E. Bertel

Adsorption dynamics for the system hydrogen/palladium and its relation to the surface electronic structure

Abstract We have determined differential sticking coefficients for a monoenergetic nozzle beam of hydrogen on Pd(111) and Pd(110). In particular the energy dependence and the angular variation of the initial sticking coefficient were measured. The results indicate that adsorption of hydrogen on palladium occurs in parallel processes through a direct path with an activation barrier of perhaps 50 meV or less and a precursor path. There is relatively little difference in the adsorption properties of the (111) and the (110) plane. The appearance of a molecular precursor on the (111) plane can be related to the electronic structure of palladium, in particular to the absence of occupied Shockley surface states, as compared to Ni (111) and Pt (111). Pre-adsorbed potassium on a (110) plane acts as an inhibitor to adsorption. Different inhibiting mechanisms are observed for the direct adsorption path and the precursor path. At high potassium coverage the precursor path is completely suppressed.

Promotors, poisons and surfactants: Electronic effects of surface doping on metals

The interaction of adsorbates with metal surfaces is discussed. It is shown that the evanescent charge density produced by occupied sp derived surface states yields a considerable contribution to the Pauli repulsion experienced by adsorbate particles with a closed-shell electronic structure, e.g. rare-gases or molecules such as H2 or N2. For rare-gases this results in a reduction of the binding energy in the presence of occupied surface states, for molecules this gives rise to an additional contribution to the dissociation barrier. Suitable surface dopants are able to depopulate surface states and thereby to reduce the dissociation barrier. Such dopants can substantially promote catalytic reactions in which the dissociation from the gas phase or a physisorbed precursor is the rate limiting step. In contrast to closed-shell systems the bonding interaction for metal adsorbates on metal substrates is enhanced by occupied surface states. This leads to an extra diffusion barrier at steps, because the surface state amplitude drops to zero at upper step edges. The additional step-edge barrier, which is a kinetic hindrance for layer-by-layer growth, can be reduced by surface dopants depopulating the corresponding surface state. Such dopants promote layer-by-layer growth and act therefore as surfactants. It is concluded that the effect of promoters in catalysis and of surfactants in metal epitaxy is in part due to the same basic mechanism, namely the depopulation of surface states.

Inverse photoemission study of carbon monoxide bonding to transition metals

Abstract The unoccupied CO derived bands for the densely packed CO monolayer on Ni(110), Pd(llO), and Pt(110) have been investigated by inverse photoemission. In all three cases four CO derived bands can be observed in the region of the 2π ∗ and 5σ ∗ orbitals. For Ni and Pd the band dispersions show the signature of π bands. No evidence is found for unoccupied σ contributions, i.e. for 5σ donation. However, in Pt the band dispersions are significantly different and hint at a significant 5σ donation. The energy position of the CO derived manifold is rather similar for all three metals. These results on the electronic structure are consistent with thermodynamical and vibrational data reported in the literature for the three systems.

The interaction of atomic hydrogen with Cu(110)

Abstract The interaction of atomic hydrogen with Cu(110) has been investigated by thermal desorption spectroscopy and by surface state spectroscopy using ultraviolet photoemission and inverse photoemission. Atomic H first forms a chemisorption layer, which saturates at a coverage of ∼ 0.5 monolayers. At low temperature further exposure leads to a thermal desorption peak which was not observed hitherto. We attribute it to H absorption into subsurface sites. The surface state shifts caused by the adsorption of hydrogen on Cu(110) show striking parallels to those observed upon hydrogen adsorption on Ni(110). The (1 × 2) reconstruction of the H/Cu(110) surface gives rise to a surface state shift which is characteristic for a missing or added row phase. The intensity variation of the surface states during the formation of the (1 × 2) phase indicates the formation of large islands. An occupied surface state on H/Cu(110) contributes to the stabilization of these islands.

Observation of an sp-derived surface resonance on Pt(111) indicating the crucial role of surface states in physisorption

Abstract High-resolution angle-resolved photoemission of Pt(111) reveals the presence of an sp-derived surface resonance close to EF in the centre of the surface Brillouin zone. Similar surface states are found on all (111) faces of the d9 transition and the noble metals with the only exception of Pd(111). This finding is used to explain previous observations of an exceptional physisorption behaviour of Pd(111) towards Xe and H2. The results strongly suggest that surface states crucially influence the physisorption potential on metal surfaces.

Surface states, local bonding, and surface reconstruction: Na on Cu(110)

Abstract Occupied and unoccupied surface states on Na/Cu(110) have been investigated by photoemission and inverse photoemission. By measuring surface state positions at the same adsorbate coverage on the reconstructed and the unreconstructed surface it is demonstrated that the surface states contribute to the total energy balance of the Na induced missing row reconstruction. During the onset of the reconstruction an up-shift of the surface states depopulates an initially occupied Cuue5f8Cu bonding surface state and consequently destabilizes the topmost Cu atoms. The surface state wavefunctions are qualitatively analyzed in a thight-binding LCAO picture.

Unoccupied electronic states in adsorbate systems

Experimental work on unoccupied electronic states in adsorbate systems on metallic substrates is reviewed with emphasis on recent developments. The first part is devoted to molecular adsorbates. Weakly chemisorbed hydrocarbons are briefly discussed. An exhaustive inverse photoemission (IPE) study of the CO bond to the transition metals Ni, Pb, and Pt is presented. Adsorbed NO is taken as an example to demonstrate the persisting discrepancies in the interpretation of IPE spectra. Atomic adsorbates are discussed in the second part. The quantum well state model is applied to interpret the surface states in reconstructing and non-reconstructing adsorption systems of alkali metals and hydrogen. A recent controversy on the unoccupied electronic states of the Cu(110)/O p(2×1) surface is critically reviewed. The quantum well state model is then compared to tight binding and local-density-functional calculations of the unoccupied bands and the deficiencies of the various approaches are pointed out. Finally, the relation between the surface state model and more chemically oriented models of surface bonding is briefly discussed.

Compressed CO adsorption layers on the (110) surfaces of Ni, Pd, and Pt. An inverse photoemission study of the unoccupied electronic band structure

Abstract The unoccupied band structure of a CO monolayer on Ni(110), Pd(110), and Pt(110), respectively, has been investigated by inverse photoemission. For Ni and Pd the CO-derived band structures exhibit only marginal differences. The observed bands are interpreted in terms of 2π x - and 2π y -derived COue5f8CO bonding and antibonding bands. The level ordering in the center of the surface Brillouin zone, as obtained from a polarization analysis, revealed a similar splitting for the 2π x and the 2π y levels at normal incidence. This indicates a COue5f8CO interaction of approximately the same magnitude in the x as well as the y direction. Consequently, the CO molecules seem to be arranged in a nearly hexagonal structure at monolayer coverage which in turn is due to the COue5f8CO repulsion dominating over the CO-substrate interaction in the tightly packed monolayer. The observed level ordering of the 2π y bands is consistent with such a geometry. On Pt(110), a distinctly different electronic structure indicates a different bonding of the CO to the metal substrate, namely a reduced 2π backbonding and a more prominent role of the 5σ dative bonding. This may also give rise to a slightly different adsorption geometry.

On the adsorption site of ethylene at the Ni(110) surface : a combined experimental and theoretical study involving the unoccupied band structure

The adsorption of ethylene on Ni(110) was investigated by angle resolved inverse photoemission (ARIPE) spectroscopy as well as by detailed density functional model cluster and slab model band structure calculations to clarify the preferred adsorption site. Cluster model calculations both at the local density as well as at the gradient corrected level of theory gave a slight preference for the di-σ over the π coordinated geometry on top of the ridges, but no or, at best, a very weak binding over the troughs. A dispersionless band in the ARIPE spectra about 1.8 eV above EF is assigned to the band derived from the lowest unoccupied ethylene orbital, 1b3g(π∗). The surface state feature of the clean Ni(110) surface connecting the image state at gG and the dyz state S2 lowered almost uniformly by 0.8 eV through the interaction with the adsorbate. For adsorption on top of the ridges in the so-called “half-bridge” position intermediate between the short bridge (di-σ) and the top site (π), the symmetry requirements imposed by the surface state band are ideally met by the second lowest unoccupied band of the adsorbate monolayer which changes its character from ethylene 4ag at Γ′ to 2b3u at Y′. A similarly strong and uniform interaction is not possible when ethylene adsorbs above the troughs. Taking all experimental and theoretical evidence together, the adsorption site in the densely packed c(2×4) C2H4Ni(110) adsorption system is identified as the half-bridge position on top of the ridges.

One- and two-dimensional surface states on metals

Abstract The influence of surface states on the physics and chemistry of metal surfaces is demonstrated by a few examples. An introduction is given to surface state models and general symmetry properties of their wavefunctions with focus on Shockley surface states of fcc transition and noble metal surfaces. Their role in the adsorbate induced reconstruction is studied on the system Na Cu (110) . The existence of one-dimensional (1D) surface states in the 1D-disordered system H Ni (110) is demonstrated and mechanisms for the localization are discussed. Finally, it is shown that surface states influence the charge density outside a surface and thereby determine the strength of the physisorption potential and the barrier height for dissociative adsorption. As a consequence, a new mechanism is proposed for the promotion of catalytic reactions.