D.P. Woodruff

The Solid–Liquid Interface

Preface 1. Interfacial free energy and the y-ploy 2. The experimental determination of the solid-liquid interfacial free energy ySL 3. The structure of the solid-liquid interface 4. Non-structural views of the solid-liquid interface 5. Morphological stability 6. Dendritic growth 7. Eutectic growth 8. Mechanisms and kinetics of crystal growth.

Leed structure analysis of the Ni{100} (2 × 2)C (p4g) structure; A case of adsorbate-induced substrate distortion

Abstract A detailed study has been undertaken of the Ni{100} (2 × 2)C structure formed by cracking ethylene on a clean Ni{100} surface. The LEED pattern shows characteristic missing spots which can be attributed to the presence of glide lines and indicate a space group symmetry of p4g. We show that this can be readily interpreted in terms of a distortion of the top nickel layer both parallel and perpendicular to the surface, which accompanies the carbon adsorption. Detailed comparisons of LEED intensity data with dynamical calculations indicate that the top layer nickel atoms are displaced 0.35 ± 0.05 A parallel to the surface, 0.20 ± 0.05 A outwards from the surface, and that the carbon atoms are in 4-fold hollows (now distorted) at a spacing of 0.1 ± 0.1 A from the surface. These conclusions lead to a nickel-carbon nearest neighbour spacing of 1.803 ± 0.015 A.

Dipole coupling and chemical shifts in IRAS of CO adsorbed on Cu(110)

Results are reported of an experimental study using infra-red reflection absorption spectroscopy of mixed isotopic layers of 12C18O and 12C16O adsorbed on Cu(110). It is shown that dipole coupling leads to an upward shift of the frequency of the 12C16O layer by 50 cm−1 as the coverage is increased to saturation from an initial value of 2088 cm−1 but this is counteracted by a downward chemical shift of 44 cm−1. A general discussion of the method of separating these terms using the isotopic dilution limit is presented and some special problems involved in this separation for CO on copper are discussed. A model for the origin of the chemical shift involving through space interactions between the CO molecules due to overlap of 2 πb levels is proposed which takes into account other data on Cu, Pt and Pd surfaces.

An iras study of formic acid and surface formate adsorbed on Cu(110)

The adsorption and decomposition of formic acid on Cu(110) has been studied using infrared reflection-absorption spectroscopy and temperature-programmed desorption. A broad double peak at ∼ 1635 cm−1 was found to be characteristic of the carbonyl stretch ν(CO) of the acid monolayer after adsorption at 120 K. In agreement with earlier data, deprotonation occurred at ~ 270 K to give a surface formate species. The TPD measurements gave an estimate of formate coverage (θsat = 0.25 ± 0.05) and thus a determination of the reaction probability for deprotonation. The symmetric O-C-O vibration νs(COO) (1348–1358 cm−1) and the C-H stretch vibration ν(CH) (2891–2900 cm−1) were measured as a function of formate coverage. A vibrational band corresponding to νa(COO) was not observed under any experimental conditions; the formate species is thus thought to have C2v or Cs(1) symmetry. For the first time in IR spectroscopy of adsorbates a combination band νcomb at 2950 cm−1 was also observed. Its components (νa(COO) and δ(CH)) were not observed individually, since they have dynamic dipoles parallel to the surface. The combination band shifts at low coverages to lower frequencies, which is due to lateral interactions between parallel dynamic dipoles, and has an intensity comparable to that of the neighbouring C-H stretch, which may be due to a Fermi resonance.

The structure of the formate species on copper surfaces: new photoelectron diffraction results and sexafs data reassessed

Abstract Photoelectron diffraction from the C 1s and O 1s levels of the surface formate species (HCOO) on Cu{100} and Cu{110} has been measured and shows almost identical modulation structure indicating that the adsorption site is the same on both surfaces. Calculations show that the molecule adsorbs on the (short) bridge site in each case with the oxygen atoms close to atop positions and with a CuO nearest-neighbour distance of 1.98±0.04A and also indicate that the OCO bond angle may be increased to 134°. The bond lengthsagree with previous SEXAFS results but the sites differ and the reason for the discrepancies are discussed by reference to the results from model calculations of the SEXAFS data.

Determination of the local structure of glycine adsorbed on Cu(110)

Abstract Scanned-energy mode N 1s and O 1s photoelectron diffraction has been used to determine the local geometry of glycine adsorbed on Cu(110) in an ordered (3 × 2) phase. The results are consistent with a molecular geometry in which the CC axis lies approximately parallel to the surface and the molecule bonds across a pair of [110] Cu surface rows through the two oxygen atoms of the carboxyl group and the N atom of the amino group. The N atom is displaced by 0.24 ± 0.10 A off an atop site along the [110] rows with a CuN nearest-neighbour distance of 2.04 ± 0.02 A . The carboxyl bonds to two Cu atoms in a [110] row, with the O atoms displaced 0.80 ± 0.07 A from atop in [001] towards the amino group with a CuO nearest-neighbour bond length of 2.03 ± 0.03 A and a resulting tilt of the OCu bond relative to the surface normal of 23 ± 2°. A specific structural model comprising two molecular moieties per unit mesh which has the space group p1g1, consistent with the qualitative LEED observations, is proposed on the basis of these data.

Normal incidence X-ray standing wave determination of adsorbate structures

The technique of X-ray standing wavefield absorption applied to the determination of adsorbate structures at the solid-vacuum interface is reviewed with particular emphasis on those studies exploiting normal incidence to the Bragg scatterer planes which allows the method to be used to study adsorption structures at the surfaces of typical metal as well as semiconductor single crystals. The general theory of the XSW technique is described, as is the connection between the structural parameters obtained from the method, the so-called coherent position and coherent fraction, and the actual distribution of adsorbed atoms at the surface. Some general conclusions concerning the significance of low and high coherent fractions are drawn, with especial emphasis on adsorption at fcc (111) surfaces as model systems, but considering high and low symmetry local adsorption sites, and simple and complex commensurate structures as well as incommensurate overlayer phases. Extensions of the method to obtain structural information on clean surface structures and adsorbate-induced modification of substrate surfaces, primarily concerning surface layer relaxations, are also described. The special advantages and problems of photoemission monitoring of the X-ray standing wavefield at adsorbate sites are considered, including the ability to obtain chemical state-specific as well as element-specific structural information and the special problems encountered when non-dipole effects contribute significantly to the photoemission process.

Structure determination of Ni(111)c(4 × 2)-CO and its implications for the interpretation of vibrational spectroscopic data

Abstract A detailed quantitative structure determination of the Ni(111)c(4 × 2)-CO structure has been undertaken using scanned energy mode photoelectron diffraction from the C 1s state over a wide range of emission angles. Analyses of these data by approximate direct methods, and by two independent multiple scattering trial-and-error fitting optimisations lead to a consistent structure in which the CO occupies both types of hollow site on the surface in equal amounts with a C-Ni top layer spacing of 1.29 ± 0.05 A. This structure is therefore essentially the same as that for Ni(111)c(4 × 2)-NO, and provides further evidence that simple use of the intramolecular stretching frequencies of such adsorbed molecules, which had been interpreted in both cases as indicative of bridge site adsorption, is not always a reliable indicator of local adsorption site.

Determination of the adsorption structure for formate on Cu(110) using SEXAFS and NEXAFS

Abstract Near-edge X-ray absorption fine structure (NEXAFS) is used to establish the CO bond length (1.25±0.05 A ) , the OCO angle (124±15°) and the azimuthal orientation (along [1 1 0]) of the molecular plane of the formate catalytic intermediate adsorbed on Cu(110). Extended X-ray absorption fine structure (EXAFS) is then used to determine the CuO bond length (1.98±0.07 A ) and the adsorption site (atop copper atoms in the top (ridge) rows, with oxygen atoms near short bridge sites). The problems of obtaining reliable quantitative results from NEXAFS are discussed. The adsorption site and bond length, which agrees well with that for anhydrous copper formate, is contrasted with the much longer bond length found for this species adsorbed on Cu(100) by Stohr et al. [Phys. Rev. Letters 54 (1985) 1256].

Missing spots in low energy electron diffraction

Abstract Recent attempts to justify the use, after appropriate data averaging, of kinematical theory in surface structure determination by LEED, make it important to be able to distinguish truly kinematical features from those which can also be predicted from entirely general symmetry arguments. Such arguments are therefore developed formally into a set of rules determining the consequences of symmetry for LEED patterns. In particular the conditions for missing spots due to the presence of glide planes are given. It is pointed out that in some circumstances the isotropic scatterer model leads to spurious selection rules, that will not be satisfied in general.