(510), has been studied using electron energy loss spectroscopy, EELS. At a surface temperature of ~100 K both dissociative and molecular chemisorption is found. For small doses hydrogen atoms adsorb in both hollow sites on the terraces and in low symmetry bridge sites at the steps with a preference for the step sites. For larger exposures, 0.5–0.9 L H 2 , a dense layer of hydrogen atoms is formed on the terraces and the coverage increases at the steps where a second kind of step site gets occupied. Finally, for exposures larger than 0.9 L H 2 , hydrogen molecules chemisorb at sites above the steps.

Coadsorption of hydrogen and carbon monoxide on Ni(100) and Pd(100) - hydrogen recombination, dissolution and desorption

Abstract The adsorption of carbon monoxide on hydrogen covered Ni(100) and Pd(100) surfaces has been investigated using electron energy loss Spectroscopy (HREELS), low energy electron diffraction (LEED) and thermal desorption Spectroscopy (TDS). On the Ni surface, part of the adsorbed H recombine to form chemisorbed H 2 and part of the CO molecules change adsorption sites. The H 2 molecules seem to be stabilized by the specific CO environment. The Pd surface shows a more complex behaviour. The hydrogen is partly displaced subsurface. For a CO coverage around 0.5 monolayers an intermediate state preceding H-dissolution can be isolated.

Step site adsorption and ordering of CO on Ni(510) and Pd(510)

Using HREELS and LEED we have observed profound differences in step site adsorption and ordering of CO on two stepped fcc (100) surfaces, Ni(510) and Pd(510). On Ni(510) we find that CO initially decorates the steps. This seems to prohibit the formation of any globally ordered structure, although the local structure at high coverages resembles that observed on the flat Ni(100) surface. Two distinct adsorption sites associated with the near step region are observed. On Pd(510) adsorption initially occurs on the terraces, and a well-ordered structure corresponding to 0.5 monolayers is formed. More dense structures also develop, but are somewhat disordered. A specific step site is occupied at large CO coverages and low substrate temperature.

Adsorption of water on Cu(100) and Pd(100) at low temperatures: observation of monomeric water

Abstract The adsorption of water on Cu(100) and Pd(100) has been studied with use of electron energy loss spectroscopy (EELS). At 10 K and low coverage, water is found to adsorb associatively as monomers on the Cu(100), Pd(100) and Pd(100)p(1×1)H surfaces. The molecular axis is tilted ~ 60° away from the surface normal. At 110 K water adsorbs on Pd(100) as an intermolecularly hydrogen bonded ice layer. The presence of oxygen on the surface prevents the formation of hydrogen bonded water for small exposures.

Adsorption of hydrogen on stepped Ni and Pd surfaces - observation of chemisorbed hydrogen molecules

Abstract We report the observation of chemisorbed hydrogen molecules at the step sites of Ni(510) and Pd(510) surfaces which are saturated with atomic hydrogen. No such states have been observed on the corresponding flat (100)-surfaces. For Ni four dipole active vibrational modes related to the H 2 molecule are observed. The mode assignments and the intensities of the loss peaks indicate that the molecule is side-on bonded at the step edge. The internal stretch mode is shifted down to 398 meV (gas-phase value: 516 meV). In the case of Pd only the internal stretch mode is observed at 414 meV. Adsorption deeper into the step may explain the difficulty of observing any other H 2 modes. We suggest that these H 2 chemisorption states can be stably formed at the step sites due to residual unfilled d-states and the simultaneous blocking of the dissociative channel by the saturated atomic adsorbate layer.

Soft X-ray appearance potential spectroscopy of O chemisorbed on Ti, Cr, Fe and Ni

Abstract We wish to report the observation of O 1 s level soft X-ray appearance potential spectra for O adsorbed on clean polycrystalline surfaces of Ti, Cr, Fe and Ni. The spectra are detectable in the chemisorption regime and are then characterized by one single peak at the threshold. The peak is thought to be related to an enhanced local density of unfilled valence states on the oxygen atoms. These simple spectra are in sharp contrast to the multipeaked O 1 s level spectra observed from the surface oxides grown after prolonged oxygen exposures.

The coadsorption of hydrogen and nitric oxide on Pd(100) studied by EELS

Abstract High resolution electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED) have been used to study the coadsorption of hydrogen and nitric oxide on Pd(lOO). The NO molecules adsorb in the same sites on the Pd(100)p(1 × 1)H surface as on the clean Pd(100) surface. The NO adlayer forces the hydrogen atoms to move deeper into the substrate.