Photoelectron Diffraction Experiments Research Articles

Break of symmetry at the surface of IrTe2 upon phase transition measured by x-ray photoelectron diffraction

IrTe2 undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Full hemispherical x-ray photoelectron diffraction (XPD) experiments have been performed to investigate the atomic position changes undergone near the surface of 1T-IrTe2 in the first-order phase transition, from the (1 × 1) phase to the (5 × 1) phase. Comparison between experiment and simulation allows us to identify the consequence of the dimerization on the Ir atoms local environment. We report that XPD permits to unveil the break of symmetry of IrTe2 trigonal to a monoclinic unit cell and confirm the occurrence of the (5 × 1) reconstruction within the first few layers below the surface with a staircase-like stacking of dimers.

Key role of rotated domains in oxygen intercalation at graphene on Ni(1 1 1)

Herein, we provide a complete description of the intercalation of oxygen at the strongly interacting graphene on Ni(1 1 1), highlighting the role of rotated graphene domains in triggering the intercalation. High-resolution core-level photoelectron spectroscopy provided a full characterization of the interface at each stage of the intercalation, revealing the formation of an oxide layer between graphene and the metal substrate. Angle-resolved photoemission spectroscopy measurements showed that the oxide decouples efficiently graphene from the substrate, restoring the Dirac cone and providing a slight n-doping. Photoelectron diffraction experiments revealed that graphene domains not aligned with the Ni substrate are the first to be intercalated with oxygen and are preferential regions under which the oxygen is retained during the deintercalation.

Imaging molecular structure through femtosecond photoelectron diffraction on aligned and oriented gas-phase molecules.

This paper gives an account of our progress towards performing femtosecond time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe setup combining optical lasers and an X-ray free-electron laser. We present results of two experiments aimed at measuring photoelectron angular distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C(8)H(5)F) and dissociating, laser-aligned 1,4-dibromobenzene (C(6)H(4)Br(2)) molecules and discuss them in the larger context of photoelectron diffraction on gas-phase molecules. We also show how the strong nanosecond laser pulse used for adiabatically laser-aligning the molecules influences the measured electron and ion spectra and angular distributions, and discuss how this may affect the outcome of future time-resolved photoelectron diffraction experiments.

Preparation and investigation of the A-site and B-site terminated SrTiO3(001) surface: A combined experimental and theoretical x-ray photoelectron diffraction study

We have investigated surface composition, structure, and termination of SrTiO3(001) samples (STO) following typical steps of standard ex situ and in situ preparation procedures by performing x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD) experiments, and multiple scattering cluster (MSC) calculations. XPS has confirmed the presence of Sr2+, Ti4+, and O2-related to STO. Well-developed diffraction features have been observed in all XPD polar scans demonstrating the good structural ordering of the investigated surfaces. In order to permit a detailed comparison of XPD experiment and theory, we have especially taken care of effects due to the angular dependent instrument function by applying a new ratio procedure. Thus, we achieved a very good agreement between results of MSC calculations and experimental angular distributions. A fingerprint region has been identified in Sr3d polar scans, which has exhibited both evident dependencies on surface preparation steps in the experiments and clear sensitivity to surface termination in the theoretical modeling. In this manner, we have been able to describe as-received samples and in situ-treated samples as being TiO2- and SrO-terminated, respectively. Standard buffered hydrofluoric acid-treated samples turned out to be mostly TiO2-terminated but with a non-negligible admixture of different, likely SrO-type termination.

High-resolution core-level spectroscopy study of the ultrathin aluminum oxide film on NiAl(110)

We have studied the ultrathin aluminum oxide film on NiAl(110) by a combination of high-resolution core-level spectroscopy and density functional theory calculations. Energy-dependent core-level data from the O 1$s$ and Al 2$p$ levels allows for a distinction between oxygen and aluminum atoms residing at the surface or inside the aluminum oxide film. A comparison to calculated core-level binding energies from the recent model by Kresse et al. [Science 308, 1440 (2005)] reveals good agreement with experiment, and the complex spectroscopic signature of the thin Al oxide on NiAl(110) can be explained. Our assignment of a shifted component in the O $1s$ spectra to oxygen atoms at the surface with a particular Al and oxygen coordination may have implications for the interpretation of photoelectron-diffraction experiments from similar ultrathin aluminum oxide films.

STM and LEED observations of a c(2 × 2) Ge overlayer on Pt(1 0 0)

We have investigated surface structures formed by deposition of 0.2 and 0.5-ML Ge on Pt(1 0 0) by using scanning tunneling microscopy (STM) and low electron energy diffraction (LEED). In addition, their temperature dependence and reactivity to CO have been studied. We observed the formation of disordered domains for Ge adatom coverages below 0.25-ML and complete c(2 × 2) structures at 0.25 to 0.5-ML Ge after annealing at 600–1200 K. Deposition of 0.2-ML Ge on a clean, hexagonally reconstructed (5 × 20)-Pt(1 0 0) substrate at 400 K lifts the reconstruction and ejects excess Pt atoms from the first layer into the adlayer. After annealing this surface to 600 K, the deposited Ge formed Ge adatoms on flat terraces and on round Pt adislands with incomplete c(2 × 2) structures, in addition to the presence of clean (1 × 1)-Pt(1 0 0) domains that were several nanometers across. Some domains of the unreconstructed (5 × 20)-Pt(1 0 0) surface still remained. After the deposition of 0.5-ML Ge and annealing at 600 K, disordered Ge domains disappeared and a c(2 × 2) Ge overlayer was produced all over the surface. Square terraces with square domains of the clean (1 × 1)-Pt(1 0 0) surface extended for nanometers. Annealing this surface to 900 K produced disordered Ge domains, and this was associated with an increase in Ge vacancies. When surfaces with 0.2-ML Ge were heated to 900 or 1200 K, or when a surface with 0.5-ML Ge was heated to 1200 K, larger domains of (5 × 20)-Pt(1 0 0) were formed with the agglomeration of disordered Ge adatoms. Pt clusters were observed in the Ge domains, and we consider these to be composed of those excess Pt atoms formed by lifting the reconstruction of the (5 × 20)-Pt(1 0 0) surface upon Ge agglomeration during cooling. A paper published elsewhere [T. Matsumoto, C. Ho, M. Batzill, B.E. Koel, Physical Review B, submitted for publication.] describes Na +-ion scattering spectroscopy (Na +-ISS) and X-ray photoelectron diffraction (XPD) experiments that distinguish between Ge present in an overlayer from incorporation into the top Pt layer to form a surface alloy for the surface structures reported here. Furthermore, these investigations revealed that disordered Ge adatoms observed herein might be associated with incomplete c(2 × 2) structures. Therefore, our observations of the formation of complete and incomplete domains of c(2 × 2) Ge adatoms indicate that interactions between Ge adatoms are repulsive at nearest neighbor distances and attractive at second-nearest neighbor distances. Regarding the reactivity of these surfaces, CO does not chemisorb on a Pt(1 0 0) surface with a c(2 × 2)-Ge overlayer and no measurable CO uptake was observed under UHV conditions at 220 K.

Local coordination of Mn atoms at the Mn:Ge(111) interface from photoelectron diffraction experiments

X-ray photoelectron diffraction (PED) experiment on the metallic Mn:Ge(111) surface alloy has been carried out in the initial stage of mixed phase growth. Our findings show that the ${\mathrm{Mn}}_{5}{\mathrm{Ge}}_{3}$ phase is not yet formed for a 1.3 ML (monolayer) coverage, as well as for a 2 ML coverage, followed by an annealing at temperature $(\ensuremath{\sim}300\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$. Rather, we observed the formation of an ordered surface alloy by Mn occupation of the hollow ${\mathrm{H}}_{3}$ sites in the topmost layer, while about 10% of Mn atoms are found in subsurface layers, partially confirming the theoretical expectations by Zhu et al. [Phys. Rev. Lett. 93, 126102 (2004)]. However, the contribution to PED patterns from subsurface Mn atoms is found compatible with the occupation of interstitial sites only for the 1.3 ML coverage and with the occupation of both interstitial and unexpected substitutional sites for the 2 ML coverage. These findings thus open questions about the determination of the kinetic path to Mn subsurface migration in diluted alloys.

Defects at the TiO 2(1 0 0) surface probed by resonant photoelectron diffraction

We report photoelectron diffraction (PED) experiments of weakly sub-stoichiometric TiO 2(1 0 0) rutile surfaces. Apart from standard core-level PED from the Ti-2p3/2 line, we have studied valence band PED from the defect induced Ti-3d states in the insulating band gap. For maximum yield, the latter were resonantly excited at the Ti-2p absorption edge. The PED patterns have been analyzed within the forward scattering approximation as well as by comparison with simulated PED patterns obtained in multiple scattering calculations. The analysis shows that the defect induced Ti-3d charge is mainly located on the second layer Ti atoms.

Single-particle excitation spectra ofC60molecules and monolayers

In this paper we present calculations of single-particle excitation spectra of neutral and three-electron-doped Hubbard C$_{60}$ molecules and monolayers from large-scale quantum Monte Carlo simulations and cluster perturbation theory. By a comparison to experimental photoemission, inverse photoemission, and angle-resolved photoemission data, we estimate the intermolecular hopping integrals and the C$_{60}$ molecular orientation angle, finding agreement with recent X-ray photoelectron diffraction (XPD) experiments. Our results demonstrate that a simple effective Hubbard model, with intermediate coupling, $U=4t$, provides a reasonable basis for modeling the properties of C$_{60}$ compounds.

X-ray photoelectron diffraction study of ultrathin PbTiO3 films

Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate at the atomic level ultrathin epitaxial c-axis oriented PbTiO3 (PTO) films grown on Nb-doped SrTiO3 substrates. Comparison between experiment and theory allows us to identify a preferential ferroelectric polarization state in a 60 A -thick PTO film. Multiple scattering theory based on a cluster-model [ Phys. Rev. B $\textbf{63}$ , 075404 (2001)] is used to simulate the experiments.

Mg(0001) surface oxidation: A two-dimensional oxide phase

First-principles electronic structure calculations and angle-scanned x-ray photoelectron diffraction experiments reveal the geometrical structure of the Mg(0001) surface upon oxidation. In contrast to previous studies of Mg(0001) oxidation and unlike the case of aluminum oxidation we determine a surface oxide structure, consisting of mixed oxygen-magnesium layers on top of an almost unchanged Mg(0001) surface. This unusual, directionally bound, surface oxide is locally formed already at very low dosing, corresponding to total coverages of 0.1 monolayer (ML), and grows laterally with increasing oxygen coverage up to at least 3 ML total coverage.

O adsorption and incipient oxidation of the Mg(0001) surface

First-principles density-functional calculations are used to study the early oxidation stages of the Mg(0001) surface for oxygen coverages $1/16<~\ensuremath{\Theta}<~3$ monolayers. It is found that at very low coverages O is incorporated below the topmost Mg layer in tetrahedral sites. At higher oxygen load the binding in on-surface sites is increased but at one monolayer coverage the on-surface binding is still about 60 meV weaker than for subsurface sites. The subsurface octahedral sites are found to be unfavorable compared to subsurface tetrahedral sites and to on-surface sites. At higher coverages oxygen adsorbs both under the surface and up. Our calculations predict island formation and clustering of incorporated and adsorbed oxygen in agreement with previous calculations. The calculated configurations are compared with results from angle-scanned x-ray photoelectron diffraction experiments to determine the geometrical structure of the oxidized Mg(0001) surface.

Assessment of heterochiral and homochiral glycine adlayers on Cu(1 1 0) using density functional theory

We have used plane wave density functional theory (DFT) to determine the structure and relative energy of two possible (3 × 2) glycine adlayers on Cu(1 1 0), namely, heterochiral and homochiral adlayers. Our calculated heterochiral structure is in quantitative agreement with previous X-ray photoelectron diffraction experiments, but our calculated homochiral structure is inconsistent with this experimental data. Our calculations also indicate that the heterochiral (3 × 2) adlayer is significantly more energetically stable than the homochiral (3 × 2) adlayer. Our results indicate that large regions of the homochiral (3 × 2) glycine adlayer are unlikely to exist on Cu(1 1 0).

Bond lengths and bond strengths in weak and strong chemisorption: N2, CO, and CO/H on nickel surfaces.

New chemical-state-specific scanned-energy mode photoelectron diffraction experiments and density functional theory calculations, applied to CO, CO/H, and N2 adsorption on Ni(100), show that chemisorption bond length changes associated with large changes in bond strength are small, but those associated with changes in bond order are much larger, and are similar to those found in molecular systems. Specifically, halving the bond strength of atop CO to Ni increases the Ni-C distance by 0.06 A, but halving the bond order (atop to bridge site) at fixed bond strength causes an increase of 0.16 A.

An XPD and LEED study of highly strained ultrathin Ni films on Pd(1 0 0)

The epitaxial growth of ultrathin Ni films on the Pd(100) surface was studied by means of X-ray photoelectron diffraction (XPD) and LEED experiments. In excellent numerical agreement with the predictions of elasticity theory, the data indicate the formation of tetragonally strained Ni epitaxial layers, which subsequently turns into a bulk-like Ni structure as the thickness of approximately 12MLE is exceeded. This study demonstrates that LEED and XPD methodologies are rather complementary in order to have a detailed picture of the evolution of the overlayer structure in different thickness regimes.

Structure of highly strained ultrathin Ni films on Pd( [formula omitted

The epitaxial growth of ultrathin Ni films on the Pd(1 0 0) surface was studied by means of XPS, angle-scanned photoelectron diffraction (XPD) and LEED experiments. The XPD and LEED data indicate that the early stages of deposition are characterised by the formation of a tetragonally strained Ni fcc phase, which subsequently develops into a bulk-like Ni fcc structure as the critical thickness of approximately 12 ML eq is exceeded. The highly strained structure below the critical thickness is associated with subtle electronic structure changes with respect to Ni bulk, as evidenced by the analysis of the satellite structure in the Ni 2p XPS region.

Study of the isotropic contribution to the analysis of photoelectron diffraction experiments at the ALOISA beamline

The angular distribution of the intensity in photoemission experiments is affected by electron diffraction patterns and by a smoothly varying contribution originated by both intrumental details and physical properties of the samples. The peculiar variety of scattering configurations available at the ALOISA beamline experimental station in Trieste stimulated the developement of an analytical description for the smooth angular dependence sustaining the diffraction features. We present here the basic formulae and their application to experimental data taken on the Fe/Cu 3Au(001) system in order to highlight the role of the various parameters included in the distribution function. A specific model for the surface illumination has been developed, and the overlayer thickness and surface roughness have been considered.

X-ray Photoelectron Diffraction: Probing Atom Positions and Molecular Orientation at Surfaces

With the growing need to gain control over atomic scale objects in nanosciences new tools have to be developed. Quantitative structural information is fundamental to the understanding of the physical properties, e.g. for the interpretation of spectroscopic results or as a starting point for theoretical calculations. This review will focus on the angle-resolved photoemission experiment to probe the geometrical structure on the atomic scale. Using soft X-rays shallow core levels can be excited for a chemical analysis of the surface. Photoelectrons emitted from a particular atom are mapped as a function of emission angle in the so-called photoelectron diffraction experiment. From the interference between the photoelectron wave directly reaching the detector and the waves scattered from the neighboring atoms we obtain very direct knowledge of the local real-space environment of the emitting atom. Photoelectron diffraction is able to determine atom positions at surfaces, to distinguish between different crystallographic surface structures, to identify impurity or dopant site positions or to unravel molecular orientation.

A soft X-ray beamline for surface chemistry at the Photon Factory

A new soft X-ray (50–1500 eV) beamline has been constructed at a bending magnet station at the Photon Factory in order to perform photoemission spectroscopy (PES), X-ray absorption fine structure (XAFS), photoelectron diffraction (PED) and X-ray magnetic circular dichroism (XMCD) experiments in surface chemistry. Approximately 10 11 photons/s can be obtained with a medium resolution ( E/Δ E∼1000), while E/Δ E∼8000 can be attained at the N K edge with a photon flux of ∼10 9 photons/s. In addition to conventional XAFS, a novel technique ‘energy dispersive NEXAFS (near-edge X-ray absorption fine structure)’ has been successfully developed. The performance of the new beamline is reported and typical examples of its application to surface chemistry are demonstrated.

IMAGING OF DICHROISM IN PHOTOEMISSION ELECTRON MICROSCOPY AT NONMAGNETIC MATERIALS USING CIRCULARLY POLARIZED SOFT X-RAYS

A new approach for investigations of circular dichroism in the angular distribution of photoelectrons (CDAD) is presented. The image contrast using a photoemission line of a certain material is combined with imaging of the angular distribution pattern using a photoemission electron microscope (PEEM). CDAD can be used to investigate pure scattering information by means of the same instrument in microscopically selected regions on a surface. The experiment combines angle-resolved XPS imaging with the indirect mapping of the local environment of atoms by means of CDAD holography. In a conventional photoelectron diffraction or photoelectron holography experiment, it is necessary to move the sample and/or the detector; for example, a rotatable electron analyzer is used to map a full angular pattern. In the present approach, the diffraction plane is mapped for a certain kinetic energy of electrons in the backfocal plane. An imaging high-pass energy filter has been used in combination with the PEEM in order to acquire surface images and/or diffraction pattern maps. Direct photoemission from the W-4f core levels was observed at different photon polarization to obtain two images and two diffraction patterns of the W-4f emission line using light of opposite helicity. The agreement of calculations with the experimental result is satisfactory. This constitutes a very encouraging starting point for the use of the CDAD holography together with the PEEM technique providing mesoscopic and atomic as well as chemical resolution in microselected areas on solid surfaces.