M. Kerkar

Adsorption site determination for oxygen on Al(111) using normal incidence standing X-ray wavefield absorption

Abstract The structure of the chemisorption phase of oxygen on Al(111) has been studied using normal incidence standing X-ray wavefield absorption (NISXW) at the (111) and the ( 1 11) Bragg reflection conditions. The resulting OAl layer spacings in these two directions gives a measure of both the absorbate-substrate layer spacing perpendicular the surface, and the adsorption site. At the lowest coverages of oxygen, the results indicate that the adsorbed oxygen atoms occupy “fcc” hollow sites (i.e. directly above Al atoms in the third layer of the substrate) at a layer spacing of 0.70 A. This result agrees well with previous SEXAFS results and with later LEED work, and reinforces the view that early LEED results giving much larger layer spacings were in error. The new results also indicate that this site is occupied at the lowest coverages of the chemisorbed phase, and that there is no evidence for a previously proposed precursor underlayer. Similar NISXW measurements taken from a surface containing a significant fraction of the subsequent surface oxide phase indicate that the oxygen atoms in this structure retain the same coordination, as well as bondlength, relative to the Al substrate, and favour a model in which the majority of the overlayer oxygen atoms of the chemisorbed phase retain their overlayer sites.

Structural determination of the Cu(111) -(√3 × √3) R30°-ClBr surface using the normal incidence X-ray standing wave method

Abstract A full structural determination has been made of the Cu (111)(√3 × √3)- R 30°- Cl Br surface, which comprises a mixed Cl Br overlayer with a nominal coverage of one third of a monolayer and a 1:1 stoichiometry, using the normal incidence X-ray standing wave (NIXSW) method. Two Bragg reflections were used, the (111) which uses reflecting planes parallel to, and the (111) which are tilted at an angle of 70.5° to, the (111) surface. Using the (111) data, adsorbate-substrate outermost layer spacings were found to be 1.81 ± 0.05 A for chlorine and 2.00 ± 0.05 A for bromine assuming zero substrate relaxation. These results, coupled with the (111) data, are consistent with a model in which both halogens, each of which has a coverage of 1 6 monolayer, occupy three-fold coordinated hollow sites on the surface. The adsorbates occupy primarily fcc hollows (directly above third-layer Cu atoms) but some occupation of hcp hollows (above second-layer Cu atoms) is also observed. For chlorine the fcc:hcp population ratio is 75%:25% (± 10%), while for bromine the rato is 80%:20% (± 10%). Possible origins of the minor species hcp hollow site occupation are discussed.

The structure of sodium adsorption phases on Al(111)

Abstract The structures formed by the adsorption of Na and Al(111) have been studied by normal incidence standing X-ray wavefield absorption using both the (111) and (111) Bragg reflections in order to obtain not only the Na-Al layer spacings, but also the adsorption sites. In the case of the ordered (√3 × √3)R30°-Na phase, at a nominal coverage of 0.33 ML, and at lower coverages down to approximately 0.12 ML, Na is found to adsorb in a coverage independent site which involves a substitution of some of the top layer Al atoms; if the substrate layer spacings remain unchanged by this reconstruction, the effective radius of the adsorbed Na species is found to be 1.67 A. The higher coverage (2 × 2)-Na phase is found to involve (at least) two distinct Na adsorption sites, but each of these has a very similar layer spacing relative to the extended (111) substrate scatterer lattice planes. A specific model of this phase, involving two reconstructed layers each of stoichiometry NaAl 2 is proposed, which accounts in a quantitative way for the present X-ray standing wave data and previous X-ray absorption data, and in a qualitative way for published high resolution soft X-ray photoelectron spectroscopy results. As Al and Na are essentially immiscible in the bulk, this two-layer structure appears to be an entirely new kind of surface phase.

The structure of sulphur adsorption phases on Ni(111) studied by X-ray standing wavefield absorption

Abstract The technique of normal-incidence X-ray standing wavefield (NIXSW) absorption has been applied to an investigation of several structural phases of S on Ni(111), specifically the (2 × 2), (√3 × √3)R30° and (5√3 × 2)rect. phases, with a view in particular to clarifying recent controversy over the structure of the last of these. Absolute adsorbate site determination has been effected through real-space triangulation using both (111) and (111) Bragg reflections. For the (2 × 2) phase the results are consistent with a simple overlayer structure with S atoms occupying the “fcc” hollow sites (directly above third-layer Ni atoms), in agreement with earlier determinations by LEED, ion scattering and SEXAFS. The same local site is found to be occupied in the (√3 × √3)R30° phase, although the NISXW data indicate poorer local order. For the (5√3 × 2)rect. phase, a simple undistorted missing-row model is found to be inconsistent with the data, as is any ideal “coincidence lattice” structure, including a simple undistorted pseudo-(100) surface reconstruction. A modification of this latter model, in which some S atoms penetrate the pseudo-(100) layer to bond to the underlying (111) substrate does, however, contain the main ingredients needed to fit the data. By using not only (111) and (111) NIXSW but also (111) and (111) absorption profiles, recorded from a near single-domain structure formed on a miscut crystal surface, specific information on some of the structural parameters of this modified reconstruction model is obtained and compared with complementary data from other published studies.

The growth of thin Ti and TiOx films on Pt(111): Morphology and oxidation states

Abstract The initial stages of the growth of Ti and TiO x films on Pt(111), grown by evaporation in ultra-high vacuum or under controlled pressure of oxygen, has been studied using X-ray absorption fine structure (XAFS) taken above the TiK-edge, and by ion scattering and X-ray photoelectron spectroscopy. Extended XAFS spectra of the metallic films reveal that growth occurs from the earliest stages as islands. Analysis of the EXAFS amplitudes and the low-energy ion scattering data indicate that the islands retain a morphology in which the thickness and lateral extent are similar as they coarsen through continued growth. Room-temperature oxidation of these metallic films leads to mixed oxidation states, but evaporation in oxygen atmospheres allows films of higher, and more homogeneous, oxidation state to be prepared, as judged by near-edge XAFS and Ti 2p level photoemission.

An unusual adsorption site for methoxy on Al(111) surfaces

The adsorption site of the methoxy (CH3O) species on Al(111) has been investigated using the technique of normal incidence standing X-ray wavefield absorption. By recording the X-ray absorption of the O atom of this species at both the (111) and (111)-normal incidence Bragg scattering conditions (at normal and 70.5 degrees incidence to the surface) the adsorption site was obtained by simple real-space triangulation. The species is adsorbed in a three-fold symmetry hollow site with an O-Al layer spacing of 0.70+or-0.10 AA, (as found for chemisorbed oxygen), but the site occupied is the HCP hollow directly above an Al atom in the second layer, and not the FCC site (above an Al atom in the third layer) which is occupied by chemisorbed oxygen. This appears to be the first example of an adsorbate with a clear preference for this HCP hollow site on an otherwise clean FCC (111) surface.

Surface scienceStructural determination of the Cu(111) -(√3 × √3) R30°-ClBr surface using the normal incidence X-ray standing wave method

Abstract A full structural determination has been made of the Cu (111)(√3 × √3)- R 30°- Cl Br surface, which comprises a mixed Cl Br overlayer with a nominal coverage of one third of a monolayer and a 1:1 stoichiometry, using the normal incidence X-ray standing wave (NIXSW) method. Two Bragg reflections were used, the (111) which uses reflecting planes parallel to, and the (111) which are tilted at an angle of 70.5° to, the (111) surface. Using the (111) data, adsorbate-substrate outermost layer spacings were found to be 1.81 ± 0.05 A for chlorine and 2.00 ± 0.05 A for bromine assuming zero substrate relaxation. These results, coupled with the (111) data, are consistent with a model in which both halogens, each of which has a coverage of 1 6 monolayer, occupy three-fold coordinated hollow sites on the surface. The adsorbates occupy primarily fcc hollows (directly above third-layer Cu atoms) but some occupation of hcp hollows (above second-layer Cu atoms) is also observed. For chlorine the fcc:hcp population ratio is 75%:25% (± 10%), while for bromine the rato is 80%:20% (± 10%). Possible origins of the minor species hcp hollow site occupation are discussed.

Structural study of 1,2-dichloroethane on Cu(111) using X-ray absorption and standing waves

Abstract The structure of 1,2-dichloroethane (DCE) adsorbed on Cu(111) in a single monolayer form has been investigated using ClK-edge near-edge X-ray absorption and normal-incidence standing X-ray wavefield absorption at the (111) reflection. These measurements indicate a small tilt (25°) of the ClC axis from parallel to the surface, and two different equally occupied ClCu layer spacings relative to the extended lattice planes of 1.0 and 1.6 A. These resutls are found to be best reconciled with a structural model in which the DCE bonds with the ClCCCl plane of the trans conformer perpendicular to the surface and the Cl atoms over atop adsorption sites, the CuCl bondlength being compatible with the van der Waals radius for Cl. The possible role of a frustrated rotational mode of the adsorbed molecule leading to a “flipping” between two equivalent states is considered.

Structure determination for PF3 absorption on Ni(111)

The local adsorption structure of PF3 on Ni(111) has been investigated using a combination of P K-edge near-edge and surface extended X-ray absorption fine structure (NEXAFS and SEXAFS) and normal incidence standing X-ray wavefield absorption (NISXW) at both the P and F atoms in order to determine the adsorption site, the adsorbate-substrate bond-length, the molecular orientation and the internal structure of the adsorbed species. The molecule is found to be adsorbed atop a top layer Ni atom at a P-Ni nearest-neighbour distance of 2.07+or-0.03 AA, with its C3v symmetry axis perpendicular to the surface, and with an internal structure (both bond-lengths and bond angles) unchanged from that of the free molecule. The results also indicate that there is no change in the Ni(111) substrate structure induced by the adsorption, and the PF3 has a much larger vibrational amplitude parallel to the surface than perpendicular to the surface, presumably associated with a Ni-PF3 wagging mode.

A structural study of the Al(111)( square root 3* square root 3)R30 degrees -Rb phase at different temperatures

Normal-incidence standing x-ray wavefield (NISXW) measurements have been made of the local adsorption site of Rb on Al(111) surfaces, particularly in an ordered ( square root 3* square root 3)R30 degrees phase, as a function of the sample temperature during adsorption or subsequent annealing. The results confirm the a top-site occupation for low-temperature (around 150 K) preparation, but show that room-temperature preparation leads to a structure having Rb atoms in surface substitutional sites. The overall structural situation is therefore essentially the same as that found previously by low-energy electron diffraction LEED for the Al(111)( square root 3* square root 3)R30 degrees -K phases. However, experiments involving annealing of the low-temperature prepared surface to room temperature indicate that only a small part of the surface easily transforms to the higher-temperature form, and indeed there is evidence that even in room-temperature preparations some fraction of the adsorbed atoms may remain in atop sites. The apparent conflict of this result with that from recent photoemission core-level shift and LEED data is discussed.