A. Glebov

Orientational ordering of two-dimensional ice on Pt(111)

Two highly ordered, epitaxially rotated phases of bilayer ice are observed on Pt(111) in high resolution helium atom diffraction. Analysis of helium diffraction patterns shows that the two phases differ slightly in their density and alignment with respect to the surface. The lack of any isotope effect for both phases indicates that the ice bilayers have structures and hydrogen bond lengths very similar to bulk ice.

Observation of the second ordered phase of water on the MgO(100) surface: Low energy electron diffraction and helium atom scattering studies

The interaction of water with MgO (100) single crystal surfaces cleaved in situ has been studied by low energy electron diffraction and helium atom scattering in the temperature range 80 K–230 K. At Tcrystal=100–180 K water forms a layer with a c(4×2) symmetry in good agreement with previous spot profile analysis of low energy electron diffraction experiments. Adsorption at Tcrystal=185–221 K leads to the formation of a new ordered phase. The results of the low energy electron diffraction and elastic helium atom scattering experiments show that this high‐temperature phase has a (3×2) symmetry, and that the unit cell contains a glide plane. The isosteric heat of adsorption at half coverage Qst=85.3 kJ/mol has been determined from equilibrium adsorption isotherms measured between 210 and 221 K.

A helium atom scattering study of the structure and phonon dynamics of the ice surface

The structure and phonons of an ordered ice surface, prepared in situ under ultra high vacuum conditions, have been studied by high resolution helium atom scattering. The angular distributions are dominated by sharp hexagonal (1×1) diffraction peaks characteristic of a full bilayer terminated ice Ih crystal. Additional, very broad and weak, p(2.1×2.1) peaks may indicate the presence of small domains of antiphase oriented molecules. An eikonal analysis of the 1×1 peaks is compatible with either a proton disordered or a proton ordered surface with corrugations of 0.76 A and 0.63 A, respectively. Inelastic time-of-flight spectra reveal not only a dispersionless phonon branch reported previously at 5.9 meV, but also the first evidence for the surface Rayleigh phonons, which are reproduced well by a Born–von Karman simulation of a full bilayer terminated ice surface using the unmodified force constants derived from neutron scattering bulk phonon measurements. Since the lattice dynamics simulations do not repro...

A helium atom scattering study of water adsorption on the NaCl(100) single crystal surface

Structural and dynamic properties of H2O adsorbed on an in situ cleaved NaCl(100) crystal surface were studied by He atom scattering in the range of temperatures and partial pressures of 80–160 K and 5×10−7–5×10−10 mbar, respectively. A well‐ordered 2D H2O layer was formed during adsorption as well as by heating of 3D ice. The isosteric heat of adsorption in the monolayer regime was found to be in the range 58–63 kJ/mol. The structure of the 2D condensed phase was investigated for both modes of preparation and in both cases a (1×1) diffraction pattern was found. Time‐of‐flight spectra of the monolayer H2O/NaCl were measured in the 〈100〉 direction and phonon dispersion curves were determined. The observed dispersion‐free vibrational mode, with ℏω=5.5 meV, was identified as a frustrated translation of the water molecule parallel to the surface, via calculations for physisorption models of the H2O/NaCl interaction. Other salient features of the adsorption, the large heat of adsorption, and the magnitude of t...

Structure and dynamics of COMgO(001): a helium atom scattering study

The structure and dynamics of CO adsorbed on MgO(001) surfaces cleaved in situ under UHV has been studied in the temperature range from 36 to 59 K by means of helium atom scattering. The monolayer was prepared at T sample = 36 K under a CO pressure of 1 × 10 −8 mbar. Two stable adsorbate phases were observed. For the lower temperature phase < 40 K a c(4 × 2) pattern was found. Increasing the temperature to 51 K leads to a (1 × 1) symmetry. The change in the diffraction pattern is accompanied by desorption of CO from the MgO surface. The observed phase transition is reversible only if gaseous CO is offered during cooling down to < 40 K again. Thus, the measured (1 × 1) symmetry corresponds to a coverage of Θ < 1. While in time-of-flight experiments only one dispersion-free mode at 9 meV was found for the «(1 × 1)» phase, an additional mode at 10.5 meV was measured for the low-temperature c(4 × 2) structure. From the intensity ratio of both Einstein modes a c(4 × 2) unit cell containing six partially tilted molecules is suggested, while the «(1 × 1)» is attributed to a CO lattice gas

The properties of a two-dimensional water layer on MgO(001)

Abstract Elastic helium-atom scattering (HAS) has been employed to study the adsorption behavior of water on the in-site cleaved MgO(001) surface at 100 ≤ T crystal ≤ 180 K. At these temperatures, water forms a dense phase which consists of orthogonal domains each with a c(4 × 2) unit mesh. No differences in the structures of the H 2 O and D 2 O low-temperature phases were observed. Heating of the crystal covered with the c(4 × 2) water phase to T crystal > 185 K results in the partial desorption of the water layer and the formation of a less dense p(3 × 2) water phase. A thermodynamic study of the high-temperature water phase using low-energy electron diffraction (LEED) to measure the equilibrium adsorption isobars and isotherms enabled us to determine the isosteric heat of adsorption Q st = 85.3 ± 2.1 kJ mol −1 as well as the lateral interaction energy Q ‖ = 35.1 ± 9.6 kJ mol −1 .

Structure and dynamics of a monolayer OCS on NaCl(001): a combined FTIRS, HAS and LEED study

Monolayer films of carbonyl sulfide (OCS) on well-defined NaCl(001) single-crystal surfaces have been studied using Fouriertransform infrared spectroscopy (FTIRS) with polarized light, helium atom scattering (HAS) and low-energy electron diffraction (LEED). The monolayer growth at Tsample = 77–80 K and p(OCS) = 1 × 10−8mbar proceeds via large islands which have the structure of the completed monolayer. For the latter, both HAS and LEED indicate a (2 × 1) superstructure, most probably with two molecules in each adsorbate unit cell. In the infrared spectrum of the OCS asymmetric stretching vibration v3, three intense and three weak absorptions are observed (FWHM < 1 cm−1). Due to the non-degeneracy of the v3 mode this observation would suggest at least three molecules in each unit cell, and challenges the conclusions drawn from HAS, LEED and the integrated infrared absorption. HAS time-of-flight spectroscopy was used to investigate the adsorbate phonon dispersion curves. Eight modes with energies below 80 cm−1 were observed along the 〈100〉 direction. The observability of some phonon modes in certain parts of reciprocal space suggests a close relation between phonon intensity and phonon symmetry.

Rotational-disordering phase transition of C60(111) epitaxial films grown on GeS(001)

The surface structures of C60 films, epitaxially grown on the GeS(001) surface, were investigated from 90 to 350 K by helium atom scattering (HAS) diffraction. The present HAS results indicate a step-flow growth mode that is consistent with the results of previous x-ray scattering studies. By monitoring the diffraction intensities, the orientational-disordering phase transition is found to be completed at Tc=235 K, which is about 25 K lower than the bulk transition temperature. This surface phase transition appears to be preempted by rotational disordering of C60 molecules at defect sites, already initiated at Ts=130 K.

Evidence for a deep bound-state level in the 4HeNaCl(001) potential

Abstract Through a systematic high-resolution study of the elastic bound state resonances observed in the inelastic background of the angular distributions of 4He scattering from the NaCl(001) surface, carried out at 90 K, we obtained clear and conclusive evidence that a deep level in the 4HeNaCl(001) interaction potential exists at −7.07 meV. Important deviations from the free-atom dispersion are observed, as expected for deep bound-state levels. We discuss the physical reasons why deep-level resonances, which are observed in inelastic scattering, may not be detectable in elastic scattering experiments. This finding appears to be crucial with respect to the theory of atom-surface potentials since former quantum-mechanical and close-coupling calculations excluded the existence of a deep level in 4HeNaCl(001) in agreement with elastic scattering studies but at odds with phenomenological predictions.

Structural and vibrational properties of the carbonyl sulfide (OCS) covered NaCl(100) surface

Abstract The structure and dynamics of a physisorbed OCS monolayer on the in situ cleaved NaCl(100) surface have been studied by elastic as well as inelastic helium atom scattering (HAS) in the temperature range 30–100 K. OCS adsorbs on NaCl(100) via the formation of large islands. The commensurate overlayer consists of domains which are rotated 90° with respect to each other and have (2 × 1) unit meshes. Desorption spectra with simultaneous surface coverage monitoring by HAS were used to roughly estimate the surface-adsorbate binding energy to be about 0.26 eV. Phonon spectra from an OCS monolayer were measured in the 〈100〉 direction. At least eight modes with energies below 10 meV were observed. The phonon peak intensities were analyzed as a function of parallel momentum transfer in order to distinguish modes with predominantly transversal or longitudinal polarization.